# Learning the Basics

version 8.10.2

## Dependency Management

Software projects rarely work in isolation. Projects often rely on reusable functionality from libraries. Some projects organize unrelated functionality into separate parts of a modular system.

Dependency management is an automated technique for declaring, resolving, and using functionality required by a project.

| For an overview of dependency management terms, see Dependency Management Terminology.

### Dependency Management in Gradle

Figure 1. Dependencies management at a glance

Gradle has built-in support for dependency management.

Let’s explore the main concepts with the help of a theoretical but common project:

- This project builds Java source code.

- Some Java source files import classes from the Google Guava library.

- This project uses JUnit for testing.

The Gradle build file might look as follows:

Example 1. Gradle build file with dependencies
build.gradle.kts

    plugins {
        `java-library`
    }

    repositories { (1)
        google()
        mavenCentral()
    }

    val customConfiguration by configurations.creating (3)

    dependencies { (2)
        implementation("com.google.guava:guava:32.1.2-jre")
        testImplementation("junit:junit:4.13.2")
        customConfiguration("org.eclipse.jgit:org.eclipse.jgit:4.9.2.201712150930-r")

        constraints { (4)
            api("org.apache.juneau:juneau-marshall:8.2.0")
        }
    }

build.gradle

    plugins {
        id 'java-library'
    }

    repositories { (1)
        google()
        mavenCentral()
    }

    configurations { (3)
        customConfiguration
    }

    dependencies { (2)
        implementation 'com.google.guava:guava:32.1.2-jre'
        testImplementation 'junit:junit:4.13.2'
        customConfiguration 'org.eclipse.jgit:org.eclipse.jgit:4.9.2.201712150930-r'

        constraints { (4)
            api 'org.apache.juneau:juneau-marshall:8.2.0'
        }
    }

1 | Here we define repositories for the project.
2 | Here we declare remote and local repositories for dependency locations. You can declare repositories to tell Gradle where to fetch local or remote dependencies. In this example, Gradle fetches dependencies from the Maven Central and Google repositories. During a build, Gradle locates and downloads the dependencies, a process called dependency resolution. Gradle then stores resolved dependencies in a local cache called the dependency cache. Subsequent builds use this cache to avoid unnecessary network calls and speed up the build process.
3 | Here we define dependencies used by the project.
4 | Here we declare the specific dependency name and version within a scope. You can add code to your Java project from an external library such as `com.google.common.base` (a Guava package) which becomes a dependency. In this example, the theoretical project uses Guava version 32.1.2-jre and JUnit 4.13.2 as dependencies. A build engineer can declare dependencies for different scopes. For example, you can declare dependencies that are only used at compile time. Gradle calls the scope of a dependency a configuration.

Repositories offer dependencies in multiple formats. For information about the formats supported by Gradle, see dependency types.

Metadata describes dependencies. Some examples of metadata include:

- coordinates for finding the dependency in a repository

- information about the project that created the dependency

- the authors of the dependency

- other dependencies required for a dependency to work properly, known as transitive dependencies

You can customize Gradle’s handling of transitive dependencies based on the requirements of a project.

Projects with hundreds of declared dependencies can be difficult to debug. Gradle provides tools to visualize and analyze a project’s dependency graph (i.e. dependency tree). You can use a Build Scan™ or built-in tasks.

Figure 2. Build scan dependencies report

## 3. Declaring repositories

Gradle needs to know where it can download the dependencies used in the project.

For example, the `com.google.guava:guava:30.0-jre` dependency can be downloaded from the public Maven Central repository `mavenCentral()`. Gradle will find and download the `guava` source code (as a `jar`) from Maven Central and use it build the project.

You can add any number of repositories for your dependencies by configuring the `repositories` block in your `build.gradle(.kts)` file:

build.gradle.kts

    repositories {
        mavenCentral()  (1)
        maven {         (2)
            url = uri("https://company/com/maven2")
        }
        mavenLocal()    (3)
        flatDir {       (4)
            dirs("libs")
        }
    }

1 | Public repository
2 | Private/Custom repository
3 | Local repository
4 | File location

build.gradle

    repositories {
        mavenCentral()  (1)
        maven {         (2)
            url = uri("https://company/com/maven2")
        }
        mavenLocal()    (3)
        flatDir {       (4)
            dirs "libs"
        }
    }

1 | Public repository
2 | Private/Custom repository
3 | Local repository
4 | File location

Gradle can resolve dependencies from one or many repositories based on Maven, Ivy or flat directory formats.

If a library is available from more than one of the listed repositories, Gradle will simply pick the first one.

### Declaring a public repository

Organizations building software may want to leverage public binary repositories to download and consume open source dependencies. Popular public repositories include Maven Central and the Google Android repository.

Gradle provides built-in shorthand notations for these widely-used repositories:

build.gradle.kts

    repositories {
        mavenCentral()
        google()
        gradlePluginPortal()
    }

build.gradle

    repositories {
        mavenCentral()
        google()
        gradlePluginPortal()
    }

Under the covers Gradle resolves dependencies from the respective URL of the public repository defined by the shorthand notation. All shorthand notations are available via the RepositoryHandler API.

### Declaring a private or custom repository

Most enterprise projects establish a binary repository accessible only within their intranet. In-house repositories allow teams to publish internal binaries, manage users and security, and ensure uptime and availability.

Specifying a custom URL is useful for declaring less popular but publicly-available repositories. Repositories with custom URLs can be specified as Maven or Ivy repositories by calling the corresponding methods available on the RepositoryHandler API:

build.gradle.kts

    repositories {
        maven {
            url = uri("https://maven-central.storage.apis.com")
        }
        ivy {
            url = uri("https://github.com/ivy-rep/")
        }
    }

build.gradle

    repositories {
        maven {
            url = uri("https://maven-central.storage.apis.com")
        }
        ivy {
            url = uri("https://github.com/ivy-rep/")
        }
    }

### Declaring a local repository

Gradle can consume dependencies available in a local Maven repository.

To declare the local Maven cache as a repository, add this to your build script:

build.gradle.kts

    repositories {
        mavenLocal()
    }

build.gradle

    repositories {
        mavenLocal()
    }

### Understanding supported repository types

Gradle supports a wide range of sources for dependencies, both in terms of format and in terms of connectivity. You may resolve dependencies from:

- Different formats

  - a Maven compatible artifact repository (e.g: Maven Central)

  - an Ivy compatible artifact repository (including custom layouts)

  - local (flat) directories

- with different connectivity

  - authenticated repositories

  - a wide variety of remote protocols such as HTTPS, SFTP, AWS S3 and Google Cloud Storage based on the presence of artifacts.

Here is a quick snapshot:

build.gradle

    repositories {

        // Ivy Repository with Custom Layout
        ivy {
            url 'https://your.ivy.repo/url'
            layout 'pattern', {
                ivy '[organisation]/[module]/[revision]/[type]s/[artifact]-[revision].[ext]'
                artifact '[organisation]/[module]/[revision]/[type]s/[artifact]-[revision].[ext]'
            }
        }

        // Authenticated HTTPS Maven Repository
        maven {
            url 'https://your.secure.repo/url'
            credentials {
                username = 'your-username'
                password = 'your-password'
            }
        }

        // SFTP Repository
        maven {
            url 'sftp://your.sftp.repo/url'
            credentials {
                username = 'your-username'
                password = 'your-password'
            }
        }

        // AWS S3 Repository
        maven {
            url "s3://your-bucket/repository-path"
            credentials(AwsCredentials) {
                accessKey = 'your-access-key'
                secretKey = 'your-secret-key'
            }
        }

        // Google Cloud Storage Repository
        maven {
            url "gcs://your-bucket/repository-path"
        }
    }

Next Step: Learn about Centralizing Dependencies >>

## 1. Declaring dependencies

Declaring dependencies in Gradle involves specifying libraries or files that your project depends on.

### Understanding producers and consumers

In dependency management, it is essential to understand the distinction between producers and consumers.

When you build a library, you are acting as a producer, creating artifacts that will be consumed by others, the consumers.

When you depend on that library, you are acting as a consumer. Consumers can be broadly defined as:

- Projects that depend on other projects.

- Configurations that declare dependencies on specific artifacts.

The decisions we make in dependency management often depend on the type of project we are building, specifically, what kind of consumer we are.

### Adding a dependency

To add a dependency in Gradle, you use the `dependencies{}` block in your build script.

The `dependencies` block allows you to specify various types of dependencies such as external libraries, local JAR files, or other projects within a multi-project build.

External dependencies in Gradle are declared using a configuration name (e.g., `implementation`, `compileOnly`, `testImplementation`) followed by the dependency notation, which includes the group ID (group), artifact ID (name), and version.

build.gradle

    dependencies {
        // Configuration Name + Dependency Notation - GroupID : ArtifactID (Name) : Version
        configuration('<group>:<name>:<version>')
    }

Note:

1. Gradle automatically includes transitive dependencies, which are dependencies of your dependencies.

2. Gradle offers several configuration options for dependencies, which define the scope in which dependencies are used, such as compile-time, runtime, or test-specific scenarios.

3. You can specify the repositories where Gradle should look for dependencies in your build file.

### Understanding types of dependencies

There are three kinds of dependencies, module dependencies, project dependencies, and file dependencies.

#### 1. Module dependencies

Module dependencies are the most common dependencies. They refer to a module in a repository:

build.gradle.kts

    dependencies {
        implementation("org.codehaus.groovy:groovy:3.0.5")
        implementation("org.codehaus.groovy:groovy-json:3.0.5")
        implementation("org.codehaus.groovy:groovy-nio:3.0.5")
    }

build.gradle

    dependencies {
        implementation 'org.codehaus.groovy:groovy:3.0.5'
        implementation 'org.codehaus.groovy:groovy-json:3.0.5'
        implementation 'org.codehaus.groovy:groovy-nio:3.0.5'
    }

#### 2. Project dependencies

Project dependencies allow you to declare dependencies on other projects within the same build. This is useful in multi-project builds where multiple projects are part of the same Gradle build.

Project dependencies are declared by referencing the project path:

build.gradle.kts

    dependencies {
        implementation(project(":utils"))
        implementation(project(":api"))
    }

build.gradle

    dependencies {
        implementation project(':utils')
        implementation project(':api')
    }

#### 3. File dependencies

In some projects, you might not rely on binary repository products like JFrog Artifactory or Sonatype Nexus for hosting and resolving external dependencies. Instead, you might host these dependencies on a shared drive or to check them into version control alongside the project source code.

These are known as file dependencies because they represent files without any metadata (such as information about transitive dependencies, origin, or author) attached to them.

To add files as dependencies for a configuration, you simply pass a file collection as a dependency:

build.gradle.kts

    dependencies {
        runtimeOnly(files("libs/a.jar", "libs/b.jar"))
        runtimeOnly(fileTree("libs") { include("*.jar") })
    }

build.gradle

    dependencies {
        runtimeOnly files('libs/a.jar', 'libs/b.jar')
        runtimeOnly fileTree('libs') { include '*.jar' }
    }

| It is recommended to use project dependencies or external dependencies over file dependencies.

### Looking at an example

Let’s imagine an example for a Java application which uses Guava, a set of core Java libraries from Google:

The Java app contains the following Java class:

InitializeCollection.java

    package org.example;

    import com.google.common.collect.ImmutableMap;  // Comes from the Guava library

    public class InitializeCollection {
        public static void main(String[] args) {
            ImmutableMap<String, Integer> immutableMap
                = ImmutableMap.of("coin", 3, "glass", 4, "pencil", 1);
        }
    }

To add the Guava library to your Gradle project as a dependency, you must add the following line to your build file:

build.gradle.kts

    dependencies {
        implementation("com.google.guava:guava:23.0")
    }

build.gradle

Where:

- `implementation` is the configuration.

- `com.google.guava:guava:23.0` specifies the group, name, and version of the library:

  - `com.google.guava` is the group ID.

  - `guava` is the artifact ID (i.e., name).

  - `23.0` is the version.

Take a quick look at the Guava page in Maven Central as a reference.

### Listing project dependencies

The `dependencies` task provides an overview of the dependencies of your project. It helps you understand what dependencies are being used, how they are resolved, and their relationships, including any transitive dependencies by rendering a dependency tree from the command line.

This task can be particularly useful for debugging dependency issues, such as version conflicts or missing dependencies.

For example, let’s say our `app` project contains the follow lines in its build script:

build.gradle.kts

    dependencies {
        implementation("com.google.guava:guava:30.0-jre")
        runtimeOnly("org.apache.commons:commons-lang3:3.14.0")
    }

build.gradle

    dependencies {
        implementation("com.google.guava:guava:30.0-jre")
        runtimeOnly("org.apache.commons:commons-lang3:3.14.0")
    }

Running the `dependencies` task on the `app` project yields the following:

    $ ./gradlew app:dependencies

    > Task :app:dependencies

    ------------------------------------------------------------
    Project ':app'
    ------------------------------------------------------------

    implementation - Implementation dependencies for the 'main' feature. (n)
    \--- com.google.guava:guava:30.0-jre (n)

    runtimeClasspath - Runtime classpath of source set 'main'.
    +--- com.google.guava:guava:30.0-jre
    |    +--- com.google.guava:failureaccess:1.0.1
    |    +--- com.google.guava:listenablefuture:9999.0-empty-to-avoid-conflict-with-guava
    |    +--- com.google.code.findbugs:jsr305:3.0.2
    |    +--- org.checkerframework:checker-qual:3.5.0
    |    +--- com.google.errorprone:error_prone_annotations:2.3.4
    |    \--- com.google.j2objc:j2objc-annotations:1.3
    \--- org.apache.commons:commons-lang3:3.14.0

    runtimeOnly - Runtime-only dependencies for the 'main' feature. (n)
    \--- org.apache.commons:commons-lang3:3.14.0 (n)

We can clearly see that for the `implementation` configuration, the `com.google.guava:guava:30.0-jre` dependency has been added. As for the `runtimeOnly` configuration, the `org.org.apache.commons:commons-lang3:3.14.0` dependency has been added.

We also see a list of transitive dependencies for `com.google.guava:guava:30.0-jre` (which are the dependencies for the `guava` library), such as `com.google.guava:failureaccess:1.0.1` in the `runtimeClasspath` configuration.

Next Step: Learn about Dependency Configurations >>

## Understanding the difference between libraries and applications

### Producers vs consumers

A key concept in dependency management with Gradle is the difference between consumers and producers.

When you build a library, you are effectively on the producer side: you are producing artifacts which are going to be consumed by someone else, the consumer.

A lot of problems with traditional build systems is that they don’t make the difference between a producer and a consumer.

A consumer needs to be understood in the large sense:

- a project that depends on another project is a consumer

- a task that depends on an artifact is a finer grained consumer

In dependency management, a lot of the decisions we make depend on the type of project we are building, that is to say, what kind of consumer we are.

### Producer variants

A producer may want to generate different artifacts for different kinds of consumers: for the same source code, different binaries are produced. Or, a project may produce artifacts which are for consumption by other projects (same repository) but not for external use.

A typical example in the Java world is the Guava library which is published in different versions: one for Java projects, and one for Android projects.

However, it’s the consumer responsibility to tell what version to use, and it’s the dependency management engine responsibility to ensure consistency of the graph (for example making sure that you don’t end up with both Java and Android versions of Guava on your classpath). This is where the variant model of Gradle comes into play.

In Gradle, producer variants are exposed via consumable configurations.

### Strong encapsulation

In order for a producer to compile a library, it needs all its implementation dependencies on the compile classpath. There are dependencies which are only required as an implementation detail of the library and there are libraries which are effectively part of the API.

However, a library depending on this produced library only needs to "see" the public API of your library and therefore the dependencies of this API. It’s a subset of the compile classpath of the producer: this is strong encapsulation of dependencies.

The consequence is that a dependency which is assigned to the `implementation` configuration of a library does not end up on the compile classpath of the consumer. On the other hand, a dependency which is assigned to the `api` configuration of a library would end up on the compile classpath of the consumer. At runtime, however, all dependencies are required. Gradle makes the difference between different kinds of consumer even within a single project: the Java compile task, for example, is a different consumer than the Java exec task.

More details on the segregation of API and runtime dependencies in the Java world can be found here.

### Being respectful of consumers

Whenever, as a developer, you decide to include a dependency, you must understand that there are consequences for your consumers. For example, if you add a dependency to your project, it becomes a transitive dependency of your consumers, and therefore may participate in conflict resolution if the consumer needs a different version.

A lot of the problems Gradle handles are about fixing the mismatch between the expectations of a consumer and a producer.

However, some projects are easier than others:

- if you are at the end of the consumption chain, that is to say you build an application, then there are effectively no consumer of your project (apart from final customers): adding exclusions will have no other consequence than fixing your problem.

- however if you are a library, adding exclusions may prevent consumers from working properly, because they would exercise a path of the code that you don’t

Always keep in mind that the solution you choose to fix a problem can "leak" to your consumers. This documentation aims at guiding you to find the right solution to the right problem, and more importantly, make decisions which help the resolution engine to take the right decisions in case of conflicts.

## View and Debug Dependencies

Gradle provides tooling to navigate dependency graphs and mitigate dependency hell. Users can render the full graph of dependencies as well as identify the selection reason and origin for a dependency. Dependencies can originate through build script declared dependencies or transitive dependencies. You can visualize dependencies with:

- the built-in Gradle CLI `dependencies` task

- the built-in Gradle CLI `dependencyInsight` task

- build scans

### List Project Dependencies

Gradle provides the built-in `dependencies` task to render a dependency tree from the command line. By default, the dependency tree renders dependencies for all configurations within a single project. The dependency tree indicates the selected version of each dependency. It also displays information about dependency conflict resolution.

The `dependencies` task can be especially helpful for issues related to transitive dependencies. Your build file lists direct dependencies, but the `dependencies` task can help you understand which transitive dependencies resolve during your build.

| Graph of dependencies declared in the `buildscript` `classpath` configuration can be rendered using task `buildEnvironment`.

#### Output Annotations

The `dependencies` task marks dependency trees with the following annotations:

- `(*)`: Indicates repeated occurrences of a transitive dependency subtree. Gradle expands transitive dependency subtrees only once per project; repeat occurrences only display the root of the subtree, followed by this annotation.

- `(c)`: This element is a dependency constraint, not a dependency. Look for the matching dependency elsewhere in the tree.

- `(n)`: A dependency or dependency configuration that cannot be resolved.

#### Specify a Dependency Configuration

To focus on the information about one dependency configuration, provide the optional parameter `--configuration`. Just like project and task names, Gradle accepts abbreviated names to select a dependency configuration. For example, you can specify `tRC` instead of `testRuntimeClasspath` if the pattern matches to a single dependency configuration. Both of the following examples show dependencies in the `testRuntimeClasspath` dependency configuration of a Java project:

    > gradle -q dependencies --configuration testRuntimeClasspath

    > gradle -q dependencies --configuration tRC

To see a list of all the configurations available in a project, including those added by any plugins, you can run a `resolvableConfigurations` report.

For more info, see that plugin’s documentation (for instance, the Java Plugin is documented here).

#### Example

Consider a project that uses the JGit library to execute Source Control Management (SCM) operations for a release process. You can declare dependencies for external tooling with the help of a custom dependency configuration. This avoids polluting other contexts, such as the compilation classpath for your production source code.

The following example declares a custom dependency configuration named "scm" that contains the JGit dependency:

build.gradle.kts

    configurations {
        create("scm")
    }

    dependencies {
        "scm"("org.eclipse.jgit:org.eclipse.jgit:4.9.2.201712150930-r")
    }

build.gradle

    configurations {
        scm
    }

    dependencies {
        scm 'org.eclipse.jgit:org.eclipse.jgit:4.9.2.201712150930-r'
    }

Use the following command to view a dependency tree for the `scm` dependency configuration:

    > gradle -q dependencies --configuration scm

    ------------------------------------------------------------
    Root project 'dependencies-report'
    ------------------------------------------------------------

    scm
    \--- org.eclipse.jgit:org.eclipse.jgit:4.9.2.201712150930-r
         +--- com.jcraft:jsch:0.1.54
         +--- com.googlecode.javaewah:JavaEWAH:1.1.6
         +--- org.apache.httpcomponents:httpclient:4.3.6
         |    +--- org.apache.httpcomponents:httpcore:4.3.3
         |    +--- commons-logging:commons-logging:1.1.3
         |    \--- commons-codec:commons-codec:1.6
         \--- org.slf4j:slf4j-api:1.7.2

    A web-based, searchable dependency report is available by adding the --scan option.

### Identify the Dependency Version Selected

A project may request two different versions of the same dependency either directly or transitively. Gradle applies version conflict resolution to ensure that only one version of the dependency exists in the dependency graph. The following example introduces a conflict with `commons-codec:commons-codec`, added both as a direct dependency and a transitive dependency of JGit:

build.gradle.kts

    repositories {
        mavenCentral()
    }

    configurations {
        create("scm")
    }

    dependencies {
        "scm"("org.eclipse.jgit:org.eclipse.jgit:4.9.2.201712150930-r")
        "scm"("commons-codec:commons-codec:1.7")
    }

build.gradle

    repositories {
        mavenCentral()
    }

    configurations {
        scm
    }

    dependencies {
        scm 'org.eclipse.jgit:org.eclipse.jgit:4.9.2.201712150930-r'
        scm 'commons-codec:commons-codec:1.7'
    }

The dependency tree in a build scan shows information about conflicts. Click on a dependency and select the "Required By" tab to see the selection reason and origin of the dependency.

#### Dependency Insights

Gradle provides the built-in `dependencyInsight` task to render a dependency insight report from the command line. Dependency insights provide information about a single dependency within a single configuration. Given a dependency, you can identify the selection reason and origin.

`dependencyInsight` accepts the following parameters:

`--dependency <dependency>` (mandatory)

  The dependency to investigate. You can supply a complete `group:name`, or part of it. If multiple dependencies match, Gradle generates a report covering all matching dependencies.

`--configuration <name>` (mandatory)

  The dependency configuration which resolves the given dependency. This parameter is optional for projects that use the Java plugin, since the plugin provides a default value of `compileClasspath`.

`--single-path` (optional)

  Render only a single path to the dependency.

`--all-variants` (optional)

  Render information about all variants, not only the selected variant.

The following code snippet demonstrates how to run a dependency insight report for all paths to a dependency named "commons-codec" within the "scm" configuration:

    > gradle -q dependencyInsight --dependency commons-codec --configuration scm
    commons-codec:commons-codec:1.7
      Variant default:
        | Attribute Name    | Provided | Requested |
        |-------------------|----------|-----------|
        | org.gradle.status | release  |           |
       Selection reasons:
          - By conflict resolution: between versions 1.7 and 1.6

    commons-codec:commons-codec:1.7
    \--- scm

    commons-codec:commons-codec:1.6 -> 1.7
    \--- org.apache.httpcomponents:httpclient:4.3.6
         \--- org.eclipse.jgit:org.eclipse.jgit:4.9.2.201712150930-r
              \--- scm

    A web-based, searchable dependency report is available by adding the --scan option.

For more information about configurations, see the dependency configuration documentation.

##### Selection Reasons

The "Selection reasons" section of the dependency insight report lists the reasons why a dependency was selected. Have a look at the table below to understand the meaning of the different terms used:

Table 1. Terminology
Reason | Meaning
--- | ---
(Absent) | No reason other than a reference, direct or transitive, was present.
Was requested : <text> | The dependency appears in the graph, and the inclusion came with a `because` text.
Was requested : didn’t match versions <versions> | The dependency appears with a dynamic version which did not include the listed versions. May be followed by a `because` text.
Was requested : reject version <versions> | The dependency appears with a rich version containing one or more `reject`. May be followed by a `because` text.
By conflict resolution : between versions <version> | The dependency appeared multiple times, with different version requests. This resulted in conflict resolution to select the most appropriate version.
By constraint | A dependency constraint participated in the version selection. May be followed by a `because` text.
By ancestor | There is a rich version with a `strictly` which enforces the version of this dependency.
Selected by rule | A dependency resolution rule overruled the default selection process. May be followed by a `because` text.
Rejection : <version> by rule because <text> | A `ComponentSelection.reject` rejected the given version of the dependency.
Rejection: version <version>: <attributes information> | The dependency has a dynamic version and some versions did not match the requested attributes.
Forced | The build enforces the version of the dependency through an enforced platform or resolution strategy.

If multiple selection reasons exist, the insight report lists all of them.

### Troubleshooting

#### Version Conflicts

If the selected version does not match your expectation, Gradle offers a series of tools to help you control transitive dependencies.

#### Variant Selection Errors

Sometimes a selection error happens at the variant selection level. Have a look at the dedicated section to understand these errors and how to resolve them.

#### Unsafe Configuration Resolution Errors

Resolving a configuration can have side effects on Gradle’s project model. As a result, Gradle must manage access to each project’s configurations. There are a number of ways a configuration might be resolved unsafely. For example:

- A task from one project directly resolves a configuration in another project in the task’s action.

- A task specifies a configuration from another project as an input file collection.

- A build script for one project resolves a configuration in another project during evaluation.

- Project configurations are resolved in the settings file.

Gradle produces a deprecation warning for each unsafe access. Unsafe access can cause indeterminate errors. You should fix unsafe access warnings in your build.

In most cases, you can resolve unsafe accesses by creating a cross-project dependency on the other project. See the documentation for sharing outputs between projects for more information.

If you find a use case that can’t be resolved using these techniques, please let us know by filing a GitHub Issue.

## Understanding dependency resolution

This chapter covers the way dependency resolution works inside Gradle. After covering how you can declare repositories and dependencies, it makes sense to explain how these declarations come together during dependency resolution.

Dependency resolution is a process that consists of two phases, which are repeated until the dependency graph is complete:

- When a new dependency is added to the graph, perform conflict resolution to determine which version should be added to the graph.

- When a specific dependency, that is a module with a version, is identified as part of the graph, retrieve its metadata so that its dependencies can be added in turn.

The following section will describe what Gradle identifies as conflicts and how it can resolve them automatically. After that, the retrieval of metadata will be covered, explaining how Gradle can follow dependency links.

### How Gradle handles conflicts?

When doing dependency resolution, Gradle handles two types of conflicts:

Version conflicts

  That is when two or more dependencies require a given dependency but with different versions.

Implementation conflicts

  That is when the dependency graph contains multiple modules that provide the same implementation, or capability in Gradle terminology.

The following sections will explain in detail how Gradle attempts to resolve these conflicts.

The dependency resolution process is highly customizable to meet enterprise requirements. For more information, see the chapter on Controlling transitive dependencies.

### Version conflict resolution

A version conflict occurs when two components:

- Depend on the same module, let’s say `com.google.guava:guava`

- But on different versions, let’s say `20.0` and `25.1-android`

  - Our project itself depends on `com.google.guava:guava:20.0`

  - Our project also depends on `com.google.inject:guice:4.2.2` which itself depends on `com.google.guava:guava:25.1-android`

#### Resolution strategy

Given the conflict above, there exist multiple ways to handle it, either by selecting a version or failing the resolution. Different tools that handle dependency management have different ways of handling these type of conflicts.

Apache Maven uses a nearest first strategy.

Maven will take the shortest path to a dependency and use that version. In case there are multiple paths of the same length, the first one wins.

This means that in the example above, the version of `guava` will be `20.0` because the direct dependency is closer than the `guice` dependency.

The main drawback of this method is that it is ordering dependent. Keeping order in a very large graph can be a challenge. For example, what if the new version of a dependency ends up having its own dependency declarations in a different order than the previous version?

With Maven, this could have unwanted impact on resolved versions.

| Apache Ivy is a very flexible dependency management tool. It offers the possibility to customize dependency resolution, including conflict resolution. This flexibility comes with the price of making it hard to reason about.

Gradle will consider all requested versions, wherever they appear in the dependency graph. By default, it will select the highest version. More information on version ordering here.

As you have seen, Gradle supports a concept of rich version declaration, so what is the highest version depends on the way versions were declared:

- If no ranges are involved, then the highest version that is not rejected will be selected.

  - If a version declared as `strictly` is lower than that version, selection will fail.

- If ranges are involved:

  - If there is a non range version that falls within the specified ranges or is higher than their upper bound, it will be selected.

  - If there are only ranges, the selection will depend on the intersection of ranges:

    - If all the ranges intersect, then the highest existing version of the intersection will be selected.

    - If there is no clear intersection between all the ranges, the highest existing version will be selected from the highest range. If there is no version available for the highest range, the resolution will fail.

  - If a version declared as `strictly` is lower than that version, selection will fail.

Note that in the case where ranges come into play, Gradle requires metadata to determine which versions do exist for the considered range. This causes an intermediate lookup for metadata, as described in How Gradle retrieves dependency metadata?.

#### Qualifiers

There is a caveat to comparing versions when it comes to selecting the highest one. All the rules of version ordering still apply, but the conflict resolver has a bias towards versions without qualifiers.

The "qualifier" of a version, if it exists, is the tail end of the version string, starting at the first non-dot separator found in it. The other (first) part of the version string is called the "base form" of the version. Here are some examples to illustrate:

Original version | Base version | Qualifier
--- | --- | ---
1.2.3 | 1.2.3 | <none>
1.2-3 | 1.2 | 3
1_alpha | 1 | alpha
abc | abc | <none>
1.2b3 | 1.2 | b3
abc.1+3 | abc.1 | 3
b1-2-3.3 | b | 1-2-3.3

As you can see separators are any of the `.`, `-`, `_`, `+` characters, plus the empty string when a numeric and a non-numeric part of the version are next to each-other.

When resolving the conflict between competing versions, the following logic applies:

- first the versions with the highest base version are selected, the rest are discarded

- if there are still multiple competing versions left, then one is picked with a preference for not having a qualifier or having release status.

### Implementation conflict resolution

Gradle uses variants and capabilities to identify what a module provides.

This is a unique feature that deserves its own chapter to understand what it means and enables.

A conflict occurs the moment two modules either:

- Attempt to select incompatible variants,

- Declare the same capability

Learn more about handling these type of conflicts in Selecting between candidates.

### How Gradle retrieves dependency metadata?

Gradle requires metadata about the modules included in your dependency graph. That information is required for two main points:

- Determine the existing versions of a module when the declared version is dynamic.

- Determine the dependencies of the module for a given version.

#### Discovering versions

Faced with a dynamic version, Gradle needs to identify the concrete matching versions:

- Each repository is inspected, Gradle does not stop on the first one returning some metadata. When multiple are defined, they are inspected in the order they were added.

- For Maven repositories, Gradle will use the `maven-metadata.xml` which provides information about the available versions.

- For Ivy repositories, Gradle will resort to directory listing.

This process results in a list of candidate versions that are then matched to the dynamic version expressed. At this point, version conflict resolution is resumed.

Note that Gradle caches the version information, more information can be found in the section Controlling dynamic version caching.

#### Obtaining module metadata

Given a required dependency, with a version, Gradle attempts to resolve the dependency by searching for the module the dependency points at.

- Each repository is inspected in order.

  - Depending on the type of repository, Gradle looks for metadata files describing the module (`.module`, `.pom` or `ivy.xml` file) or directly for artifact files.

  - Modules that have a module metadata file (`.module`, `.pom` or `ivy.xml` file) are preferred over modules that have an artifact file only.

  - Once a repository returns a metadata result, following repositories are ignored.

- Metadata for the dependency is retrieved and parsed, if found

  - If the module metadata is a POM file that has a parent POM declared, Gradle will recursively attempt to resolve each of the parent modules for the POM.

- All of the artifacts for the module are then requested from the same repository that was chosen in the process above.

- All of that data, including the repository source and potential misses are then stored in the The Dependency Cache.

| The penultimate point above is what can make the integration with Maven Local problematic. As it is a cache for Maven, it will sometimes miss some artifacts of a given module. If Gradle is sourcing such a module from Maven Local, it will consider the missing artifacts to be missing altogether.

#### Repository disabling

When Gradle fails to retrieve information from a repository, it will disable it for the duration of the build and fail all dependency resolution.

That last point is important for reproducibility. If the build was allowed to continue, ignoring the faulty repository, subsequent builds could have a different result once the repository is back online.

##### HTTP Retries

Gradle will make several attempts to connect to a given repository before disabling it. If connection fails, Gradle will retry on certain errors which have a chance of being transient, increasing the amount of time waiting between each retry.

Blacklisting happens when the repository cannot be contacted, either because of a permanent error or because the maximum retries was reached.

### The Dependency Cache

Gradle contains a highly sophisticated dependency caching mechanism, which seeks to minimise the number of remote requests made in dependency resolution, while striving to guarantee that the results of dependency resolution are correct and reproducible.

The Gradle dependency cache consists of two storage types located under `$GRADLE_USER_HOME/caches`:

- A file-based store of downloaded artifacts, including binaries like jars as well as raw downloaded meta-data like POM files and Ivy files. The storage path for a downloaded artifact includes the SHA1 checksum, meaning that 2 artifacts with the same name but different content can easily be cached.

- A binary store of resolved module metadata, including the results of resolving dynamic versions, module descriptors, and artifacts.

The Gradle cache does not allow the local cache to hide problems and create other mysterious and difficult to debug behavior. Gradle enables reliable and reproducible enterprise builds with a focus on bandwidth and storage efficiency.

#### Separate metadata cache

Gradle keeps a record of various aspects of dependency resolution in binary format in the metadata cache. The information stored in the metadata cache includes:

- The result of resolving a dynamic version (e.g. `1.+`) to a concrete version (e.g. `1.2`).

- The resolved module metadata for a particular module, including module artifacts and module dependencies.

- The resolved artifact metadata for a particular artifact, including a pointer to the downloaded artifact file.

- The absence of a particular module or artifact in a particular repository, eliminating repeated attempts to access a resource that does not exist.

Every entry in the metadata cache includes a record of the repository that provided the information as well as a timestamp that can be used for cache expiry.

#### Repository caches are independent

As described above, for each repository there is a separate metadata cache. A repository is identified by its URL, type and layout. If a module or artifact has not been previously resolved from this repository, Gradle will attempt to resolve the module against the repository. This will always involve a remote lookup on the repository, however in many cases no download will be required.

Dependency resolution will fail if the required artifacts are not available in any repository specified by the build, even if the local cache has a copy of this artifact which was retrieved from a different repository. Repository independence allows builds to be isolated from each other in an advanced way that no build tool has done before. This is a key feature to create builds that are reliable and reproducible in any environment.

#### Artifact reuse

Before downloading an artifact, Gradle tries to determine the checksum of the required artifact by downloading the sha file associated with that artifact. If the checksum can be retrieved, an artifact is not downloaded if an artifact already exists with the same id and checksum. If the checksum cannot be retrieved from the remote server, the artifact will be downloaded (and ignored if it matches an existing artifact).

As well as considering artifacts downloaded from a different repository, Gradle will also attempt to reuse artifacts found in the local Maven Repository. If a candidate artifact has been downloaded by Maven, Gradle will use this artifact if it can be verified to match the checksum declared by the remote server.

#### Checksum based storage

It is possible for different repositories to provide a different binary artifact in response to the same artifact identifier. This is often the case with Maven SNAPSHOT artifacts, but can also be true for any artifact which is republished without changing its identifier. By caching artifacts based on their SHA1 checksum, Gradle is able to maintain multiple versions of the same artifact. This means that when resolving against one repository Gradle will never overwrite the cached artifact file from a different repository. This is done without requiring a separate artifact file store per repository.

#### Cache Locking

The Gradle dependency cache uses file-based locking to ensure that it can safely be used by multiple Gradle processes concurrently. The lock is held whenever the binary metadata store is being read or written, but is released for slow operations such as downloading remote artifacts.

This concurrent access is only supported if the different Gradle processes can communicate together. This is usually not the case for containerized builds.

##### Cache Cleanup

Gradle keeps track of which artifacts in the dependency cache are accessed. Using this information, the cache is periodically (at most every 24 hours) scanned for artifacts that have not been used for more than 30 days. Obsolete artifacts are then deleted to ensure the cache does not grow indefinitely.

### Dealing with ephemeral builds

It’s a common practice to run builds in ephemeral containers. A container is typically spawned to only execute a single build before it is destroyed. This can become a practical problem when a build depends on a lot of dependencies which each container has to re-download. To help with this scenario, Gradle provides a couple of options:

- copying the dependency cache into each container

- sharing a read-only dependency cache between multiple containers

#### Copying and reusing the cache

The dependency cache, both the file and metadata parts, are fully encoded using relative paths. This means that it is perfectly possible to copy a cache around and see Gradle benefit from it.

The path that can be copied is `$GRADLE_USER_HOME/caches/modules-<version>`. The only constraint is placing it using the same structure at the destination, where the value of `GRADLE_USER_HOME` can be different.

Do not copy the `*.lock` or `gc.properties` files if they exist.

Note that creating the cache and consuming it should be done using compatible Gradle version, as shown in the table below. Otherwise, the build might still require some interactions with remote repositories to complete missing information, which might be available in a different version. If multiple incompatible Gradle versions are in play, all should be used when seeding the cache.

Table 2. Dependency cache compatibility
Module cache version | File cache version | Metadata cache version | Gradle version(s)
--- | --- | --- | ---
`modules-2` | `files-2.1` | `metadata-2.95` | Gradle 6.1 to Gradle 6.3
`modules-2` | `files-2.1` | `metadata-2.96` | Gradle 6.4 to Gradle 6.7
`modules-2` | `files-2.1` | `metadata-2.97` | Gradle 6.8 to Gradle 7.4
`modules-2` | `files-2.1` | `metadata-2.99` | Gradle 7.5 to Gradle 7.6.1
`modules-2` | `files-2.1` | `metadata-2.101` | Gradle 7.6.2
`modules-2` | `files-2.1` | `metadata-2.100` | Gradle 8.0
`modules-2` | `files-2.1` | `metadata-2.105` | Gradle 8.1
`modules-2` | `files-2.1` | `metadata-2.106` | Gradle 8.2 and above

#### Sharing the dependency cache with other Gradle instances

Instead of copying the dependency cache into each container, it’s possible to mount a shared, read-only directory that will act as a dependency cache for all containers. This cache, unlike the classical dependency cache, is accessed without locking, making it possible for multiple builds to read from the cache concurrently. It’s important that the read-only cache is not written to when other builds may be reading from it.

When using the shared read-only cache, Gradle looks for dependencies (artifacts or metadata) in both the writable cache in the local Gradle User Home directory and the shared read-only cache. If a dependency is present in the read-only cache, it will not be downloaded. If a dependency is missing from the read-only cache, it will be downloaded and added to the writable cache. In practice, this means that the writable cache will only contain dependencies that are unavailable in the read-only cache.

The read-only cache should be sourced from a Gradle dependency cache that already contains some of the required dependencies. The cache can be incomplete; however, an empty shared cache will only add overhead.

| The shared read-only dependency cache is an incubating feature.

The first step in using a shared dependency cache is to create one by copying of an existing local cache. For this you need to follow the instructions above.

Then set the `GRADLE_RO_DEP_CACHE` environment variable to point to the directory containing the cache:

    $GRADLE_RO_DEP_CACHE
       |-- modules-2 : the read-only dependency cache, should be mounted with read-only privileges

    $GRADLE_HOME
       |-- caches
             |-- modules-2 : the container specific dependency cache, should be writable
             |-- ...
       |-- ...

In a CI environment, it’s a good idea to have one build which "seeds" a Gradle dependency cache, which is then copied to a different directory. This directory can then be used as the read-only cache for other builds. You shouldn’t use an existing Gradle installation cache as the read-only cache, because this directory may contain locks and may be modified by the seeding build.

### Accessing the resolution result programmatically

While most users only need access to a "flat list" of files, there are cases where it can be interesting to reason on a graph and get more information about the resolution result:

- for tooling integration, where a model of the dependency graph is required

- for tasks generating a visual representation (image, `.dot` file, …​) of a dependency graph

- for tasks providing diagnostics (similar to the `dependencyInsight` task)

- for tasks which need to perform dependency resolution at execution time (e.g, download files on demand)

For those use cases, Gradle provides lazy, thread-safe APIs, accessible by calling the Configuration.getIncoming() method:

- the ResolutionResult API gives access to a resolved dependency graph, whether the resolution was successful or not.

- the artifacts API provides a simple access to the resolved artifacts, untransformed, but with lazy download of artifacts (they would only be downloaded on demand).

- the artifact view API provides an advanced, filtered view of artifacts, possibly transformed.

| See the documentation on using dependency resolution results for more details on how to consume the results in a task.

## Verifying dependencies

Working with external dependencies and plugins published on third-party repositories puts your build at risk. In particular, you need to be aware of what binaries are brought in transitively and if they are legit. To mitigate the security risks and avoid integrating compromised dependencies in your project, Gradle supports dependency verification.

Dependency verification is, by nature, an inconvenient feature to use. It means that whenever you’re going to update a dependency, builds are likely to fail. It means that merging branches are going to be harder because each branch can have different dependencies. It means that you will be tempted to switch it off.

So why should you bother?

Dependency verification is about trust in what you get and what you ship.

Without dependency verification it’s easy for an attacker to compromise your supply chain. There are many real world examples of tools compromised by adding a malicious dependency. Dependency verification is meant to protect yourself from those attacks, by forcing you to ensure that the artifacts you include in your build are the ones that you expect. It is not meant, however, to prevent you from including vulnerable dependencies.

Finding the right balance between security and convenience is hard but Gradle will try to let you choose the "right level" for you.

Dependency verification consists of two different and complementary operations:

- checksum verification, which allows asserting the integrity of a dependency

- signature verification, which allows asserting the provenance of a dependency

Gradle supports both checksum and signature verification out of the box but performs no dependency verification by default. This section will guide you into configuring dependency verification properly for your needs.

This feature can be used for:

- detecting compromised dependencies

- detecting compromised plugins

- detecting tampered dependencies in the local dependency caches

### Enabling dependency verification

#### The verification metadata file

| Currently the only source of dependency verification metadata is this XML configuration file. Future versions of Gradle may include other sources (for example via external services).

Dependency verification is automatically enabled once the configuration file for dependency verification is discovered. This configuration file is located at `$PROJECT_ROOT/gradle/verification-metadata.xml`. This file minimally consists of the following:

    <?xml version="1.0" encoding="UTF-8"?>
    <verification-metadata xmlns="https://schema.gradle.org/dependency-verification"
          xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
          xsi:schemaLocation="https://schema.gradle.org/dependency-verification https://schema.gradle.org/dependency-verification/dependency-verification-1.3.xsd">
       <configuration>
          <verify-metadata>true</verify-metadata>
          <verify-signatures>false</verify-signatures>
        </configuration>
    </verification-metadata>

Doing so, Gradle will verify all artifacts using checksums, but will not verify signatures. Gradle will verify any artifact downloaded using its dependency management engine, which includes, but is not limited to:

- artifact files (e.g jar files, zips, …​) used during a build

- metadata artifacts (POM files, Ivy descriptors, Gradle Module Metadata)

- plugins (both project and settings plugins)

- artifacts resolved using the advanced dependency resolution APIs

Gradle will not verify changing dependencies (in particular `SNAPSHOT` dependencies) nor locally produced artifacts (typically jars produced during the build itself) as by nature their checksums and signatures would always change.

With such a minimal configuration file, a project using any external dependency or plugin would immediately start failing because it doesn’t contain any checksum to verify.

#### Scope of the dependency verification

A dependency verification configuration is global: a single file is used to configure verification of the whole build. In particular, the same file is used for both the (sub)projects and `buildSrc`.

If an included build is used:

- the configuration file of the current build is used for verification

- so if the included build itself uses verification, its configuration is ignored in favor of the current one

- which means that including a build works similarly to upgrading a dependency: it may require you to update your current verification metadata

An easy way to get started is therefore to generate the minimal configuration for an existing build.

#### Configuring the console output

By default, if dependency verification fails, Gradle will generate a small summary about the verification failure as well as an HTML report containing the full information about the failures. If your environment prevents you from reading this HTML report file (for example if you run a build on CI and that it’s not easy to fetch the remote artifacts), Gradle provides a way to opt-in a verbose console report. For this, you need to add this Gradle property to your `gradle.properties` file:

    org.gradle.dependency.verification.console=verbose

#### Bootstrapping dependency verification

It’s worth mentioning that while Gradle can generate a dependency verification file for you, you should always check whatever Gradle generated for you because your build may already contain compromised dependencies without you knowing about it. Please refer to the appropriate checksum verification or signature verification section for more information.

If you plan on using signature verification, please also read the corresponding section of the docs.

Bootstrapping can either be used to create a file from the beginning, or also to update an existing file with new information. Therefore, it’s recommended to always use the same parameters once you started bootstrapping.

The dependency verification file can be generated with the following CLI instructions:

    gradle --write-verification-metadata sha256 help

The `write-verification-metadata` flag requires the list of checksums that you want to generate or `pgp` for signatures.

Executing this command line will cause Gradle to:

- resolve all resolvable configurations, which includes:

  - configurations from the root project

  - configurations from all subprojects

  - configurations from `buildSrc`

  - included builds configurations

  - configurations used by plugins

- download all artifacts discovered during resolution

- compute the requested checksums and possibly verify signatures depending on what you asked

- At the end of the build, generate the configuration file which will contain the inferred verification metadata

As a consequence, the `verification-metadata.xml` file will be used in subsequent builds to verify dependencies.

There are dependencies that Gradle cannot discover this way. In particular, you will notice that the CLI above uses the `help` task. If you don’t specify any task, Gradle will automatically run the default task and generate a configuration file at the end of the build too.

The difference is that Gradle may discover more dependencies and artifacts depending on the tasks you execute. As a matter of fact, Gradle cannot automatically discover detached configurations, which are basically dependency graphs resolved as an internal implementation detail of the execution of a task: they are not, in particular, declared as an input of the task because they effectively depend on the configuration of the task at execution time.

A good way to start is just to use the simplest task, `help`, which will discover as much as possible, and if subsequent builds fail with a verification error, you can re-execute generation with the appropriate tasks to "discover" more dependencies.

Gradle won’t verify either checksums or signatures of plugins which use their own HTTP clients. Only plugins which use the infrastructure provided by Gradle for performing requests will see their requests verified.

#### Using generation for incremental updates

The verification file generated by Gradle has a strict ordering for all its content. It also uses the information from the existing state to limit changes to the strict minimum.

This means that generation is actually a convenient tool for updating a verification file:

- Checksum entries generated by Gradle will have a clear `origin` that starts with "Generated by Gradle", which is a good indicator that an entry needs to be reviewed,

- Entries added by hand will immediately be accounted for, and appear at the right location after writing the file,

- The header comments of the file will be preserved, i.e. comments before the root XML node. This allows you to have a license header or instructions on which tasks and which parameters to use for generating that file.

With the above benefits, it is really easy to account for new dependencies or dependency versions by simply generating the file again and reviewing the changes.

#### Using dry mode

By default, bootstrapping is incremental, which means that if you run it multiple times, information is added to the file and in particular you can rely on your VCS to check the diffs. There are situations where you would just want to see what the generated verification metadata file would look like without actually changing the existing one or overwriting it.

For this purpose, you can just add `--dry-run`:

    gradle --write-verification-metadata sha256 help --dry-run

Then instead of generating the `verification-metadata.xml` file, a new file will be generated, called `verification-metadata.dryrun.xml`.

| Because `--dry-run` doesn’t execute tasks, this would be much faster, but it will miss any resolution happening at task execution time.

#### Disabling metadata verification

By default, Gradle will not only verify artifacts (jars, …​) but also the metadata associated with those artifacts (typically POM files). Verifying this ensures the maximum level of security: metadata files typically tell what transitive dependencies will be included, so a compromised metadata file may cause the introduction of undesired dependencies in the graph. However, because all artifacts are verified, such artifacts would in general easily be discovered by you, because they would cause a checksum verification failure (checksums would be missing from verification metadata). Because metadata verification can significantly increase the size of your configuration file, you may therefore want to disable verification of metadata. If you understand the risks of doing so, set the `<verify-metadata>` flag to `false` in the configuration file:

    <?xml version="1.0" encoding="UTF-8"?>
    <verification-metadata xmlns="https://schema.gradle.org/dependency-verification"
          xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
          xsi:schemaLocation="https://schema.gradle.org/dependency-verification https://schema.gradle.org/dependency-verification/dependency-verification-1.3.xsd">
       <configuration>
          <verify-metadata>false</verify-metadata>
          <verify-signatures>false</verify-signatures>
        </configuration>
        <!-- the rest of this file doesn't need to declare anything about metadata files -->
    </verification-metadata>

### Verifying dependency checksums

Checksum verification allows you to ensure the integrity of an artifact. This is the simplest thing that Gradle can do for you to make sure that the artifacts you use are un-tampered.

Gradle supports MD5, SHA1, SHA-256 and SHA-512 checksums. However, only SHA-256 and SHA-512 checksums are considered secure nowadays.

#### Adding the checksum for an artifact

External components are identified by GAV coordinates, then each of the artifacts by their file names. To declare the checksums of an artifact, you need to add the corresponding section in the verification metadata file. For example, to declare the checksum for Apache PDFBox. The GAV coordinates are:

- group `org.apache.pdfbox`

- name `pdfbox`

- version `2.0.17`

Using this dependency will trigger the download of 2 different files:

- `pdfbox-2.0.17.jar` which is the main artifact

- `pdfbox-2.0.17.pom` which is the metadata file associated with this artifact

As a consequence, you need to declare the checksums for both of them (unless you disabled metadata verification):

    <?xml version="1.0" encoding="UTF-8"?>
    <verification-metadata xmlns="https://schema.gradle.org/dependency-verification"
          xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
          xsi:schemaLocation="https://schema.gradle.org/dependency-verification https://schema.gradle.org/dependency-verification/dependency-verification-1.3.xsd">
       <configuration>
          <verify-metadata>true</verify-metadata>
          <verify-signatures>false</verify-signatures>
       </configuration>
       <components>
          <component group="org.apache.pdfbox" name="pdfbox" version="2.0.17">
             <artifact name="pdfbox-2.0.17.jar">
                <sha512 value="7e11e54a21c395d461e59552e88b0de0ebaf1bf9d9bcacadf17b240d9bbc29bf6beb8e36896c186fe405d287f5d517b02c89381aa0fcc5e0aa5814e44f0ab331" origin="PDFBox Official site (https://pdfbox.apache.org/download.cgi)"/>
             </artifact>
             <artifact name="pdfbox-2.0.17.pom">
                <sha512 value="82de436b38faf6121d8d2e71dda06e79296fc0f7bc7aba0766728c8d306fd1b0684b5379c18808ca724bf91707277eba81eb4fe19518e99e8f2a56459b79742f" origin="Generated by Gradle"/>
             </artifact>
          </component>
       </components>
    </verification-metadata>

#### Where to get checksums from?

In general, checksums are published alongside artifacts on public repositories. However, if a dependency is compromised in a repository, it’s likely its checksum will be too, so it’s a good practice to get the checksum from a different place, usually the website of the library itself.

In fact, it’s a good security practice to publish the checksums of artifacts on a different server than the server where the artifacts themselves are hosted: it’s harder to compromise a library both on the repository and the official website.

In the example above, the checksum was published on the website for the JAR, but not the POM file. This is why it’s usually easier to let Gradle generate the checksums and verify by reviewing the generated file carefully.

In this example, not only could we check that the checksum was correct, but we could also find it on the official website, which is why we changed the value of the of `origin` attribute on the `sha512` element from `Generated by Gradle` to `PDFBox Official site`. Changing the `origin` gives users a sense of how trustworthy your build it.

Interestingly, using `pdfbox` will require much more than those 2 artifacts, because it will also bring in transitive dependencies. If the dependency verification file only included the checksums for the main artifacts you used, the build would fail with an error like this one:

    Execution failed for task ':compileJava'.
    > Dependency verification failed for configuration ':compileClasspath':
        - On artifact commons-logging-1.2.jar (commons-logging:commons-logging:1.2) in repository 'MavenRepo': checksum is missing from verification metadata.
        - On artifact commons-logging-1.2.pom (commons-logging:commons-logging:1.2) in repository 'MavenRepo': checksum is missing from verification metadata.

What this indicates is that your build requires `commons-logging` when executing `compileJava`, however the verification file doesn’t contain enough information for Gradle to verify the integrity of the dependencies, meaning you need to add the required information to the verification metadata file.

See troubleshooting dependency verification for more insights on what to do in this situation.

#### What checksums are verified?

If a dependency verification metadata file declares more than one checksum for a dependency, Gradle will verify all of them and fail if any of them fails. For example, the following configuration would check both the `md5` and `sha1` checksums:

    <component group="org.apache.pdfbox" name="pdfbox" version="2.0.17">
       <artifact name="pdfbox-2.0.17.jar">
          <md5 value="c713a8e252d0add65e9282b151adf6b4" origin="official site"/>
          <sha1 value="b5c8dff799bd967c70ccae75e6972327ae640d35" origin="official site" reason="Additional check for this artifact"/>
       </artifact>
    </component>

There are multiple reasons why you’d like to do so:

1. an official site doesn’t publish secure checksums (SHA-256, SHA-512) but publishes multiple insecure ones (MD5, SHA1). While it’s easy to fake a MD5 checksum and hard but possible to fake a SHA1 checksum, it’s harder to fake both of them for the same artifact.

2. you might want to add generated checksums to the list above

3. when updating dependency verification file with more secure checksums, you don’t want to accidentally erase checksums

### Verifying dependency signatures

In addition to checksums, Gradle supports verification of signatures. Signatures are used to assess the provenance of a dependency (it tells who signed the artifacts, which usually corresponds to who produced it).

As enabling signature verification usually means a higher level of security, you might want to replace checksum verification with signature verification.

| Signatures can also be used to assess the integrity of a dependency similarly to checksums. Signatures are signatures of the hash of artifacts, not artifacts themselves. This means that if the signature is done on an unsafe hash (even SHA1), then you’re not correctly assessing the integrity of a file. For this reason, if you care about both, you need to add both signatures and checksums to your verification metadata.

However:

- Gradle only supports verification of signatures published on remote repositories as ASCII-armored PGP files

- Not all artifacts are published with signatures

- A good signature doesn’t mean that the signatory was legit

As a consequence, signature verification will often be used alongside checksum verification.

About expired keys

It’s very common to find artifacts which are signed with an expired key. This is not a problem for verification: key expiry is mostly used to avoid signing with a stolen key. If an artifact was signed before expiry, it’s still valid.

#### Enabling signature verification

Because verifying signatures is more expensive (both I/O and CPU wise) and harder to check manually, it’s not enabled by default.

Enabling it requires you to change the configuration option in the `verification-metadata.xml` file:

    <?xml version="1.0" encoding="UTF-8"?>
    <verification-metadata xmlns="https://schema.gradle.org/dependency-verification"
          xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
          xsi:schemaLocation="https://schema.gradle.org/dependency-verification https://schema.gradle.org/dependency-verification/dependency-verification-1.3.xsd">
       <configuration>
          <verify-signatures>true</verify-signatures>
       </configuration>
    </verification-metadata>

### Understanding signature verification

Once signature verification is enabled, for each artifact, Gradle will:

- try to download the corresponding `.asc` file

- if it’s present

  - automatically download the keys required to perform verification of the signature

  - verify the artifact using the downloaded public keys

  - if signature verification passes, perform additional requested checksum verification

- if it’s absent, fallback to checksum verification

That is to say that Gradle’s verification mechanism is much stronger if signature verification is enabled than just with checksum verification. In particular:

- if an artifact is signed with multiple keys, all of them must pass validation or the build will fail

- if an artifact passes verification, any additional checksum configured for the artifact will also be checked

However, it’s not because an artifact passes signature verification that you can trust it: you need to trust the keys.

In practice, it means you need to list the keys that you trust for each artifact, which is done by adding a `pgp` entry instead of a `sha1` for example:

    <component group="com.github.javaparser" name="javaparser-core" version="3.6.11">
       <artifact name="javaparser-core-3.6.11.jar">
          <pgp value="8756c4f765c9ac3cb6b85d62379ce192d401ab61"/>
       </artifact>
    </component>

| For the `pgp` and `trusted-key` elements, Gradle requires full fingerprint IDs (e.g. `b801e2f8ef035068ec1139cc29579f18fa8fd93b` instead of a long ID `29579f18fa8fd93b`). This minimizes the chance of a collision attack. At the time, V4 key fingerprints are of 160-bit (40 characters) length. We accept longer keys to be future-proof in case a longer key fingerprint is introduced. In `ignore-key` elements, either fingerprints or long (64-bit) IDs can be used. A shorter ID can only result in a bigger range of exclusion, therefore, it’s safe to use.

This effectively means that you trust `com.github.javaparser:javaparser-core:3.6.11` if it’s signed with the key `8756c4f765c9ac3cb6b85d62379ce192d401ab61`.

Without this, the build would fail with this error:

    > Dependency verification failed for configuration ':compileClasspath':
        - On artifact javaparser-core-3.6.11.jar (com.github.javaparser:javaparser-core:3.6.11) in repository 'MavenRepo': Artifact was signed with key '8756c4f765c9ac3cb6b85d62379ce192d401ab61' (Bintray (by JFrog) <****>) and passed verification but the key isn't in your trusted keys list.

| The key IDs that Gradle shows in error messages are the key IDs found in the signature file it tries to verify. It doesn’t mean that it’s necessarily the keys that you should trust. In particular, if the signature is correct but done by a malicious entity, Gradle wouldn’t tell you.

#### Trusting keys globally

Signature verification has the advantage that it can make the configuration of dependency verification easier by not having to explicitly list all artifacts like for checksum verification only. In fact, it’s common that the same key can be used to sign several artifacts. If this is the case, you can move the trusted key from the artifact level to the global configuration block:

    <?xml version="1.0" encoding="UTF-8"?>
    <verification-metadata xmlns="https://schema.gradle.org/dependency-verification"
          xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
          xsi:schemaLocation="https://schema.gradle.org/dependency-verification https://schema.gradle.org/dependency-verification/dependency-verification-1.3.xsd">
       <configuration>
          <verify-metadata>true</verify-metadata>
          <verify-signatures>true</verify-signatures>
          <trusted-keys>
             <trusted-key id="8756c4f765c9ac3cb6b85d62379ce192d401ab61" group="com.github.javaparser"/>
          </trusted-keys>
       </configuration>
       <components/>
    </verification-metadata>

The configuration above means that for any artifact belonging to the group `com.github.javaparser`, we trust it if it’s signed with the `8756c4f765c9ac3cb6b85d62379ce192d401ab61` fingerprint.

The `trusted-key` element works similarly to the trusted-artifact element:

- `group`, the group of the artifact to trust

- `name`, the name of the artifact to trust

- `version`, the version of the artifact to trust

- `file`, the name of the artifact file to trust

- `regex`, a boolean saying if the `group`, `name`, `version` and `file` attributes need to be interpreted as regular expressions (defaults to `false`)

You should be careful when trusting a key globally.

Try to limit it to the appropriate groups or artifacts:

- a valid key may have been used to sign artifact `A` which you trust

- later on, the key is stolen and used to sign artifact `B`

It means you can trust the key `A` for the first artifact, probably only up to the released version before the key was stolen, but not for `B`.

Remember that anybody can put an arbitrary name when generating a PGP key, so never trust the key solely based on the key name. Verify if the key is listed at the official site. For example, Apache projects typically provide a KEYS.txt file that you can trust.

#### Specifying key servers and ignoring keys

Gradle will automatically download the public keys required to verify a signature. For this it uses a list of well known and trusted key servers (the list may change between Gradle versions, please refer to the implementation to figure out what servers are used by default).

You can explicitly set the list of key servers that you want to use by adding them to the configuration:

    <?xml version="1.0" encoding="UTF-8"?>
    <verification-metadata xmlns="https://schema.gradle.org/dependency-verification"
          xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
          xsi:schemaLocation="https://schema.gradle.org/dependency-verification https://schema.gradle.org/dependency-verification/dependency-verification-1.3.xsd">
       <configuration>
          <verify-metadata>true</verify-metadata>
          <verify-signatures>true</verify-signatures>
          <key-servers>
             <key-server uri="hkp://my-key-server.org"/>
             <key-server uri="https://my-other-key-server.org"/>
          </key-servers>
       </configuration>
    </verification-metadata>

Despite this, it’s possible that a key is not available:

- because it wasn’t published to a public key server

- because it was lost

In this case, you can ignore a key in the configuration block:

    <?xml version="1.0" encoding="UTF-8"?>
    <verification-metadata xmlns="https://schema.gradle.org/dependency-verification"
          xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
          xsi:schemaLocation="https://schema.gradle.org/dependency-verification https://schema.gradle.org/dependency-verification/dependency-verification-1.3.xsd">
       <configuration>
          <verify-metadata>true</verify-metadata>
          <verify-signatures>true</verify-signatures>
          <ignored-keys>
             <ignored-key id="abcdef1234567890" reason="Key is not available in any key server"/>
          </ignored-keys>
       </configuration>
    </verification-metadata>

As soon as a key is ignored, it will not be used for verification, even if the signature file mentions it. However, if the signature cannot be verified with at least one other key, Gradle will mandate that you provide a checksum.

| If Gradle cannot download a key while bootstrapping, it will mark it as ignored. If you can find the key but Gradle does not, you can manually add it to the keyring file.

#### Exporting keys for faster verification

Gradle automatically downloads the required keys but this operation can be quite slow and requires everyone to download the keys. To avoid this, Gradle offers the ability to use a local keyring file containing the required public keys. Note that only public key packets and a single userId per key are stored and used. All other information (user attributes, signatures, etc.) is stripped from downloaded or exported keys.

Gradle supports 2 different file formats for keyrings: a binary format (`.gpg` file) and a plain text format (`.keys`), also known as ASCII-armored format.

There are pros and cons for each of the formats: the binary format is more compact and can be updated directly via GPG commands, but is completely opaque (binary). On the opposite, the ASCII-armored format is human-readable, can be easily updated by hand and makes it easier to do code reviews thanks to readable diffs.

You can configure which file type would be used by adding the `keyring-format` configuration option:

    <?xml version="1.0" encoding="UTF-8"?>
    <verification-metadata xmlns="https://schema.gradle.org/dependency-verification"
          xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
          xsi:schemaLocation="https://schema.gradle.org/dependency-verification https://schema.gradle.org/dependency-verification/dependency-verification-1.3.xsd">
       <configuration>
          <verify-metadata>true</verify-metadata>
          <verify-signatures>true</verify-signatures>
          <keyring-format>armored</keyring-format>
       </configuration>
    </verification-metadata>

Available options for keyring format are `armored` and `binary`.

Without `keyring-format`, if the `gradle/verification-keyring.gpg` or `gradle/verification-keyring.keys` file is present, Gradle will search for keys there in priority. The plain text file will be ignored if there’s already a `.gpg` file (the binary version takes precedence).

You can ask Gradle to export all keys it used for verification of this build to the keyring during bootstrapping:

    ./gradlew --write-verification-metadata pgp,sha256 --export-keys

Unless `keyring-format` is specified, this command will generate both the binary version and the ASCII-armored file. Use this option to choose the preferred format. You should only pick one for your project.

It’s a good idea to commit this file to VCS (as long as you trust your VCS). If you use git and use the binary version, make sure to make it treat this file as binary, by adding this to your `.gitattributes` file:

    *.gpg           binary

You can also ask Gradle to export all trusted keys without updating the verification metadata file:

    ./gradlew --export-keys

| This command will not report verification errors, only export keys.

#### Bootstrapping and signature verification

| Signature verification bootstrapping takes an optimistic point of view that signature verification is enough. Therefore, if you also care about integrity, you must first bootstrap using checksum verification, then with signature verification.

Similarly to bootstrapping for checksums, Gradle provides a convenience for bootstrapping a configuration file with signature verification enabled. For this, just add the `pgp` option to the list of verifications to generate. However, because there might be verification failures, missing keys or missing signature files, you must provide a fallback checksum verification algorithm:

    ./gradlew --write-verification-metadata pgp,sha256

this means that Gradle will verify the signatures and fallback to SHA-256 checksums when there’s a problem.

When bootstrapping, Gradle performs optimistic verification and therefore assumes a sane build environment. It will therefore:

- automatically add the trusted keys as soon as verification passes

- automatically add ignored keys for keys which couldn’t be downloaded from public key servers. See here how to manually add keys if needed

- automatically generate checksums for artifacts without signatures or ignored keys

If, for some reason, verification fails during the generation, Gradle will automatically generate an ignored key entry but warn you that you must absolutely check what happens.

This situation is common as explained for this section: a typical case is when the POM file for a dependency differs from one repository to the other (often in a non-meaningful way).

In addition, Gradle will try to group keys automatically and generate the `trusted-keys` block which reduced the configuration file size as much as possible.

### Forcing use of local keyrings only

The local keyring files (`.gpg` or `.keys`) can be used to avoid reaching out to key servers whenever a key is required to verify an artifact. However, it may be that the local keyring doesn’t contain a key, in which case Gradle would use the key servers to fetch the missing key. If the local keyring file isn’t regularly updated, using key export, then it may be that your CI builds, for example, would reach out to key servers too often (especially if you use disposable containers for builds).

To avoid this, Gradle offers the ability to disallow use of key servers altogether: only the local keyring file would be used, and if a key is missing from this file, the build will fail.

To enable this mode, you need to disable key servers in the configuration file:

    <?xml version="1.0" encoding="UTF-8"?>
    <verification-metadata xmlns="https://schema.gradle.org/dependency-verification"
          xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
          xsi:schemaLocation="https://schema.gradle.org/dependency-verification https://schema.gradle.org/dependency-verification/dependency-verification-1.3.xsd">
       <configuration>
          <key-servers enabled="false"/>
          ...
       </configuration>
       ...
    </verification-metadata>

| If you are asking Gradle to generate a verification metadata file and that an existing verification metadata file sets `enabled` to `false`, then this flag will be ignored, so that potentially missing keys are downloaded.

### Disabling verification or making it lenient

Dependency verification can be expensive, or sometimes verification could get in the way of day to day development (because of frequent dependency upgrades, for example).

Alternatively, you might want to enable verification on CI servers but not on local machines.

Gradle actually provides 3 different verification modes:

- `strict`, which is the default. Verification fails as early as possible, in order to avoid the use of compromised dependencies during the build.

- `lenient`, which will run the build even if there are verification failures. The verification errors will be displayed during the build without causing a build failure.

- `off` when verification is totally ignored.

All those modes can be activated on the CLI using the `--dependency-verification` flag, for example:

    ./gradlew --dependency-verification lenient build

Alternatively, you can set the `org.gradle.dependency.verification` system property, either on the CLI:

    ./gradlew -Dorg.gradle.dependency.verification=lenient build

or in a `gradle.properties` file:

    org.gradle.dependency.verification=lenient

### Disabling dependency verification for some configurations only

In order to provide the strongest security level possible, dependency verification is enabled globally. This will ensure, for example, that you trust all the plugins you use. However, the plugins themselves may need to resolve additional dependencies that it doesn’t make sense to ask the user to accept. For this purpose, Gradle provides an API which allows disabling dependency verification on some specific configurations.

| Disabling dependency verification, if you care about security, is not a good idea. This API mostly exist for cases where it doesn’t make sense to check dependencies. However, in order to be on the safe side, Gradle will systematically print a warning whenever verification has been disabled for a specific configuration.

As an example, a plugin may want to check if there are newer versions of a library available and list those versions. It doesn’t make sense, in this context, to ask the user to put the checksums of the POM files of the newer releases because by definition, they don’t know about them. So the plugin might need to run its code independently of the dependency verification configuration.

To do this, you need to call the `ResolutionStrategy#disableDependencyVerification` method:

Example 2. Disabling dependency verification
build.gradle.kts

    configurations {
        "myPluginClasspath" {
            resolutionStrategy {
                disableDependencyVerification()
            }
        }
    }

build.gradle

    configurations {
        myPluginClasspath {
            resolutionStrategy {
                disableDependencyVerification()
            }
        }
    }

It’s also possible to disable verification on detached configurations like in the following example:

Example 3. Disabling dependency verification
build.gradle.kts

    tasks.register("checkDetachedDependencies") {
        val detachedConf: FileCollection = configurations.detachedConfiguration(dependencies.create("org.apache.commons:commons-lang3:3.3.1")).apply {
            resolutionStrategy.disableDependencyVerification()
        }
        doLast {
            println(detachedConf.files)
        }
    }

build.gradle

    tasks.register("checkDetachedDependencies") {
        def detachedConf = configurations.detachedConfiguration(dependencies.create("org.apache.commons:commons-lang3:3.3.1"))
        detachedConf.resolutionStrategy.disableDependencyVerification()
        doLast {
            println(detachedConf.files)
        }
    }

### Trusting some particular artifacts

You might want to trust some artifacts more than others. For example, it’s legitimate to think that artifacts produced in your company and found in your internal repository only are safe, but you want to check every external component.

| This is a typical company policy. In practice, nothing prevents your internal repository from being compromised, so it’s a good idea to check your internal artifacts too!

For this purpose, Gradle offers a way to automatically trust some artifacts. You can trust all artifacts in a group by adding this to your configuration:

    <?xml version="1.0" encoding="UTF-8"?>
    <verification-metadata xmlns="https://schema.gradle.org/dependency-verification"
          xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
          xsi:schemaLocation="https://schema.gradle.org/dependency-verification https://schema.gradle.org/dependency-verification/dependency-verification-1.3.xsd">
       <configuration>
          <trusted-artifacts>
             <trust group="com.mycompany" reason="We trust mycompany artifacts"/>
          </trusted-artifacts>
       </configuration>
    </verification-metadata>

This means that all components which group is `com.mycompany` will automatically be trusted. Trusted means that Gradle will not perform any verification whatsoever.

The `trust` element accepts those attributes:

- `group`, the group of the artifact to trust

- `name`, the name of the artifact to trust

- `version`, the version of the artifact to trust

- `file`, the name of the artifact file to trust

- `regex`, a boolean saying if the `group`, `name`, `version` and `file` attributes need to be interpreted as regular expressions (defaults to `false`)

- `reason`, an optional reason, why matched artifacts are trusted

In the example above it means that the trusted artifacts would be artifacts in `com.mycompany` but not `com.mycompany.other`. To trust all artifacts in `com.mycompany` and all subgroups, you can use:

    <?xml version="1.0" encoding="UTF-8"?>
    <verification-metadata xmlns="https://schema.gradle.org/dependency-verification"
          xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
          xsi:schemaLocation="https://schema.gradle.org/dependency-verification https://schema.gradle.org/dependency-verification/dependency-verification-1.3.xsd">
       <configuration>
          <trusted-artifacts>
             <trust group="^com[.]mycompany($|([.].*))" regex="true" reason="We trust all mycompany artifacts"/>
          </trusted-artifacts>
       </configuration>
    </verification-metadata>

### Trusting multiple checksums for an artifact

It’s quite common to have different checksums for the same artifact in the wild. How is that possible? Despite progress, it’s often the case that developers publish, for example, to Maven Central and another repository separately, using different builds. In general, this is not a problem but sometimes it means that the metadata files would be different (different timestamps, additional whitespaces, …​). Add to this that your build may use several repositories or repository mirrors and it makes it quite likely that a single build can "see" different metadata files for the same component! In general, it’s not malicious (but you must verify that the artifact is actually correct), so Gradle lets you declare the additional artifact checksums. For example:

          <component group="org.apache" name="apache" version="13">
             <artifact name="apache-13.pom">
                <sha256 value="2fafa38abefe1b40283016f506ba9e844bfcf18713497284264166a5dbf4b95e">
                   <also-trust value="ff513db0361fd41237bef4784968bc15aae478d4ec0a9496f811072ccaf3841d"/>
                </sha256>
             </artifact>
          </component>

You can have as many `also-trust` entries as needed, but in general you shouldn’t have more than 2.

### Skipping Javadocs and sources

By default Gradle will verify all downloaded artifacts, which includes Javadocs and sources. In general this is not a problem but you might face an issue with IDEs which automatically try to download them during import: if you didn’t set the checksums for those too, importing would fail.

To avoid this, you can configure Gradle to trust automatically all javadocs/sources:

    <trusted-artifacts>
       <trust file=".*-javadoc[.]jar" regex="true"/>
       <trust file=".*-sources[.]jar" regex="true"/>
    </trusted-artifacts>

### Adding keys manually to the keyring

#### Adding keys to the ASCII-armored keyring

The added key must be ASCII-armored formatted and can be simply added at the end of the file. If you already downloaded the key in the right format, you can simply append it to the file.

Or you can amend an existing KEYS file by issuing the following commands:

    $ gpg --no-default-keyring --keyring /tmp/keyring.gpg --recv-keys 8756c4f765c9ac3cb6b85d62379ce192d401ab61

    gpg: keybox '/tmp/keyring.gpg' created
    gpg: key 379CE192D401AB61: public key "Bintray (by JFrog) <****>" imported
    gpg: Total number processed: 1
    gpg:               imported: 1

    # Write its ASCII-armored version
    $ gpg --keyring /tmp/keyring.gpg --export --armor 8756c4f765c9ac3cb6b85d62379ce192d401ab61 > gradle/verification-keyring.keys

Once done, make sure to run the generation command again so that the key is processed by Gradle. This will do the following:

- Add a standard header to the key

- Rewrite the key using Gradle’s own format, which trims the key to the bare minimum

- Move the key to its sorted location, keeping the file reproducible

#### Adding keys to the binary keyring

You can add keys to the binary version using GPG, for example issuing the following commands (syntax may depend on the tool you use):

    $ gpg --no-default-keyring --keyring gradle/verification-keyring.gpg --recv-keys 8756c4f765c9ac3cb6b85d62379ce192d401ab61

    gpg: keybox 'gradle/verification-keyring.gpg' created
    gpg: key 379CE192D401AB61: public key "Bintray (by JFrog) <****>" imported
    gpg: Total number processed: 1
    gpg:               imported: 1

    $ gpg --no-default-keyring --keyring gradle/verification-keyring.gpg --recv-keys 6f538074ccebf35f28af9b066a0975f8b1127b83

    gpg: key 0729A0AFF8999A87: public key "Kotlin Release <****>" imported
    gpg: Total number processed: 1
    gpg:               imported: 1

### Dealing with a verification failure

Dependency verification can fail in different ways, this section explains how you should deal with the various cases.

#### Missing verification metadata

The simplest failure you can have is when verification metadata is missing from the dependency verification file. This is the case for example if you use checksum verification, then you update a dependency and new versions of the dependency (and potentially its transitive dependencies) are brought in.

Gradle will tell you what metadata is missing:

    Execution failed for task ':compileJava'.
    > Dependency verification failed for configuration ':compileClasspath':
        - On artifact commons-logging-1.2.jar (commons-logging:commons-logging:1.2) in repository 'MavenRepo': checksum is missing from verification metadata.

- the missing module group is `commons-logging`, it’s artifact name is `commons-logging` and its version is `1.2`. The corresponding artifact is `commons-logging-1.2.jar` so you need to add the following entry to the verification file:

    <component group="commons-logging" name="commons-logging" version="1.2">
       <artifact name="commons-logging-1.2.jar">
          <sha256 value="daddea1ea0be0f56978ab3006b8ac92834afeefbd9b7e4e6316fca57df0fa636" origin="official distribution"/>
       </artifact>
    </component>

Alternatively, you can ask Gradle to generate the missing information by using the bootstrapping mechanism: existing information in the metadata file will be preserved, Gradle will only add the missing verification metadata.

#### Incorrect checksums

A more problematic issue is when the actual checksum verification fails:

    Execution failed for task ':compileJava'.
    > Dependency verification failed for configuration ':compileClasspath':
        - On artifact commons-logging-1.2.jar (commons-logging:commons-logging:1.2) in repository 'MavenRepo': expected a 'sha256' checksum of '91f7a33096ea69bac2cbaf6d01feb934cac002c48d8c8cfa9c240b40f1ec21df' but was 'daddea1ea0be0f56978ab3006b8ac92834afeefbd9b7e4e6316fca57df0fa636'

This time, Gradle tells you what dependency is at fault, what was the expected checksum (the one you declared in the verification metadata file) and the one which was actually computed during verification.

Such a failure indicates that a dependency may have been compromised. At this stage, you must perform manual verification and check what happens. Several things can happen:

- a dependency was tampered in the local dependency cache of Gradle. This is usually harmless: erase the file from the cache and Gradle would redownload the dependency.

- a dependency is available in multiple sources with slightly different binaries (additional whitespace, …​)

  - please inform the maintainers of the library that they have such an issue

  - you can use `also-trust` to accept the additional checksums

- the dependency was compromised

  - immediately inform the maintainers of the library

  - notify the repository maintainers of the compromised library

Note that a variation of a compromised library is often name squatting, when a hacker would use GAV coordinates which look legit but are actually different by one character, or repository shadowing, when a dependency with the official GAV coordinates is published in a malicious repository which comes first in your build.

#### Untrusted signatures

If you have signature verification enabled, Gradle will perform verification of the signatures but will not trust them automatically:

    > Dependency verification failed for configuration ':compileClasspath':
        - On artifact javaparser-core-3.6.11.jar (com.github.javaparser:javaparser-core:3.6.11) in repository 'MavenRepo': Artifact was signed with key '379ce192d401ab61' (Bintray (by JFrog) <****>) and passed verification but the key isn't in your trusted keys list.

In this case it means you need to check yourself if the key that was used for verification (and therefore the signature) can be trusted, in which case refer to this section of the documentation to figure out how to declare trusted keys.

#### Failed signature verification

If Gradle fails to verify a signature, you will need to take action and verify artifacts manually because this may indicate a compromised dependency.

If such a thing happens, Gradle will fail with:

    > Dependency verification failed for configuration ':compileClasspath':
        - On artifact javaparser-core-3.6.11.jar (com.github.javaparser:javaparser-core:3.6.11) in repository 'MavenRepo': Artifact was signed with key '379ce192d401ab61' (Bintray (by JFrog) <****>) but signature didn't match

There are several options:

1. signature was wrong in the first place, which happens frequently with dependencies published on different repositories.

2. the signature is correct but the artifact has been compromised (either in the local dependency cache or remotely)

The right approach here is to go to the official site of the dependency and see if they publish signatures for their artifacts. If they do, verify that the signature that Gradle downloaded matches the one published.

If you have checked that the dependency is not compromised and that it’s "only" the signature which is wrong, you should declare an artifact level key exclusion:

       <components>
           <component group="com.github.javaparser" name="javaparser-core" version="3.6.11">
              <artifact name="javaparser-core-3.6.11.pom">
                 <ignored-keys>
                    <ignored-key id="379ce192d401ab61" reason="internal repo has corrupted POM"/>
                 </ignored-keys>
              </artifact>
           </component>
       </components>

However, if you only do so, Gradle will still fail because all keys for this artifact will be ignored and you didn’t provide a checksum:

       <components>
           <component group="com.github.javaparser" name="javaparser-core" version="3.6.11">
              <artifact name="javaparser-core-3.6.11.pom">
                 <ignored-keys>
                    <ignored-key id="379ce192d401ab61" reason="internal repo has corrupted POM"/>
                 </ignored-keys>
                 <sha256 value="a2023504cfd611332177f96358b6f6db26e43d96e8ef4cff59b0f5a2bee3c1e1"/>
              </artifact>
           </component>
       </components>

#### Manual verification of a dependency

You will likely face a dependency verification failure (either checksum verification or signature verification) and will need to figure out if the dependency has been compromised or not.

In this section we give an example how you can manually check if a dependency was compromised.

For this we will take this example failure:

    > Dependency verification failed for configuration ':compileClasspath':
    - On artifact j2objc-annotations-1.1.jar (com.google.j2objc:j2objc-annotations:1.1) in repository 'MyCompany Mirror': Artifact was signed with key '29579f18fa8fd93b' but signature didn't match

This error message gives us the GAV coordinates of the problematic dependency, as well as an indication of where the dependency was fetched from. Here, the dependency comes from `MyCompany Mirror`, which is a repository declared in our build.

The first thing to do is therefore to download the artifact and its signature manually from the mirror:

    $ curl https://my-company-mirror.com/repo/com/google/j2objc/j2objc-annotations/1.1/j2objc-annotations-1.1.jar --output j2objc-annotations-1.1.jar
    $ curl https://my-company-mirror.com/repo/com/google/j2objc/j2objc-annotations/1.1/j2objc-annotations-1.1.jar.asc --output j2objc-annotations-1.1.jar.asc

Then we can use the key information provided in the error message to import the key locally:

    $ gpg --recv-keys B801E2F8EF035068EC1139CC29579F18FA8FD93B

And perform verification:

    $ gpg --verify j2objc-annotations-1.1.jar.asc
    gpg: assuming signed data in 'j2objc-annotations-1.1.jar'
    gpg: Signature made Thu 19 Jan 2017 12:06:51 AM CET
    gpg:                using RSA key 29579F18FA8FD93B
    gpg: BAD signature from "Tom Ball <****>" [unknown]

What this tells us is that the problem is not on the local machine: the repository already contains a bad signature.

The next step is to do the same by downloading what is actually on Maven Central:

    $ curl https://my-company-mirror.com/repo/com/google/j2objc/j2objc-annotations/1.1/j2objc-annotations-1.1.jar  --output central-j2objc-annotations-1.1.jar
    $ curl https://my-company-mirror.com/repo/com/google/j2objc/j2objc-annotations/1/1/j2objc-annotations-1.1.jar.asc  --output central-j2objc-annotations-1.1.jar.asc

And we can now check the signature again:

    $ gpg --verify central-j2objc-annotations-1.1.jar.asc

    gpg: assuming signed data in 'central-j2objc-annotations-1.1.jar'
    gpg: Signature made Thu 19 Jan 2017 12:06:51 AM CET
    gpg:                using RSA key 29579F18FA8FD93B
    gpg: Good signature from "Tom Ball <****>" [unknown]
    gpg: WARNING: This key is not certified with a trusted signature!
    gpg:          There is no indication that the signature belongs to the owner.
    Primary key fingerprint: B801 E2F8 EF03 5068 EC11  39CC 2957 9F18 FA8F D93B

This indicates that the dependency is valid on Maven Central. At this stage, we already know that the problem lives in the mirror, it may have been compromised, but we need to verify.

A good idea is to compare the 2 artifacts, which you can do with a tool like diffoscope.

We then figure out that the intent wasn’t malicious but that somehow a build has been overwritten with a newer version (the version in Central is newer than the one in our repository).

In this case, you can decide to:

- ignore the signature for this artifact and trust the different possible checksums (both for the old artifact and the new version)

- or cleanup your mirror so that it contains the same version as in Maven Central

It’s worth noting that if you choose to delete the version from your repository, you will also need to remove it from the local Gradle cache.

This is facilitated by the fact the error message tells you were the file is located:

    > Dependency verification failed for configuration ':compileClasspath':
        - On artifact j2objc-annotations-1.1.jar (com.google.j2objc:j2objc-annotations:1.1) in repository 'MyCompany Mirror': Artifact was signed with key '29579f18fa8fd93b' but signature didn't match

      This can indicate that a dependency has been compromised. Please carefully verify the signatures and checksums.

      For your information here are the path to the files which failed verification:
        - $<<directory_layout.adoc#dir:gradle_user_home,GRADLE_USER_HOME>>/caches/modules-2/files-2.1/com.google.j2objc/j2objc-annotations/1.1/976d8d30bebc251db406f2bdb3eb01962b5685b3/j2objc-annotations-1.1.jar (signature: GRADLE_USER_HOME/caches/modules-2/files-2.1/com.google.j2objc/j2objc-annotations/1.1/82e922e14f57d522de465fd144ec26eb7da44501/j2objc-annotations-1.1.jar.asc)

      GRADLE_USER_HOME = /home/jiraya/.gradle

You can safely delete the artifact file as Gradle would automatically re-download it:

    rm -rf ~/.gradle/caches/modules-2/files-2.1/com.google.j2objc/j2objc-annotations/1.1

### Cleaning up the verification file

If you do nothing, the dependency verification metadata will grow over time as you add new dependencies or change versions: Gradle will not automatically remove unused entries from this file. The reason is that there’s no way for Gradle to know upfront if a dependency will effectively be used during the build or not.

As a consequence, adding dependencies or changing dependency version can easily lead to more entries in the file, while leaving unnecessary entries out there.

One option to cleanup the file is to move the existing `verification-metadata.xml` file to a different location and call Gradle with the `--dry-run` mode: while not perfect (it will not notice dependencies only resolved at configuration time), it generates a new file that you can compare with the existing one.

We need to move the existing file because both the bootstrapping mode and the dry-run mode are incremental: they copy information from the existing metadata verification file (in particular, trusted keys).

### Refreshing missing keys

Gradle caches missing keys for 24 hours, meaning it will not attempt to re-download the missing keys for 24 hours after failing.

If you want to retry immediately, you can run with the `--refresh-keys` CLI flag:

    ./gradlew build --refresh-keys

See here how to manually add keys if Gradle keeps failing to download them.

# Declaring Versions

## Declaring Versions and Ranges

You can declare dependencies with specific versions or version ranges to define the acceptable versions of a dependency that your project can use:

    dependencies {
        implementation 'org.springframework:spring-core:5.3.8'
        implementation 'org.springframework:spring-core:5.3.+'
        implementation 'org.springframework:spring-core:latest.release'
        implementation 'org.springframework:spring-core:[5.2.0, 5.3.8]'
        implementation 'org.springframework:spring-core:[5.2.0,)'
    }

### Understanding version declaration

The simplest version declaration is a simple string representing the version to use. Gradle supports different ways of declaring a version string:

Version | Example | Note
--- | --- | ---
An exact version | `1.3`, `1.3.0-beta3`, `1.0-20150201.131010-1` |
A Maven-style version range | `[1.0,)`, `[1.1, 2.0)`, `(1.2, 1.5]` | The `[` and `]` symbols indicate an inclusive bound; `(` and `)` indicate an exclusive bound. When the upper or lower bound is missing, the range has no upper or lower bound. The symbol `]` can be used instead of `(` for an exclusive lower bound and `[` instead of `)` for an exclusive upper bound. e.g. `]1.0, 2.0[`. An upper bound exclude acts as a prefix exclude.
A prefix version range | `1.+`, `1.3.+` | Only versions exactly matching the portion before the `+` are included. The range `+` on its own will include any version.
A `latest-status` version | `latest.integration`, `latest.release` | Will match the highest versioned module with the specified status. See ComponentMetadata.getStatus().
A Maven `SNAPSHOT` version identifier | `1.0-SNAPSHOT`, `1.4.9-beta1-SNAPSHOT` |

### Understanding version ordering

Versions have an implicit ordering. Version ordering is used to:

- Determine if a particular version is included in a range.

- Determine which version is 'newest' when performing conflict resolution (watch out though, conflict resolution uses "base versions").

Versions are ordered based on the following rules:

- Each version is split into it’s constituent "parts":

  - The characters `[. - _ +]` are used to separate the different "parts" of a version.

  - Any part that contains both digits and letters is split into separate parts for each: `1a1 == 1.a.1`

  - Only the parts of a version are compared. The actual separator characters are not significant: `1.a.1 == 1-a+1 == 1.a-1 == 1a1` (watch out though, in the context of conflict resolution there are exceptions to this rule).

- The equivalent parts of 2 versions are compared using the following rules:

  - If both parts are numeric, the highest numeric value is higher: `1.1` < `1.2`

  - If one part is numeric, it is considered higher than the non-numeric part: `1.a` < `1.1`

  - If both are non-numeric, the parts are compared alphabetically, in a case-sensitive manner: `1.A` < `1.B` < `1.a` < `1.b`

  - A version with an extra numeric part is considered higher than a version without (even when it’s zero): `1.1` < `1.1.0`

  - A version with an extra non-numeric part is considered lower than a version without: `1.1.a` < `1.1`

- Certain non-numeric parts have special meaning for ordering:

  - `dev` is consider lower than any other non-numeric part: `1.0-dev` < `1.0-ALPHA` < `1.0-alpha` < `1.0-rc`.

  - The strings `rc`, `snapshot`, `final`, `ga`, `release` and `sp` are considered higher than any other string part (sorted in this order): `1.0-zeta` < `1.0-rc` < `1.0-snapshot` < `1.0-final` < `1.0-ga` < `1.0-release` < `1.0-sp` < `1.0`.

  - These special values are NOT case sensitive, as opposed to regular string parts, and they do not depend on the separator used around them: `1.0-RC-1` == `1.0.rc.1`

### Understanding version declaration semantics

When you declare a version using the shorthand notation, then the version is considered a required version:

build.gradle.kts

    dependencies {
        implementation("org.slf4j:slf4j-api:1.7.15")
    }

build.gradle

    dependencies {
        implementation('org.slf4j:slf4j-api:1.7.15')
    }

This means it should minimally be `1.7.15` but can be upgraded by the engine (optimistic upgrade).

There is, however, a shorthand notation for strict versions, using the `!!` notation:

build.gradle.kts

    dependencies {
        // short-hand notation with !!
        implementation("org.slf4j:slf4j-api:1.7.15!!")
        // is equivalent to
        implementation("org.slf4j:slf4j-api") {
            version {
               strictly("1.7.15")
            }
        }

        // or...
        implementation("org.slf4j:slf4j-api:[1.7, 1.8[!!1.7.25")
        // is equivalent to
        implementation("org.slf4j:slf4j-api") {
            version {
               strictly("[1.7, 1.8[")
               prefer("1.7.25")
            }
        }
    }

build.gradle

    dependencies {
        // short-hand notation with !!
        implementation('org.slf4j:slf4j-api:1.7.15!!')
        // is equivalent to
        implementation("org.slf4j:slf4j-api") {
            version {
               strictly '1.7.15'
            }
        }

        // or...
        implementation('org.slf4j:slf4j-api:[1.7, 1.8[!!1.7.25')
        // is equivalent to
        implementation('org.slf4j:slf4j-api') {
            version {
               strictly '[1.7, 1.8['
               prefer '1.7.25'
            }
        }
    }

A strict version cannot be upgraded and takes precedence over any transitive dependencies that specify a different version. It is recommended to use version ranges when defining strict versions.

The notation `[1.7, 1.8[!!1.7.25` above is equivalent to:

- strictly `[1.7, 1.8[`

- prefer `1.7.25`

This means that the engine must select a version between 1.7 (included) and 1.8 (excluded) and that if no other component in the graph needs a different version, it should prefer `1.7.25`.

### Declaring a dependency without version

A recommended practice for larger projects is to declare dependencies without versions and use dependency constraints for version declaration.

The advantage is that dependency constraints allow you to manage versions of all dependencies, including transitive ones, in one place:

build.gradle.kts

    dependencies {
        implementation("org.springframework:spring-web")
    }

    dependencies {
        constraints {
            implementation("org.springframework:spring-web:5.0.2.RELEASE")
        }
    }

build.gradle

    dependencies {
        implementation 'org.springframework:spring-web'
    }

    dependencies {
        constraints {
            implementation 'org.springframework:spring-web:5.0.2.RELEASE'
        }
    }

## Declaring Rich Versions

Gradle supports a rich model for declaring versions, which allows you to combine different levels of version information.

The terms and their meaning are explained below, from the strongest to the weakest:

`strictly`

  Any version not matched by this version notation will be excluded. This is the strongest version declaration. On a declared dependency, a `strictly` can downgrade a version. For a transitive dependency, dependency resolution will fail if no acceptable version can be selected. See overriding dependency version for details. This term supports dynamic versions.

  When defined, this overrides any previous `require` declaration and clears previous `reject`.

`require`

  Implies that the selected version cannot be lower than what `require` accepts but could be higher through conflict resolution, even if higher has an exclusive higher bound. This is what a direct dependency translates to. This term supports dynamic versions.

  When defined, this overrides any previous `strictly` declaration and clears previous `reject`.

`prefer`

  This is a very soft version declaration. It applies only if there is no stronger non-dynamic opinion on a version of the module. This term does not support dynamic versions.

  Definition can complement `strictly` or `require`.

  When defined, this overrides any previous `prefer` declaration and clears previous `reject`.

There is also an additional term outside of the level hierarchy:

`reject`

  Declares that specific version(s) are not accepted for the module. This will cause dependency resolution to fail if the selected version is rejected. This term supports dynamic versions.

The following table illustrates several use cases and how to combine the different terms for rich version declaration:

Which version(s) of this dependency are acceptable? | `strictly` | `require` | `prefer` | `rejects` | Selection result
--- | --- | --- | --- | --- | ---
Tested with version `1.5`; believe all future versions should work. |  | 1.5 |  |  | Any version starting from `1.5`, equivalent to `org:foo:1.5`. An upgrade to `2.4` is accepted.
Tested with `1.5`, soft constraint upgrades according to semantic versioning. |  | [1.0, 2.0[ | 1.5 |  | Any version between `1.0` and `2.0`, `1.5` if nobody else cares. An upgrade to `2.4` is accepted. 🔒
Tested with `1.5`, but follows semantic versioning. | [1.0, 2.0[ |  | 1.5 |  | Any version between `1.0` and `2.0` (exclusive), `1.5` if nobody else cares. Overwrites versions from transitive dependencies. 🔒
Same as above, with `1.4` known broken. | [1.0, 2.0[ |  | 1.5 | 1.4 | Any version between `1.0` and `2.0` (exclusive) except for `1.4`, `1.5` if nobody else cares. Overwrites versions from transitive dependencies. 🔒
No opinion, works with `1.5`. |  |  | 1.5 |  | `1.5` if no other opinion, any otherwise.
No opinion, prefer the latest release. |  |  | `latest.release` |  | The latest release at build time. 🔒
On the edge, latest release, no downgrade. |  | `latest.release` |  |  | The latest release at build time. 🔒
No other version than 1.5. | 1.5 |  |  |  | 1.5, or failure if another `strict` or higher `require` constraint disagrees. Overwrites versions from transitive dependencies.
`1.5` or a patch version of it exclusively. | [1.5,1.6[ |  |  |  | Latest `1.5.x` patch release, or failure if another `strict` or higher `require` constraint disagrees. Overwrites versions from transitive dependencies. 🔒

Lines annotated with a lock (🔒) indicate that leveraging dependency locking makes sense in this context. Another concept related to rich version declaration is the ability to publish resolved versions instead of declared ones.

Using `strictly`, especially for a library, must be a well-thought-out process as it impacts downstream consumers. At the same time, if used correctly, it will help consumers understand what combination of libraries does not work together in their context. See overriding dependency version for more information.

| Rich version information will be preserved in the Gradle Module Metadata format. However conversion to Ivy or Maven metadata formats will be lossy. The highest level will be published, that is `strictly` or `require` over `prefer`. In addition, any `reject` will be ignored.

Rich version declaration is accessed through the `version` DSL method on a dependency or constraint declaration, which gives access to MutableVersionConstraint:

build.gradle.kts

    dependencies {
        implementation("org.slf4j:slf4j-api") {
            version {
                strictly("[1.7, 1.8[")
                prefer("1.7.25")
            }
        }

        constraints {
            add("implementation", "org.springframework:spring-core") {
                version {
                    require("4.2.9.RELEASE")
                    reject("4.3.16.RELEASE")
                }
            }
        }
    }

build.gradle

    dependencies {
        implementation('org.slf4j:slf4j-api') {
            version {
                strictly '[1.7, 1.8['
                prefer '1.7.25'
            }
        }

        constraints {
            implementation('org.springframework:spring-core') {
                version {
                    require '4.2.9.RELEASE'
                    reject '4.3.16.RELEASE'
                }
            }
        }
    }

## Handling dynamic versions

There are many situations when you might need to use the latest version of a specific module dependency or the latest within a range of versions. This is often necessary during development or when creating a library that is compatible with various dependency versions.

You can easily depend on these constantly changing dependencies by using a dynamic version.

A dynamic version can be either a version range (e.g., `2.+`) or a placeholder for the latest version available, e.g., `latest.integration`.

| For reproducible builds, use dependency locking when declaring dependencies with dynamic versions.

Alternatively, the module you request can change even for the same version, which is known as a changing version. An example of a changing module is a Maven `SNAPSHOT` module, which always points to the latest artifact published. In other words, a standard Maven snapshot is a module that is continually evolving; it is a "changing module".

| Using dynamic versions and changing modules can lead to unreproducible builds. As new versions of a particular module are published, its API may become incompatible with your source code. Use this feature with caution.

### Declaring a dynamic version

Projects might adopt a more aggressive approach for consuming dependencies to modules. For example, you might want to integrate the latest version of a dependency to consume cutting-edge features at any given time. A dynamic version allows for resolving the latest version or the latest version of a version range for a given module.

| Using dynamic versions in a build can break it. As soon as a new version of the dependency that contains an incompatible API change is released, your source code might stop compiling.

build.gradle.kts

    plugins {
        `java-library`
    }

    repositories {
        mavenCentral()
    }

    dependencies {
        implementation("org.springframework:spring-web:5.+")
    }

build.gradle

    plugins {
        id 'java-library'
    }

    repositories {
        mavenCentral()
    }

    dependencies {
        implementation 'org.springframework:spring-web:5.+'
    }

A Build Scan can effectively visualize dynamic dependency versions and their respective selected versions:

Gradle caches dynamic versions of dependencies for 24 hours by default, during which it does not attempt to resolve newer versions from the declared repositories. You can adjust the caching threshold to resolve new versions sooner.

### Declaring a changing version

A team may implement a series of features before releasing a new version of the application or library. A common strategy to allow consumers to integrate an unfinished version of their artifacts early and often is to release a module with a so-called changing version. A changing version indicates that the feature set is still under active development and hasn’t released a stable version for general availability yet.

In Maven repositories, changing versions are commonly referred to as snapshot versions. Snapshot versions contain the suffix `-SNAPSHOT`.

The following example demonstrates how to declare a snapshot version on the Spring dependency:

build.gradle.kts

    plugins {
        `java-library`
    }

    repositories {
        mavenCentral()
        maven {
            url = uri("https://repo.spring.io/snapshot/")
        }
    }

    dependencies {
        implementation("org.springframework:spring-web:5.0.3.BUILD-SNAPSHOT")
    }

build.gradle

    plugins {
        id 'java-library'
    }

    repositories {
        mavenCentral()
        maven {
            url 'https://repo.spring.io/snapshot/'
        }
    }

    dependencies {
        implementation 'org.springframework:spring-web:5.0.3.BUILD-SNAPSHOT'
    }

By default, Gradle caches changing versions of dependencies for 24 hours. Within this time frame, Gradle does not try to resolve newer versions from the declared repositories. The threshold is configurable.

Gradle is flexible enough to treat any version as a changing version, e.g., if you want to model snapshot behavior for an Ivy module. All you need to do is to set the property ExternalModuleDependency.setChanging(boolean) to `true`.

### Controlling dynamic version caching

By default, Gradle caches dynamic versions and changing modules for 24 hours. Gradle does not contact any of the declared remote repositories for new versions during that time. You must change the time to live (TTL) threshold if you want Gradle to check the remote repository more frequently or with every build execution.

| Using a short TTL threshold for dynamic or changing versions may result in longer build times due to increased HTTP(s) calls.

You can override the default cache modes using command line options. You can also change the cache expiry times in your build programmatically using the resolution strategy.

### Controlling dependency caching programmatically

You can fine-tune certain aspects of caching programmatically using the ResolutionStrategy for a configuration. The programmatic approach is useful if you want to change the settings permanently.

By default, Gradle caches dynamic versions for 24 hours. To change how long Gradle will cache the resolved version for a dynamic version, use:

build.gradle.kts

    configurations.all {
        resolutionStrategy.cacheDynamicVersionsFor(10, "minutes")
    }

build.gradle

    configurations.all {
        resolutionStrategy.cacheDynamicVersionsFor 10, 'minutes'
    }

By default, Gradle caches changing modules for 24 hours. To change how long Gradle will cache the meta-data and artifacts for a changing module, use:

build.gradle.kts

    configurations.all {
        resolutionStrategy.cacheChangingModulesFor(4, "hours")
    }

build.gradle

    configurations.all {
        resolutionStrategy.cacheChangingModulesFor 4, 'hours'
    }

### Controlling dependency caching from the command line

#### Avoiding network access with offline mode

The `--offline` command-line switch instructs Gradle to use dependency modules from the cache, regardless of whether they are due to be checked again. When running with `offline`, Gradle will not attempt to access the network for dependency resolution. If the required modules are not in the dependency cache, the build will fail.

#### Refreshing dependencies

You can control the behavior of dependency caching for a distinct build invocation from the command line. Command line options help make a selective, ad-hoc choice for a single build execution.

At times, the Gradle Dependency Cache can become out of sync with the actual state of the configured repositories. Perhaps a repository was initially misconfigured, or maybe a "non-changing" module was published incorrectly. To refresh all dependencies in the dependency cache, use the `--refresh-dependencies` option on the command line.

The `--refresh-dependencies` option tells Gradle to ignore all cached entries for resolved modules and artifacts. A fresh resolve will be performed against all configured repositories, with dynamic versions recalculated, modules refreshed, and artifacts downloaded. However, where possible Gradle will check if the previously downloaded artifacts are valid before downloading again. This is done by comparing published SHA1 values in the repository with the SHA1 values for existing downloaded artifacts.

- new versions of dynamic dependencies

- new versions of changing modules (modules that use the same version string but can have different contents)

Refreshing dependencies will cause Gradle to invalidate its listing caches. However:

- it will perform HTTP HEAD requests on metadata files but will not re-download them if they are identical

- it will perform HTTP HEAD requests on artifact files but will not re-download them if they are identical

In other words, refreshing dependencies only has an impact if you actually use dynamic dependencies or that you have changing dependencies that you were not aware of (in which case it is your responsibility to declare them correctly to Gradle as changing dependencies).

It’s a common misconception to think that using `--refresh-dependencies` will force the download of dependencies. This is not the case: Gradle will only perform what is strictly required to refresh the dynamic dependencies. This may involve downloading new listings, metadata files, or even artifacts, but the impact is minimal if nothing changed.

### Using component selection rules

Component selection rules may influence which component instance should be selected when multiple versions are available that match a version selector. Rules are applied against every available version and allow the version to be explicitly rejected. This allows Gradle to ignore any component instance that does not satisfy conditions set by the rule. Examples include:

- For a dynamic version like `1.+` certain versions may be explicitly rejected from selection.

- For a static version like `1.4` an instance may be rejected based on extra component metadata such as the Ivy branch attribute, allowing an instance from a subsequent repository to be used.

Rules are configured via the ComponentSelectionRules object. Each rule configured will be called with a ComponentSelection object as an argument that contains information about the candidate version being considered. Calling ComponentSelection.reject(java.lang.String) causes the given candidate version to be explicitly rejected, in which case the candidate will not be considered for the selector.

The following example shows a rule that disallows a particular version of a module but allows the dynamic version to choose the next best candidate:

build.gradle.kts

    configurations {
        create("rejectConfig") {
            resolutionStrategy {
                componentSelection {
                    // Accept the highest version matching the requested version that isn't '1.5'
                    all {
                        if (candidate.group == "org.sample" && candidate.module == "api" && candidate.version == "1.5") {
                            reject("version 1.5 is broken for 'org.sample:api'")
                        }
                    }
                }
            }
        }
    }

    dependencies {
        "rejectConfig"("org.sample:api:1.+")
    }

build.gradle

    configurations {
        rejectConfig {
            resolutionStrategy {
                componentSelection {
                    // Accept the highest version matching the requested version that isn't '1.5'
                    all { ComponentSelection selection ->
                        if (selection.candidate.group == 'org.sample' && selection.candidate.module == 'api' && selection.candidate.version == '1.5') {
                            selection.reject("version 1.5 is broken for 'org.sample:api'")
                        }
                    }
                }
            }
        }
    }

    dependencies {
        rejectConfig "org.sample:api:1.+"
    }

Note that version selection is applied starting with the highest version first. The version selected will be the first version found that all component selection rules accept. A version is considered accepted if no rule explicitly rejects it.

Similarly, rules can be targeted at specific modules. Modules must be specified in the form of `group:module`:

build.gradle.kts

    configurations {
        create("targetConfig") {
            resolutionStrategy {
                componentSelection {
                    withModule("org.sample:api") {
                        if (candidate.version == "1.5") {
                            reject("version 1.5 is broken for 'org.sample:api'")
                        }
                    }
                }
            }
        }
    }

build.gradle

    configurations {
        targetConfig {
            resolutionStrategy {
                componentSelection {
                    withModule("org.sample:api") { ComponentSelection selection ->
                        if (selection.candidate.version == "1.5") {
                            selection.reject("version 1.5 is broken for 'org.sample:api'")
                        }
                    }
                }
            }
        }
    }

Component selection rules can also consider component metadata when selecting a version. Possible additional metadata that can be considered are ComponentMetadata and IvyModuleDescriptor. Note that this extra information may not always be available and thus should be checked for `null` values:

build.gradle.kts

    configurations {
        create("metadataRulesConfig") {
            resolutionStrategy {
                componentSelection {
                    // Reject any versions with a status of 'experimental'
                    all {
                        if (candidate.group == "org.sample" && metadata?.status == "experimental") {
                            reject("don't use experimental candidates from 'org.sample'")
                        }
                    }
                    // Accept the highest version with either a "release" branch or a status of 'milestone'
                    withModule("org.sample:api") {
                        if (getDescriptor(IvyModuleDescriptor::class)?.branch != "release" && metadata?.status != "milestone") {
                            reject("'org.sample:api' must have testing branch or milestone status")
                        }
                    }
                }
            }
        }
    }

build.gradle

    configurations {
        metadataRulesConfig {
            resolutionStrategy {
                componentSelection {
                    // Reject any versions with a status of 'experimental'
                    all { ComponentSelection selection ->
                        if (selection.candidate.group == 'org.sample' && selection.metadata?.status == 'experimental') {
                            selection.reject("don't use experimental candidates from 'org.sample'")
                        }
                    }
                    // Accept the highest version with either a "release" branch or a status of 'milestone'
                    withModule('org.sample:api') { ComponentSelection selection ->
                        if (selection.getDescriptor(IvyModuleDescriptor)?.branch != "release" && selection.metadata?.status != 'milestone') {
                            selection.reject("'org.sample:api' must be a release branch or have milestone status")
                        }
                    }
                }
            }
        }
    }

Note that a ComponentSelection argument is always required as a parameter when declaring a component selection rule.

## Locking dependency versions

Use of dynamic dependency versions (e.g. `1.+` or `[1.0,2.0)`) makes builds non-deterministic. This causes builds to break without any obvious change, and worse, can be caused by a transitive dependency that the build author has no control over.

To achieve reproducible builds, it is necessary to lock versions of dependencies and transitive dependencies such that a build with the same inputs will always resolve the same module versions. This is called dependency locking.

It enables several key scenarios, including:

- Companies managing multiple repositories no longer need to rely on `-SNAPSHOT` or changing dependencies, which can result in cascading failures when a dependency introduces a bug or incompatibility.

- Teams using the latest dependencies can use dynamic versions, locking their dependencies only for releases. The release tag will contain the lock states, ensuring the build is fully reproducible when bug fixes need to be developed.

Combined with publishing resolved versions, you can replace the declared dynamic versions at publication time. Consumers will see the resolved versions that your release used.

Locking is enabled per dependency configuration. Once enabled, you must create an initial lock state. This will cause Gradle to verify that resolution results do not change, ensuring the same dependencies are selected even if newer versions are available. Modifications to your build that impact the resolved set of dependencies will cause it to fail. This ensures that changes in published dependencies or build definitions do not alter resolution without updating the lock state.

| Dependency locking is effective only with dynamic versions. It has no impact on changing versions (e.g., `-SNAPSHOT`), where the coordinates remain the same but the content may change. Gradle will emit a warning when persisting the lock state if changing dependencies are present in the resolution result.

### Locking specific configurations

Locking of a configuration happens through the ResolutionStrategy:

build.gradle.kts

    configurations {
        compileClasspath {
            resolutionStrategy.activateDependencyLocking()
        }
    }

build.gradle

    configurations {
        compileClasspath {
            resolutionStrategy.activateDependencyLocking()
        }
    }

Only configurations that can be resolved will have lock state attached to them. Applying locking on non resolvable-configurations is a no-op.

The following locks all configurations:

build.gradle.kts

    dependencyLocking {
        lockAllConfigurations()
    }

build.gradle

    dependencyLocking {
        lockAllConfigurations()
    }

| The above will lock all project configurations, but not the buildscript ones.

You can also disable locking on a specific configuration. This can be useful if a plugin configured locking on all configurations, but you happen to add one that should not be locked:

build.gradle.kts

    configurations.compileClasspath {
        resolutionStrategy.deactivateDependencyLocking()
    }

build.gradle

    configurations {
        compileClasspath {
            resolutionStrategy.deactivateDependencyLocking()
        }
    }

#### Locking buildscript classpath configuration

If you apply plugins to your build, you may want to leverage dependency locking there as well. To lock the `classpath` configuration used for script plugins:

build.gradle.kts

    buildscript {
        configurations.classpath {
            resolutionStrategy.activateDependencyLocking()
        }
    }

build.gradle

    buildscript {
        configurations.classpath {
            resolutionStrategy.activateDependencyLocking()
        }
    }

### Generating and updating dependency locks

In order to generate or update lock state, you specify the `--write-locks` command line argument in addition to the normal tasks that would trigger configurations to be resolved. This will cause the creation of lock state for each resolved configuration in that build execution. If lock state existed previously, it is overwritten.

| Gradle will not write the lock state to disk if the build fails. This prevents persisting possibly invalid state.

#### Lock all configurations in one build execution

When locking multiple configurations, you may want to lock them all at once, during a single build execution.

For this, you have two options:

- Run `gradle dependencies --write-locks`. This will effectively lock all resolvable configurations that have locking enabled. Note that in a multi project setup, `dependencies` only is executed on one project, the root one in this case.

- Declare a custom task that resolves all configurations. This does not work for Android projects.

build.gradle.kts

    tasks.register("resolveAndLockAll") {
        notCompatibleWithConfigurationCache("Filters configurations at execution time")
        doFirst {
            require(gradle.startParameter.isWriteDependencyLocks) { "$path must be run from the command line with the `--write-locks` flag" }
        }
        doLast {
            configurations.filter {
                // Add any custom filtering on the configurations to be resolved
                it.isCanBeResolved
            }.forEach { it.resolve() }
        }
    }

build.gradle

    tasks.register('resolveAndLockAll') {
        notCompatibleWithConfigurationCache("Filters configurations at execution time")
        doFirst {
            assert gradle.startParameter.writeDependencyLocks : "$path must be run from the command line with the `--write-locks` flag"
        }
        doLast {
            configurations.findAll {
                // Add any custom filtering on the configurations to be resolved
                it.canBeResolved
            }.each { it.resolve() }
        }
    }

That second option, with proper selection of configurations, can be the only option in the native world, where not all configurations can be resolved on a single platform.

### Lock state location and format

A lockfile is a file that stores the exact versions of dependencies used in a project, preventing unexpected changes in dependencies when a project is built on different machines or at different times.

Lock state will be preserved in a file located at the root of the project or subproject directory. Each file is named `gradle.lockfile`.

The one exception to this rule is for the lock file for the buildscript itself. In that case the file will be named `buildscript-gradle.lockfile`.

For the following dependency declaration:

build.gradle.kts

    configurations {
        compileClasspath {
            resolutionStrategy.activateDependencyLocking()
        }
        runtimeClasspath {
            resolutionStrategy.activateDependencyLocking()
        }
        annotationProcessor {
            resolutionStrategy.activateDependencyLocking()
        }
    }

    dependencies {
        implementation("org.springframework:spring-beans:[5.0,6.0)")
    }

build.gradle

    configurations {
        compileClasspath {
            resolutionStrategy.activateDependencyLocking()
        }
        runtimeClasspath {
            resolutionStrategy.activateDependencyLocking()
        }
        annotationProcessor {
            resolutionStrategy.activateDependencyLocking()
        }
    }

    dependencies {
        implementation 'org.springframework:spring-beans:[5.0,6.0)'
    }

The lockfile will have the following content:

gradle.lockfile

    # This is a Gradle generated file for dependency locking.
    # Manual edits can break the build and are not advised.
    # This file is expected to be part of source control.
    org.springframework:spring-beans:5.0.5.RELEASE=compileClasspath, runtimeClasspath
    org.springframework:spring-core:5.0.5.RELEASE=compileClasspath, runtimeClasspath
    org.springframework:spring-jcl:5.0.5.RELEASE=compileClasspath, runtimeClasspath
    empty=annotationProcessor

Where:

- Each line represents a single dependency in the `group:artifact:version` notation

- It then lists all configurations that contain the given dependency

- Module and configurations are ordered alphabetically, to ease diffs

- The last line of the file lists all empty configurations, that is configurations known to have no dependencies

#### Migrating from the lockfile per configuration format

If your project uses the legacy lock file format of a file per locked configuration, follow these instructions to migrate to the new format:

1. Follow the documentation for writing or updating dependency lock state.

2. Upon writing the single lock file per project, Gradle will also delete all lock files per configuration for which the state was transferred.

| Migration can be done one configuration at a time. Gradle will keep sourcing the lock state from the per configuration files as long as there is no information for that configuration in the single lock file.

#### Configuring the lock file name and location

When using a single lock file per project, you can configure its name and location.

This capability allows you to specify a file name based on project properties, enabling a single project to store different lock states for different execution contexts.

For example, in the JVM ecosystem, the Scala version is often included in artifact coordinates:

build.gradle.kts

    val scalaVersion = "2.12"
    dependencyLocking {
        lockFile = file("$projectDir/locking/gradle-${scalaVersion}.lockfile")
    }

build.gradle

    def scalaVersion = "2.12"
    dependencyLocking {
        lockFile = file("$projectDir/locking/gradle-${scalaVersion}.lockfile")
    }

### Running a build with lock state present

The moment a build needs to resolve a configuration that has locking enabled and it finds a matching lock state, it will use it to verify that the given configuration still resolves the same versions.

A successful build indicates that the same dependencies are used as stored in the lock state, regardless if new versions matching the dynamic selector have been produced.

The complete validation is as follows:

- Existing entries in the lock state must be matched in the build

  - A version mismatch or missing resolved module causes a build failure

- Resolution result must not contain extra dependencies compared to the lock state

#### Fine tuning dependency locking behaviour with lock mode

While the default lock mode behaves as described above, two other modes are available:

Strict mode

  In this mode, in addition to the validations above, dependency locking will fail if a configuration marked as locked does not have lock state associated with it.

Lenient mode

  In this mode, dependency locking will still pin dynamic versions but otherwise changes to the dependency resolution are no longer errors.

The lock mode can be controlled from the `dependencyLocking` block as shown below:

build.gradle.kts

    dependencyLocking {
        lockMode = LockMode.STRICT
    }

build.gradle

    dependencyLocking {
        lockMode = LockMode.STRICT
    }

### Updating lock state entries selectively

In order to update only specific modules of a configuration, you can use the `--update-locks` command line flag. It takes a comma (`,`) separated list of module notations. In this mode, the existing lock state is still used as input to resolution, filtering out the modules targeted by the update.

    ❯ gradle dependencies --update-locks org.apache.commons:commons-lang3,org.slf4j:slf4j-api

Wildcards, indicated with `*`, can be used in the group or module name. They can be the only character or appear at the end of the group or module respectively. The following wildcard notation examples are valid:

- `org.apache.commons:*`: will let all modules belonging to group `org.apache.commons` update

- `*:guava`: will let all modules named `guava`, whatever their group, update

- `org.springframework.spring*:spring*`: will let all modules having their group starting with `org.springframework.spring` and name starting with `spring` update

| The resolution may cause other module versions to update, as dictated by the Gradle resolution rules.

### Disabling dependency locking

1. Make sure that the configuration for which you no longer want locking is not configured with locking.

2. Next time you update the save lock state, Gradle will automatically clean up all stale lock state from it.

Gradle needs to resolve a configuration, no longer marked as locked, to detect that associated lock state can be dropped.

### Ignoring specific dependencies from the lock state

Dependency locking can be used in cases where reproducibility is not the main goal.

As a build author, you may want to have different frequency of dependency version updates, based on their origin for example. In that case, it might be convenient to ignore some dependencies because you always want to use the latest version for those. An example is the internal dependencies in an organization which should always use the latest version as opposed to third party dependencies which have a different upgrade cycle.

| This feature can break reproducibility and should be used with caution. There are scenarios that are better served with leveraging different lock modes or using different names for lock files.

You can configure ignored dependencies in the `dependencyLocking` project extension:

build.gradle.kts

    dependencyLocking {
        ignoredDependencies.add("com.example:*")
    }

build.gradle

    dependencyLocking {
        ignoredDependencies.add('com.example:*')
    }

The notation is a `<group>:<name>` dependency notation, where `*` can be used as a trailing wildcard. See the description on updating lock files for more details. Note that the value `*:*` is not accepted as it is equivalent to disabling locking.

Ignoring dependencies will have the following effects:

- An ignored dependency applies to all locked configurations. The setting is project scoped.

- Ignoring a dependency does not mean lock state ignores its transitive dependencies.

- There is no validation that an ignored dependency is present in any configuration resolution.

- If the dependency is present in lock state, loading it will filter out the dependency.

- If the dependency is present in the resolution result, it will be ignored when validating that resolution matches the lock state.

- Finally, if the dependency is present in the resolution result and the lock state is persisted, it will be absent from the written lock state.

### Understanding locking limitations

- Locking cannot yet be applied to source dependencies.

# Controlling Transitive Dependencies

## Upgrading versions of transitive dependencies

### Direct dependencies vs dependency constraints

A component may have two different kinds of dependencies:

- direct dependencies are directly required by the component. A direct dependency is also referred to as a first level dependency. For example, if your project source code requires Guava, Guava should be declared as direct dependency.

- transitive dependencies are dependencies that your component needs, but only because another dependency needs them.

It’s quite common that issues with dependency management are about transitive dependencies. Often developers incorrectly fix transitive dependency issues by adding direct dependencies. To avoid this, Gradle provides the concept of dependency constraints.

### Adding constraints on transitive dependencies

Dependency constraints allow you to define the version or the version range of both dependencies declared in the build script and transitive dependencies. It is the preferred method to express constraints that should be applied to all dependencies of a configuration. When Gradle attempts to resolve a dependency to a module version, all dependency declarations with version, all transitive dependencies and all dependency constraints for that module are taken into consideration. The highest version that matches all conditions is selected. If no such version is found, Gradle fails with an error showing the conflicting declarations. If this happens you can adjust your dependencies or dependency constraints declarations, or make other adjustments to the transitive dependencies if needed. Similar to dependency declarations, dependency constraint declarations are scoped by configurations and can therefore be selectively defined for parts of a build. If a dependency constraint influenced the resolution result, any type of dependency resolve rules may still be applied afterwards.

Example 4. Define dependency constraints
build.gradle.kts

    dependencies {
        implementation("org.apache.httpcomponents:httpclient")
        constraints {
            implementation("org.apache.httpcomponents:httpclient:4.5.3") {
                because("previous versions have a bug impacting this application")
            }
            implementation("commons-codec:commons-codec:1.11") {
                because("version 1.9 pulled from httpclient has bugs affecting this application")
            }
        }
    }

build.gradle

    dependencies {
        implementation 'org.apache.httpcomponents:httpclient'
        constraints {
            implementation('org.apache.httpcomponents:httpclient:4.5.3') {
                because 'previous versions have a bug impacting this application'
            }
            implementation('commons-codec:commons-codec:1.11') {
                because 'version 1.9 pulled from httpclient has bugs affecting this application'
            }
        }
    }

In the example, all versions are omitted from the dependency declaration. Instead, the versions are defined in the constraints block. The version definition for `commons-codec:1.11` is only taken into account if `commons-codec` is brought in as transitive dependency, since `commons-codec` is not defined as dependency in the project. Otherwise, the constraint has no effect. Dependency constraints can also define a rich version constraint and support strict versions to enforce a version even if it contradicts with the version defined by a transitive dependency (e.g. if the version needs to be downgraded).

| Dependency constraints are only published when using Gradle Module Metadata. This means that currently they are only fully supported if Gradle is used for publishing and consuming (i.e. they are 'lost' when consuming modules with Maven or Ivy).

Dependency constraints themselves can also be added transitively.

## Downgrading versions and excluding dependencies

### Overriding transitive dependency versions

Gradle resolves any dependency version conflicts by selecting the latest version found in the dependency graph. Some projects might need to divert from the default behavior and enforce an earlier version of a dependency e.g. if the source code of the project depends on an older API of a dependency than some of the external libraries.

| Forcing a version of a dependency requires a conscious decision. Changing the version of a transitive dependency might lead to runtime errors if external libraries do not properly function without them. Consider upgrading your source code to use a newer version of the library as an alternative approach.

In general, forcing dependencies is done to downgrade a dependency. There might be different use cases for downgrading:

- a bug was discovered in the latest release

- your code depends on a lower version which is not binary compatible

- your code doesn’t depend on the code paths which need a higher version of a dependency

In all situations, this is best expressed saying that your code strictly depends on a version of a transitive. Using strict versions, you will effectively depend on the version you declare, even if a transitive dependency says otherwise.

| Strict dependencies are to some extent similar to Maven’s nearest first strategy, but there are subtle differences: strict dependencies don’t suffer an ordering problem: they are applied transitively to the subgraph, and it doesn’t matter in which order dependencies are declared. conflicting strict dependencies will trigger a build failure that you have to resolve strict dependencies can be used with rich versions, meaning that it’s better to express the requirement in terms of a strict range combined with a single preferred version.

Let’s say a project uses the HttpClient library for performing HTTP calls. HttpClient pulls in Commons Codec as transitive dependency with version 1.10. However, the production source code of the project requires an API from Commons Codec 1.9 which is not available in 1.10 anymore. A dependency version can be enforced by declaring it as strict it in the build script:

Example 5. Setting a strict version
build.gradle.kts

    dependencies {
        implementation("org.apache.httpcomponents:httpclient:4.5.4")
        implementation("commons-codec:commons-codec") {
            version {
                strictly("1.9")
            }
        }
    }

build.gradle

    dependencies {
        implementation 'org.apache.httpcomponents:httpclient:4.5.4'
        implementation('commons-codec:commons-codec') {
            version {
                strictly '1.9'
            }
        }
    }

#### Consequences of using strict versions

Using a strict version must be carefully considered, in particular by library authors. As the producer, a strict version will effectively behave like a force: the version declaration takes precedence over whatever is found in the transitive dependency graph. In particular, a strict version will override any other strict version on the same module found transitively.

However, for consumers, strict versions are still considered globally during graph resolution and may trigger an error if the consumer disagrees.

For example, imagine that your project `B` strictly depends on `C:1.0`. Now, a consumer, `A`, depends on both `B` and `C:1.1`.

Then this would trigger a resolution error because `A` says it needs `C:1.1` but `B`, within its subgraph, strictly needs `1.0`. This means that if you choose a single version in a strict constraint, then the version can no longer be upgraded, unless the consumer also sets a strict version constraint on the same module.

In the example above, `A` would have to say it strictly depends on 1.1.

For this reason, a good practice is that if you use strict versions, you should express them in terms of ranges and a preferred version within this range. For example, `B` might say, instead of `strictly 1.0`, that it strictly depends on the `[1.0, 2.0[` range, but prefers `1.0`. Then if a consumer chooses 1.1 (or any other version in the range), the build will no longer fail (constraints are resolved).

#### Forced dependencies vs strict dependencies

If the project requires a specific version of a dependency at the configuration-level this can be achieved by calling the method ResolutionStrategy.force(java.lang.Object[]).

Example 6. Enforcing a dependency version on the configuration-level
build.gradle.kts

    configurations {
        "compileClasspath" {
            resolutionStrategy.force("commons-codec:commons-codec:1.9")
        }
    }

    dependencies {
        implementation("org.apache.httpcomponents:httpclient:4.5.4")
    }

build.gradle

    configurations {
        compileClasspath {
            resolutionStrategy.force 'commons-codec:commons-codec:1.9'
        }
    }

    dependencies {
        implementation 'org.apache.httpcomponents:httpclient:4.5.4'
    }

### Excluding transitive dependencies

While the previous section showed how you can enforce a certain version of a transitive dependency, this section covers excludes as a way to remove a transitive dependency completely.

| Similar to forcing a version of a dependency, excluding a dependency completely requires a conscious decision. Excluding a transitive dependency might lead to runtime errors if external libraries do not properly function without them. If you use excludes, make sure that you do not utilise any code path requiring the excluded dependency by sufficient test coverage.

Transitive dependencies can be excluded on the level of a declared dependency. Exclusions are spelled out as a key/value pair via the attributes `group` and/or `module` as shown in the example below. For more information, refer to ModuleDependency.exclude(java.util.Map).

Example 7. Excluding a transitive dependency for a particular dependency declaration
build.gradle.kts

    dependencies {
        implementation("commons-beanutils:commons-beanutils:1.9.4") {
            exclude(group = "commons-collections", module = "commons-collections")
        }
    }

build.gradle

    dependencies {
        implementation('commons-beanutils:commons-beanutils:1.9.4') {
            exclude group: 'commons-collections', module: 'commons-collections'
        }
    }

In this example, we add a dependency to `commons-beanutils` but exclude the transitive dependency `commons-collections`. In our code, shown below, we only use one method from the beanutils library, `PropertyUtils.setSimpleProperty()`. Using this method for existing setters does not require any functionality from `commons-collections` as we verified through test coverage.

Example 8. Using a utility from the beanutils library
src/main/java/Main.java

    import org.apache.commons.beanutils.PropertyUtils;

    public class Main {
        public static void main(String[] args) throws Exception {
            Object person = new Person();
            PropertyUtils.setSimpleProperty(person, "name", "Bart Simpson");
            PropertyUtils.setSimpleProperty(person, "age", 38);
        }
    }

Effectively, we are expressing that we only use a subset of the library, which does not require the `commons-collection` library. This can be seen as implicitly defining a feature variant that has not been explicitly declared by `commons-beanutils` itself. However, the risk of breaking an untested code path increased by doing this.

For example, here we use the `setSimpleProperty()` method to modify properties defined by setters in the `Person` class, which works fine. If we would attempt to set a property not existing on the class, we should get an error like `Unknown property on class Person`. However, because the error handling path uses a class from `commons-collections`, the error we now get is `NoClassDefFoundError: org/apache/commons/collections/FastHashMap`. So if our code would be more dynamic, and we would forget to cover the error case sufficiently, consumers of our library might be confronted with unexpected errors.

This is only an example to illustrate potential pitfalls. In practice, larger libraries or frameworks can bring in a huge set of dependencies. If those libraries fail to declare features separately and can only be consumed in a "all or nothing" fashion, excludes can be a valid method to reduce the library to the feature set actually required.

On the upside, Gradle’s exclude handling is, in contrast to Maven, taking the whole dependency graph into account. So if there are multiple dependencies on a library, excludes are only exercised if all dependencies agree on them. For example, if we add `opencsv` as another dependency to our project above, which also depends on `commons-beanutils`, `commons-collection` is no longer excluded as `opencsv` itself does not exclude it.

Example 9. Excludes only apply if all dependency declarations agree on an exclude
build.gradle.kts

    dependencies {
        implementation("commons-beanutils:commons-beanutils:1.9.4") {
            exclude(group = "commons-collections", module = "commons-collections")
        }
        implementation("com.opencsv:opencsv:4.6") // depends on 'commons-beanutils' without exclude and brings back 'commons-collections'
    }

build.gradle

    dependencies {
        implementation('commons-beanutils:commons-beanutils:1.9.4') {
            exclude group: 'commons-collections', module: 'commons-collections'
        }
        implementation 'com.opencsv:opencsv:4.6' // depends on 'commons-beanutils' without exclude and brings back 'commons-collections'
    }

If we still want to have `commons-collections` excluded, because our combined usage of `commons-beanutils` and `opencsv` does not need it, we need to exclude it from the transitive dependencies of `opencsv` as well.

Example 10. Excluding a transitive dependency for multiple dependency declaration
build.gradle.kts

    dependencies {
        implementation("commons-beanutils:commons-beanutils:1.9.4") {
            exclude(group = "commons-collections", module = "commons-collections")
        }
        implementation("com.opencsv:opencsv:4.6") {
            exclude(group = "commons-collections", module = "commons-collections")
        }
    }

build.gradle

    dependencies {
        implementation('commons-beanutils:commons-beanutils:1.9.4') {
            exclude group: 'commons-collections', module: 'commons-collections'
        }
        implementation('com.opencsv:opencsv:4.6') {
            exclude group: 'commons-collections', module: 'commons-collections'
        }
    }

Historically, excludes were also used as a band aid to fix other issues not supported by some dependency management systems. Gradle however, offers a variety of features that might be better suited to solve a certain use case. You may consider to look into the following features:

- Update or downgrade dependency versions: If versions of dependencies clash, it is usually better to adjust the version through a dependency constraint, instead of attempting to exclude the dependency with the undesired version.

- Component Metadata Rules: If a library’s metadata is clearly wrong, for example if it includes a compile time dependency which is never needed at compile time, a possible solution is to remove the dependency in a component metadata rule. By this, you tell Gradle that a dependency between two modules is never needed — i.e. the metadata was wrong — and therefore should never be considered. If you are developing a library, you have to be aware that this information is not published, and so sometimes an exclude can be the better alternative.

- Resolving mutually exclusive dependency conflicts: Another situation that you often see solved by excludes is that two dependencies cannot be used together because they represent two implementations of the same thing (the same capability). Some popular examples are clashing logging API implementations (like `log4j` and `log4j-over-slf4j`) or modules that have different coordinates in different versions (like `com.google.collections` and `guava`). In these cases, if this information is not known to Gradle, it is recommended to add the missing capability information via component metadata rules as described in the declaring component capabilities section. Even if you are developing a library, and your consumers will have to deal with resolving the conflict again, it is often the right solution to leave the decision to the final consumers of libraries. I.e. you as a library author should not have to decide which logging implementation your consumers use in the end.

## Sharing dependency versions between projects

### Central declaration of dependencies

#### Using a version catalog

A version catalog is a list of dependencies, represented as dependency coordinates, that a user can pick from when declaring dependencies in a build script.

For example, instead of declaring a dependency using a string notation, the dependency coordinates can be picked from a version catalog:

Example 11. Using a library declared in a version catalog
build.gradle.kts

    dependencies {
        implementation(libs.groovy.core)
    }

build.gradle

    dependencies {
        implementation(libs.groovy.core)
    }

In this context, `libs` is a catalog and `groovy` represents a dependency available in this catalog. A version catalog provides a number of advantages over declaring the dependencies directly in build scripts:

- For each catalog, Gradle generates type-safe accessors so that you can easily add dependencies with autocompletion in the IDE.

- Each catalog is visible to all projects of a build. It is a central place to declare a version of a dependency and to make sure that a change to that version applies to every subproject.

- Catalogs can declare dependency bundles, which are "groups of dependencies" that are commonly used together.

- Catalogs can separate the group and name of a dependency from its actual version and use version references instead, making it possible to share a version declaration between multiple dependencies.

Adding a dependency using the `libs.someLib` notation works exactly like if you had hardcoded the group, artifact and version directly in the build script.

| A dependency catalog doesn’t enforce the version of a dependency: like a regular dependency notation, it declares the requested version or a rich version. That version is not necessarily the version that is selected during conflict resolution.

#### Declaring a version catalog

Version catalogs can be declared in the `settings.gradle(.kts)` file. In the example above, in order to make `groovy` available via the `libs` catalog, we need to associate an alias with GAV (group, artifact, version) coordinates:

Example 12. Declaring a version catalog
settings.gradle.kts

    dependencyResolutionManagement {
        versionCatalogs {
            create("libs") {
                library("groovy-core", "org.codehaus.groovy:groovy:3.0.5")
                library("groovy-json", "org.codehaus.groovy:groovy-json:3.0.5")
                library("groovy-nio", "org.codehaus.groovy:groovy-nio:3.0.5")
                library("commons-lang3", "org.apache.commons", "commons-lang3").version {
                    strictly("[3.8, 4.0[")
                    prefer("3.9")
                }
            }
        }
    }

settings.gradle

    dependencyResolutionManagement {
        versionCatalogs {
            libs {
                library('groovy-core', 'org.codehaus.groovy:groovy:3.0.5')
                library('groovy-json', 'org.codehaus.groovy:groovy-json:3.0.5')
                library('groovy-nio', 'org.codehaus.groovy:groovy-nio:3.0.5')
                library('commons-lang3', 'org.apache.commons', 'commons-lang3').version {
                    strictly '[3.8, 4.0['
                    prefer '3.9'
                }
            }
        }
    }

##### Aliases and their mapping to type safe accessors

Aliases must consist of a series of identifiers separated by a dash (`-`, recommended), an underscore (`_`) or a dot (`.`). Identifiers themselves must consist of ascii characters, preferably lowercase, eventually followed by numbers.

For example:

- `guava` is a valid alias

- `groovy-core` is a valid alias

- `commons-lang3` is a valid alias

- `androidx.awesome.lib` is also a valid alias

- but `this.#is.not!`

Then type safe accessors are generated for each subgroup. For example, given the following aliases in a version catalog named `libs`:

`guava`, `groovy-core`, `groovy-xml`, `groovy-json`, `androidx.awesome.lib`

We would generate the following type-safe accessors:

- `libs.guava`

- `libs.groovy.core`

- `libs.groovy.xml`

- `libs.groovy.json`

- `libs.androidx.awesome.lib`

Where the `libs` prefix comes from the version catalog name.

In case you want to avoid the generation of a subgroup accessor, we recommend relying on case to differentiate. For example the aliases `groovyCore`, `groovyJson` and `groovyXml` would be mapped to the `libs.groovyCore`, `libs.groovyJson` and `libs.groovyXml` accessors respectively.

When declaring aliases, it’s worth noting that any of the `-`, `_` and `.` characters can be used as separators, but the generated catalog will have all normalized to `.`: for example `foo-bar` as an alias is converted to `foo.bar` automatically.

Some keywords are reserved, so they cannot be used as an alias. Next words cannot be used as an alias:

- extensions

- class

- convention

Additional to that next words cannot be used as a first subgroup of an alias for dependencies (for bundles, versions and plugins this restriction doesn’t apply):

- bundles

- versions

- plugins

So for example for dependencies an alias `versions-dependency` is not valid, but `versionsDependency` or `dependency-versions` are valid.

##### Dependencies with same version numbers

In the first example in declaring a version catalog, we can see that we declare 3 aliases for various components of the `groovy` library and that all of them share the same version number.

Instead of repeating the same version number, we can declare a version and reference it:

Example 13. Declaring versions separately from libraries
settings.gradle.kts

    dependencyResolutionManagement {
        versionCatalogs {
            create("libs") {
                version("groovy", "3.0.5")
                version("checkstyle", "8.37")
                library("groovy-core", "org.codehaus.groovy", "groovy").versionRef("groovy")
                library("groovy-json", "org.codehaus.groovy", "groovy-json").versionRef("groovy")
                library("groovy-nio", "org.codehaus.groovy", "groovy-nio").versionRef("groovy")
                library("commons-lang3", "org.apache.commons", "commons-lang3").version {
                    strictly("[3.8, 4.0[")
                    prefer("3.9")
                }
            }
        }
    }

settings.gradle

    dependencyResolutionManagement {
        versionCatalogs {
            libs {
                version('groovy', '3.0.5')
                version('checkstyle', '8.37')
                library('groovy-core', 'org.codehaus.groovy', 'groovy').versionRef('groovy')
                library('groovy-json', 'org.codehaus.groovy', 'groovy-json').versionRef('groovy')
                library('groovy-nio', 'org.codehaus.groovy', 'groovy-nio').versionRef('groovy')
                library('commons-lang3', 'org.apache.commons', 'commons-lang3').version {
                    strictly '[3.8, 4.0['
                    prefer '3.9'
                }
            }
        }
    }

Versions declared separately are also available via type-safe accessors, making them usable for more use cases than dependency versions, in particular for tooling:

Example 14. Using a version declared in a version catalog
build.gradle.kts

    checkstyle {
        // will use the version declared in the catalog
        toolVersion = libs.versions.checkstyle.get()
    }

build.gradle

    checkstyle {
        // will use the version declared in the catalog
        toolVersion = libs.versions.checkstyle.get()
    }

If the alias of a declared version is also a prefix of some more specific alias, as in `libs.versions.zinc` and `libs.versions.zinc.apiinfo`, then the value of the more generic version is available via `asProvider()` on the type-safe accessor:

Example 15. Using a version from a version catalog when there are more specific aliases
build.gradle.kts

    scala {
        zincVersion = libs.versions.zinc.asProvider().get()
    }

build.gradle

    scala {
        zincVersion = libs.versions.zinc.asProvider().get()
    }

Dependencies declared in a catalog are exposed to build scripts via an extension corresponding to their name. In the example above, because the catalog declared in settings is named `libs`, the extension is available via the name `libs` in all build scripts of the current build. Declaring dependencies using the following notation…​

Example 16. Dependency notation correspondance
build.gradle.kts

    dependencies {
        implementation(libs.groovy.core)
        implementation(libs.groovy.json)
        implementation(libs.groovy.nio)
    }

build.gradle

    dependencies {
        implementation libs.groovy.core
        implementation libs.groovy.json
        implementation libs.groovy.nio
    }

…​has exactly the same effect as writing:

Example 17. Dependency notation correspondance
build.gradle.kts

    dependencies {
        implementation("org.codehaus.groovy:groovy:3.0.5")
        implementation("org.codehaus.groovy:groovy-json:3.0.5")
        implementation("org.codehaus.groovy:groovy-nio:3.0.5")
    }

build.gradle

    dependencies {
        implementation 'org.codehaus.groovy:groovy:3.0.5'
        implementation 'org.codehaus.groovy:groovy-json:3.0.5'
        implementation 'org.codehaus.groovy:groovy-nio:3.0.5'
    }

Versions declared in the catalog are rich versions. Please refer to the version catalog builder API for the full version declaration support documentation.

##### Dependency bundles

Because it’s frequent that some dependencies are systematically used together in different projects, a version catalog offers the concept of a "dependency bundle". A bundle is basically an alias for several dependencies. For example, instead of declaring 3 individual dependencies like above, you could write:

Example 18. Using a dependency bundle
build.gradle.kts

    dependencies {
        implementation(libs.bundles.groovy)
    }

build.gradle

    dependencies {
        implementation libs.bundles.groovy
    }

The bundle named `groovy` needs to be declared in the catalog:

Example 19. Declaring a dependency bundle
settings.gradle.kts

    dependencyResolutionManagement {
        versionCatalogs {
            create("libs") {
                version("groovy", "3.0.5")
                version("checkstyle", "8.37")
                library("groovy-core", "org.codehaus.groovy", "groovy").versionRef("groovy")
                library("groovy-json", "org.codehaus.groovy", "groovy-json").versionRef("groovy")
                library("groovy-nio", "org.codehaus.groovy", "groovy-nio").versionRef("groovy")
                library("commons-lang3", "org.apache.commons", "commons-lang3").version {
                    strictly("[3.8, 4.0[")
                    prefer("3.9")
                }
                bundle("groovy", listOf("groovy-core", "groovy-json", "groovy-nio"))
            }
        }
    }

settings.gradle

    dependencyResolutionManagement {
        versionCatalogs {
            libs {
                version('groovy', '3.0.5')
                version('checkstyle', '8.37')
                library('groovy-core', 'org.codehaus.groovy', 'groovy').versionRef('groovy')
                library('groovy-json', 'org.codehaus.groovy', 'groovy-json').versionRef('groovy')
                library('groovy-nio', 'org.codehaus.groovy', 'groovy-nio').versionRef('groovy')
                library('commons-lang3', 'org.apache.commons', 'commons-lang3').version {
                    strictly '[3.8, 4.0['
                    prefer '3.9'
                }
                bundle('groovy', ['groovy-core', 'groovy-json', 'groovy-nio'])
            }
        }
    }

The semantics are again equivalent: adding a single bundle is equivalent to adding all dependencies which are part of the bundle individually.

##### Plugins

In addition to libraries, version catalog supports declaring plugin versions. While libraries are represented by their group, artifact and version coordinates, Gradle plugins are identified by their id and version only. Therefore, they need to be declared separately:

| You cannot use a plugin declared in a version catalog in your settings file or settings plugin (because catalogs are defined in settings themselves, it would be a chicken and egg problem).

Example 20. Declaring a plugin version
settings.gradle.kts

    dependencyResolutionManagement {
        versionCatalogs {
            create("libs") {
                plugin("versions", "com.github.ben-manes.versions").version("0.45.0")
            }
        }
    }

settings.gradle

    dependencyResolutionManagement {
        versionCatalogs {
            libs {
                plugin('versions', 'com.github.ben-manes.versions').version('0.45.0')
            }
        }
    }

Then the plugin is accessible in the `plugins` block and can be consumed in any project of the build using:

Example 21. Using a plugin declared in a catalog
build.gradle.kts

    plugins {
        `java-library`
        checkstyle
        alias(libs.plugins.versions)
    }

build.gradle

    plugins {
        id 'java-library'
        id 'checkstyle'
        // Use the plugin `versions` as declared in the `libs` version catalog
        alias(libs.plugins.versions)
    }

##### Using multiple catalogs

Aside from the conventional `libs` catalog, you can declare any number of catalogs through the `Settings` API. This allows you to separate dependency declarations in multiple sources in a way that makes sense for your projects.

Example 22. Using a custom catalog
settings.gradle.kts

    dependencyResolutionManagement {
        versionCatalogs {
            create("testLibs") {
                val junit5 = version("junit5", "5.7.1")
                library("junit-api", "org.junit.jupiter", "junit-jupiter-api").versionRef(junit5)
                library("junit-engine", "org.junit.jupiter", "junit-jupiter-engine").versionRef(junit5)
            }
        }
    }

settings.gradle

    dependencyResolutionManagement {
        versionCatalogs {
            testLibs {
                def junit5 = version('junit5', '5.7.1')
                library('junit-api', 'org.junit.jupiter', 'junit-jupiter-api').versionRef(junit5)
                library('junit-engine', 'org.junit.jupiter', 'junit-jupiter-engine').versionRef(junit5)
            }
        }
    }

| Each catalog will generate an extension applied to all projects for accessing its content. As such it makes sense to reduce the chance of collisions by picking a name that reduces the potential conflicts. As an example, one option is to pick a name that ends with `Libs`.

#### The libs.versions.toml file

In addition to the settings API above, Gradle offers a conventional file to declare a catalog. If a `libs.versions.toml` file is found in the `gradle` subdirectory of the root build, then a catalog will be automatically declared with the contents of this file.

Declaring a `libs.versions.toml` file doesn’t make it the single source of truth for dependencies: it’s a conventional location where dependencies can be declared. As soon as you start using catalogs, it’s strongly recommended to declare all your dependencies in a catalog and not hardcode group/artifact/version strings in build scripts. Be aware that it may happen that plugins add dependencies, which are dependencies defined outside of this file.

Just like `src/main/java` is a convention to find the Java sources, which doesn’t prevent additional source directories to be declared (either in a build script or a plugin), the presence of the `libs.versions.toml` file doesn’t prevent the declaration of dependencies elsewhere.

The presence of this file does, however, suggest that most dependencies, if not all, will be declared in this file. Therefore, updating a dependency version, for most users, should only consists of changing a line in this file.

By default, the `libs.versions.toml` file will be an input to the `libs` catalog. It is possible to change the name of the default catalog, for example if you already have an extension with the same name:

Example 23. Changing the default extension name
settings.gradle.kts

    dependencyResolutionManagement {
        defaultLibrariesExtensionName = "projectLibs"
    }

settings.gradle

    dependencyResolutionManagement {
        defaultLibrariesExtensionName = 'projectLibs'
    }

##### The version catalog TOML file format

The TOML file consists of 4 major sections:

- the `[versions]` section is used to declare versions which can be referenced by dependencies

- the `[libraries]` section is used to declare the aliases to coordinates

- the `[bundles]` section is used to declare dependency bundles

- the `[plugins]` section is used to declare plugins

For example:

The libs.versions.toml file

    [versions]
    groovy = "3.0.5"
    checkstyle = "8.37"

    [libraries]
    groovy-core = { module = "org.codehaus.groovy:groovy", version.ref = "groovy" }
    groovy-json = { module = "org.codehaus.groovy:groovy-json", version.ref = "groovy" }
    groovy-nio = { module = "org.codehaus.groovy:groovy-nio", version.ref = "groovy" }
    commons-lang3 = { group = "org.apache.commons", name = "commons-lang3", version = { strictly = "[3.8, 4.0[", prefer="3.9" } }

    [bundles]
    groovy = ["groovy-core", "groovy-json", "groovy-nio"]

    [plugins]
    versions = { id = "com.github.ben-manes.versions", version = "0.45.0" }

Versions can be declared either as a single string, in which case they are interpreted as a required version, or as a rich versions:

    [versions]
    my-lib = { strictly = "[1.0, 2.0[", prefer = "1.2" }

Supported members of a version declaration are:

- `require`: the required version

- `strictly`: the strict version

- `prefer`: the preferred version

- `reject`: the list of rejected versions

- `rejectAll`: a boolean to reject all versions

Dependency declaration can either be declared as a simple string, in which case they are interpreted as `group:artifact:version` coordinates, or separating the version declaration from the group and name:

| For aliases, the rules described in the section aliases and their mapping to type safe accessors apply as well.

Different dependency notations

    [versions]
    common = "1.4"

    [libraries]
    my-lib = "com.mycompany:mylib:1.4"
    my-lib-no-version.module = "com.mycompany:mylib"
    my-other-lib = { module = "com.mycompany:other", version = "1.4" }
    my-other-lib2 = { group = "com.mycompany", name = "alternate", version = "1.4" }
    mylib-full-format = { group = "com.mycompany", name = "alternate", version = { require = "1.4" } }

    [plugins]
    short-notation = "some.plugin.id:1.4"
    long-notation = { id = "some.plugin.id", version = "1.4" }
    reference-notation = { id = "some.plugin.id", version.ref = "common" }

In case you want to reference a version declared in the `[versions]` section, you should use the `version.ref` property:

    [versions]
    some = "1.4"

    [libraries]
    my-lib = { group = "com.mycompany", name="mylib", version.ref="some" }

The TOML file format is very lenient and lets you write "dotted" properties as shortcuts to full object declarations. For example, this:

    a.b.c="d"

is equivalent to:

    a.b = { c = "d" }

or

    a = { b = { c = "d" } }

See the TOML specification for details.

#### Type unsafe API

Version catalogs can be accessed through a type unsafe API. This API is available in situations where generated accessors are not. It is accessed through the version catalog extension:

build.gradle.kts

    val versionCatalog = versionCatalogs.named("libs")
    println("Library aliases: ${versionCatalog.libraryAliases}")
    dependencies {
        versionCatalog.findLibrary("groovy-json").ifPresent {
            implementation(it)
        }
    }

build.gradle

    def versionCatalog = versionCatalogs.named("libs")
    println "Library aliases: ${versionCatalog.libraryAliases}"
    dependencies {
        versionCatalog.findLibrary("groovy-json").ifPresent {
            implementation(it)
        }
    }

Check the version catalog API for all supported methods.

### Sharing catalogs

Version catalogs are used in a single build (possibly multi-project build) but may also be shared between builds. For example, an organization may want to create a catalog of dependencies that different projects, from different teams, may use.

#### Importing a catalog from a TOML file

The version catalog builder API supports including a model from an external file. This makes it possible to reuse the catalog of the main build for `buildSrc`, if needed. For example, the `buildSrc/settings.gradle(.kts)` file can include this file using:

Example 24. Sharing the dependency catalog with buildSrc
settings.gradle.kts

    dependencyResolutionManagement {
        versionCatalogs {
            create("libs") {
                from(files("../gradle/libs.versions.toml"))
            }
        }
    }

settings.gradle

    dependencyResolutionManagement {
        versionCatalogs {
            libs {
                from(files("../gradle/libs.versions.toml"))
            }
        }
    }

| Only a single file will be accepted when using the VersionCatalogBuilder.from(Object dependencyNotation) method. This means that notations like Project.files(java.lang.Object…​) must refer to a single file, otherwise the build will fail. If a more complicated structure is required (version catalogs imported from multiple files), it’s advisable to use a code-based approach, instead of TOML file.

This technique can therefore be used to declare multiple catalogs from different files:

Example 25. Declaring additional catalogs
settings.gradle.kts

    dependencyResolutionManagement {
        versionCatalogs {
            // declares an additional catalog, named 'testLibs', from the 'test-libs.versions.toml' file
            create("testLibs") {
                from(files("gradle/test-libs.versions.toml"))
            }
        }
    }

settings.gradle

    dependencyResolutionManagement {
        versionCatalogs {
            // declares an additional catalog, named 'testLibs', from the 'test-libs.versions.toml' file
            testLibs {
                from(files('gradle/test-libs.versions.toml'))
            }
        }
    }

#### The version catalog plugin

While importing catalogs from local files is convenient, it doesn’t solve the problem of sharing a catalog in an organization or for external consumers. One option to share a catalog is to write a settings plugin, publish it on the Gradle plugin portal or an internal repository, and let the consumers apply the plugin on their settings file.

Alternatively, Gradle offers a version catalog plugin, which offers the ability to declare, then publish a catalog.

To do this, you need to apply the `version-catalog` plugin:

Example 26. Applying the version catalog plugin
build.gradle.kts

    plugins {
        `version-catalog`
        `maven-publish`
    }

build.gradle

    plugins {
        id 'version-catalog'
        id 'maven-publish'
    }

This plugin will then expose the catalog extension that you can use to declare a catalog:

Example 27. Definition of a catalog
build.gradle.kts

    catalog {
        // declare the aliases, bundles and versions in this block
        versionCatalog {
            library("my-lib", "com.mycompany:mylib:1.2")
        }
    }

build.gradle

    catalog {
        // declare the aliases, bundles and versions in this block
        versionCatalog {
            library('my-lib', 'com.mycompany:mylib:1.2')
        }
    }

Such a catalog can then be published by applying either the `maven-publish` or `ivy-publish` plugin and configuring the publication to use the `versionCatalog` component:

Example 28. Publishing a catalog
build.gradle.kts

    publishing {
        publications {
            create<MavenPublication>("maven") {
                from(components["versionCatalog"])
            }
        }
    }

build.gradle

    publishing {
        publications {
            maven(MavenPublication) {
                from components.versionCatalog
            }
        }
    }

When publishing such a project, a `libs.versions.toml` file will automatically be generated (and uploaded), which can then be consumed from other Gradle builds.

#### Importing a published catalog

A catalog produced by the version catalog plugin can be imported via the settings API:

Example 29. Using a published catalog
settings.gradle.kts

    dependencyResolutionManagement {
        versionCatalogs {
            create("libs") {
                from("com.mycompany:catalog:1.0")
            }
        }
    }

settings.gradle

    dependencyResolutionManagement {
        versionCatalogs {
            libs {
                from("com.mycompany:catalog:1.0")
            }
        }
    }

##### Overwriting catalog versions

In case a catalog declares a version, you can overwrite the version when importing the catalog:

Example 30. Overwriting versions declared in a published catalog
settings.gradle.kts

    dependencyResolutionManagement {
        versionCatalogs {
            create("amendedLibs") {
                from("com.mycompany:catalog:1.0")
                // overwrite the "groovy" version declared in the imported catalog
                version("groovy", "3.0.6")
            }
        }
    }

settings.gradle

    dependencyResolutionManagement {
        versionCatalogs {
            amendedLibs {
                from("com.mycompany:catalog:1.0")
                // overwrite the "groovy" version declared in the imported catalog
                version("groovy", "3.0.6")
            }
        }
    }

In the example above, any dependency which was using the `groovy` version as reference will be automatically updated to use `3.0.6`.

| Again, overwriting a version doesn’t mean that the actual resolved dependency version will be the same: this only changes what is imported, that is to say what is used when declaring a dependency. The actual version will be subject to traditional conflict resolution, if any.

### Using a platform to control transitive versions

A platform is a special software component which can be used to control transitive dependency versions. In most cases it’s exclusively composed of dependency constraints which will either suggest dependency versions or enforce some versions. As such, this is a perfect tool whenever you need to share dependency versions between projects. In this case, a project will typically be organized this way:

- a `platform` project which defines constraints for the various dependencies found in the different sub-projects

- a number of sub-projects which depend on the platform and declare dependencies without version

In the Java ecosystem, Gradle provides a plugin for this purpose.

It’s also common to find platforms published as Maven BOMs which Gradle supports natively.

A dependency on a platform is created using the `platform` keyword:

Example 31. Getting versions declared in a platform
build.gradle.kts

    dependencies {
        // get recommended versions from the platform project
        api(platform(project(":platform")))
        // no version required
        api("commons-httpclient:commons-httpclient")
    }

build.gradle

    dependencies {
        // get recommended versions from the platform project
        api platform(project(':platform'))
        // no version required
        api 'commons-httpclient:commons-httpclient'
    }

This `platform` notation is a short-hand notation which actually performs several operations under the hood:

- it sets the org.gradle.category attribute to `platform`, which means that Gradle will select the platform component of the dependency.

- it sets the endorseStrictVersions behavior by default, meaning that if the platform declares strict dependencies, they will be enforced.

This means that by default, a dependency to a platform triggers the inheritance of all strict versions defined in that platform, which can be useful for platform authors to make sure that all consumers respect their decisions in terms of versions of dependencies. This can be turned off by explicitly calling the `doNotEndorseStrictVersions` method.

### Importing Maven BOMs

Gradle provides support for importing bill of materials (BOM) files, which are effectively `.pom` files that use `<dependencyManagement>` to control the dependency versions of direct and transitive dependencies. The BOM support in Gradle works similar to using `<scope>import</scope>` when depending on a BOM in Maven. In Gradle however, it is done via a regular dependency declaration on the BOM:

Example 32. Depending on a BOM to import its dependency constraints
build.gradle.kts

    dependencies {
        // import a BOM
        implementation(platform("org.springframework.boot:spring-boot-dependencies:1.5.8.RELEASE"))
        // define dependencies without versions
        implementation("com.google.code.gson:gson")
        implementation("dom4j:dom4j")
    }

build.gradle

    dependencies {
        // import a BOM
        implementation platform('org.springframework.boot:spring-boot-dependencies:1.5.8.RELEASE')
        // define dependencies without versions
        implementation 'com.google.code.gson:gson'
        implementation 'dom4j:dom4j'
    }

In the example, the versions of `gson` and `dom4j` are provided by the Spring Boot BOM. This way, if you are developing for a platform like Spring Boot, you do not have to declare any versions yourself but can rely on the versions the platform provides.

Gradle treats all entries in the `<dependencyManagement>` block of a BOM similar to Gradle’s dependency constraints. This means that any version defined in the `<dependencyManagement>` block can impact the dependency resolution result. In order to qualify as a BOM, a `.pom` file needs to have `<packaging>pom</packaging>` set.

However often BOMs are not only providing versions as recommendations, but also a way to override any other version found in the graph. You can enable this behavior by using the `enforcedPlatform` keyword, instead of `platform`, when importing the BOM:

Example 33. Importing a BOM, making sure the versions it defines override any other version found
build.gradle.kts

    dependencies {
        // import a BOM. The versions used in this file will override any other version found in the graph
        implementation(enforcedPlatform("org.springframework.boot:spring-boot-dependencies:1.5.8.RELEASE"))

        // define dependencies without versions
        implementation("com.google.code.gson:gson")
        implementation("dom4j:dom4j")

        // this version will be overridden by the one found in the BOM
        implementation("org.codehaus.groovy:groovy:1.8.6")
    }

build.gradle

    dependencies {
        // import a BOM. The versions used in this file will override any other version found in the graph
        implementation enforcedPlatform('org.springframework.boot:spring-boot-dependencies:1.5.8.RELEASE')

        // define dependencies without versions
        implementation 'com.google.code.gson:gson'
        implementation 'dom4j:dom4j'

        // this version will be overridden by the one found in the BOM
        implementation 'org.codehaus.groovy:groovy:1.8.6'
    }

| Using `enforcedPlatform` needs to be considered with care if your software component can be consumed by others. This declaration is effectively transitive and so will apply to the dependency graph of your consumers. Unfortunately they will have to use `exclude` if they happen to disagree with one of the forced versions. Instead, if your reusable software component has a strong opinion on some third party dependency versions, consider using a rich version declaration with a `strictly`.

### Should I use a platform or a catalog?

Because platforms and catalogs both talk about dependency versions and can both be used to share dependency versions in a project, there might be a confusion regarding what to use and if one is preferable to the other.

In short, you should:

- use catalogs to only define dependencies and their versions for projects and to generate type-safe accessors

- use platform to apply versions to dependency graph and to affect dependency resolution

A catalog helps with centralizing the dependency versions and is only, as it name implies, a catalog of dependencies you can pick from. We recommend using it to declare the coordinates of your dependencies, in all cases. It will be used by Gradle to generate type-safe accessors, present short-hand notations for external dependencies and it allows sharing those coordinates between different projects easily. Using a catalog will not have any kind of consequence on downstream consumers: it’s transparent to them.

A platform is a more heavyweight construct: it’s a component of a dependency graph, like any other library. If you depend on a platform, that platform is itself a component in the graph. It means, in particular, that:

- Constraints defined in a platform can influence transitive dependencies, not only the direct dependencies of your project.

- A platform is versioned, and a transitive dependency in the graph can depend on a different version of the platform, causing various dependency upgrades.

- A platform can tie components together, and in particular can be used as a construct for aligning versions.

- A dependency on a platform is "inherited" by the consumers of your dependency: it means that a dependency on a platform can influence what versions of libraries would be used by your consumers even if you don’t directly, or transitively, depend on components the platform references.

In summary, using a catalog is always a good engineering practice as it centralizes common definitions, allows sharing of dependency versions or plugin versions, but it is an "implementation detail" of the build: it will not be visible to consumers and unused elements of a catalog are just ignored.

A platform is meant to influence the dependency resolution graph, for example by adding constraints on transitive dependencies: it’s a solution for structuring a dependency graph and influencing the resolution result.

In practice, your project can both use a catalog and declare a platform which itself uses the catalog:

Example 34. Using a catalog within a platform definition
build.gradle.kts

    plugins {
        `java-platform`
    }

    dependencies {
        constraints {
            api(libs.mylib)
        }
    }

build.gradle

    plugins {
        id 'java-platform'
    }

    dependencies {
        constraints {
            api(libs.mylib)
        }
    }

## Aligning dependency versions

Dependency version alignment allows different modules belonging to the same logical group (a platform) to have identical versions in a dependency graph.

### Handling inconsistent module versions

Gradle supports aligning versions of modules which belong to the same "platform". It is often preferable, for example, that the API and implementation modules of a component are using the same version. However, because of the game of transitive dependency resolution, it is possible that different modules belonging to the same platform end up using different versions. For example, your project may depend on the `jackson-databind` and `vert.x` libraries, as illustrated below:

Example 35. Declaring dependencies
build.gradle.kts

    dependencies {
        // a dependency on Jackson Databind
        implementation("com.fasterxml.jackson.core:jackson-databind:2.8.9")

        // and a dependency on vert.x
        implementation("io.vertx:vertx-core:3.5.3")
    }

build.gradle

    dependencies {
        // a dependency on Jackson Databind
        implementation 'com.fasterxml.jackson.core:jackson-databind:2.8.9'

        // and a dependency on vert.x
        implementation 'io.vertx:vertx-core:3.5.3'
    }

Because `vert.x` depends on `jackson-core`, we would actually resolve the following dependency versions:

- `jackson-core` version `2.9.5` (brought by `vertx-core`)

- `jackson-databind` version `2.9.5` (by conflict resolution)

- `jackson-annotation` version `2.9.0` (dependency of `jackson-databind:2.9.5`)

It’s easy to end up with a set of versions which do not work well together. To fix this, Gradle supports dependency version alignment, which is supported by the concept of platforms. A platform represents a set of modules which "work well together". Either because they are actually published as a whole (when one of the members of the platform is published, all other modules are also published with the same version), or because someone tested the modules and indicates that they work well together (typically, the Spring Platform).

### Aligning versions natively with Gradle

Gradle natively supports alignment of modules produced by Gradle. This is a direct consequence of the transitivity of dependency constraints. So if you have a multi-project build, and you wish that consumers get the same version of all your modules, Gradle provides a simple way to do this using the Java Platform Plugin.

For example, if you have a project that consists of 3 modules:

- `lib`

- `utils`

- `core`, depending on `lib` and `utils`

And a consumer that declares the following dependencies:

- `core` version 1.0

- `lib` version 1.1

Then by default resolution would select `core:1.0` and `lib:1.1`, because `lib` has no dependency on `core`. We can fix this by adding a new module in our project, a platform, that will add constraints on all the modules of your project:

Example 36. The platform module
build.gradle.kts

    plugins {
        `java-platform`
    }

    dependencies {
        // The platform declares constraints on all components that
        // require alignment
        constraints {
            api(project(":core"))
            api(project(":lib"))
            api(project(":utils"))
        }
    }

build.gradle

    plugins {
        id 'java-platform'
    }

    dependencies {
        // The platform declares constraints on all components that
        // require alignment
        constraints {
            api(project(":core"))
            api(project(":lib"))
            api(project(":utils"))
        }
    }

Once this is done, we need to make sure that all modules now depend on the platform, like this:

Example 37. Declaring a dependency on the platform
build.gradle.kts

    dependencies {
        // Each project has a dependency on the platform
        api(platform(project(":platform")))

        // And any additional dependency required
        implementation(project(":lib"))
        implementation(project(":utils"))
    }

build.gradle

    dependencies {
        // Each project has a dependency on the platform
        api(platform(project(":platform")))

        // And any additional dependency required
        implementation(project(":lib"))
        implementation(project(":utils"))
    }

It is important that the platform contains a constraint on all the components, but also that each component has a dependency on the platform. By doing this, whenever Gradle will add a dependency to a module of the platform on the graph, it will also include constraints on the other modules of the platform. This means that if we see another module belonging to the same platform, we will automatically upgrade to the same version.

In our example, it means that we first see `core:1.0`, which brings a platform `1.0` with constraints on `lib:1.0` and `lib:1.0`. Then we add `lib:1.1` which has a dependency on `platform:1.1`. By conflict resolution, we select the `1.1` platform, which has a constraint on `core:1.1`. Then we conflict resolve between `core:1.0` and `core:1.1`, which means that `core` and `lib` are now aligned properly.

| This behavior is enforced for published components only if you use Gradle Module Metadata.

### Aligning versions of modules not published with Gradle

Whenever the publisher doesn’t use Gradle, like in our Jackson example, we can explain to Gradle that all Jackson modules "belong to" the same platform and benefit from the same behavior as with native alignment. There are two options to express that a set of modules belong to a platform:

1. A platform is published as a BOM and can be used: For example, `com.fasterxml.jackson:jackson-bom` can be used as platform. The information missing to Gradle in that case is that the platform should be added to the dependencies if one of its members is used.

2. No existing platform can be used. Instead, a virtual platform should be created by Gradle: In this case, Gradle builds up the platform itself based on all the members that are used.

To provide the missing information to Gradle, you can define component metadata rules as explained in the following.

#### Align versions of modules using a published BOM

Example 38. A dependency version alignment rule
build.gradle.kts

    abstract class JacksonBomAlignmentRule: ComponentMetadataRule {
        override fun execute(ctx: ComponentMetadataContext) {
            ctx.details.run {
                if (id.group.startsWith("com.fasterxml.jackson")) {
                    // declare that Jackson modules belong to the platform defined by the Jackson BOM
                    belongsTo("com.fasterxml.jackson:jackson-bom:${id.version}", false)
                }
            }
        }
    }

build.gradle

    abstract class JacksonBomAlignmentRule implements ComponentMetadataRule {
        void execute(ComponentMetadataContext ctx) {
            ctx.details.with {
                if (id.group.startsWith("com.fasterxml.jackson")) {
                    // declare that Jackson modules belong to the platform defined by the Jackson BOM
                    belongsTo("com.fasterxml.jackson:jackson-bom:${id.version}", false)
                }
            }
        }
    }

By using the `belongsTo` with `false` (not virtual), we declare that all modules belong to the same published platform. In this case, the platform is `com.fasterxml.jackson:jackson-bom` and Gradle will look for it, as for any other module, in the declared repositories.

Example 39. Making use of a dependency version alignment rule
build.gradle.kts

    dependencies {
        components.all<JacksonBomAlignmentRule>()
    }

build.gradle

    dependencies {
        components.all(JacksonBomAlignmentRule)
    }

Using the rule, the versions in the example above align to whatever the selected version of `com.fasterxml.jackson:jackson-bom` defines. In this case, `com.fasterxml.jackson:jackson-bom:2.9.5` will be selected as `2.9.5` is the highest version of a module selected. In that BOM, the following versions are defined and will be used: `jackson-core:2.9.5`, `jackson-databind:2.9.5` and `jackson-annotation:2.9.0`. The lower versions of `jackson-annotation` here might be the desired result as it is what the BOM recommends.

| This behavior is working reliable since Gradle 6.1. Effectively, it is similar to a component metadata rule that adds a platform dependency to all members of the platform using `withDependencies`.

#### Align versions of modules without a published platform

Example 40. A dependency version alignment rule
build.gradle.kts

    abstract class JacksonAlignmentRule: ComponentMetadataRule {
        override fun execute(ctx: ComponentMetadataContext) {
            ctx.details.run {
                if (id.group.startsWith("com.fasterxml.jackson")) {
                    // declare that Jackson modules all belong to the Jackson virtual platform
                    belongsTo("com.fasterxml.jackson:jackson-virtual-platform:${id.version}")
                }
            }
        }
    }

build.gradle

    abstract class JacksonAlignmentRule implements ComponentMetadataRule {
        void execute(ComponentMetadataContext ctx) {
            ctx.details.with {
                if (id.group.startsWith("com.fasterxml.jackson")) {
                    // declare that Jackson modules all belong to the Jackson virtual platform
                    belongsTo("com.fasterxml.jackson:jackson-virtual-platform:${id.version}")
                }
            }
        }
    }

By using the `belongsTo` keyword without further parameter (platform is virtual), we declare that all modules belong to the same virtual platform, which is treated specially by the engine. A virtual platform will not be retrieved from a repository. The identifier, in this case `com.fasterxml.jackson:jackson-virtual-platform`, is something you as the build author define yourself. The "content" of the platform is then created by Gradle on the fly by collecting all `belongsTo` statements pointing at the same virtual platform.

Example 41. Making use of a dependency version alignment rule
build.gradle.kts

    dependencies {
        components.all<JacksonAlignmentRule>()
    }

build.gradle

    dependencies {
        components.all(JacksonAlignmentRule)
    }

Using the rule, all versions in the example above would align to `2.9.5`. In this case, also `jackson-annotation:2.9.5` will be taken, as that is how we defined our local virtual platform.

For both published and virtual platforms, Gradle lets you override the version choice of the platform itself by specifying an enforced dependency on the platform:

Example 42. Forceful platform downgrade
build.gradle.kts

    dependencies {
        // Forcefully downgrade the virtual Jackson platform to 2.8.9
        implementation(enforcedPlatform("com.fasterxml.jackson:jackson-virtual-platform:2.8.9"))
    }

build.gradle

    dependencies {
        // Forcefully downgrade the virtual Jackson platform to 2.8.9
        implementation enforcedPlatform('com.fasterxml.jackson:jackson-virtual-platform:2.8.9')
    }

## Handling mutually exclusive dependencies

### Introduction to component capabilities

Often a dependency graph would accidentally contain multiple implementations of the same API. This is particularly common with logging frameworks, where multiple bindings are available, and that one library chooses a binding when another transitive dependency chooses another. Because those implementations live at different GAV coordinates, the build tool has usually no way to find out that there’s a conflict between those libraries. To solve this, Gradle provides the concept of capability.

It’s illegal to find two components providing the same capability in a single dependency graph. Intuitively, it means that if Gradle finds two components that provide the same thing on classpath, it’s going to fail with an error indicating what modules are in conflict. In our example, it means that different bindings of a logging framework provide the same capability.

### Capability coordinates

A capability is defined by a `(group, module, version)` triplet. Each component defines an implicit capability corresponding to its GAV coordinates (group, artifact, version). For example, the `org.apache.commons:commons-lang3:3.8` module has an implicit capability with group `org.apache.commons`, name `commons-lang3` and version `3.8`. It is important to realize that capabilities are versioned.

### Declaring component capabilities

By default, Gradle will fail if two components in the dependency graph provide the same capability. Because most modules are currently published without Gradle Module Metadata, capabilities are not always automatically discovered by Gradle. It is however interesting to use rules to declare component capabilities in order to discover conflicts as soon as possible, during the build instead of runtime.

A typical example is whenever a component is relocated at different coordinates in a new release. For example, the ASM library lived at `asm:asm` coordinates until version `3.3.1`, then changed to `org.ow2.asm:asm` since `4.0`. It is illegal to have both ASM <= 3.3.1 and 4.0+ on the classpath, because they provide the same feature, it’s just that the component has been relocated. Because each component has an implicit capability corresponding to its GAV coordinates, we can "fix" this by having a rule that will declare that the `asm:asm` module provides the `org.ow2.asm:asm` capability:

Example 43. Conflict resolution by capability
build.gradle.kts

    class AsmCapability : ComponentMetadataRule {
        override
        fun execute(context: ComponentMetadataContext) = context.details.run {
            if (id.group == "asm" && id.name == "asm") {
                allVariants {
                    withCapabilities {
                        // Declare that ASM provides the org.ow2.asm:asm capability, but with an older version
                        addCapability("org.ow2.asm", "asm", id.version)
                    }
                }
            }
        }
    }

build.gradle

    @CompileStatic
    class AsmCapability implements ComponentMetadataRule {
        void execute(ComponentMetadataContext context) {
            context.details.with {
                if (id.group == "asm" && id.name == "asm") {
                    allVariants {
                        it.withCapabilities {
                            // Declare that ASM provides the org.ow2.asm:asm capability, but with an older version
                            it.addCapability("org.ow2.asm", "asm", id.version)
                        }
                    }
                }
            }
        }
    }

Now the build is going to fail whenever the two components are found in the same dependency graph.

| At this stage, Gradle will only make more builds fail. It will not automatically fix the problem for you, but it helps you realize that you have a problem. It is recommended to write such rules in plugins which are then applied to your builds. Then, users have to express their preferences, if possible, or fix the problem of having incompatible things on the classpath, as explained in the following section.

### Selecting between candidates

At some point, a dependency graph is going to include either incompatible modules, or modules which are mutually exclusive. For example, you may have different logger implementations and you need to choose one binding. Capabilities help realizing that you have a conflict, but Gradle also provides tools to express how to solve the conflicts.

#### Selecting between different capability candidates

In the relocation example above, Gradle was able to tell you that you have two versions of the same API on classpath: an "old" module and a "relocated" one. Now we can solve the conflict by automatically choosing the component which has the highest capability version:

Example 44. Conflict resolution by capability versioning
build.gradle.kts

    configurations.all {
        resolutionStrategy.capabilitiesResolution.withCapability("org.ow2.asm:asm") {
            selectHighestVersion()
        }
    }

build.gradle

    configurations.all {
        resolutionStrategy.capabilitiesResolution.withCapability('org.ow2.asm:asm') {
            selectHighestVersion()
        }
    }

However, fixing by choosing the highest capability version conflict resolution is not always suitable. For a logging framework, for example, it doesn’t matter what version of the logging frameworks we use, we should always select Slf4j.

In this case, we can fix it by explicitly selecting slf4j as the winner:

Example 45. Substitute log4j with slf4j
build.gradle.kts

    configurations.all {
        resolutionStrategy.capabilitiesResolution.withCapability("log4j:log4j") {
            val toBeSelected = candidates.firstOrNull { it.id.let { id -> id is ModuleComponentIdentifier && id.module == "log4j-over-slf4j" } }
            if (toBeSelected != null) {
                select(toBeSelected)
            }
            because("use slf4j in place of log4j")
        }
    }

build.gradle

    configurations.all {
        resolutionStrategy.capabilitiesResolution.withCapability("log4j:log4j") {
            def toBeSelected = candidates.find { it.id instanceof ModuleComponentIdentifier && it.id.module == 'log4j-over-slf4j' }
            if (toBeSelected != null) {
                select(toBeSelected)
            }
            because 'use slf4j in place of log4j'
        }
    }

Note that this approach works also well if you have multiple Slf4j bindings on the classpath: bindings are basically different logger implementations and you need only one. However, the selected implementation may depend on the configuration being resolved. For example, for tests, `slf4j-simple` may be enough but for production, `slf4-over-log4j` may be better.

Resolution can only be made in favor of a module found in the graph.

The `select` method only accepts a module found in the current candidates. If the module you want to select is not part of the conflict, you can abstain from performing a selection, effectively not resolving this conflict. It might be that another conflict exists in the graph for the same capability and will have the module you want to select.

If no resolution is given for all conflicts on a given capability, the build will fail given the module chosen for resolution was not part of the graph at all.

In addition `select(null)` will result in an error and so should be avoided.

For more information, check out the the capabilities resolution API.

## Fixing metadata with component metadata rules

Each module that is pulled from a repository has metadata associated with it, such as its group, name, version as well as the different variants it provides with their artifacts and dependencies. Sometimes, this metadata is incomplete or incorrect. To manipulate such incomplete metadata from within the build script, Gradle offers an API to write component metadata rules. These rules take effect after a module’s metadata has been downloaded, but before it is used in dependency resolution.

### Basics of writing a component metadata rule

Component metadata rules are applied in the components (ComponentMetadataHandler) section of the dependencies block (DependencyHandler) of a build script or in the settings script. The rules can be defined in two different ways:

1. As an action directly when they are applied in the components section

2. As an isolated class implementing the ComponentMetadataRule interface

While defining rules inline as action can be convenient for experimentation, it is generally recommended to define rules as separate classes. Rules that are written as isolated classes can be annotated with `@CacheableRule` to cache the results of their application such that they do not need to be re-executed each time dependencies are resolved.

Example 46. Example of a configurable component metadata rule
build.gradle.kts

    @CacheableRule
    abstract class TargetJvmVersionRule @Inject constructor(val jvmVersion: Int) : ComponentMetadataRule {
        @get:Inject abstract val objects: ObjectFactory

        override fun execute(context: ComponentMetadataContext) {
            context.details.withVariant("compile") {
                attributes {
                    attribute(TargetJvmVersion.TARGET_JVM_VERSION_ATTRIBUTE, jvmVersion)
                    attribute(Usage.USAGE_ATTRIBUTE, objects.named(Usage.JAVA_API))
                }
            }
        }
    }
    dependencies {
        components {
            withModule<TargetJvmVersionRule>("commons-io:commons-io") {
                params(7)
            }
            withModule<TargetJvmVersionRule>("commons-collections:commons-collections") {
                params(8)
            }
        }
        implementation("commons-io:commons-io:2.6")
        implementation("commons-collections:commons-collections:3.2.2")
    }

build.gradle

    @CacheableRule
    abstract class TargetJvmVersionRule implements ComponentMetadataRule {
        final Integer jvmVersion
        @Inject TargetJvmVersionRule(Integer jvmVersion) {
            this.jvmVersion = jvmVersion
        }

        @Inject abstract ObjectFactory getObjects()

        void execute(ComponentMetadataContext context) {
            context.details.withVariant("compile") {
                attributes {
                    attribute(TargetJvmVersion.TARGET_JVM_VERSION_ATTRIBUTE, jvmVersion)
                    attribute(Usage.USAGE_ATTRIBUTE, objects.named(Usage, Usage.JAVA_API))
                }
            }
        }
    }
    dependencies {
        components {
            withModule("commons-io:commons-io", TargetJvmVersionRule) {
                params(7)
            }
            withModule("commons-collections:commons-collections", TargetJvmVersionRule) {
                params(8)
            }
        }
        implementation("commons-io:commons-io:2.6")
        implementation("commons-collections:commons-collections:3.2.2")
    }

As can be seen in the examples above, component metadata rules are defined by implementing ComponentMetadataRule which has a single `execute` method receiving an instance of ComponentMetadataContext as parameter. In this example, the rule is also further configured through an ActionConfiguration. This is supported by having a constructor in your implementation of `ComponentMetadataRule` accepting the parameters that were configured and the services that need injecting.

Gradle enforces isolation of instances of `ComponentMetadataRule`. This means that all parameters must be `Serializable` or known Gradle types that can be isolated.

In addition, Gradle services can be injected into your `ComponentMetadataRule`. Because of this, the moment you have a constructor, it must be annotated with `@javax.inject.Inject`. A commonly required service is ObjectFactory to create instances of strongly typed value objects like a value for setting an Attribute. A service which is helpful for advanced usage of component metadata rules with custom metadata is the RepositoryResourceAccessor.

A component metadata rule can be applied to all modules — `all(rule)` — or to a selected module — `withModule(groupAndName, rule)`. Usually, a rule is specifically written to enrich metadata of one specific module and hence the `withModule` API should be preferred.

#### Declaring rules in a central place

| Declaring component metadata rules in settings is an incubating feature

Instead of declaring rules for each subproject individually, it is possible to declare rules in the `settings.gradle(.kts)` file for the whole build. Rules declared in settings are the conventional rules applied to each project: if the project doesn’t declare any rules, the rules from the settings script will be used.

Example 47. Declaring a rule in settings
settings.gradle.kts

    dependencyResolutionManagement {
        components {
            withModule<GuavaRule>("com.google.guava:guava")
        }
    }

settings.gradle

    dependencyResolutionManagement {
        components {
            withModule("com.google.guava:guava", GuavaRule)
        }
    }

By default, rules declared in a project will override whatever is declared in settings. It is possible to change this default, for example to always prefer the settings rules:

Example 48. Preferring rules declared in settings
settings.gradle.kts

    dependencyResolutionManagement {
        rulesMode = RulesMode.PREFER_SETTINGS
    }

settings.gradle

    dependencyResolutionManagement {
        rulesMode = RulesMode.PREFER_SETTINGS
    }

If this method is called and that a project or plugin declares rules, a warning will be issued. You can make this a failure instead by using this alternative:

Example 49. Enforcing rules declared in settings
settings.gradle.kts

    dependencyResolutionManagement {
        rulesMode = RulesMode.FAIL_ON_PROJECT_RULES
    }

settings.gradle

    dependencyResolutionManagement {
        rulesMode = RulesMode.FAIL_ON_PROJECT_RULES
    }

The default behavior is equivalent to calling this method:

Example 50. Preferring rules declared in projects
settings.gradle.kts

    dependencyResolutionManagement {
        rulesMode = RulesMode.PREFER_PROJECT
    }

settings.gradle

    dependencyResolutionManagement {
        rulesMode = RulesMode.PREFER_PROJECT
    }

### Which parts of metadata can be modified?

The component metadata rules API is oriented at the features supported by Gradle Module Metadata and the dependencies API in build scripts. The main difference between writing rules and defining dependencies and artifacts in the build script is that component metadata rules, following the structure of Gradle Module Metadata, operate on variants directly. On the contrary, in build scripts you often influence the shape of multiple variants at once (e.g. an api dependency is added to the api and runtime variant of a Java library, the artifact produced by the jar task is also added to these two variants).

Variants can be addressed for modification through the following methods:

- `allVariants`: modify all variants of a component

- `withVariant(name)`: modify a single variant identified by its name

- `addVariant(name)` or `addVariant(name, base)`: add a new variant to the component either from scratch or by copying the details of an existing variant (base)

The following details of each variant can be adjusted:

- The attributes that identify the variant — `attributes {}` block

- The capabilities the variant provides — `withCapabilities { }` block

- The dependencies of the variant, including rich versions — `withDependencies {}` block

- The dependency constraints of the variant, including rich versions — `withDependencyConstraints {}` block

- The location of the published files that make up the actual content of the variant — `withFiles { }` block

There are also a few properties of the whole component that can be changed:

- The component level attributes, currently the only meaningful attribute there is `org.gradle.status`

- The status scheme to influence interpretation of the `org.gradle.status` attribute during version selection

- The belongsTo property for version alignment through virtual platforms

Depending on the format of the metadata of a module, it is mapped differently to the variant-centric representation of the metadata:

- If the module has Gradle Module Metadata, the data structure the rule operates on is very similar to what you find in the module’s `.module` file.

- If the module was published only with `.pom` metadata, a number of fixed variants is derived as explained in the mapping of POM files to variants section.

- If the module was published only with an `ivy.xml` file, the Ivy configurations defined in the file can be accessed instead of variants. Their dependencies, dependency constraints and files can be modified. Additionally, the `addVariant(name, baseVariantOrConfiguration) { }` API can be used to derive variants from Ivy configurations if desired (for example, compile and runtime variants for the Java library plugin can be defined with this).

### When to use Component Metadata Rules?

In general, if you consider using component metadata rules to adjust the metadata of a certain module, you should check first if that module was published with Gradle Module Metadata (`.module` file) or traditional metadata only (`.pom` or `ivy.xml`).

If a module was published with Gradle Module Metadata, the metadata is likely complete although there can still be cases where something is just plainly wrong. For these modules you should only use component metadata rules if you have clearly identified a problem with the metadata itself. If you have an issue with the dependency resolution result, you should first check if you can solve the issue by declaring dependency constraints with rich versions. In particular, if you are developing a library that you publish, you should remember that dependency constraints, in contrast to component metadata rules, are published as part of the metadata of your own library. So with dependency constraints, you automatically share the solution of dependency resolution issues with your consumers, while component metadata rules are only applied to your own build.

If a module was published with traditional metadata (`.pom` or `ivy.xml` only, no `.module` file) it is more likely that the metadata is incomplete as features such as variants or dependency constraints are not supported in these formats. Still, conceptually such modules can contain different variants or might have dependency constraints they just omitted (or wrongly defined as dependencies). In the next sections, we explore a number existing oss modules with such incomplete metadata and the rules for adding the missing metadata information.

As a rule of thumb, you should contemplate if the rule you are writing also works out of context of your build. That is, does the rule still produce a correct and useful result if applied in any other build that uses the module(s) it affects?

### Fixing wrong dependency details

Let’s consider as an example the publication of the Jaxen XPath Engine on Maven central. The pom of version 1.1.3 declares a number of dependencies in the compile scope which are not actually needed for compilation. These have been removed in the 1.1.4 pom. Assuming that we need to work with 1.1.3 for some reason, we can fix the metadata with the following rule:

Example 51. Rule to remove unused dependencies of Jaxen metadata
build.gradle.kts

    @CacheableRule
    abstract class JaxenDependenciesRule: ComponentMetadataRule {
        override fun execute(context: ComponentMetadataContext) {
            context.details.allVariants {
                withDependencies {
                    removeAll { it.group in listOf("dom4j", "jdom", "xerces",  "maven-plugins", "xml-apis", "xom") }
                }
            }
        }
    }

build.gradle

    @CacheableRule
    abstract class JaxenDependenciesRule implements ComponentMetadataRule {
        void execute(ComponentMetadataContext context) {
            context.details.allVariants {
                withDependencies {
                    removeAll { it.group in ["dom4j", "jdom", "xerces",  "maven-plugins", "xml-apis", "xom"] }
                }
            }
        }
    }

Within the `withDependencies` block you have access to the full list of dependencies and can use all methods available on the Java collection interface to inspect and modify that list. In addition, there are `add(notation, configureAction)` methods accepting the usual notations similar to declaring dependencies in the build script. Dependency constraints can be inspected and modified the same way in the `withDependencyConstraints` block.

If we take a closer look at the Jaxen 1.1.4 pom, we observe that the dom4j, jdom and xerces dependencies are still there but marked as optional. Optional dependencies in poms are not automatically processed by Gradle nor Maven. The reason is that they indicate that there are optional feature variants provided by the Jaxen library which require one or more of these dependencies, but the information what these features are and which dependency belongs to which is missing. Such information cannot be represented in pom files, but in Gradle Module Metadata through variants and capabilities. Hence, we can add this information in a rule as well.

Example 52. Rule to add optional feature to Jaxen metadata
build.gradle.kts

    @CacheableRule
    abstract class JaxenCapabilitiesRule: ComponentMetadataRule {
        override fun execute(context: ComponentMetadataContext) {
            context.details.addVariant("runtime-dom4j", "runtime") {
                withCapabilities {
                    removeCapability("jaxen", "jaxen")
                    addCapability("jaxen", "jaxen-dom4j", context.details.id.version)
                }
                withDependencies {
                    add("dom4j:dom4j:1.6.1")
                }
            }
        }
    }

build.gradle

    @CacheableRule
    abstract class JaxenCapabilitiesRule implements ComponentMetadataRule {
        void execute(ComponentMetadataContext context) {
            context.details.addVariant("runtime-dom4j", "runtime") {
                withCapabilities {
                    removeCapability("jaxen", "jaxen")
                    addCapability("jaxen", "jaxen-dom4j", context.details.id.version)
                }
                withDependencies {
                    add("dom4j:dom4j:1.6.1")
                }
            }
        }
    }

Here, we first use the `addVariant(name, baseVariant)` method to create an additional variant, which we identify as feature variant by defining a new capability jaxen-dom4j to represent the optional dom4j integration feature of Jaxen. This works similar to defining optional feature variants in build scripts. We then use one of the `add` methods for adding dependencies to define which dependencies this optional feature needs.

In the build script, we can then add a dependency to the optional feature and Gradle will use the enriched metadata to discover the correct transitive dependencies.

Example 53. Applying and utilising rules for Jaxen metadata
build.gradle.kts

    dependencies {
        components {
            withModule<JaxenDependenciesRule>("jaxen:jaxen")
            withModule<JaxenCapabilitiesRule>("jaxen:jaxen")
        }
        implementation("jaxen:jaxen:1.1.3")
        runtimeOnly("jaxen:jaxen:1.1.3") {
            capabilities { requireCapability("jaxen:jaxen-dom4j") }
        }
    }

build.gradle

    dependencies {
        components {
            withModule("jaxen:jaxen", JaxenDependenciesRule)
            withModule("jaxen:jaxen", JaxenCapabilitiesRule)
        }
        implementation("jaxen:jaxen:1.1.3")
        runtimeOnly("jaxen:jaxen:1.1.3") {
            capabilities { requireCapability("jaxen:jaxen-dom4j") }
        }
    }

### Making variants published as classified jars explicit

While in the previous example, all variants, "main variants" and optional features, were packaged in one jar file, it is common to publish certain variants as separate files. In particular, when the variants are mutual exclusive — i.e. they are not feature variants, but different variants offering alternative choices. One example all pom-based libraries already have are the runtime and compile variants, where Gradle can choose only one depending on the task at hand. Another of such alternatives discovered often in the Java ecosystems are jars targeting different Java versions.

As example, we look at version 0.7.9 of the asynchronous programming library Quasar published on Maven central. If we inspect the directory listing, we discover that a `quasar-core-0.7.9-jdk8.jar` was published, in addition to `quasar-core-0.7.9.jar`. Publishing additional jars with a classifier (here jdk8) is common practice in maven repositories. And while both Maven and Gradle allow you to reference such jars by classifier, they are not mentioned at all in the metadata. Thus, there is no information that these jars exist and if there are any other differences, like different dependencies, between the variants represented by such jars.

In Gradle Module Metadata, this variant information would be present and for the already published Quasar library, we can add it using the following rule:

Example 54. Rule to add JDK 8 variants to Quasar metadata
build.gradle.kts

    @CacheableRule
    abstract class QuasarRule: ComponentMetadataRule {
        override fun execute(context: ComponentMetadataContext) {
            listOf("compile", "runtime").forEach { base ->
                context.details.addVariant("jdk8${base.capitalize()}", base) {
                    attributes {
                        attribute(TargetJvmVersion.TARGET_JVM_VERSION_ATTRIBUTE, 8)
                    }
                    withFiles {
                        removeAllFiles()
                        addFile("${context.details.id.name}-${context.details.id.version}-jdk8.jar")
                    }
                }
                context.details.withVariant(base) {
                    attributes {
                        attribute(TargetJvmVersion.TARGET_JVM_VERSION_ATTRIBUTE, 7)
                    }
                }
            }
        }
    }

build.gradle

    @CacheableRule
    abstract class QuasarRule implements ComponentMetadataRule {
        void execute(ComponentMetadataContext context) {
            ["compile", "runtime"].each { base ->
                context.details.addVariant("jdk8${base.capitalize()}", base) {
                    attributes {
                        attribute(TargetJvmVersion.TARGET_JVM_VERSION_ATTRIBUTE, 8)
                    }
                    withFiles {
                        removeAllFiles()
                        addFile("${context.details.id.name}-${context.details.id.version}-jdk8.jar")
                    }
                }
                context.details.withVariant(base) {
                    attributes {
                        attribute(TargetJvmVersion.TARGET_JVM_VERSION_ATTRIBUTE, 7)
                    }
                }
            }
        }
    }

In this case, it is pretty clear that the classifier stands for a target Java version, which is a known Java ecosystem attribute. Because we also need both a compile and runtime for Java 8, we create two new variants but use the existing compile and runtime variants as base. This way, all other Java ecosystem attributes are already set correctly and all dependencies are carried over. Then we set the `TARGET_JVM_VERSION_ATTRIBUTE` to `8` for both variants, remove any existing file from the new variants with `removeAllFiles()`, and add the jdk8 jar file with `addFile()`. The `removeAllFiles()` is needed, because the reference to the main jar `quasar-core-0.7.5.jar` is copied from the corresponding base variant.

We also enrich the existing compile and runtime variants with the information that they target Java 7 — `attribute(TARGET_JVM_VERSION_ATTRIBUTE, 7)`.

Now, we can request a Java 8 versions for all of our dependencies on the compile classpath in the build script and Gradle will automatically select the best fitting variant for each library. In the case of Quasar this will now be the jdk8Compile variant exposing the `quasar-core-0.7.9-jdk8.jar`.

Example 55. Applying and utilising rule for Quasar metadata
build.gradle.kts

    configurations["compileClasspath"].attributes {
        attribute(TargetJvmVersion.TARGET_JVM_VERSION_ATTRIBUTE, 8)
    }
    dependencies {
        components {
            withModule<QuasarRule>("co.paralleluniverse:quasar-core")
        }
        implementation("co.paralleluniverse:quasar-core:0.7.9")
    }

build.gradle

    configurations.compileClasspath.attributes {
        attribute(TargetJvmVersion.TARGET_JVM_VERSION_ATTRIBUTE, 8)
    }
    dependencies {
        components {
            withModule("co.paralleluniverse:quasar-core", QuasarRule)
        }
        implementation("co.paralleluniverse:quasar-core:0.7.9")
    }

### Making variants encoded in versions explicit

Another solution to publish multiple alternatives for the same library is the usage of a versioning pattern as done by the popular Guava library. Here, each new version is published twice by appending the classifier to the version instead of the jar artifact. In the case of Guava 28 for example, we can find a 28.0-jre (Java 8) and 28.0-android (Java 6) version on Maven central. The advantage of using this pattern when working only with pom metadata is that both variants are discoverable through the version. The disadvantage is that there is no information what the different version suffixes mean semantically. So in the case of conflict, Gradle would just pick the highest version when comparing the version strings.

Turning this into proper variants is a bit more tricky, as Gradle first selects a version of a module and then selects the best fitting variant. So the concept that variants are encoded as versions is not supported directly. However, since both variants are always published together we can assume that the files are physically located in the same repository. And since they are published with Maven repository conventions, we know the location of each file if we know module name and version. We can write the following rule:

Example 56. Rule to add JDK 6 and JDK 8 variants to Guava metadata
build.gradle.kts

    @CacheableRule
    abstract class GuavaRule: ComponentMetadataRule {
        override fun execute(context: ComponentMetadataContext) {
            val variantVersion = context.details.id.version
            val version = variantVersion.substring(0, variantVersion.indexOf("-"))
            listOf("compile", "runtime").forEach { base ->
                mapOf(6 to "android", 8 to "jre").forEach { (targetJvmVersion, jarName) ->
                    context.details.addVariant("jdk$targetJvmVersion${base.capitalize()}", base) {
                        attributes {
                            attributes.attribute(TargetJvmVersion.TARGET_JVM_VERSION_ATTRIBUTE, targetJvmVersion)
                        }
                        withFiles {
                            removeAllFiles()
                            addFile("guava-$version-$jarName.jar", "../$version-$jarName/guava-$version-$jarName.jar")
                        }
                    }
                }
            }
        }
    }

build.gradle

    @CacheableRule
    abstract class GuavaRule implements ComponentMetadataRule {
        void execute(ComponentMetadataContext context) {
            def variantVersion = context.details.id.version
            def version = variantVersion.substring(0, variantVersion.indexOf("-"))
            ["compile", "runtime"].each { base ->
                [6: "android", 8: "jre"].each { targetJvmVersion, jarName ->
                    context.details.addVariant("jdk$targetJvmVersion${base.capitalize()}", base) {
                        attributes {
                            attributes.attribute(TargetJvmVersion.TARGET_JVM_VERSION_ATTRIBUTE, targetJvmVersion)
                        }
                        withFiles {
                            removeAllFiles()
                            addFile("guava-$version-${jarName}.jar", "../$version-$jarName/guava-$version-${jarName}.jar")
                        }
                    }
                }
            }
        }
    }

Similar to the previous example, we add runtime and compile variants for both Java versions. In the `withFiles` block however, we now also specify a relative path for the corresponding jar file which allows Gradle to find the file no matter if it has selected a -jre or -android version. The path is always relative to the location of the metadata (in this case `pom`) file of the selection module version. So with this rules, both Guava 28 "versions" carry both the jdk6 and jdk8 variants. So it does not matter to which one Gradle resolves. The variant, and with it the correct jar file, is determined based on the requested `TARGET_JVM_VERSION_ATTRIBUTE` value.

Example 57. Applying and utilising rule for Guava metadata
build.gradle.kts

    configurations["compileClasspath"].attributes {
        attribute(TargetJvmVersion.TARGET_JVM_VERSION_ATTRIBUTE, 6)
    }
    dependencies {
        components {
            withModule<GuavaRule>("com.google.guava:guava")
        }
        // '23.3-android' and '23.3-jre' are now the same as both offer both variants
        implementation("com.google.guava:guava:23.3+")
    }

build.gradle

    configurations.compileClasspath.attributes {
        attribute(TargetJvmVersion.TARGET_JVM_VERSION_ATTRIBUTE, 6)
    }
    dependencies {
        components {
            withModule("com.google.guava:guava", GuavaRule)
        }
        // '23.3-android' and '23.3-jre' are now the same as both offer both variants
        implementation("com.google.guava:guava:23.3+")
    }

### Adding variants for native jars

Jars with classifiers are also used to separate parts of a library for which multiple alternatives exists, for example native code, from the main artifact. This is for example done by the Lightweight Java Game Library (LWGJ), which publishes several platform specific jars to Maven central from which always one is needed, in addition to the main jar, at runtime. It is not possible to convey this information in pom metadata as there is no concept of putting multiple artifacts in relation through the metadata. In Gradle Module Metadata, each variant can have arbitrary many files and we can leverage that by writing the following rule:

Example 58. Rule to add native runtime variants to LWGJ metadata
build.gradle.kts

    @CacheableRule
    abstract class LwjglRule: ComponentMetadataRule {
        data class NativeVariant(val os: String, val arch: String, val classifier: String)

        private val nativeVariants = listOf(
            NativeVariant(OperatingSystemFamily.LINUX,   "arm32",  "natives-linux-arm32"),
            NativeVariant(OperatingSystemFamily.LINUX,   "arm64",  "natives-linux-arm64"),
            NativeVariant(OperatingSystemFamily.WINDOWS, "x86",    "natives-windows-x86"),
            NativeVariant(OperatingSystemFamily.WINDOWS, "x86-64", "natives-windows"),
            NativeVariant(OperatingSystemFamily.MACOS,   "x86-64", "natives-macos")
        )

        @get:Inject abstract val objects: ObjectFactory

        override fun execute(context: ComponentMetadataContext) {
            context.details.withVariant("runtime") {
                attributes {
                    attributes.attribute(OperatingSystemFamily.OPERATING_SYSTEM_ATTRIBUTE, objects.named("none"))
                    attributes.attribute(MachineArchitecture.ARCHITECTURE_ATTRIBUTE, objects.named("none"))
                }
            }
            nativeVariants.forEach { variantDefinition ->
                context.details.addVariant("${variantDefinition.classifier}-runtime", "runtime") {
                    attributes {
                        attributes.attribute(OperatingSystemFamily.OPERATING_SYSTEM_ATTRIBUTE, objects.named(variantDefinition.os))
                        attributes.attribute(MachineArchitecture.ARCHITECTURE_ATTRIBUTE, objects.named(variantDefinition.arch))
                    }
                    withFiles {
                        addFile("${context.details.id.name}-${context.details.id.version}-${variantDefinition.classifier}.jar")
                    }
                }
            }
        }
    }

build.gradle

    @CacheableRule
    abstract class LwjglRule implements ComponentMetadataRule { //val os: String, val arch: String, val classifier: String)
        private def nativeVariants = [
            [os: OperatingSystemFamily.LINUX,   arch: "arm32",  classifier: "natives-linux-arm32"],
            [os: OperatingSystemFamily.LINUX,   arch: "arm64",  classifier: "natives-linux-arm64"],
            [os: OperatingSystemFamily.WINDOWS, arch: "x86",    classifier: "natives-windows-x86"],
            [os: OperatingSystemFamily.WINDOWS, arch: "x86-64", classifier: "natives-windows"],
            [os: OperatingSystemFamily.MACOS,   arch: "x86-64", classifier: "natives-macos"]
        ]

        @Inject abstract ObjectFactory getObjects()

        void execute(ComponentMetadataContext context) {
            context.details.withVariant("runtime") {
                attributes {
                    attributes.attribute(OperatingSystemFamily.OPERATING_SYSTEM_ATTRIBUTE, objects.named(OperatingSystemFamily, "none"))
                    attributes.attribute(MachineArchitecture.ARCHITECTURE_ATTRIBUTE, objects.named(MachineArchitecture, "none"))
                }
            }
            nativeVariants.each { variantDefinition ->
                context.details.addVariant("${variantDefinition.classifier}-runtime", "runtime") {
                    attributes {
                        attributes.attribute(OperatingSystemFamily.OPERATING_SYSTEM_ATTRIBUTE, objects.named(OperatingSystemFamily, variantDefinition.os))
                        attributes.attribute(MachineArchitecture.ARCHITECTURE_ATTRIBUTE, objects.named(MachineArchitecture, variantDefinition.arch))
                    }
                    withFiles {
                        addFile("${context.details.id.name}-${context.details.id.version}-${variantDefinition.classifier}.jar")
                    }
                }
            }
        }
    }

This rule is quite similar to the Quasar library example above. Only this time we have five different runtime variants we add and nothing we need to change for the compile variant. The runtime variants are all based on the existing runtime variant and we do not change any existing information. All Java ecosystem attributes, the dependencies and the main jar file stay part of each of the runtime variants. We only set the additional attributes `OPERATING_SYSTEM_ATTRIBUTE` and `ARCHITECTURE_ATTRIBUTE` which are defined as part of Gradle’s native support. And we add the corresponding native jar file so that each runtime variant now carries two files: the main jar and the native jar.

In the build script, we can now request a specific variant and Gradle will fail with a selection error if more information is needed to make a decision.

Gradle is able to understand the common case where a single attribute is missing that would have removed the ambiguity. In this case, rather than listing information about all attributes on all available variants, Gradle helpfully lists only possible values for that attribute along with the variants each value would select.

Example 59. Applying and utilising rule for LWGJ metadata
build.gradle.kts

    configurations["runtimeClasspath"].attributes {
        attribute(OperatingSystemFamily.OPERATING_SYSTEM_ATTRIBUTE, objects.named("windows"))
    }
    dependencies {
        components {
            withModule<LwjglRule>("org.lwjgl:lwjgl")
        }
        implementation("org.lwjgl:lwjgl:3.2.3")
    }

build.gradle

    configurations["runtimeClasspath"].attributes {
        attribute(OperatingSystemFamily.OPERATING_SYSTEM_ATTRIBUTE, objects.named(OperatingSystemFamily, "windows"))
    }
    dependencies {
        components {
            withModule("org.lwjgl:lwjgl", LwjglRule)
        }
        implementation("org.lwjgl:lwjgl:3.2.3")
    }

Gradle fails to select a variant because a machine architecture needs to be chosen

    > Could not resolve all files for configuration ':runtimeClasspath'.
       > Could not resolve org.lwjgl:lwjgl:3.2.3.
         Required by:
             project :
          > The consumer was configured to find a library for use during runtime, compatible with Java 11, packaged as a jar, preferably optimized for standard JVMs, and its dependencies declared externally, as well as attribute 'org.gradle.native.operatingSystem' with value 'windows'. There are several available matching variants of org.lwjgl:lwjgl:3.2.3
           The only attribute distinguishing these variants is 'org.gradle.native.architecture'. Add this attribute to the consumer's configuration to resolve the ambiguity:
              - Value: 'x86-64' selects variant: 'natives-windows-runtime'
              - Value: 'x86' selects variant: 'natives-windows-x86-runtime'

### Making different flavors of a library available through capabilities

Because it is difficult to model optional feature variants as separate jars with pom metadata, libraries sometimes compose different jars with a different feature set. That is, instead of composing your flavor of the library from different feature variants, you select one of the pre-composed variants (offering everything in one jar). One such library is the well-known dependency injection framework Guice, published on Maven central, which offers a complete flavor (the main jar) and a reduced variant without aspect-oriented programming support (`guice-4.2.2-no_aop.jar`). That second variant with a classifier is not mentioned in the pom metadata. With the following rule, we create compile and runtime variants based on that file and make it selectable through a capability named `com.google.inject:guice-no_aop`.

Example 60. Rule to add no_aop feature variant to Guice metadata
build.gradle.kts

    @CacheableRule
    abstract class GuiceRule: ComponentMetadataRule {
        override fun execute(context: ComponentMetadataContext) {
            listOf("compile", "runtime").forEach { base ->
                context.details.addVariant("noAop${base.capitalize()}", base) {
                    withCapabilities {
                        addCapability("com.google.inject", "guice-no_aop", context.details.id.version)
                    }
                    withFiles {
                        removeAllFiles()
                        addFile("guice-${context.details.id.version}-no_aop.jar")
                    }
                    withDependencies {
                        removeAll { it.group == "aopalliance" }
                    }
                }
            }
        }
    }

build.gradle

    @CacheableRule
    abstract class GuiceRule implements ComponentMetadataRule {
        void execute(ComponentMetadataContext context) {
            ["compile", "runtime"].each { base ->
                context.details.addVariant("noAop${base.capitalize()}", base) {
                    withCapabilities {
                        addCapability("com.google.inject", "guice-no_aop", context.details.id.version)
                    }
                    withFiles {
                        removeAllFiles()
                        addFile("guice-${context.details.id.version}-no_aop.jar")
                    }
                    withDependencies {
                        removeAll { it.group == "aopalliance" }
                    }
                }
            }
        }
    }

The new variants also have the dependency on the standardized aop interfaces library `aopalliance:aopalliance` removed, as this is clearly not needed by these variants. Again, this is information that cannot be expressed in pom metadata. We can now select a `guice-no_aop` variant and will get the correct jar file and the correct dependencies.

Example 61. Applying and utilising rule for Guice metadata
build.gradle.kts

    dependencies {
        components {
            withModule<GuiceRule>("com.google.inject:guice")
        }
        implementation("com.google.inject:guice:4.2.2") {
            capabilities { requireCapability("com.google.inject:guice-no_aop") }
        }
    }

build.gradle

    dependencies {
        components {
            withModule("com.google.inject:guice", GuiceRule)
        }
        implementation("com.google.inject:guice:4.2.2") {
            capabilities { requireCapability("com.google.inject:guice-no_aop") }
        }
    }

### Adding missing capabilities to detect conflicts

Another usage of capabilities is to express that two different modules, for example `log4j` and `log4j-over-slf4j`, provide alternative implementations of the same thing. By declaring that both provide the same capability, Gradle only accepts one of them in a dependency graph. This example, and how it can be tackled with a component metadata rule, is described in detail in the feature modelling section.

### Making Ivy modules variant-aware

Modules with Ivy metadata, do not have variants by default. However, Ivy configurations can be mapped to variants as the `addVariant(name, baseVariantOrConfiguration)` accepts any Ivy configuration that was published as base. This can be used, for example, to define runtime and compile variants. An example of a corresponding rule can be found here. Ivy details of Ivy configurations (e.g. dependencies and files) can also be modified using the `withVariant(configurationName)` API. However, modifying attributes or capabilities on Ivy configurations has no effect.

For very Ivy specific use cases, the component metadata rules API also offers access to other details only found in Ivy metadata. These are available through the IvyModuleDescriptor interface and can be accessed using `getDescriptor(IvyModuleDescriptor)` on the ComponentMetadataContext.

Example 62. Ivy component metadata rule
build.gradle.kts

    @CacheableRule
    abstract class IvyComponentRule : ComponentMetadataRule {
        override fun execute(context: ComponentMetadataContext) {
            val descriptor = context.getDescriptor(IvyModuleDescriptor::class)
            if (descriptor != null && descriptor.branch == "testing") {
                context.details.status = "rc"
            }
        }
    }

build.gradle

    @CacheableRule
    abstract class IvyComponentRule implements ComponentMetadataRule {
        void execute(ComponentMetadataContext context) {
            def descriptor = context.getDescriptor(IvyModuleDescriptor)
            if (descriptor != null && descriptor.branch == "testing") {
                context.details.status = "rc"
            }
        }
    }

### Filter using Maven metadata

For Maven specific use cases, the component metadata rules API also offers access to other details only found in POM metadata. These are available through the PomModuleDescriptor interface and can be accessed using `getDescriptor(PomModuleDescriptor)` on the ComponentMetadataContext.

Example 63. Access pom packaging type in component metadata rule
build.gradle.kts

    @CacheableRule
    abstract class MavenComponentRule : ComponentMetadataRule {
        override fun execute(context: ComponentMetadataContext) {
            val descriptor = context.getDescriptor(PomModuleDescriptor::class)
            if (descriptor != null && descriptor.packaging == "war") {
                // ...
            }
        }
    }

build.gradle

    @CacheableRule
    abstract class MavenComponentRule implements ComponentMetadataRule {
        void execute(ComponentMetadataContext context) {
            def descriptor = context.getDescriptor(PomModuleDescriptor)
            if (descriptor != null && descriptor.packaging == "war") {
                // ...
            }
        }
    }

### Modifying metadata on the component level for alignment

While all the examples above made modifications to variants of a component, there is also a limited set of modifications that can be done to the metadata of the component itself. This information can influence the version selection process for a module during dependency resolution, which is performed before one or multiple variants of a component are selected.

The first API available on the component is `belongsTo()` to create virtual platforms for aligning versions of multiple modules without Gradle Module Metadata. It is explained in detail in the section on aligning versions of modules not published with Gradle.

### Modifying metadata on the component level for version selection based on status

Gradle and Gradle Module Metadata also allow attributes to be set on the whole component instead of a single variant. Each of these attributes carries special semantics as they influence version selection which is done before variant selection. While variant selection can handle any custom attribute, version selection only considers attributes for which specific semantics are implemented. At the moment, the only attribute with meaning here is `org.gradle.status`. It is therefore recommended to only modify this attribute, if any, on the component level. A dedicated API `setStatus(value)` is available for this. To modify another attribute for all variants of a component `withAllVariants { attributes {} }` should be utilised instead.

A module’s status is taken into consideration when a latest version selector is resolved. Specifically, `latest.someStatus` will resolve to the highest module version that has status `someStatus` or a more mature status. For example, `latest.integration` will select the highest module version regardless of its status (because `integration` is the least mature status as explained below), whereas `latest.release` will select the highest module version with status `release`.

The interpretation of the status can be influenced by changing a module’s status scheme through the `setStatusScheme(valueList)` API. This concept models the different levels of maturity that a module transitions through over time with different publications. The default status scheme, ordered from least to most mature status, is `integration`, `milestone`, `release`. The `org.gradle.status` attribute must be set, to one of the values in the components status scheme. Thus each component always has a status which is determined from the metadata as follows:

- Gradle Module Metadata: the value that was published for the `org.gradle.status` attribute on the component

- Ivy metadata: `status` defined in the ivy.xml, defaults to `integration` if missing

- Pom metadata: `integration` for modules with a SNAPSHOT version, `release` for all others

The following example demonstrates `latest` selectors based on a custom status scheme declared in a component metadata rule that applies to all modules:

Example 64. Custom status scheme
build.gradle.kts

    @CacheableRule
    abstract class CustomStatusRule : ComponentMetadataRule {
        override fun execute(context: ComponentMetadataContext) {
            context.details.statusScheme = listOf("nightly", "milestone", "rc", "release")
            if (context.details.status == "integration") {
                context.details.status = "nightly"
            }
        }
    }

    dependencies {
        components {
            all<CustomStatusRule>()
        }
        implementation("org.apache.commons:commons-lang3:latest.rc")
    }

build.gradle

    @CacheableRule
    abstract class CustomStatusRule implements ComponentMetadataRule {
        void execute(ComponentMetadataContext context) {
            context.details.statusScheme = ["nightly", "milestone", "rc", "release"]
            if (context.details.status == "integration") {
                context.details.status = "nightly"
            }
        }
    }

    dependencies {
        components {
            all(CustomStatusRule)
        }
        implementation("org.apache.commons:commons-lang3:latest.rc")
    }

Compared to the default scheme, the rule inserts a new status `rc` and replaces `integration` with `nightly`. Existing modules with the state `integration` are mapped to `nightly`.

## Customizing resolution of a dependency directly

This section covers mechanisms Gradle offers to directly influence the behavior of the dependency resolution engine. In contrast to the other concepts covered in this chapter, like dependency constraints or component metadata rules, which are all inputs to resolution, the following mechanisms allow you to write rules which are directly injected into the resolution engine. Because of this, they can be seen as brute force solutions, that may hide future problems (e.g. if new dependencies are added). Therefore, the general advice is to only use the following mechanisms if other means are not sufficient. If you are authoring a library, you should always prefer dependency constraints as they are published for your consumers.

### Using dependency resolve rules

A dependency resolve rule is executed for each resolved dependency, and offers a powerful api for manipulating a requested dependency prior to that dependency being resolved. The feature currently offers the ability to change the group, name and/or version of a requested dependency, allowing a dependency to be substituted with a completely different module during resolution.

Dependency resolve rules provide a very powerful way to control the dependency resolution process, and can be used to implement all sorts of advanced patterns in dependency management. Some of these patterns are outlined below. For more information and code samples see the ResolutionStrategy class in the API documentation.

#### Implementing a custom versioning scheme

In some corporate environments, the list of module versions that can be declared in Gradle builds is maintained and audited externally. Dependency resolve rules provide a neat implementation of this pattern:

- In the build script, the developer declares dependencies with the module group and name, but uses a placeholder version, for example: `default`.

- The `default` version is resolved to a specific version via a dependency resolve rule, which looks up the version in a corporate catalog of approved modules.

This rule implementation can be neatly encapsulated in a corporate plugin, and shared across all builds within the organisation.

Example 65. Using a custom versioning scheme
build.gradle.kts

    configurations.all {
        resolutionStrategy.eachDependency {
            if (requested.version == "default") {
                val version = findDefaultVersionInCatalog(requested.group, requested.name)
                useVersion(version.version)
                because(version.because)
            }
        }
    }

    data class DefaultVersion(val version: String, val because: String)

    fun findDefaultVersionInCatalog(group: String, name: String): DefaultVersion {
        //some custom logic that resolves the default version into a specific version
        return DefaultVersion(version = "1.0", because = "tested by QA")
    }

build.gradle

    configurations.all {
        resolutionStrategy.eachDependency { DependencyResolveDetails details ->
            if (details.requested.version == 'default') {
                def version = findDefaultVersionInCatalog(details.requested.group, details.requested.name)
                details.useVersion version.version
                details.because version.because
            }
        }
    }

    def findDefaultVersionInCatalog(String group, String name) {
        //some custom logic that resolves the default version into a specific version
        [version: "1.0", because: 'tested by QA']
    }

#### Denying a particular version with a replacement

Dependency resolve rules provide a mechanism for denying a particular version of a dependency and providing a replacement version. This can be useful if a certain dependency version is broken and should not be used, where a dependency resolve rule causes this version to be replaced with a known good version. One example of a broken module is one that declares a dependency on a library that cannot be found in any of the public repositories, but there are many other reasons why a particular module version is unwanted and a different version is preferred.

In example below, imagine that version `1.2.1` contains important fixes and should always be used in preference to `1.2`. The rule provided will enforce just this: any time version `1.2` is encountered it will be replaced with `1.2.1`. Note that this is different from a forced version as described above, in that any other versions of this module would not be affected. This means that the 'newest' conflict resolution strategy would still select version `1.3` if this version was also pulled transitively.

Example 66. Example: Blacklisting a version with a replacement
build.gradle.kts

    configurations.all {
        resolutionStrategy.eachDependency {
            if (requested.group == "org.software" && requested.name == "some-library" && requested.version == "1.2") {
                useVersion("1.2.1")
                because("fixes critical bug in 1.2")
            }
        }
    }

build.gradle

    configurations.all {
        resolutionStrategy.eachDependency { DependencyResolveDetails details ->
            if (details.requested.group == 'org.software' && details.requested.name == 'some-library' && details.requested.version == '1.2') {
                details.useVersion '1.2.1'
                details.because 'fixes critical bug in 1.2'
            }
        }
    }

| There’s a difference with using the reject directive of rich version constraints: rich versions will cause the build to fail if a rejected version is found in the graph, or select a non rejected version when using dynamic dependencies. Here, we manipulate the requested versions in order to select a different version when we find a rejected one. In other words, this is a solution to rejected versions, while rich version constraints allow declaring the intent (you should not use this version).

### Using module replacement rules

It is preferable to express module conflicts in terms of capabilities conflicts. However, if there’s no such rule declared or that you are working on versions of Gradle which do not support capabilities, Gradle provides tooling to work around those issues.

Module replacement rules allow a build to declare that a legacy library has been replaced by a new one. A good example when a new library replaced a legacy one is the `google-collections` -> `guava` migration. The team that created google-collections decided to change the module name from `com.google.collections:google-collections` into `com.google.guava:guava`. This is a legal scenario in the industry: teams need to be able to change the names of products they maintain, including the module coordinates. Renaming of the module coordinates has impact on conflict resolution.

To explain the impact on conflict resolution, let’s consider the `google-collections` -> `guava` scenario. It may happen that both libraries are pulled into the same dependency graph. For example, our project depends on `guava` but some of our dependencies pull in a legacy version of `google-collections`. This can cause runtime errors, for example during test or application execution. Gradle does not automatically resolve the `google-collections` -> `guava` conflict because it is not considered as a version conflict. It’s because the module coordinates for both libraries are completely different and conflict resolution is activated when `group` and `module` coordinates are the same but there are different versions available in the dependency graph (for more info, refer to the section on conflict resolution). Traditional remedies to this problem are:

- Declare exclusion rule to avoid pulling in `google-collections` to graph. It is probably the most popular approach.

- Avoid dependencies that pull in legacy libraries.

- Upgrade the dependency version if the new version no longer pulls in a legacy library.

- Downgrade to `google-collections`. It’s not recommended, just mentioned for completeness.

Traditional approaches work but they are not general enough. For example, an organisation wants to resolve the `google-collections` -> `guava` conflict resolution problem in all projects. It is possible to declare that certain module was replaced by other. This enables organisations to include the information about module replacement in the corporate plugin suite and resolve the problem holistically for all Gradle-powered projects in the enterprise.

Example 67. Declaring a module replacement
build.gradle.kts

    dependencies {
        modules {
            module("com.google.collections:google-collections") {
                replacedBy("com.google.guava:guava", "google-collections is now part of Guava")
            }
        }
    }

build.gradle

    dependencies {
        modules {
            module("com.google.collections:google-collections") {
                replacedBy("com.google.guava:guava", "google-collections is now part of Guava")
            }
        }
    }

For more examples and detailed API, refer to the DSL reference for ComponentMetadataHandler.

What happens when we declare that `google-collections` is replaced by `guava`? Gradle can use this information for conflict resolution. Gradle will consider every version of `guava` newer/better than any version of `google-collections`. Also, Gradle will ensure that only guava jar is present in the classpath / resolved file list. Note that if only `google-collections` appears in the dependency graph (e.g. no `guava`) Gradle will not eagerly replace it with `guava`. Module replacement is an information that Gradle uses for resolving conflicts. If there is no conflict (e.g. only `google-collections` or only `guava` in the graph) the replacement information is not used.

Currently it is not possible to declare that a given module is replaced by a set of modules. However, it is possible to declare that multiple modules are replaced by a single module.

### Using dependency substitution rules

Dependency substitution rules work similarly to dependency resolve rules. In fact, many capabilities of dependency resolve rules can be implemented with dependency substitution rules. They allow project and module dependencies to be transparently substituted with specified replacements. Unlike dependency resolve rules, dependency substitution rules allow project and module dependencies to be substituted interchangeably.

Adding a dependency substitution rule to a configuration changes the timing of when that configuration is resolved. Instead of being resolved on first use, the configuration is instead resolved when the task graph is being constructed. This can have unexpected consequences if the configuration is being further modified during task execution, or if the configuration relies on modules that are published during execution of another task.

To explain:

- A `Configuration` can be declared as an input to any Task, and that configuration can include project dependencies when it is resolved.

- If a project dependency is an input to a Task (via a configuration), then tasks to build the project artifacts must be added to the task dependencies.

- In order to determine the project dependencies that are inputs to a task, Gradle needs to resolve the `Configuration` inputs.

- Because the Gradle task graph is fixed once task execution has commenced, Gradle needs to perform this resolution prior to executing any tasks.

In the absence of dependency substitution rules, Gradle knows that an external module dependency will never transitively reference a project dependency. This makes it easy to determine the full set of project dependencies for a configuration through simple graph traversal. With this functionality, Gradle can no longer make this assumption, and must perform a full resolve in order to determine the project dependencies.

#### Substituting an external module dependency with a project dependency

One use case for dependency substitution is to use a locally developed version of a module in place of one that is downloaded from an external repository. This could be useful for testing a local, patched version of a dependency.

The module to be replaced can be declared with or without a version specified.

Example 68. Substituting a module with a project
build.gradle.kts

    configurations.all {
        resolutionStrategy.dependencySubstitution {
            substitute(module("org.utils:api"))
                .using(project(":api")).because("we work with the unreleased development version")
            substitute(module("org.utils:util:2.5")).using(project(":util"))
        }
    }

build.gradle

    configurations.all {
        resolutionStrategy.dependencySubstitution {
            substitute module("org.utils:api") using project(":api") because "we work with the unreleased development version"
            substitute module("org.utils:util:2.5") using project(":util")
        }
    }

Note that a project that is substituted must be included in the multi-project build (via `settings.gradle`). Dependency substitution rules take care of replacing the module dependency with the project dependency and wiring up any task dependencies, but do not implicitly include the project in the build.

#### Substituting a project dependency with a module replacement

Another way to use substitution rules is to replace a project dependency with a module in a multi-project build. This can be useful to speed up development with a large multi-project build, by allowing a subset of the project dependencies to be downloaded from a repository rather than being built.

The module to be used as a replacement must be declared with a version specified.

Example 69. Substituting a project with a module
build.gradle.kts

    configurations.all {
        resolutionStrategy.dependencySubstitution {
            substitute(project(":api"))
                .using(module("org.utils:api:1.3")).because("we use a stable version of org.utils:api")
        }
    }

build.gradle

    configurations.all {
        resolutionStrategy.dependencySubstitution {
            substitute project(":api") using module("org.utils:api:1.3") because "we use a stable version of org.utils:api"
        }
    }

When a project dependency has been replaced with a module dependency, that project is still included in the overall multi-project build. However, tasks to build the replaced dependency will not be executed in order to resolve the depending `Configuration`.

#### Conditionally substituting a dependency

A common use case for dependency substitution is to allow more flexible assembly of sub-projects within a multi-project build. This can be useful for developing a local, patched version of an external dependency or for building a subset of the modules within a large multi-project build.

The following example uses a dependency substitution rule to replace any module dependency with the group `org.example`, but only if a local project matching the dependency name can be located.

Example 70. Conditionally substituting a dependency
build.gradle.kts

    configurations.all {
        resolutionStrategy.dependencySubstitution.all {
            requested.let {
                if (it is ModuleComponentSelector && it.group == "org.example") {
                    val targetProject = findProject(":${it.module}")
                    if (targetProject != null) {
                        useTarget(targetProject)
                    }
                }
            }
        }
    }

build.gradle

    configurations.all {
        resolutionStrategy.dependencySubstitution.all { DependencySubstitution dependency ->
            if (dependency.requested instanceof ModuleComponentSelector && dependency.requested.group == "org.example") {
                def targetProject = findProject(":${dependency.requested.module}")
                if (targetProject != null) {
                    dependency.useTarget targetProject
                }
            }
        }
    }

Note that a project that is substituted must be included in the multi-project build (via `settings.gradle`). Dependency substitution rules take care of replacing the module dependency with the project dependency, but do not implicitly include the project in the build.

#### Substituting a dependency with another variant

Gradle’s dependency management engine is variant-aware meaning that for a single component, the engine may select different artifacts and transitive dependencies.

What to select is determined by the attributes of the consumer configuration and the attributes of the variants found on the producer side. It is, however, possible that some specific dependencies override attributes from the configuration itself. This is typically the case when using the Java Platform plugin: this plugin builds a special kind of component which is called a "platform" and can be addressed by setting the component category attribute to `platform`, in opposition to typical dependencies which are targetting libraries.

Therefore, you may face situations where you want to substitute a platform dependency with a regular dependency, or the other way around.

##### Substituting a dependency with attributes

Let’s imagine that you want to substitute a platform dependency with a regular dependency. This means that the library you are consuming declared something like this:

Example 71. An incorrect dependency on a platform
lib/build.gradle.kts

    dependencies {
        // This is a platform dependency but you want the library
        implementation(platform("com.google.guava:guava:28.2-jre"))
    }

lib/build.gradle

    dependencies {
        // This is a platform dependency but you want the library
        implementation platform('com.google.guava:guava:28.2-jre')
    }

The `platform` keyword is actually a short-hand notation for a dependency with attributes. If we want to substitute this dependency with a regular dependency, then we need to select precisely the dependencies which have the `platform` attribute.

This can be done by using a substitution rule:

Example 72. Substitute a platform dependency with a regular dependency
consumer/build.gradle.kts

    configurations.all {
        resolutionStrategy.dependencySubstitution {
            substitute(platform(module("com.google.guava:guava:28.2-jre")))
                .using(module("com.google.guava:guava:28.2-jre"))
        }
    }

consumer/build.gradle

    configurations.all {
        resolutionStrategy.dependencySubstitution {
            substitute(platform(module('com.google.guava:guava:28.2-jre'))).
                using module('com.google.guava:guava:28.2-jre')
        }
    }

The same rule without the `platform` keyword would try to substitute regular dependencies with a regular dependency, which is not what you want, so it’s important to understand that the substitution rules apply on a dependency specification: it matches the requested dependency (`substitute XXX`) with a substitute (`using YYY`).

You can have attributes on both the requested dependency or the substitute and the substitution is not limited to `platform`: you can actually specify the whole set of dependency attributes using the `variant` notation. The following rule is strictly equivalent to the rule above:

Example 73. Substitute a platform dependency with a regular dependency using the variant notation
consumer/build.gradle.kts

    configurations.all {
        resolutionStrategy.dependencySubstitution {
            substitute(variant(module("com.google.guava:guava:28.2-jre")) {
                attributes {
                    attribute(Category.CATEGORY_ATTRIBUTE, objects.named(Category.REGULAR_PLATFORM))
                }
            }).using(module("com.google.guava:guava:28.2-jre"))
        }
    }

consumer/build.gradle

    configurations.all {
        resolutionStrategy.dependencySubstitution {
            substitute variant(module('com.google.guava:guava:28.2-jre')) {
                attributes {
                    attribute(Category.CATEGORY_ATTRIBUTE, objects.named(Category, Category.REGULAR_PLATFORM))
                }
            } using module('com.google.guava:guava:28.2-jre')
        }
    }

Please refer to the Substitution DSL API docs for a complete reference of the variant substitution API.

| In composite builds, the rule that you have to match the exact requested dependency attributes is not applied: when using composites, Gradle will automatically match the requested attributes. In other words, it is implicit that if you include another build, you are substituting all variants of the substituted module with an equivalent variant in the included build.

##### Substituting a dependency with a dependency with capabilities

Similarly to attributes substitution, Gradle lets you substitute a dependency with or without capabilities with another dependency with or without capabilities.

For example, let’s imagine that you need to substitute a regular dependency with its test fixtures instead. You can achieve this by using the following dependency substitution rule:

Example 74. Substitute a dependency with its test fixtures
build.gradle.kts

    configurations.testCompileClasspath {
        resolutionStrategy.dependencySubstitution {
            substitute(module("com.acme:lib:1.0")).using(variant(module("com.acme:lib:1.0")) {
                capabilities {
                    requireCapability("com.acme:lib-test-fixtures")
                }
            })
        }
    }

build.gradle

    configurations.testCompileClasspath {
        resolutionStrategy.dependencySubstitution {
            substitute(module('com.acme:lib:1.0'))
                .using variant(module('com.acme:lib:1.0')) {
                capabilities {
                    requireCapability('com.acme:lib-test-fixtures')
                }
            }
        }
    }

Capabilities which are declared in a substitution rule on the requested dependency constitute part of the dependency match specification, and therefore dependencies which do not require the capabilities will not be matched.

Please refer to the Substitution DSL API docs for a complete reference of the variant substitution API.

#### Substituting a dependency with a classifier or artifact

While external modules are in general addressed via their group/artifact/version coordinates, it is common that such modules are published with additional artifacts that you may want to use in place of the main artifact. This is typically the case for classified artifacts, but you may also need to select an artifact with a different file type or extension. Gradle discourages use of classifiers in dependencies and prefers to model such artifacts as additional variants of a module. There are lots of advantages of using variants instead of classified artifacts, including, but not only, a different set of dependencies for those artifacts.

However, in order to help bridging the two models, Gradle provides means to change or remove a classifier in a substitution rule.

Example 75. Dependencies which will lead to a resolution error
consumer/build.gradle.kts

    dependencies {
        implementation("com.google.guava:guava:28.2-jre")
        implementation("co.paralleluniverse:quasar-core:0.8.0")
        implementation(project(":lib"))
    }

consumer/build.gradle

    dependencies {
        implementation 'com.google.guava:guava:28.2-jre'
        implementation 'co.paralleluniverse:quasar-core:0.8.0'
        implementation project(':lib')
    }

In the example above, the first level dependency on `quasar` makes us think that Gradle would resolve `quasar-core-0.8.0.jar` but it’s not the case: the build would fail with this message:

    Execution failed for task ':resolve'.
    > Could not resolve all files for configuration ':runtimeClasspath'.
       > Could not find quasar-core-0.8.0-jdk8.jar (co.paralleluniverse:quasar-core:0.8.0).
         Searched in the following locations:
             https://repo1.maven.org/maven2/co/paralleluniverse/quasar-core/0.8.0/quasar-core-0.8.0-jdk8.jar

That’s because there’s a dependency on another project, `lib`, which itself depends on a different version of `quasar-core`:

Example 76. A "classified" dependency
lib/build.gradle.kts

    dependencies {
        implementation("co.paralleluniverse:quasar-core:0.7.10:jdk8")
    }

lib/build.gradle

    dependencies {
        implementation "co.paralleluniverse:quasar-core:0.7.10:jdk8"
    }

What happens is that Gradle would perform conflict resolution between `quasar-core` 0.8.0 and `quasar-core` 0.7.10. Because 0.8.0 is higher, we select this version, but the dependency in `lib` has a classifier, `jdk8` and this classifier doesn’t exist anymore in release 0.8.0.

To fix this problem, you can ask Gradle to resolve both dependencies without classifier:

Example 77. A resolution rule to disable selection of a classifier
consumer/build.gradle.kts

    configurations.all {
        resolutionStrategy.dependencySubstitution {
            substitute(module("co.paralleluniverse:quasar-core"))
                .using(module("co.paralleluniverse:quasar-core:0.8.0"))
                .withoutClassifier()
        }
    }

consumer/build.gradle

    configurations.all {
        resolutionStrategy.dependencySubstitution {
            substitute module('co.paralleluniverse:quasar-core') using module('co.paralleluniverse:quasar-core:0.8.0') withoutClassifier()
        }
    }

This rule effectively replaces any dependency on `quasar-core` found in the graph with a dependency without classifier.

Alternatively, it’s possible to select a dependency with a specific classifier or, for more specific use cases, substitute with a very specific artifact (type, extension and classifier).

For more information, please refer to the following API documentation:

- artifact selection via the Substitution DSL

- artifact selection via the DependencySubstitution API

- artifact selection via the ResolutionStrategy API

### Disabling transitive resolution

By default Gradle resolves all transitive dependencies specified by the dependency metadata. Sometimes this behavior may not be desirable e.g. if the metadata is incorrect or defines a large graph of transitive dependencies. You can tell Gradle to disable transitive dependency management for a dependency by setting ModuleDependency.setTransitive(boolean) to `false`. As a result only the main artifact will be resolved for the declared dependency.

Example 78. Disabling transitive dependency resolution for a declared dependency
build.gradle.kts

    dependencies {
        implementation("com.google.guava:guava:23.0") {
            isTransitive = false
        }
    }

build.gradle

    dependencies {
        implementation('com.google.guava:guava:23.0') {
            transitive = false
        }
    }

| Disabling transitive dependency resolution will likely require you to declare the necessary runtime dependencies in your build script which otherwise would have been resolved automatically. Not doing so might lead to runtime classpath issues.

A project can decide to disable transitive dependency resolution completely. You either don’t want to rely on the metadata published to the consumed repositories or you want to gain full control over the dependencies in your graph. For more information, see Configuration.setTransitive(boolean).

Example 79. Disabling transitive dependency resolution on the configuration-level
build.gradle.kts

    configurations.all {
        isTransitive = false
    }

    dependencies {
        implementation("com.google.guava:guava:23.0")
    }

build.gradle

    configurations.all {
        transitive = false
    }

    dependencies {
        implementation 'com.google.guava:guava:23.0'
    }

### Lazily influencing resolved dependencies

At times, a plugin may want to influence dependencies of a configuration lazily. Use cases include:

- Adding a dependency to a configuration based on some condition.

- Setting a preferred version of a dependency if the user has not specified a version.

Consider the following examples that demonstrate these use cases.

Example 80. Lazily adding a dependency to a configuration based on some condition
build.gradle.kts

    configurations {
        implementation {
            dependencies.addLater(project.provider {
                val dependencyNotation = conditionalLogic()
                if (dependencyNotation != null) {
                    project.dependencies.create(dependencyNotation)
                } else {
                    null
                }
            })
        }
    }

build.gradle

    configurations {
        implementation {
            dependencies.addLater(project.provider {
                def dependencyNotation = conditionalLogic()
                if (dependencyNotation != null) {
                    return project.dependencies.create(dependencyNotation)
                } else {
                    return null
                }
            })
        }
    }

Example 81. Preferring a default version of a dependency if the user has not specified a version
build.gradle.kts

    dependencies {
        implementation("org:foo")

        // Can indiscriminately be added by build logic
        constraints {
            implementation("org:foo:1.0") {
                version {
                    // Applied to org:foo if no other version is specified
                    prefer("1.0")
                }
            }
        }
    }

build.gradle

    dependencies {
        implementation("org:foo")

        // Can indiscriminately be added by build logic
        constraints {
            implementation("org:foo:1.0") {
                version {
                    // Applied to org:foo if no other version is specified
                    prefer("1.0")
                }
            }
        }
    }

### Setting default configuration dependencies

A configuration can be configured with default dependencies to be used if no dependencies are explicitly set for the configuration. A primary use case of this functionality is for developing plugins that make use of versioned tools that the user might override. By specifying default dependencies, the plugin can use a default version of the tool only if the user has not specified a particular version to use.

Example 82. Specifying default dependencies on a configuration
build.gradle.kts

    configurations {
        create("pluginTool") {
            defaultDependencies {
                add(project.dependencies.create("org.gradle:my-util:1.0"))
            }
        }
    }

build.gradle

    configurations {
        pluginTool {
            defaultDependencies { dependencies ->
                dependencies.add(project.dependencies.create("org.gradle:my-util:1.0"))
            }
        }
    }

### Excluding a dependency from a configuration completely

Similar to excluding a dependency in a dependency declaration, you can exclude a transitive dependency for a particular configuration completely by using Configuration.exclude(java.util.Map). This will automatically exclude the transitive dependency for all dependencies declared on the configuration.

Example 83. Excluding transitive dependency for a particular configuration
build.gradle.kts

    configurations {
        "implementation" {
            exclude(group = "commons-collections", module = "commons-collections")
        }
    }

    dependencies {
        implementation("commons-beanutils:commons-beanutils:1.9.4")
        implementation("com.opencsv:opencsv:4.6")
    }

build.gradle

    configurations {
        implementation {
            exclude group: 'commons-collections', module: 'commons-collections'
        }
    }

    dependencies {
        implementation 'commons-beanutils:commons-beanutils:1.9.4'
        implementation 'com.opencsv:opencsv:4.6'
    }

### Matching dependencies to repositories

Gradle exposes an API to declare what a repository may or may not contain. This feature offers a fine grained control on which repository serve which artifacts, which can be one way of controlling the source of dependencies.

Head over to the section on repository content filtering to know more about this feature.

### Enabling Ivy dynamic resolve mode

Gradle’s Ivy repository implementations support the equivalent to Ivy’s dynamic resolve mode. Normally, Gradle will use the `rev` attribute for each dependency definition included in an `ivy.xml` file. In dynamic resolve mode, Gradle will instead prefer the `revConstraint` attribute over the `rev` attribute for a given dependency definition. If the `revConstraint` attribute is not present, the `rev` attribute is used instead.

To enable dynamic resolve mode, you need to set the appropriate option on the repository definition. A couple of examples are shown below. Note that dynamic resolve mode is only available for Gradle’s Ivy repositories. It is not available for Maven repositories, or custom Ivy `DependencyResolver` implementations.

Example 84. Enabling dynamic resolve mode
build.gradle.kts

    // Can enable dynamic resolve mode when you define the repository
    repositories {
        ivy {
            url = uri("http://repo.mycompany.com/repo")
            resolve.isDynamicMode = true
        }
    }

    // Can use a rule instead to enable (or disable) dynamic resolve mode for all repositories
    repositories.withType<IvyArtifactRepository> {
        resolve.isDynamicMode = true
    }

build.gradle

    // Can enable dynamic resolve mode when you define the repository
    repositories {
        ivy {
            url "http://repo.mycompany.com/repo"
            resolve.dynamicMode = true
        }
    }

    // Can use a rule instead to enable (or disable) dynamic resolve mode for all repositories
    repositories.withType(IvyArtifactRepository) {
        resolve.dynamicMode = true
    }

# Producing and Consuming Variants of Libraries

## Declaring Capabilities of a Library

### Capabilities as first-level concept

Components provide a number of features which are often orthogonal to the software architecture used to provide those features. For example, a library may include several features in a single artifact. However, such a library would be published at single GAV (group, artifact and version) coordinates. This means that, at single coordinates, potentially co-exist different "features" of a component.

With Gradle it becomes interesting to explicitly declare what features a component provides. For this, Gradle provides the concept of capability.

A feature is often built by combining different capabilities.

In an ideal world, components shouldn’t declare dependencies on explicit GAVs, but rather express their requirements in terms of capabilities:

- "give me a component which provides logging"

- "give me a scripting engine"

- "give me a scripting engine that supports Groovy"

By modeling capabilities, the dependency management engine can be smarter and tell you whenever you have incompatible capabilities in a dependency graph, or ask you to choose whenever different modules in a graph provide the same capability.

### Declaring capabilities for external modules

It’s worth noting that Gradle supports declaring capabilities for components you build, but also for external components in case they didn’t.

For example, if your build file contains the following dependencies:

Example 85. A build file with an implicit conflict of logging frameworks
build.gradle.kts

    dependencies {
        // This dependency will bring log4:log4j transitively
        implementation("org.apache.zookeeper:zookeeper:3.4.9")

        // We use log4j over slf4j
        implementation("org.slf4j:log4j-over-slf4j:1.7.10")
    }

build.gradle

    dependencies {
        // This dependency will bring log4:log4j transitively
        implementation 'org.apache.zookeeper:zookeeper:3.4.9'

        // We use log4j over slf4j
        implementation 'org.slf4j:log4j-over-slf4j:1.7.10'
    }

As is, it’s pretty hard to figure out that you will end up with two logging frameworks on the classpath. In fact, `zookeeper` will bring in `log4j`, where what we want to use is `log4j-over-slf4j`. We can preemptively detect the conflict by adding a rule which will declare that both logging frameworks provide the same capability:

Example 86. A build file with an implicit conflict of logging frameworks
build.gradle.kts

    dependencies {
        // Activate the "LoggingCapability" rule
        components.all(LoggingCapability::class.java)
    }

    class LoggingCapability : ComponentMetadataRule {
        val loggingModules = setOf("log4j", "log4j-over-slf4j")

        override
        fun execute(context: ComponentMetadataContext) = context.details.run {
            if (loggingModules.contains(id.name)) {
                allVariants {
                    withCapabilities {
                        // Declare that both log4j and log4j-over-slf4j provide the same capability
                        addCapability("log4j", "log4j", id.version)
                    }
                }
            }
        }
    }

build.gradle

    dependencies {
        // Activate the "LoggingCapability" rule
        components.all(LoggingCapability)
    }

    @CompileStatic
    class LoggingCapability implements ComponentMetadataRule {
        final static Set<String> LOGGING_MODULES = ["log4j", "log4j-over-slf4j"] as Set<String>

        void execute(ComponentMetadataContext context) {
            context.details.with {
                if (LOGGING_MODULES.contains(id.name)) {
                    allVariants {
                        it.withCapabilities {
                            // Declare that both log4j and log4j-over-slf4j provide the same capability
                            it.addCapability("log4j", "log4j", id.version)
                        }
                    }
                }
            }
        }
    }

By adding this rule, we will make sure that Gradle will detect conflicts and properly fail:

    > Could not resolve all files for configuration ':compileClasspath'.
       > Could not resolve org.slf4j:log4j-over-slf4j:1.7.10.
         Required by:
             project :
          > Module 'org.slf4j:log4j-over-slf4j' has been rejected:
               Cannot select module with conflict on capability 'log4j:log4j:1.7.10' also provided by [log4j:log4j:1.2.16(compile)]
       > Could not resolve log4j:log4j:1.2.16.
         Required by:
             project : > org.apache.zookeeper:zookeeper:3.4.9
          > Module 'log4j:log4j' has been rejected:
               Cannot select module with conflict on capability 'log4j:log4j:1.2.16' also provided by [org.slf4j:log4j-over-slf4j:1.7.10(compile)]

See the capabilities section of the documentation to figure out how to fix capability conflicts.

### Declaring additional capabilities for a local component

All components have an implicit capability corresponding to the same GAV coordinates as the component. However, it is also possible to declare additional explicit capabilities for a component. This is convenient whenever a library published at different GAV coordinates is an alternate implementation of the same API:

Example 87. Declaring capabilities of a component
build.gradle.kts

    configurations {
        apiElements {
            outgoing {
                capability("com.acme:my-library:1.0")
                capability("com.other:module:1.1")
            }
        }
        runtimeElements {
            outgoing {
                capability("com.acme:my-library:1.0")
                capability("com.other:module:1.1")
            }
        }
    }

build.gradle

    configurations {
        apiElements {
            outgoing {
                capability("com.acme:my-library:1.0")
                capability("com.other:module:1.1")
            }
        }
        runtimeElements {
            outgoing {
                capability("com.acme:my-library:1.0")
                capability("com.other:module:1.1")
            }
        }
    }

Capabilities must be attached to outgoing configurations, which are consumable configurations of a component.

This example shows that we declare two capabilities:

1. `com.acme:my-library:1.0`, which corresponds to the implicit capability of the library

2. `com.other:module:1.1`, which corresponds to another capability of this library

It’s worth noting we need to do 1. because as soon as you start declaring explicit capabilities, then all capabilities need to be declared, including the implicit one.

The second capability can be specific to this library, or it can correspond to a capability provided by an external component. In that case, if `com.other:module` appears in the same dependency graph, the build will fail and consumers will have to choose what module to use.

Capabilities are published to Gradle Module Metadata. However, they have no equivalent in POM or Ivy metadata files. As a consequence, when publishing such a component, Gradle will warn you that this feature is only for Gradle consumers:

    Maven publication 'maven' contains dependencies that cannot be represented in a published pom file.
      - Declares capability com.acme:my-library:1.0
      - Declares capability com.other:module:1.1

## Modeling library features

Gradle supports the concept of features: it’s often the case that a single library can be split up into multiple related yet distinct libraries, where each feature can be used alongside the main library.

Features allow a component to expose multiple related libraries, each of which can declare its own dependencies. These libraries are exposed as variants, similar to how the main library exposes variants for its API and runtime.

This allows for a number of different scenarios (list is non-exhaustive):

- a (better) substitute for Maven optional dependencies

- a main library is built with support for different mutually-exclusive implementations of runtime features; the user must choose one, and only one, implementation of each such feature

- a main library is built with support for optional runtime features, each of which requires a different set of dependencies

- a main library comes with supplementary features like test fixtures

- a main library comes with a main artifact, and enabling an additional feature requires additional artifacts

### Selection of features via capabilities

Declaring a dependency on a component is usually done by providing a set of coordinates (group, artifact, version also known as GAV coordinates). This allows the engine to determine the component we’re looking for, but such a component may provide different variants. A variant is typically chosen based on the usage. For example, we might choose a different variant for compiling against a component (in which case we need the API of the component) or when executing code (in which case we need the runtime of the component). All variants of a component provide a number of capabilities, which are denoted similarly using GAV coordinates.

A capability is denoted by GAV coordinates, but you must think of it as feature description:

- "I provide an SLF4J binding"

- "I provide runtime support for MySQL"

- "I provide a Groovy runtime"

And in general, having two components that provide the same thing in the graph is a problem (they conflict).

This is an important concept because:

- By default, a variant provides a capability corresponding to the GAV coordinates of its component

- No two variants in a dependency graph can provide the same capability

- Multiple variants of a single component may be selected as long as they provide different capabilities

A typical component will only provide variants with the default capability. A Java library, for example, exposes two variants (API and runtime) which provide the same capability. As a consequence, it is an error to have both the API and runtime of a single component in a dependency graph.

However, imagine that you need the runtime and the test fixtures runtime of a component. Then it is allowed as long as the runtime and test fixtures runtime variant of the library declare different capabilities.

If we do so, a consumer would then have to declare two dependencies:

- one on the "main" feature, the library

- one on the "test fixtures" feature, by requiring its capability

| While the resolution engine supports multi-variant components independently of the ecosystem, features are currently only available using the Java plugins.

### Registering features

Features can be declared by applying the `java-library` plugin. The following code illustrates how to declare a feature named `mongodbSupport`:

Example 88. Registering a feature
build.gradle.kts

    sourceSets {
        create("mongodbSupport") {
            java {
                srcDir("src/mongodb/java")
            }
        }
    }

    java {
        registerFeature("mongodbSupport") {
            usingSourceSet(sourceSets["mongodbSupport"])
        }
    }

build.gradle

    sourceSets {
        mongodbSupport {
            java {
                srcDir 'src/mongodb/java'
            }
        }
    }

    java {
        registerFeature('mongodbSupport') {
            usingSourceSet(sourceSets.mongodbSupport)
        }
    }

Gradle will automatically set up a number of things for you, in a very similar way to how the Java Library Plugin sets up configurations.

Dependency scope configurations are created in the same manner as for the main feature:

- the configuration `mongodbSupportApi`, used to declare API dependencies for this feature

- the configuration `mongodbSupportImplementation`, used to declare implementation dependencies for this feature

- the configuration `mongodbSupportRuntimeOnly`, used to declare runtime-only dependencies for this feature

- the configuration `mongodbSupportCompileOnly`, used to declare compile-only dependencies for this feature

- the configuration `mongodbSupportCompileOnlyApi`, used to declare compile-only API dependencies for this feature

Furthermore, consumable configurations are created in the same manner as for the main feature:

- the configuration `mongodbSupportApiElements`, used by consumers to fetch the artifacts and API dependencies of this feature

- the configuration `mongodbSupportRuntimeElements`, used by consumers to fetch the artifacts and runtime dependencies of this feature

A feature should have a source set with the same name. Gradle will create a `Jar` task to bundle the classes built from the feature source set, using a classifier corresponding to the kebab-case name of the feature.

| Do not use the main source set when registering a feature. This behavior will be deprecated in a future version of Gradle.

Most users will only need to care about the dependency scope configurations, to declare the specific dependencies of this feature:

Example 89. Declaring dependencies of a feature
build.gradle.kts

    dependencies {
        "mongodbSupportImplementation"("org.mongodb:mongodb-driver-sync:3.9.1")
    }

build.gradle

    dependencies {
        mongodbSupportImplementation 'org.mongodb:mongodb-driver-sync:3.9.1'
    }

By convention, Gradle maps the feature name to a capability whose group and version are the same as the group and version of the main component, respectively, but whose name is the main component name followed by a `-` followed by the kebab-cased feature name.

For example, if the component’s group is `org.gradle.demo`, its name is `provider`, its version is `1.0`, and the feature is named `mongodbSupport`, the feature’s variants will have the `org.gradle.demo:provider-mongodb-support:1.0` capability.

If you choose the capability name yourself or add more capabilities to a variant, it is recommended to follow the same convention.

### Publishing features

Depending on the metadata file format, publishing features may be lossy:

- using Gradle Module Metadata, everything is published and consumers will get the full benefit of features

- using POM metadata (Maven), features are published as optional dependencies and artifacts of features are published with different classifiers

- using Ivy metadata, features are published as extra configurations, which are not extended by the `default` configuration

Publishing features is supported using the `maven-publish` and `ivy-publish` plugins only. The Java Library Plugin will take care of registering the additional variants for you, so there’s no additional configuration required, only the regular publications:

Example 90. Publishing a component with features
build.gradle.kts

    plugins {
        `java-library`
        `maven-publish`
    }
    // ...
    publishing {
        publications {
            create("myLibrary", MavenPublication::class.java) {
                from(components["java"])
            }
        }
    }

build.gradle

    plugins {
        id 'java-library'
        id 'maven-publish'
    }
    // ...
    publishing {
        publications {
            myLibrary(MavenPublication) {
                from components.java
            }
        }
    }

#### Adding javadoc and sources JARs

Similar to the main Javadoc and sources JARs, you can configure the added feature so that it produces JARs for the Javadoc and sources.

Example 91. Producing javadoc and sources JARs for features
build.gradle.kts

    java {
        registerFeature("mongodbSupport") {
            usingSourceSet(sourceSets["mongodbSupport"])
            withJavadocJar()
            withSourcesJar()
        }
    }

build.gradle

    java {
        registerFeature('mongodbSupport') {
            usingSourceSet(sourceSets.mongodbSupport)
            withJavadocJar()
            withSourcesJar()
        }
    }

### Dependencies on features

As mentioned earlier, features can be lossy when published. As a consequence, a consumer can depend on a feature only in these cases:

- with a project dependency (in a multi-project build)

- with Gradle Module Metadata available, that is the publisher MUST have published it

- within the Ivy world, by declaring a dependency on the configuration matching the feature

A consumer can specify that it needs a specific feature of a producer by declaring required capabilities. For example, if a producer declares a "MySQL support" feature like this:

Example 92. A library declaring a feature to support MySQL
build.gradle.kts

    group = "org.gradle.demo"

    sourceSets {
        create("mysqlSupport") {
            java {
                srcDir("src/mysql/java")
            }
        }
    }

    java {
        registerFeature("mysqlSupport") {
            usingSourceSet(sourceSets["mysqlSupport"])
        }
    }

    dependencies {
        "mysqlSupportImplementation"("mysql:mysql-connector-java:8.0.14")
    }

build.gradle

    group = 'org.gradle.demo'

    sourceSets {
        mysqlSupport {
            java {
                srcDir 'src/mysql/java'
            }
        }
    }

    java {
        registerFeature('mysqlSupport') {
            usingSourceSet(sourceSets.mysqlSupport)
        }
    }

    dependencies {
        mysqlSupportImplementation 'mysql:mysql-connector-java:8.0.14'
    }

Then the consumer can declare a dependency on the MySQL support feature by doing this:

Example 93. Consuming specific features in a multi-project build
build.gradle.kts

    dependencies {
        // This project requires the main producer component
        implementation(project(":producer"))

        // But we also want to use its MySQL support
        runtimeOnly(project(":producer")) {
            capabilities {
                requireCapability("org.gradle.demo:producer-mysql-support")
            }
        }
    }

build.gradle

    dependencies {
        // This project requires the main producer component
        implementation(project(":producer"))

        // But we also want to use its MySQL support
        runtimeOnly(project(":producer")) {
            capabilities {
                requireCapability("org.gradle.demo:producer-mysql-support")
            }
        }
    }

This will automatically bring the `mysql-connector-java` dependency on the runtime classpath. If there were more than one dependency, all of them would be brought, meaning that a feature can be used to group dependencies which contribute to a feature together.

Similarly, if an external library with features was published with Gradle Module Metadata, it is possible to depend on a feature provided by that library:

Example 94. Consuming specific features from an external repository
build.gradle.kts

    dependencies {
        // This project requires the main producer component
        implementation("org.gradle.demo:producer:1.0")

        // But we also want to use its MongoDB support
        runtimeOnly("org.gradle.demo:producer:1.0") {
            capabilities {
                requireCapability("org.gradle.demo:producer-mongodb-support")
            }
        }
    }

build.gradle

    dependencies {
        // This project requires the main producer component
        implementation('org.gradle.demo:producer:1.0')

        // But we also want to use its MongoDB support
        runtimeOnly('org.gradle.demo:producer:1.0') {
            capabilities {
                requireCapability("org.gradle.demo:producer-mongodb-support")
            }
        }
    }

### Handling mutually exclusive variants

The main advantage of using capabilities as a way to handle features is that you can precisely handle compatibility of variants. The rule is simple:

> No two variants in a dependency graph can provide the same capability

We can leverage this to ensure that Gradle fails whenever the user mis-configures dependencies. Consider a situation where your library supports MySQL, Postgres and MongoDB, but that it’s only allowed to choose one of those at the same time. We can model this restriction by ensuring each feature also provides the same capability, thus making it impossible for these features to be used together in the same graph.

Example 95. A producer of multiple features that are mutually exclusive
build.gradle.kts

    java {
        registerFeature("mysqlSupport") {
            usingSourceSet(sourceSets["mysqlSupport"])
            capability("org.gradle.demo", "producer-db-support", "1.0")
            capability("org.gradle.demo", "producer-mysql-support", "1.0")
        }
        registerFeature("postgresSupport") {
            usingSourceSet(sourceSets["postgresSupport"])
            capability("org.gradle.demo", "producer-db-support", "1.0")
            capability("org.gradle.demo", "producer-postgres-support", "1.0")
        }
        registerFeature("mongoSupport") {
            usingSourceSet(sourceSets["mongoSupport"])
            capability("org.gradle.demo", "producer-db-support", "1.0")
            capability("org.gradle.demo", "producer-mongo-support", "1.0")
        }
    }

    dependencies {
        "mysqlSupportImplementation"("mysql:mysql-connector-java:8.0.14")
        "postgresSupportImplementation"("org.postgresql:postgresql:42.2.5")
        "mongoSupportImplementation"("org.mongodb:mongodb-driver-sync:3.9.1")
    }

build.gradle

    java {
        registerFeature('mysqlSupport') {
            usingSourceSet(sourceSets.mysqlSupport)
            capability('org.gradle.demo', 'producer-db-support', '1.0')
            capability('org.gradle.demo', 'producer-mysql-support', '1.0')
        }
        registerFeature('postgresSupport') {
            usingSourceSet(sourceSets.postgresSupport)
            capability('org.gradle.demo', 'producer-db-support', '1.0')
            capability('org.gradle.demo', 'producer-postgres-support', '1.0')
        }
        registerFeature('mongoSupport') {
            usingSourceSet(sourceSets.mongoSupport)
            capability('org.gradle.demo', 'producer-db-support', '1.0')
            capability('org.gradle.demo', 'producer-mongo-support', '1.0')
        }
    }

    dependencies {
        mysqlSupportImplementation 'mysql:mysql-connector-java:8.0.14'
        postgresSupportImplementation 'org.postgresql:postgresql:42.2.5'
        mongoSupportImplementation 'org.mongodb:mongodb-driver-sync:3.9.1'
    }

Here, the producer declares 3 features, one for each database runtime support:

- `mysql-support` provides both the `db-support` and `mysql-support` capabilities

- `postgres-support` provides both the `db-support` and `postgres-support` capabilities

- `mongo-support` provides both the `db-support` and `mongo-support` capabilities

Then if the consumer tries to get both the `postgres-support` and `mysql-support` features (this also works transitively):

Example 96. A consumer trying to use 2 incompatible variants at the same time
build.gradle.kts

    dependencies {
        // This project requires the main producer component
        implementation(project(":producer"))

        // Let's try to ask for both MySQL and Postgres support
        runtimeOnly(project(":producer")) {
            capabilities {
                requireCapability("org.gradle.demo:producer-mysql-support")
            }
        }
        runtimeOnly(project(":producer")) {
            capabilities {
                requireCapability("org.gradle.demo:producer-postgres-support")
            }
        }
    }

build.gradle

    dependencies {
        implementation(project(":producer"))

        // Let's try to ask for both MySQL and Postgres support
        runtimeOnly(project(":producer")) {
            capabilities {
                requireCapability("org.gradle.demo:producer-mysql-support")
            }
        }
        runtimeOnly(project(":producer")) {
            capabilities {
                requireCapability("org.gradle.demo:producer-postgres-support")
            }
        }
    }

Dependency resolution would fail with the following error:

    Cannot choose between
       org.gradle.demo:producer:1.0 variant mysqlSupportRuntimeElements and
       org.gradle.demo:producer:1.0 variant postgresSupportRuntimeElements
       because they provide the same capability: org.gradle.demo:producer-db-support:1.0

## Understanding variant selection

In other dependency management engines, like Apache Maven™, dependencies and artifacts are bound to a component that is published at a particular GAV (group-artifact-version) coordinates. The set of dependencies for this component are always the same, regardless of which artifact may be used from the component.

If the component does have multiple artifacts, each one is identified by a cumbersome classifier. There are no common semantics associated with classifiers and that makes it difficult to guarantee a globally consistent dependency graph. This means that nothing prevents multiple artifacts for a single component (e.g., `jdk7` and `jdk8` classifiers) from appearing in a classpath and causing hard to diagnose problems.

### Maven component model

Figure 3. The Maven component model

### Gradle component model

Figure 4. The Gradle component model

Gradle’s dependency management engine is variant aware.

In addition to a component, Gradle has the concept of variants of a component. Variants correspond to the different ways a component can be used, such as for Java compilation or native linking or documentation. Artifacts are attached to a variant and each variant can have a different set of dependencies.

How does Gradle know which variant to choose when there’s more than one? Variants are matched by use of attributes, which provide semantics to the variants and help the engine to produce a consistent resolution result.

Gradle differentiates between two kind of components:

- local components (like projects), built from sources

- external components, published to repositories

For local components, variants are mapped to consumable configurations. For external components, variants are defined by published Gradle Module Metadata or are derived from Ivy/Maven metadata.

Variants vs configurations

Variants and configurations are sometimes used interchangeably in the documentation, DSL or API for historical reasons.

All components provide variants and those variants may be backed by a consumable configuration. Not all configurations are variants because they may be used for declaring or resolving dependencies.

### Variant attributes

Attributes are type-safe key-value pairs that are defined by the consumer (for a resolvable configuration) and the producer (for each variant).

The consumer can define any number of attributes. Each attribute helps narrow the possible variants that can be selected. Attribute values do not need to be exact matches.

The variant can also define any number of attributes. The attributes should describe how the variant is intended to be used. For example, Gradle uses an attribute named `org.gradle.usage` to describe with how a component is used by the consumer (for compilation, for runtime execution, etc). It is not unusual for a variant to have more attributes than the consumer needs to provide to select it.

### Variant attribute matching

About producer variants

The variant name is mostly for debugging purposes and error messages. The name does not participate variant matching—​only its attributes do.

There are no restrictions on the number of variants a component can define. Usually, a component has at least an implementation variant, but it could also expose test fixtures, documentation or source code. A component may also expose different variants for different consumers for the same usage. For example, when compiling, a component could have different headers for Linux vs Windows vs macOS.

Gradle performs variant aware selection by matching the attributes requested by the consumer against attributes defined by the producer. The selection algorithm is detailed in another section.

| There are two exceptions to this rule that bypass variant aware resolution: when a producer has no variants, a default artifact is chosen. when a consumer explicitly selects a configuration by name, the artifacts of the configuration are chosen.

### A simple example

Let’s consider an example where a consumer is trying to use a library for compilation.

First, the consumer needs to explain how it’s going to use the result of dependency resolution. This is done by setting attributes on the resolvable configuration of the consumer.

The consumer wants to resolve a variant that matches: `org.gradle.usage=java-api`

Second, the producer needs to expose the different variants of the component.

The producer component exposes 2 variants:

- its API (named `apiElements`) with attribute `org.gradle.usage=java-api`

- its runtime (named `runtimeElements`) with attribute `org.gradle.usage=java-runtime`

Finally, Gradle selects the appropriate variant by looking at the variant attributes:

- the consumer wants a variant with attributes `org.gradle.usage=java-api`

- the producer has a matching variant (`apiElements`)

- the producer has a non-matching variant (`runtimeElements`)

Gradle provides the artifacts and dependencies from the `apiElements` variant to the consumer.

### A more complicated example

In the real world, consumers and producers have more than one attribute.

A Java Library project in Gradle will involve several different attributes:

- `org.gradle.usage` that describes how the variant is used

- `org.gradle.dependency.bundling` that describes how the variant handles dependencies (shadow jar vs fat jar vs regular jar)

- `org.gradle.libraryelements`, that describes the packaging of the variant (classes or jar)

- `org.gradle.jvm.version` that describes the minimal version of Java this variant targets

- `org.gradle.jvm.environment` that describes the type of JVM this variant targets

Let’s consider an example where the consumer wants to run tests with a library on Java 8 and the producer supports two different Java versions (Java 8 and Java 11).

First, the consumer needs to explain which version of the Java it needs.

The consumer wants to resolve a variant that:

- can be used at runtime (has `org.gradle.usage=java-runtime`)

- can be run on at least Java 8 (`org.gradle.jvm.version=8`)

Second, the producer needs to expose the different variants of the component.

Like in the simple example, there is both a API (compilation) and runtime variant. These exist for both the Java 8 and Java 11 version of the component.

- its API for Java 8 consumers (named `apiJava8Elements`) with attribute `org.gradle.usage=java-api` and `org.gradle.jvm.version=8`

- its runtime for Java 8 consumers (named `runtime8Elements`) with attribute `org.gradle.usage=java-runtime` and `org.gradle.jvm.version=8`

- its API for Java 11 consumers (named `apiJava11Elements`) with attribute `org.gradle.usage=java-api` and `org.gradle.jvm.version=11`

- its runtime for Java 11 consumers (named `runtime11Elements`) with attribute `org.gradle.usage=java-runtime` and `org.gradle.jvm.version=11`

Finally, Gradle selects the best matching variant by looking at all of the attributes:

- the consumer wants a variant with compatible attributes to `org.gradle.usage=java-runtime` and `org.gradle.jvm.version=8`

- the variants `runtime8Elements` and `runtime11Elements` have `org.gradle.usage=java-runtime

- the variants `apiJava8Elements` and `apiJava11Elements` are incompatible

- the variant `runtime8Elements` is compatible because it can run on Java 8

- the variant `runtime11Elements` is incompatible because it cannot run on Java 8

Gradle provides the artifacts and dependencies from the `runtime8Elements` variant to the consumer.

Compatibility of variants

What if the consumer sets `org.gradle.jvm.version` to 7?

Dependency resolution would fail with an error message explaining that there’s no suitable variant. Gradle recognizes that the consumer wants a Java 7 compatible library and the minimal version of Java available on the producer is 8.

If the consumer requested `org.gradle.jvm.version=15`, then Gradle knows either the Java 8 or Java 11 variants could work. Gradle select the highest compatible Java version (11).

### Variant selection errors

When selecting the most compatible variant of a component, resolution may fail:

- when more than one variant from the producer matches the consumer attributes (ambiguity error)

- when no variants from the producer match the consumer attributes (incompatibility error)

#### Dealing with ambiguity errors

An ambiguous variant selection looks like the following:

    > Could not resolve all files for configuration ':compileClasspath'.
       > Could not resolve project :lib.
         Required by:
             project :ui
          > Cannot choose between the following variants of project :lib:
              - feature1ApiElements
              - feature2ApiElements
            All of them match the consumer attributes:
              - Variant 'feature1ApiElements' capability org.test:test-capability:1.0:
                  - Unmatched attribute:
                      - Found org.gradle.category 'library' but wasn't required.
                  - Compatible attributes:
                      - Provides org.gradle.dependency.bundling 'external'
                      - Provides org.gradle.jvm.version '11'
                      - Required org.gradle.libraryelements 'classes' and found value 'jar'.
                      - Provides org.gradle.usage 'java-api'
              - Variant 'feature2ApiElements' capability org.test:test-capability:1.0:
                  - Unmatched attribute:
                      - Found org.gradle.category 'library' but wasn't required.
                  - Compatible attributes:
                      - Provides org.gradle.dependency.bundling 'external'
                      - Provides org.gradle.jvm.version '11'
                      - Required org.gradle.libraryelements 'classes' and found value 'jar'.
                      - Provides org.gradle.usage 'java-api'

All compatible candidate variants are displayed with their attributes.

- Unmatched attributes are presented first, as they might be the missing piece in selecting the proper variant.

- Compatible attributes are presented second as they indicate what the consumer wanted and how these variants do match that request.

- There will not be any incompatible attributes as the variant would not be considered a candidate.

In the example above, the fix does not lie in attribute matching but in capability matching, which are shown next to the variant name. Because these two variants effectively provide the same attributes and capabilities, they cannot be disambiguated. So in this case, the fix is most likely to provide different capabilities on the producer side (`project :lib`) and express a capability choice on the consumer side (`project :ui`).

#### Dealing with no matching variant errors

A no matching variant error looks like the following:

    > No variants of project :lib match the consumer attributes:
      - Configuration ':lib:compile':
          - Incompatible attribute:
              - Required artifactType 'dll' and found incompatible value 'jar'.
          - Other compatible attribute:
              - Provides usage 'api'
      - Configuration ':lib:compile' variant debug:
          - Incompatible attribute:
              - Required artifactType 'dll' and found incompatible value 'jar'.
          - Other compatible attributes:
              - Found buildType 'debug' but wasn't required.
              - Provides usage 'api'
      - Configuration ':lib:compile' variant release:
          - Incompatible attribute:
              - Required artifactType 'dll' and found incompatible value 'jar'.
          - Other compatible attributes:
              - Found buildType 'release' but wasn't required.
              - Provides usage 'api'

or like:

    > No variants of project : match the consumer attributes:
       - Configuration ':myElements' declares attribute 'color' with value 'blue':
           - Incompatible because this component declares attribute 'artifactType' with value 'jar' and the consumer needed attribute 'artifactType' with value 'dll'
       - Configuration ':myElements' variant secondary declares attribute 'color' with value 'blue':
           - Incompatible because this component declares attribute 'artifactType' with value 'jar' and the consumer needed attribute 'artifactType' with value 'dll'

depending upon the stage in the variant selection algorithm where the error occurs.

All potentially compatible candidate variants are displayed with their attributes.

- Incompatible attributes are presented first, as they usually are the key in understanding why a variant could not be selected.

- Other attributes are presented second, this includes requested and compatible ones as well as all extra producer attributes that are not requested by the consumer.

Similar to the ambiguous variant error, the goal is to understand which variant should be selected. In some cases, there may not be any compatible variants from the producer (e.g., trying to run on Java 8 with a library built for Java 11).

#### Dealing with incompatible variant errors

An incompatible variant error looks like the following example, where a consumer wants to select a variant with `color=green`, but the only variant available has `color=blue`:

    > Could not resolve all dependencies for configuration ':resolveMe'.
       > Could not resolve project :.
         Required by:
             project :
          > Configuration 'mismatch' in project : does not match the consumer attributes
            Configuration 'mismatch':
              - Incompatible because this component declares attribute 'color' with value 'blue' and the consumer needed attribute 'color' with value 'green'

It occurs when Gradle cannot select a single variant of a dependency because an explicitly requested attribute value does not match (and is not compatible with) the value of that attribute on any of the variants of the dependency.

A sub-type of this failure occurs when Gradle successfully selects multiple variants of the same component, but the selected variants are incompatible with each other.

This looks like the following, where a consumer wants to select two different variants of a component, each supplying different capabilities, which is acceptable. Unfortunately one variant has `color=blue` and the other has `color=green`:

    > Could not resolve all dependencies for configuration ':resolveMe'.
       > Could not resolve project :.
         Required by:
             project :
          > Multiple incompatible variants of org.example:nyvu:1.0 were selected:
               - Variant org.example:nyvu:1.0 variant blueElementsCapability1 has attributes {color=blue}
               - Variant org.example:nyvu:1.0 variant greenElementsCapability2 has attributes {color=green}

       > Could not resolve project :.
         Required by:
             project :
          > Multiple incompatible variants of org.example:pi2e5:1.0 were selected:
               - Variant org.example:pi2e5:1.0 variant blueElementsCapability1 has attributes {color=blue}
               - Variant org.example:pi2e5:1.0 variant greenElementsCapability2 has attributes {color=green}

#### Dealing with ambiguous transformation errors

ArtifactTransforms can be used to transform artifacts from one type to another, changing their attributes. Variant selection can use the attributes available as the result of an artifact transform as a candidate variant.

If a project registers multiple artifact transforms, needs to use an artifact transform to produce a matching variant for a consumer’s request, and multiple artifact transforms could each be used to accomplish this, then Gradle will fail with an ambiguous transformation error like the following:

    > Could not resolve all dependencies for configuration ':resolveMe'.
       > Found multiple transforms that can produce a variant of project : with requested attributes:
           - color 'red'
           - shape 'round'
         Found the following transforms:
           - From 'configuration ':roundBlueLiquidElements'':
               - With source attributes:
                   - color 'blue'
                   - shape 'round'
                   - state 'liquid'
               - Candidate transform(s):
                   - Transform 'BrokenTransform' producing attributes:
                       - color 'red'
                       - shape 'round'
                       - state 'gas'
                   - Transform 'BrokenTransform' producing attributes:
                       - color 'red'
                       - shape 'round'
                       - state 'solid'

### Visualizing variant information

#### Outgoing variants report

The report task `outgoingVariants` shows the list of variants available for selection by consumers of the project. It displays the capabilities, attributes and artifacts for each variant.

This task is similar to the `dependencyInsight` reporting task.

By default, `outgoingVariants` prints information about all variants. It offers the optional parameter `--variant <variantName>` to select a single variant to display. It also accepts the `--all` flag to include information about legacy and deprecated configurations, or `--no-all` to exclude this information.

Here is the output of the `outgoingVariants` task on a freshly generated `java-library` project:

    > Task :outgoingVariants
    --------------------------------------------------
    Variant apiElements
    --------------------------------------------------
    API elements for the 'main' feature.

    Capabilities
        - new-java-library:lib:unspecified (default capability)
    Attributes
        - org.gradle.category            = library
        - org.gradle.dependency.bundling = external
        - org.gradle.jvm.version         = 11
        - org.gradle.libraryelements     = jar
        - org.gradle.usage               = java-api
    Artifacts
        - build/libs/lib.jar (artifactType = jar)

    Secondary Variants (*)

        --------------------------------------------------
        Secondary Variant classes
        --------------------------------------------------
            Description = Directories containing compiled class files for main.

            Attributes
                - org.gradle.category            = library
                - org.gradle.dependency.bundling = external
                - org.gradle.jvm.version         = 11
                - org.gradle.libraryelements     = classes
                - org.gradle.usage               = java-api
            Artifacts
                - build/classes/java/main (artifactType = java-classes-directory)

    --------------------------------------------------
    Variant mainSourceElements (i)
    --------------------------------------------------
    Description = List of source directories contained in the Main SourceSet.

    Capabilities
        - new-java-library:lib:unspecified (default capability)
    Attributes
        - org.gradle.category            = verification
        - org.gradle.dependency.bundling = external
        - org.gradle.verificationtype    = main-sources
    Artifacts
        - src/main/java (artifactType = directory)
        - src/main/resources (artifactType = directory)

    --------------------------------------------------
    Variant runtimeElements
    --------------------------------------------------
    Runtime elements for the 'main' feature.

    Capabilities
        - new-java-library:lib:unspecified (default capability)
    Attributes
        - org.gradle.category            = library
        - org.gradle.dependency.bundling = external
        - org.gradle.jvm.version         = 11
        - org.gradle.libraryelements     = jar
        - org.gradle.usage               = java-runtime
    Artifacts
        - build/libs/lib.jar (artifactType = jar)

    Secondary Variants (*)

        --------------------------------------------------
        Secondary Variant classes
        --------------------------------------------------
            Description = Directories containing compiled class files for main.

            Attributes
                - org.gradle.category            = library
                - org.gradle.dependency.bundling = external
                - org.gradle.jvm.version         = 11
                - org.gradle.libraryelements     = classes
                - org.gradle.usage               = java-runtime
            Artifacts
                - build/classes/java/main (artifactType = java-classes-directory)

        --------------------------------------------------
        Secondary Variant resources
        --------------------------------------------------
            Description = Directories containing the project's assembled resource files for use at runtime.

            Attributes
                - org.gradle.category            = library
                - org.gradle.dependency.bundling = external
                - org.gradle.jvm.version         = 11
                - org.gradle.libraryelements     = resources
                - org.gradle.usage               = java-runtime
            Artifacts
                - build/resources/main (artifactType = java-resources-directory)

    --------------------------------------------------
    Variant testResultsElementsForTest (i)
    --------------------------------------------------
    Description = Directory containing binary results of running tests for the test Test Suite's test target.

    Capabilities
        - new-java-library:lib:unspecified (default capability)
    Attributes
        - org.gradle.category              = verification
        - org.gradle.testsuite.name        = test
        - org.gradle.testsuite.target.name = test
        - org.gradle.testsuite.type        = unit-test
        - org.gradle.verificationtype      = test-results
    Artifacts
        - build/test-results/test/binary (artifactType = directory)

    (i) Configuration uses incubating attributes such as Category.VERIFICATION.
    (*) Secondary variants are variants created via the Configuration#getOutgoing(): ConfigurationPublications API which also participate in selection, in addition to the configuration itself.

From this you can see the two main variants that are exposed by a java library, `apiElements` and `runtimeElements`. Notice that the main difference is on the `org.gradle.usage` attribute, with values `java-api` and `java-runtime`. As they indicate, this is where the difference is made between what needs to be on the compile classpath of consumers, versus what’s needed on the runtime classpath.

It also shows secondary variants, which are exclusive to Gradle projects and not published. For example, the secondary variant `classes` from `apiElements` is what allows Gradle to skip the JAR creation when compiling against a `java-library` project.

##### Information about invalid consumable configurations

A project cannot have multiple configurations with the same attributes and capabilities. In that case, the project will fail to build.

In order to be able to visualize such issues, the outgoing variant reports handle those errors in a lenient fashion. This allows the report to display information about the issue.

#### Resolvable configurations report

Gradle also offers a complimentary report task called `resolvableConfigurations` that displays the resolvable configurations of a project, which are those which can have dependencies added and be resolved. The report will list their attributes and any configurations that they extend. It will also list a summary of any attributes which will be affected by Compatibility Rules or Disambiguation Rules during resolution.

By default, `resolvableConfigurations` prints information about all purely resolvable configurations. These are configurations that are marked resolvable but not marked consumable. Though some resolvable configurations are also marked consumable, these are legacy configurations that should not have dependencies added in build scripts. This report offers the optional parameter `--configuration <configurationName>` to select a single configuration to display. It also accepts the `--all` flag to include information about legacy and deprecated configurations, or `--no-all` to exclude this information. Finally, it accepts the `--recursive` flag to list in the extended configurations section those configurations which are extended transitively rather than directly. Alternatively, `--no-recursive` can be used to exclude this information.

Here is the output of the `resolvableConfigurations` task on a freshly generated `java-library` project:

    > Task :resolvableConfigurations
    --------------------------------------------------
    Configuration annotationProcessor
    --------------------------------------------------
    Description = Annotation processors and their dependencies for source set 'main'.

    Attributes
        - org.gradle.category            = library
        - org.gradle.dependency.bundling = external
        - org.gradle.jvm.environment     = standard-jvm
        - org.gradle.libraryelements     = jar
        - org.gradle.usage               = java-runtime

    --------------------------------------------------
    Configuration compileClasspath
    --------------------------------------------------
    Description = Compile classpath for source set 'main'.

    Attributes
        - org.gradle.category            = library
        - org.gradle.dependency.bundling = external
        - org.gradle.jvm.environment     = standard-jvm
        - org.gradle.jvm.version         = 11
        - org.gradle.libraryelements     = classes
        - org.gradle.usage               = java-api
    Extended Configurations
        - compileOnly
        - implementation

    --------------------------------------------------
    Configuration runtimeClasspath
    --------------------------------------------------
    Description = Runtime classpath of source set 'main'.

    Attributes
        - org.gradle.category            = library
        - org.gradle.dependency.bundling = external
        - org.gradle.jvm.environment     = standard-jvm
        - org.gradle.jvm.version         = 11
        - org.gradle.libraryelements     = jar
        - org.gradle.usage               = java-runtime
    Extended Configurations
        - implementation
        - runtimeOnly

    --------------------------------------------------
    Configuration testAnnotationProcessor
    --------------------------------------------------
    Description = Annotation processors and their dependencies for source set 'test'.

    Attributes
        - org.gradle.category            = library
        - org.gradle.dependency.bundling = external
        - org.gradle.jvm.environment     = standard-jvm
        - org.gradle.libraryelements     = jar
        - org.gradle.usage               = java-runtime

    --------------------------------------------------
    Configuration testCompileClasspath
    --------------------------------------------------
    Description = Compile classpath for source set 'test'.

    Attributes
        - org.gradle.category            = library
        - org.gradle.dependency.bundling = external
        - org.gradle.jvm.environment     = standard-jvm
        - org.gradle.jvm.version         = 11
        - org.gradle.libraryelements     = classes
        - org.gradle.usage               = java-api
    Extended Configurations
        - testCompileOnly
        - testImplementation

    --------------------------------------------------
    Configuration testRuntimeClasspath
    --------------------------------------------------
    Description = Runtime classpath of source set 'test'.

    Attributes
        - org.gradle.category            = library
        - org.gradle.dependency.bundling = external
        - org.gradle.jvm.environment     = standard-jvm
        - org.gradle.jvm.version         = 11
        - org.gradle.libraryelements     = jar
        - org.gradle.usage               = java-runtime
    Extended Configurations
        - testImplementation
        - testRuntimeOnly

    --------------------------------------------------
    Compatibility Rules
    --------------------------------------------------
    Description = The following Attributes have compatibility rules defined.

        - org.gradle.dependency.bundling
        - org.gradle.jvm.environment
        - org.gradle.jvm.version
        - org.gradle.libraryelements
        - org.gradle.plugin.api-version
        - org.gradle.usage

    --------------------------------------------------
    Disambiguation Rules
    --------------------------------------------------
    Description = The following Attributes have disambiguation rules defined.

        - org.gradle.category
        - org.gradle.dependency.bundling
        - org.gradle.jvm.environment
        - org.gradle.jvm.version
        - org.gradle.libraryelements
        - org.gradle.plugin.api-version
        - org.gradle.usage

From this you can see the two main configurations used to resolve dependencies, `compileClasspath` and `runtimeClasspath`, as well as their corresponding test configurations.

### Mapping from Maven/Ivy to Gradle variants

Neither Maven nor Ivy have the concept of variants, which are only natively supported by Gradle Module Metadata. Gradle can still work with Maven and Ivy by using different variant derivation strategies.

Relationship with Gradle Module Metadata

Gradle Module Metadata is a metadata format for modules published on Maven, Ivy and other kinds of repositories. It is similar to the `pom.xml` or `ivy.xml` metadata file, but this format contains details about variants.

See the Gradle Module Metadata specification for more information.

#### Mapping of Maven POM metadata to variants

Modules published on a Maven repository are automatically converted into variant-aware modules.

There is no way for Gradle to know which kind of component was published:

- a BOM that represents a Gradle platform

- a BOM used as a super-POM

- a POM that is both a platform and a library

The default strategy used by Java projects in Gradle is to derive 8 different variants:

- two "library" variants (attribute `org.gradle.category` = `library`)

  - the `compile` variant maps the `<scope>compile</scope>` dependencies. This variant is equivalent to the `apiElements` variant of the Java Library plugin. All dependencies of this scope are considered API dependencies.

  - the `runtime` variant maps both the `<scope>compile</scope>` and `<scope>runtime</scope>` dependencies. This variant is equivalent to the `runtimeElements` variant of the Java Library plugin. All dependencies of those scopes are considered runtime dependencies.

    - in both cases, the `<dependencyManagement>` dependencies are not converted to constraints

- a "sources" variant that represents the sources jar for the component

- a "javadoc" variant that represents the javadoc jar for the component

- four "platform" variants derived from the `<dependencyManagement>` block (attribute `org.gradle.category` = `platform`):

  - the `platform-compile` variant maps the `<scope>compile</scope>` dependency management dependencies as dependency constraints.

  - the `platform-runtime` variant maps both the `<scope>compile</scope>` and `<scope>runtime</scope>` dependency management dependencies as dependency constraints.

  - the `enforced-platform-compile` is similar to `platform-compile` but all the constraints are forced

  - the `enforced-platform-runtime` is similar to `platform-runtime` but all the constraints are forced

You can understand more about the use of platform and enforced platforms variants by looking at the importing BOMs section of the manual. By default, whenever you declare a dependency on a Maven module, Gradle is going to look for the `library` variants. However, using the `platform` or `enforcedPlatform` keyword, Gradle is now looking for one of the "platform" variants, which allows you to import the constraints from the POM files, instead of the dependencies.

#### Mapping of Ivy files to variants

Gradle has no built-in derivation strategy implemented for Ivy files. Ivy is a flexible format that allows you to publish arbitrary files and can be heavily customized.

If you want to implement a derivation strategy for compile and runtime variants for Ivy, you can do so with component metadata rule. The component metadata rules API allows you to access Ivy configurations and create variants based on them. If you know that all the Ivy modules your are consuming have been published with Gradle without further customizations of the `ivy.xml` file, you can add the following rule to your build:

Example 97. Deriving compile and runtime variants for Ivy metadata
build.gradle.kts

    abstract class IvyVariantDerivationRule @Inject internal constructor(objectFactory: ObjectFactory) : ComponentMetadataRule {
        private val jarLibraryElements: LibraryElements
        private val libraryCategory: Category
        private val javaRuntimeUsage: Usage
        private val javaApiUsage: Usage

        init {
            jarLibraryElements = objectFactory.named(LibraryElements.JAR)
            libraryCategory = objectFactory.named(Category.LIBRARY)
            javaRuntimeUsage = objectFactory.named(Usage.JAVA_RUNTIME)
            javaApiUsage = objectFactory.named(Usage.JAVA_API)
        }

        override fun execute(context: ComponentMetadataContext) {
            // This filters out any non Ivy module
            if(context.getDescriptor(IvyModuleDescriptor::class) == null) {
                return
            }

            context.details.addVariant("runtimeElements", "default") {
                attributes {
                    attribute(LibraryElements.LIBRARY_ELEMENTS_ATTRIBUTE, jarLibraryElements)
                    attribute(Category.CATEGORY_ATTRIBUTE, libraryCategory)
                    attribute(Usage.USAGE_ATTRIBUTE, javaRuntimeUsage)
                }
            }
            context.details.addVariant("apiElements", "compile") {
                attributes {
                    attribute(LibraryElements.LIBRARY_ELEMENTS_ATTRIBUTE, jarLibraryElements)
                    attribute(Category.CATEGORY_ATTRIBUTE, libraryCategory)
                    attribute(Usage.USAGE_ATTRIBUTE, javaApiUsage)
                }
            }
        }
    }

    dependencies {
        components { all<IvyVariantDerivationRule>() }
    }

build.gradle

    abstract class IvyVariantDerivationRule implements ComponentMetadataRule {
        final LibraryElements jarLibraryElements
        final Category libraryCategory
        final Usage javaRuntimeUsage
        final Usage javaApiUsage

        @Inject
        IvyVariantDerivationRule(ObjectFactory objectFactory) {
            jarLibraryElements = objectFactory.named(LibraryElements, LibraryElements.JAR)
            libraryCategory = objectFactory.named(Category, Category.LIBRARY)
            javaRuntimeUsage = objectFactory.named(Usage, Usage.JAVA_RUNTIME)
            javaApiUsage = objectFactory.named(Usage, Usage.JAVA_API)
        }

        void execute(ComponentMetadataContext context) {
            // This filters out any non Ivy module
            if(context.getDescriptor(IvyModuleDescriptor) == null) {
                return
            }

            context.details.addVariant("runtimeElements", "default") {
                attributes {
                    attribute(LibraryElements.LIBRARY_ELEMENTS_ATTRIBUTE, jarLibraryElements)
                    attribute(Category.CATEGORY_ATTRIBUTE, libraryCategory)
                    attribute(Usage.USAGE_ATTRIBUTE, javaRuntimeUsage)
                }
            }
            context.details.addVariant("apiElements", "compile") {
                attributes {
                    attribute(LibraryElements.LIBRARY_ELEMENTS_ATTRIBUTE, jarLibraryElements)
                    attribute(Category.CATEGORY_ATTRIBUTE, libraryCategory)
                    attribute(Usage.USAGE_ATTRIBUTE, javaApiUsage)
                }
            }
        }
    }

    dependencies {
        components { all(IvyVariantDerivationRule) }
    }

The rule creates an `apiElements` variant based on the `compile` configuration and a `runtimeElements` variant based on the `default` configuration of each ivy module. For each variant, it sets the corresponding Java ecosystem attributes. Dependencies and artifacts of the variants are taken from the underlying configurations. If not all consumed Ivy modules follow this pattern, the rule can be adjusted or only applied to a selected set of modules.

For all Ivy modules without variants, Gradle has a fallback selection method. Gradle does not perform variant aware resolution and instead selects either the `default` configuration or an explicitly named configuration.

## Working with Variant Attributes

As explained in the section on variant aware matching, attributes give semantics to variants and are used by Gradle’s dependency management engine to select the best matching variant.

As a user of Gradle, attributes are often hidden as implementation details. But it might be useful to understand the standard attributes defined by Gradle and its core plugins.

As a plugin author, these attributes, and the way they are defined, can serve as a basis for building your own set of attributes in your ecosystem plugin.

### Standard attributes defined by Gradle

Gradle defines a list of standard attributes used by Gradle’s core plugins.

#### Ecosystem-independent standard attributes

Table 3. Ecosystem-independent standard variant attributes
Attribute name | Description | Values | compatibility and disambiguation rules
--- | --- | --- | ---
`org.gradle.usage` | Indicates main purpose of variant | `Usage` values built from constants defined in Usage | Following ecosystem semantics (e.g. `java-runtime` can be used in place of `java-api` but not the opposite)
`org.gradle.category` | Indicates the category of this software component | `Category` values built from constants defined in Category | Following ecosystem semantics (e.g. `library` is default on the JVM, no compatibility otherwise)
`org.gradle.libraryelements` | Indicates the contents of a `org.gradle.category=library` variant | `LibraryElements` values built from constants defined in LibraryElements | Following ecosystem semantics(e.g. in the JVM world, `jar` is the default and is compatible with `classes`)
`org.gradle.docstype` | Indicates the contents of a `org.gradle.category=documentation` variant | `DocsType` values built from constants defined in DocsType | No default, no compatibility
`org.gradle.dependency.bundling` | Indicates how dependencies of a variant are accessed. | `Bundling` values built from constants defined in Bundling | Following ecosystem semantics (e.g. in the JVM world, `embedded` is compatible with `external`)
`org.gradle.verificationtype` | Indicates what kind of verification task produced this output. | `VerificationType` values built from constants defined in VerificationType | No default, no compatibility

| When the `Category` attribute is present with the incubating value `org.gradle.category=verification` on a variant, that variant is considered to be a verification-time only variant. These variants are meant to contain only the results of running verification tasks, such as test results or code coverage reports. They are not publishable, and will produce an error if added to a component which is published.

Table 4. Ecosystem-independent standard component attributes
Attribute name | Description | Values | compatibility and disambiguation rules
--- | --- | --- | ---
`org.gradle.status` | Component level attribute, derived | Based on a status scheme, with a default one existing based on the source repository. | Based on the scheme in use

#### JVM ecosystem specific attributes

In addition to the ecosystem independent attributes defined above, the JVM ecosystem adds the following attribute:

Table 5. JVM ecosystem standard component attributes
Attribute name | Description | Values | compatibility and disambiguation rules
--- | --- | --- | ---
`org.gradle.jvm.version` | Indicates the JVM version compatibility. | Integer using the version after the `1.` for Java 1.4 and before, the major version for Java 5 and beyond. | Defaults to the JVM version used by Gradle, lower is compatible with higher, prefers highest compatible.
`org.gradle.jvm.environment` | Indicates that a variant is optimized for a certain JVM environment. | Common values are `standard-jvm` and `android`. Other values are allowed. | The attribute is used to prefer one variant over another if multiple are available, but in general all values are compatible. The default is `standard-jvm`.
`org.gradle.testsuite.name` | Indicates the name of the TestSuite that produced this output. | Value is the name of the Suite. | No default, no compatibility
`org.gradle.testsuite.target.name` | Indicates the name of the TestSuiteTarget that produced this output. | Value is the name of the Target. | No default, no compatibility
`org.gradle.testsuite.type` | Indicates the type of test suite (unit test, integration test, performance test, etc.) | `TestSuiteType` values built from constants defined in TestSuiteType or other custom values for user-defined test suite types. | No default, no compatibility

The JVM ecosystem also contains a number of compatibility and disambiguation rules over the different attributes. The reader willing to know more can take a look at the code for `org.gradle.api.internal.artifacts.JavaEcosystemSupport`.

#### Native ecosystem specific attributes

In addition to the ecosystem independent attributes defined above, the native ecosystem adds the following attributes:

Table 6. Native ecosystem standard component attributes
Attribute name | Description | Values | compatibility and disambiguation rules
--- | --- | --- | ---
`org.gradle.native.debuggable` | Indicates if the binary was built with debugging symbols | Boolean | N/A
`org.gradle.native.optimized` | Indicates if the binary was built with optimization flags | Boolean | N/A
`org.gradle.native.architecture` | Indicates the target architecture of the binary | `MachineArchitecture` values built from constants defined in MachineArchitecture | None
`org.gradle.native.operatingSystem` | Indicates the target operating system of the binary | `OperatingSystemFamily` values built from constants defined in OperatingSystemFamily | None

#### Gradle plugin ecosystem specific attributes

For Gradle plugin development, the following attribute is supported since Gradle 7.0. A Gradle plugin variant can specify compatibility with a Gradle API version through this attribute.

Table 7. Gradle plugin ecosystem standard component attributes
Attribute name | Description | Values | compatibility and disambiguation rules
--- | --- | --- | ---
`org.gradle.plugin.api‑version` | Indicates the Gradle API version compatibility. | Valid Gradle version strings. | Defaults to the currently running Gradle, lower is compatible with higher, prefers highest compatible.

### Declaring custom attributes

If you are extending Gradle, e.g. by writing a plugin for another ecosystem, declaring custom attributes could be an option if you want to support variant-aware dependency management features in your plugin. However, you should be cautious if you also attempt to publish libraries. Semantics of new attributes are usually defined through a plugin, which can carry compatibility and disambiguation rules. Consequently, builds that consume libraries published for a certain ecosystem, also need to apply the corresponding plugin to interpret attributes correctly. If your plugin is intended for a larger audience, i.e. if it is openly available and libraries are published to public repositories, defining new attributes effectively extends the semantics of Gradle Module Metadata and comes with responsibilities. E.g., support for attributes that are already published should not be removed again, or should be handled in some kind of compatibility layer in future versions of the plugin.

### Creating attributes in a build script or plugin

Attributes are typed. An attribute can be created via the `Attribute<T>.of` method:

Example 98. Define attributes
build.gradle.kts

    // An attribute of type `String`
    val myAttribute = Attribute.of("my.attribute.name", String::class.java)
    // An attribute of type `Usage`
    val myUsage = Attribute.of("my.usage.attribute", Usage::class.java)

build.gradle

    // An attribute of type `String`
    def myAttribute = Attribute.of("my.attribute.name", String)
    // An attribute of type `Usage`
    def myUsage = Attribute.of("my.usage.attribute", Usage)

Attribute types support most Java primitive classes; such as `String` and `Integer`; Or anything extending `org.gradle.api.Named`. Attributes should always be declared in the attribute schema found on the `dependencies` handler:

Example 99. Registering attributes on the attributes schema
build.gradle.kts

    dependencies.attributesSchema {
        // registers this attribute to the attributes schema
        attribute(myAttribute)
        attribute(myUsage)
    }

build.gradle

    dependencies.attributesSchema {
        // registers this attribute to the attributes schema
        attribute(myAttribute)
        attribute(myUsage)
    }

Registering an attribute with the schema is required in order to use Compatibility and Disambiguation rules that can resolve ambiguity between multiple selectable variants during Attribute Matching.

Each configuration has a container of attributes. Attributes can be configured to set values:

Example 100. Setting attributes on configurations
build.gradle.kts

    configurations {
        create("myConfiguration") {
            attributes {
                attribute(myAttribute, "my-value")
            }
        }
    }

build.gradle

    configurations {
        myConfiguration {
            attributes {
                attribute(myAttribute, 'my-value')
            }
        }
    }

For attributes which type extends `Named`, the value of the attribute must be created via the object factory:

Example 101. Named attributes
build.gradle.kts

    configurations {
        "myConfiguration" {
            attributes {
                attribute(myUsage, project.objects.named(Usage::class.java, "my-value"))
            }
        }
    }

build.gradle

    configurations {
        myConfiguration {
            attributes {
                attribute(myUsage, project.objects.named(Usage, 'my-value'))
            }
        }
    }

### Attribute matching

#### Attribute compatibility rules

Attributes let the engine select compatible variants. There are cases where a producer may not have exactly what the consumer requests but has a variant that can be used.

For example, if the consumer is asking for the API of a library and the producer doesn’t have an exactly matching variant, the runtime variant could be considered compatible. This is typical of libraries published to external repositories. In this case, we know that even if we don’t have an exact match (API), we can still compile against the runtime variant (it contains more than what we need to compile but it’s still ok to use).

Gradle provides attribute compatibility rules that can be defined for each attribute. The role of a compatibility rule is to explain which attribute values are compatible based on what the consumer asked for.

Attribute compatibility rules have to be registered via the attribute matching strategy that you can obtain from the attributes schema.

#### Attribute disambiguation rules

Since multiple values for an attribute can be compatible, Gradle needs to choose the "best" candidate between all compatible candidates. This is called "disambiguation".

This is done by implementing an attribute disambiguation rule.

Attribute disambiguation rules have to be registered via the attribute matching strategy that you can obtain from the attributes schema, which is a member of DependencyHandler.

#### Variant attribute matching algorithm

Finding the best variant can get complicated when there are many different variants available for a component and many different attributes. Gradle’s dependency resolution engine performs the following algorithm when finding the best result (or failing):

1. Each candidate’s attribute value is compared to the consumer’s requested attribute value. A candidate is considered compatible if its value matches the consumer’s value exactly, passes the attribute’s compatibility rule or is not provided.

2. If only one candidate is considered compatible, that candidate wins.

3. If several candidates are compatible, but one of the candidates matches all of the same attributes as the other candidates, Gradle chooses that candidate. This is the candidate with the "longest" match.

4. If several candidates are compatible and are compatible with an equal number of attributes, Gradle needs to disambiguate the candidates.

   1. For each requested attribute, if a candidate does not have a value matching the disambiguation rule, it’s eliminated from consideration.

   2. If the attribute has a known precedence, Gradle will stop as soon as there is a single candidate remaining.

   3. If the attribute does not have a known precedence, Gradle must consider all attributes.

5. If several candidates still remain, Gradle will start to consider "extra" attributes to disambiguate between multiple candidates. Extra attributes are attributes that were not requested by the consumer but are present on at least one candidate. These extra attributes are considered in precedence order.

   1. If the attribute has a known precedence, Gradle will stop as soon as there is a single candidate remaining.

   2. After all extra attributes with precedence are considered, the remaining candidates can be chosen if they are compatible with all of the non-ordered disambiguation rules.

6. If several candidates still remain, Gradle will consider extra attributes again. A candidate can be chosen if it has the fewest number of extra attributes.

If at any step no candidates remain compatible, resolution fails. Additionally, Gradle outputs a list of all compatible candidates from step 1 to help with debugging variant matching failures.

Plugins and ecosystems can influence the selection algorithm by implementing compatibility rules, disambiguation rules and telling Gradle the precedence of attributes. Attributes with a higher precedence are used to eliminate compatible matches in order.

For example, in the Java ecosystem, the `org.gradle.usage` attribute has a higher precedence than `org.gradle.libraryelements`. This means that if two candidates were available with compatible values for both `org.gradle.usage` and `org.gradle.libraryelements`, Gradle will choose the candidate that passes the disambiguation rule for `org.gradle.usage`.

## Sharing outputs between projects

A common pattern, in multi-project builds, is that one project consumes the artifacts of another project. In general, the simplest consumption form in the Java ecosystem is that when `A` depends on `B`, then `A` would depend on the `jar` produced by project `B`. As previously described in this chapter, this is modeled by `A` depending on a variant of `B`, where the variant is selected based on the needs of `A`. For compilation, we need the API dependencies of `B`, provided by the `apiElements` variant. For runtime, we need the runtime dependencies of `B`, provided by the `runtimeElements` variant.

However, what if you need a different artifact than the main one? Gradle provides, for example, built-in support for depending on the test fixtures of another project, but sometimes the artifact you need to depend on simply isn’t exposed as a variant.

In order to be safe to share between projects and allow maximum performance (parallelism), such artifacts must be exposed via outgoing configurations.

Don’t reference other project tasks directly

A frequent anti-pattern to declare cross-project dependencies is:

    dependencies {
       // this is unsafe!
       implementation project(":other").tasks.someOtherJar
    }

This publication model is unsafe and can lead to non-reproducible and hard to parallelize builds. This section explains how to properly create cross-project boundaries by defining "exchanges" between projects by using variants.

There are two, complementary, options to share artifacts between projects. The simplified version is only suitable if what you need to share is a simple artifact that doesn’t depend on the consumer. The simple solution is also limited to cases where this artifact is not published to a repository. This also implies that the consumer does not publish a dependency to this artifact. In cases where the consumer resolves to different artifacts in different contexts (e.g., different target platforms) or that publication is required, you need to use the advanced version.

### Simple sharing of artifacts between projects

First, a producer needs to declare a configuration which is going to be exposed to consumers. As explained in the configurations chapter, this corresponds to a consumable configuration.

Let’s imagine that the consumer requires instrumented classes from the producer, but that this artifact is not the main one. The producer can expose its instrumented classes by creating a configuration that will "carry" this artifact:

Example 102. Declaring an outgoing variant
producer/build.gradle.kts

    val instrumentedJars by configurations.creating {
        isCanBeConsumed = true
        isCanBeResolved = false
        // If you want this configuration to share the same dependencies, otherwise omit this line
        extendsFrom(configurations["implementation"], configurations["runtimeOnly"])
    }

producer/build.gradle

    configurations {
        instrumentedJars {
            canBeConsumed = true
            canBeResolved = false
            // If you want this configuration to share the same dependencies, otherwise omit this line
            extendsFrom implementation, runtimeOnly
        }
    }

This configuration is consumable, which means it’s an "exchange" meant for consumers. We’re now going to add artifacts to this configuration, that consumers would get when they consume it:

Example 103. Attaching an artifact to an outgoing configuration
producer/build.gradle.kts

    artifacts {
        add("instrumentedJars", instrumentedJar)
    }

producer/build.gradle

    artifacts {
        instrumentedJars(instrumentedJar)
    }

Here the "artifact" we’re attaching is a task that actually generates a Jar. Doing so, Gradle can automatically track dependencies of this task and build them as needed. This is possible because the `Jar` task extends `AbstractArchiveTask`. If it’s not the case, you will need to explicitly declare how the artifact is generated.

Example 104. Explicitly declaring the task dependency of an artifact
producer/build.gradle.kts

    artifacts {
        add("instrumentedJars", someTask.outputFile) {
            builtBy(someTask)
        }
    }

producer/build.gradle

    artifacts {
        instrumentedJars(someTask.outputFile) {
            builtBy(someTask)
        }
    }

Now the consumer needs to depend on this configuration in order to get the right artifact:

Example 105. An explicit configuration dependency
consumer/build.gradle.kts

    dependencies {
        instrumentedClasspath(project(mapOf(
            "path" to ":producer",
            "configuration" to "instrumentedJars")))
    }

consumer/build.gradle

    dependencies {
        instrumentedClasspath(project(path: ":producer", configuration: 'instrumentedJars'))
    }

| Declaring a dependency on an explicit target configuration is not recommended. If you plan to publish the component which has this dependency, this will likely lead to broken metadata. If you need to publish the component on a remote repository, follow the instructions of the variant-aware cross publication documentation.

In this case, we’re adding the dependency to the instrumentedClasspath configuration, which is a consumer specific configuration. In Gradle terminology, this is called a resolvable configuration, which is defined this way:

Example 106. Declaring a resolvable configuration on the consumer
consumer/build.gradle.kts

    val instrumentedClasspath by configurations.creating {
        isCanBeConsumed = false
    }

consumer/build.gradle

    configurations {
        instrumentedClasspath {
            canBeConsumed = false
        }
    }

### Variant-aware sharing of artifacts between projects

In the simple sharing solution, we defined a configuration on the producer side which serves as an exchange of artifacts between the producer and the consumer. However, the consumer has to explicitly tell which configuration it depends on, which is something we want to avoid in variant aware resolution. In fact, we also have explained that it is possible for a consumer to express requirements using attributes and that the producer should provide the appropriate outgoing variants using attributes too. This allows for smarter selection, because using a single dependency declaration, without any explicit target configuration, the consumer may resolve different things. The typical example is that using a single dependency declaration `project(":myLib")`, we would either choose the `arm64` or `i386` version of `myLib` depending on the architecture.

To do this, we will add attributes to both the consumer and the producer.

It is important to understand that once configurations have attributes, they participate in variant aware resolution, which means that they are candidates considered whenever any notation like `project(":myLib")` is used. In other words, the attributes set on the producer must be consistent with the other variants produced on the same project. They must not, in particular, introduce ambiguity for the existing selection.

In practice, it means that the attribute set used on the configuration you create are likely to be dependent on the ecosystem in use (Java, C++, …​) because the relevant plugins for those ecosystems often use different attributes.

Let’s enhance our previous example which happens to be a Java Library project. Java libraries expose a couple of variants to their consumers, `apiElements` and `runtimeElements`. Now, we’re adding a 3rd one, `instrumentedJars`.

Therefore, we need to understand what our new variant is used for in order to set the proper attributes on it. Let’s look at the attributes we find on the `runtimeElements` configuration on the producer:

gradle outgoingVariants --variant runtimeElements

    Attributes
        - org.gradle.category            = library
        - org.gradle.dependency.bundling = external
        - org.gradle.jvm.version         = 11
        - org.gradle.libraryelements     = jar
        - org.gradle.usage               = java-runtime

What it tells us is that the Java Library plugin produces variants with 5 attributes:

- `org.gradle.category` tells us that this variant represents a library

- `org.gradle.dependency.bundling` tells us that the dependencies of this variant are found as jars (they are not, for example, repackaged inside the jar)

- `org.gradle.jvm.version` tells us that the minimum Java version this library supports is Java 11

- `org.gradle.libraryelements` tells us this variant contains all elements found in a jar (classes and resources)

- `org.gradle.usage` says that this variant is a Java runtime, therefore suitable for a Java compiler but also at runtime

As a consequence, if we want our instrumented classes to be used in place of this variant when executing tests, we need to attach similar attributes to our variant. In fact, the attribute we care about is `org.gradle.libraryelements` which explains what the variant contains, so we can setup the variant this way:

Example 107. Declaring the variant attributes
producer/build.gradle.kts

    val instrumentedJars by configurations.creating {
        isCanBeConsumed = true
        isCanBeResolved = false
        attributes {
            attribute(Category.CATEGORY_ATTRIBUTE, objects.named(Category.LIBRARY))
            attribute(Usage.USAGE_ATTRIBUTE, objects.named(Usage.JAVA_RUNTIME))
            attribute(Bundling.BUNDLING_ATTRIBUTE, objects.named(Bundling.EXTERNAL))
            attribute(TargetJvmVersion.TARGET_JVM_VERSION_ATTRIBUTE, JavaVersion.current().majorVersion.toInt())
            attribute(LibraryElements.LIBRARY_ELEMENTS_ATTRIBUTE, objects.named("instrumented-jar"))
        }
    }

producer/build.gradle

    configurations {
        instrumentedJars {
            canBeConsumed = true
            canBeResolved = false
            attributes {
                attribute(Category.CATEGORY_ATTRIBUTE, objects.named(Category, Category.LIBRARY))
                attribute(Usage.USAGE_ATTRIBUTE, objects.named(Usage, Usage.JAVA_RUNTIME))
                attribute(Bundling.BUNDLING_ATTRIBUTE, objects.named(Bundling, Bundling.EXTERNAL))
                attribute(TargetJvmVersion.TARGET_JVM_VERSION_ATTRIBUTE, JavaVersion.current().majorVersion.toInteger())
                attribute(LibraryElements.LIBRARY_ELEMENTS_ATTRIBUTE, objects.named(LibraryElements, 'instrumented-jar'))
            }
        }
    }

| Choosing the right attributes to set is the hardest thing in this process, because they carry the semantics of the variant. Therefore, before adding new attributes, you should always ask yourself if there isn’t an attribute which carries the semantics you need. If there isn’t, then you may add a new attribute. When adding new attributes, you must also be careful because it’s possible that it creates ambiguity during selection. Often adding an attribute means adding it to all existing variants.

What we have done here is that we have added a new variant, which can be used at runtime, but contains instrumented classes instead of the normal classes. However, it now means that for runtime, the consumer has to choose between two variants:

- `runtimeElements`, the regular variant offered by the `java-library` plugin

- `instrumentedJars`, the variant we have created

In particular, say we want the instrumented classes on the test runtime classpath. We can now, on the consumer, declare our dependency as a regular project dependency:

Example 108. Declaring the project dependency
consumer/build.gradle.kts

    dependencies {
        testImplementation("junit:junit:4.13")
        testImplementation(project(":producer"))
    }

consumer/build.gradle

    dependencies {
        testImplementation 'junit:junit:4.13'
        testImplementation project(':producer')
    }

If we stop here, Gradle will still select the `runtimeElements` variant in place of our `instrumentedJars` variant. This is because the `testRuntimeClasspath` configuration asks for a configuration which `libraryelements` attribute is `jar`, and our new `instrumented-jars` value is not compatible.

So we need to change the requested attributes so that we now look for instrumented jars:

Example 109. Changing the consumer attributes
consumer/build.gradle.kts

    configurations {
        testRuntimeClasspath {
            attributes {
                attribute(LibraryElements.LIBRARY_ELEMENTS_ATTRIBUTE, objects.named(LibraryElements::class.java, "instrumented-jar"))
            }
        }
    }

consumer/build.gradle

    configurations {
        testRuntimeClasspath {
            attributes {
                attribute(LibraryElements.LIBRARY_ELEMENTS_ATTRIBUTE, objects.named(LibraryElements, 'instrumented-jar'))
            }
        }
    }

We can look at another report on the consumer side to view exactly what attributes of each dependency will be requested:

gradle resolvableConfigurations --configuration testRuntimeClasspath

    Attributes
        - org.gradle.category            = library
        - org.gradle.dependency.bundling = external
        - org.gradle.jvm.version         = 11
        - org.gradle.libraryelements     = instrumented-jar
        - org.gradle.usage               = java-runtime

The `resolvableConfigurations` report is the complement of the `outgoingVariants` report. By running both of these reports on the consumer and producer sides of a relationship, respectively, you can see exactly what attributes are involved in matching during dependency resolution and better predict the outcome when configurations are resolved.

Now, we’re saying that whenever we’re going to resolve the test runtime classpath, what we are looking for is instrumented classes. There is a problem though: in our dependencies list, we have JUnit, which, obviously, is not instrumented. So if we stop here, Gradle is going to fail, explaining that there’s no variant of JUnit which provide instrumented classes. This is because we didn’t explain that it’s fine to use the regular jar, if no instrumented version is available. To do this, we need to write a compatibility rule:

Example 110. A compatibility rule
consumer/build.gradle.kts

    abstract class InstrumentedJarsRule: AttributeCompatibilityRule<LibraryElements> {

        override fun execute(details: CompatibilityCheckDetails<LibraryElements>) = details.run {
            if (consumerValue?.name == "instrumented-jar" && producerValue?.name == "jar") {
                compatible()
            }
        }
    }

consumer/build.gradle

    abstract class InstrumentedJarsRule implements AttributeCompatibilityRule<LibraryElements> {

        @Override
        void execute(CompatibilityCheckDetails<LibraryElements> details) {
            if (details.consumerValue.name == 'instrumented-jar' && details.producerValue.name == 'jar') {
                details.compatible()
            }
        }
    }

which we need to declare on the attributes schema:

Example 111. Making use of the compatibility rule
consumer/build.gradle.kts

    dependencies {
        attributesSchema {
            attribute(LibraryElements.LIBRARY_ELEMENTS_ATTRIBUTE) {
                compatibilityRules.add(InstrumentedJarsRule::class.java)
            }
        }
    }

consumer/build.gradle

    dependencies {
        attributesSchema {
            attribute(LibraryElements.LIBRARY_ELEMENTS_ATTRIBUTE) {
                compatibilityRules.add(InstrumentedJarsRule)
            }
        }
    }

And that’s it! Now we have:

- added a variant which provides instrumented jars

- explained that this variant is a substitute for the runtime

- explained that the consumer needs this variant only for test runtime

Gradle therefore offers a powerful mechanism to select the right variants based on preferences and compatibility. More details can be found in the variant aware plugins section of the documentation.

| By adding a value to an existing attribute like we have done, or by defining new attributes, we are extending the model. This means that all consumers have to know about this extended model. For local consumers, this is usually not a problem because all projects understand and share the same schema, but if you had to publish this new variant to an external repository, it means that external consumers would have to add the same rules to their builds for them to pass. This is in general not a problem for ecosystem plugins (e.g: the Kotlin plugin) where consumption is in any case not possible without applying the plugin, but it is a problem if you add custom values or attributes. So, avoid publishing custom variants if they are for internal use only.

### Targeting different platforms

It is common for a library to target different platforms. In the Java ecosystem, we often see different artifacts for the same library, distinguished by a different classifier. A typical example is Guava, which is published as this:

- `guava-jre` for JDK 8 and above

- `guava-android` for JDK 7

The problem with this approach is that there’s no semantics associated with the classifier. The dependency resolution engine, in particular, cannot determine automatically which version to use based on the consumer requirements. For example, it would be better to express that you have a dependency on Guava, and let the engine choose between `jre` and `android` based on what is compatible.

Gradle provides an improved model for this, which doesn’t have the weakness of classifiers: attributes.

In particular, in the Java ecosystem, Gradle provides a built-in attribute that library authors can use to express compatibility with the Java ecosystem: `org.gradle.jvm.version`. This attribute expresses the minimal version that a consumer must have in order to work properly.

When you apply the `java` or `java-library` plugins, Gradle will automatically associate this attribute to the outgoing variants. This means that all libraries published with Gradle automatically tell which target platform they use.

By default, the `org.gradle.jvm.version` is set to the value of the `release` property (or as fallback to the `targetCompatibility` value) of the main compilation task of the source set.

While this attribute is automatically set, Gradle will not, by default, let you build a project for different JVMs. If you need to do this, then you will need to create additional variants following the instructions on variant-aware matching.

| Future versions of Gradle will provide ways to automatically build for different Java platforms.

## Artifact Transforms

What if you want to adjust the JAR file of one of your dependencies before you use it?

Gradle has a built-in feature for this called Artifact Transforms. With Artifact Transforms, you can modify, extend, or reduce artifacts like JAR files before tasks or tools like the IDE use them.

### Artifact Transforms Overview

Each component exposes a set of variants, where each variant is identified by a set of attributes (i.e., key-value pairs such as `debug=true`).

When Gradle resolves a configuration, it looks at each dependency, resolves it to a component, and selects the corresponding variant from that component that matches the configuration’s request attributes. If the component does not have a matching variant, resolution fails unless Gradle finds an Artifact Transform chain that can transform one of the component’s variants' artifacts to satisfy the request attributes (without changing its transitive dependencies).

Artifact Transforms are a mechanism for converting one type of artifact into another during the build process. They provide the consumer an efficient and flexible mechanism for transforming the artifacts of a given producer to the required format without needing the producer to expose variants in that format.

Artifact Transforms are a lot like tasks. They are units of work with some inputs and outputs. Mechanisms like `UP-TO-DATE` and caching work for transforms as well.

The primary difference between tasks and transforms is how they are scheduled and put into the chain of actions Gradle executes when a build configures and runs. At a high level, transforms always run before tasks because they are executed during dependency resolution. Transforms modify artifacts BEFORE they become an input to a task.

Here’s a brief overview of how to create and use Artifact Transforms:

1. Implement a Transform: You define an artifact transform by creating a class that implements the `TransformAction` interface. This class specifies how the input artifact should be transformed into the output artifact.

2. Declare request Attributes: Attributes (key-value pairs used to describe different variants of a component) like `org.gradle.usage=java-api` and `org.gradle.usage=java-runtime` are used to specify the desired artifact format/type.

3. Register a Transform: You register the transform in your build script using the `registerTransform()` method of the `dependencies` block. This method links the input attributes to the output attributes and associates them with the transform action class.

4. Use the Transformed Artifacts: When a resolution requires an artifact matching the transform’s output attributes, Gradle automatically applies the registered transform to the input artifact and provides the transformed artifact as a result.

#### 1. Implement a Transform

A transform is usually implemented as an abstract class. The class implements the `TransformAction` interface. It can optionally have parameters defined in a separate interface.

Each transform has exactly one input artifact. It must be annotated with the `@InputArtifact` annotation.

Then, you implement the `transform(TransformOutputs)` method from the `TransformAction` interface. This is where you implement the work the transform should do when triggered. The method has the `TransformOutputs` as an argument that defines what the transform produces.

Here, `MyTransform` is the custom transform action that converts a `jar` artifact to a `transformed-jar` artifact:

build.gradle.kts

    abstract class MyTransform : TransformAction<TransformParameters.None> {
        @get:InputArtifact
        abstract val inputArtifact: Provider<FileSystemLocation>

        override fun transform(outputs: TransformOutputs) {
            val inputFile = inputArtifact.get().asFile
            val outputFile = outputs.file(inputFile.name.replace(".jar", "-transformed.jar"))
            // Perform transformation logic here
            inputFile.copyTo(outputFile, overwrite = true)
        }
    }

build.gradle

    abstract class MyTransform implements TransformAction<TransformParameters.None> {
        @InputArtifact
        abstract Provider<FileSystemLocation> getInputArtifact()

        @Override
        void transform(TransformOutputs outputs) {
            def inputFile = inputArtifact.get().asFile
            def outputFile = outputs.file(inputFile.name.replace(".jar", "-transformed.jar"))
            // Perform transformation logic here
            inputFile.withInputStream { input ->
                outputFile.withOutputStream { output ->
                    output << input
                }
            }
        }
    }

#### 2. Declare request Attributes

Attributes specify the required properties of a dependency.

Here we specify that we need the `transformed-jar` format for the `runtimeClasspath` configuration:

build.gradle.kts

    configurations.named("runtimeClasspath") {
        attributes {
            attribute(ArtifactTypeDefinition.ARTIFACT_TYPE_ATTRIBUTE, "transformed-jar")
        }
    }

build.gradle

    configurations.named("runtimeClasspath") {
        attributes {
            attribute(ArtifactTypeDefinition.ARTIFACT_TYPE_ATTRIBUTE, "transformed-jar")
        }
    }

#### 3. Register a Transform

A transform must be registered using the `dependencies.registerTransform()` method.

Here, our transform is registered with the `dependencies` block:

build.gradle.kts

    dependencies {
        registerTransform(MyTransform::class) {
            from.attribute(ArtifactTypeDefinition.ARTIFACT_TYPE_ATTRIBUTE, "jar")
            to.attribute(ArtifactTypeDefinition.ARTIFACT_TYPE_ATTRIBUTE, "transformed-jar")
        }
    }

build.gradle

    dependencies {
        registerTransform(MyTransform) {
            from.attribute(ArtifactTypeDefinition.ARTIFACT_TYPE_ATTRIBUTE, "jar")
            to.attribute(ArtifactTypeDefinition.ARTIFACT_TYPE_ATTRIBUTE, "transformed-jar")
        }
    }

#### 4. Use the Transformed Artifacts

During a build, Gradle uses registered transforms to produce a required artifact if it’s not directly available.

### Understanding Artifact Transforms

Dependencies can have different variants, essentially different versions or forms of the same dependency. These variants can differ based on their use cases, such as when compiling code or running applications.

Each variant is identified by a set of attributes. Attributes are key-value pairs that describe specific characteristics of the variant.

Let’s use the following example where an external Maven dependency has two variants:

Table 8. Maven Dependencies
Variant | Description
--- | ---
`org.gradle.usage=java-api` | Used for compiling against the dependency.
`org.gradle.usage=java-runtime` | Used for running an application with the dependency.

And a project dependency has even more variants:

Table 9. Project Dependencies
Variant | Description
--- | ---
`org.gradle.usage=java-api` `org.gradle.libraryelements=classes` | Represents classes directories.
`org.gradle.usage=java-api` `org.gradle.libraryelements=jar` | Represents a packaged JAR file, containing classes and resources.

The variants of a dependency may differ in its transitive dependencies or in the artifact itself.

For example, the `java-api` and `java-runtime` variants of the Maven dependency only differ in the transitive dependencies, and both use the same artifact — the JAR file. For the project dependency, the `java-api,classes` and the `java-api,jars` variants have the same transitive dependencies but different artifacts — the `classes` directories and the `JAR` files respectively.

When Gradle resolves a configuration, it uses the attributes defined to select the appropriate variant of each dependency. The attributes that Gradle uses to determine which variant to select are called the requested attributes.

For example, if a configuration requests `org.gradle.usage=java-api` and `org.gradle.libraryelements=classes`, Gradle will select the variant of each dependency that matches these attributes (in this case, classes directories intended for use as an API during compilation).

Sometimes, a dependency might not have the exact variant with the requested attributes. In such cases, Gradle can transform one variant into another without changing its transitive dependencies (other dependencies it relies on).

| Gradle does not try to select Artifact Transforms when a variant of the dependency matching the requested attributes already exists.

For example, if the requested variant is `java-api,classes`, but the dependency only has `java-api,jar`, Gradle can potentially transform the `JAR` file into a `classes` directory by unzipping it using an Artifact Transform that is registered with these attributes.

### Understanding Artifact Transforms Chains

When Gradle resolves a configuration and a dependency does not have a variant with the requested attributes, it attempts to find a chain of Artifact Transforms to create the desired variant. This process is called Artifact Transform selection:

Artifact Transform selection:

1. Start with requested Attributes:

   - Gradle starts with the attributes specified in the configuration.

   - It considers all registered transforms that modify these attributes.

2. Find a path to existing Variants:

   - Gradle works backwards, trying to find a path from the requested attributes to an existing variant.

For example, if the `minified` attribute has values `true` and `false`, and a transform can change `minified=false` to `minified=true`, Gradle will use this transform if only `minified=false` variants are available but `minified=true` is requested.

Gradle selects the best chain of transforms based on specific rules:

- If there is only one chain, it is selected.

- If one chain is a suffix of another, the more specific chain is selected.

- The shortest chain is preferred.

- If multiple chains are equally suitable, the selection fails, and an error is reported.

Continuing from the `minified` example above, a configuration requests `org.gradle.usage=java-runtime, org.gradle.libraryelements=jar, minified=true`. The dependencies are:

- External `guava` dependency with variants:

  - `org.gradle.usage=java-runtime, org.gradle.libraryelements=jar, minified=false`

  - `org.gradle.usage=java-api, org.gradle.libraryelements=jar, minified=false`

- Project `producer` dependency with variants:

  - `org.gradle.usage=java-runtime, org.gradle.libraryelements=jar, minified=false`

  - `org.gradle.usage=java-runtime, org.gradle.libraryelements=classes, minified=false`

  - `org.gradle.usage=java-api, org.gradle.libraryelements=jar, minified=false`

  - `org.gradle.usage=java-api, org.gradle.libraryelements=classes, minified=false`

Gradle uses the `minify` transform to convert `minified=false` variants to `minified=true`.

- For `guava`, Gradle converts

  - `org.gradle.usage=java-runtime, org.gradle.libraryelements=jar, minified=false` to

  - `org.gradle.usage=java-runtime, org.gradle.libraryelements=jar, minified=true`.

- For `producer`, Gradle converts

  - `org.gradle.usage=java-runtime, org.gradle.libraryelements=jar, minified=false` to

  - `org.gradle.usage=java-runtime, org.gradle.libraryelements=jar, minified=true`.

Then, during execution:

- Gradle downloads the `guava` JAR and minifies it.

- Gradle executes the `producer:jar` task to produce the JAR and then minifies it.

- These tasks are executed in parallel where possible.

To set up the `minified` attribute so that the above works, you need to register the new attribute in the schema, add it to all JAR artifacts, and request it on all resolvable configurations:

build.gradle.kts

    val artifactType = Attribute.of("artifactType", String::class.java)
    val minified = Attribute.of("minified", Boolean::class.javaObjectType)
    dependencies {
        attributesSchema {
            attribute(minified)                      (1)
        }
        artifactTypes.getByName("jar") {
            attributes.attribute(minified, false)    (2)
        }
    }

    configurations.all {
        afterEvaluate {
            if (isCanBeResolved) {
                attributes.attribute(minified, true) (3)
            }
        }
    }

    dependencies {
        registerTransform(Minify::class) {
            from.attribute(minified, false).attribute(artifactType, "jar")
            to.attribute(minified, true).attribute(artifactType, "jar")
        }
    }

    dependencies {                                 (4)
        implementation("com.google.guava:guava:27.1-jre")
        implementation(project(":producer"))
    }

    tasks.register<Copy>("resolveRuntimeClasspath") { (5)
        from(configurations.runtimeClasspath)
        into(layout.buildDirectory.dir("runtimeClasspath"))
    }

build.gradle

    def artifactType = Attribute.of('artifactType', String)
    def minified = Attribute.of('minified', Boolean)
    dependencies {
        attributesSchema {
            attribute(minified)                      (1)
        }
        artifactTypes.getByName("jar") {
            attributes.attribute(minified, false)    (2)
        }
    }

    configurations.all {
        afterEvaluate {
            if (canBeResolved) {
                attributes.attribute(minified, true) (3)
            }
        }
    }

    dependencies {
        registerTransform(Minify) {
            from.attribute(minified, false).attribute(artifactType, "jar")
            to.attribute(minified, true).attribute(artifactType, "jar")
        }
    }
    dependencies {                                 (4)
        implementation('com.google.guava:guava:27.1-jre')
        implementation(project(':producer'))
    }

    tasks.register("resolveRuntimeClasspath", Copy) {(5)
        from(configurations.runtimeClasspath)
        into(layout.buildDirectory.dir("runtimeClasspath"))
    }

1 | Add the attribute to the schema
2 | All JAR files are not minified
3 | Request `minified=true` on all resolvable configurations
4 | Add the dependencies which will be transformed
5 | Add task that requires the transformed artifacts

You can now see what happens when we run the `resolveRuntimeClasspath` task, which resolves the `runtimeClasspath` configuration. Gradle transforms the project dependency before the `resolveRuntimeClasspath` task starts. Gradle transforms the binary dependencies when it executes the `resolveRuntimeClasspath` task:

    $ gradle resolveRuntimeClasspath
    > Task :producer:compileJava
    > Task :producer:processResources NO-SOURCE
    > Task :producer:classes
    > Task :producer:jar

    > Transform producer.jar (project :producer) with Minify
    Nothing to minify - using producer.jar unchanged

    > Task :resolveRuntimeClasspath
    Minifying guava-27.1-jre.jar
    Nothing to minify - using listenablefuture-9999.0-empty-to-avoid-conflict-with-guava.jar unchanged
    Nothing to minify - using jsr305-3.0.2.jar unchanged
    Nothing to minify - using checker-qual-2.5.2.jar unchanged
    Nothing to minify - using error_prone_annotations-2.2.0.jar unchanged
    Nothing to minify - using j2objc-annotations-1.1.jar unchanged
    Nothing to minify - using animal-sniffer-annotations-1.17.jar unchanged
    Nothing to minify - using failureaccess-1.0.1.jar unchanged

    BUILD SUCCESSFUL in 0s
    3 actionable tasks: 3 executed

### Implementing Artifact Transforms

Similar to task types, an artifact transform consists of an action and some optional parameters. The major difference from custom task types is that the action and the parameters are implemented as two separate classes.

#### Artifact Transforms without Parameters

The implementation of the artifact transform action is a class implementing TransformAction. You must implement the `transform()` method on the action, which converts an input artifact into zero, one, or multiple output artifacts.

Most Artifact Transforms are one-to-one, so the `transform` method will transform the input artifact into exactly one output artifact.

The implementation of the artifact transform action needs to register each output artifact by calling TransformOutputs.dir() or TransformOutputs.file().

You can supply two types of paths to the `dir` or `file` methods:

- An absolute path to the input artifact or within the input artifact (for an input directory).

- A relative path.

Gradle uses the absolute path as the location of the output artifact. For example, if the input artifact is an exploded WAR, the transform action can call `TransformOutputs.file()` for all JAR files in the `WEB-INF/lib` directory. The output of the transform would then be the library JARs of the web application.

For a relative path, the `dir()` or `file()` method returns a workspace to the transform action. The transform action needs to create the transformed artifact at the location of the provided workspace.

The output artifacts replace the input artifact in the transformed variant in the order they were registered. For example, if the configuration consists of the artifacts `lib1.jar`, `lib2.jar`, `lib3.jar`, and the transform action registers a minified output artifact `<artifact-name>-min.jar` for the input artifact, then the transformed configuration consists of the artifacts `lib1-min.jar`, `lib2-min.jar`, and `lib3-min.jar`.

Here is the implementation of an `Unzip` transform, which unzips a JAR file into a `classes` directory. The `Unzip` transform does not require any parameters:

build.gradle.kts

    abstract class Unzip : TransformAction<TransformParameters.None> {          (1)
        @get:InputArtifact                                                      (2)
        abstract val inputArtifact: Provider<FileSystemLocation>

        override
        fun transform(outputs: TransformOutputs) {
            val input = inputArtifact.get().asFile
            val unzipDir = outputs.dir(input.name)                              (3)
            unzipTo(input, unzipDir)                                            (4)
        }

        private fun unzipTo(zipFile: File, unzipDir: File) {
            // implementation...
        }
    }

build.gradle

    abstract class Unzip implements TransformAction<TransformParameters.None> { (1)
        @InputArtifact                                                          (2)
        abstract Provider<FileSystemLocation> getInputArtifact()

        @Override
        void transform(TransformOutputs outputs) {
            def input = inputArtifact.get().asFile
            def unzipDir = outputs.dir(input.name)                              (3)
            unzipTo(input, unzipDir)                                            (4)
        }

        private static void unzipTo(File zipFile, File unzipDir) {
            // implementation...
        }
    }

1 | Use `TransformParameters.None` if the transform does not use parameters
2 | Inject the input artifact
3 | Request an output location for the unzipped files
4 | Do the actual work of the transform

Note how the implementation uses `@InputArtifact` to inject the artifact to transform into the action. It requests a directory for the unzipped classes by using `TransformOutputs.dir()` and then unzips the JAR file into this directory.

#### Artifact Transforms with Parameters

An artifact transform may require parameters, such as a `String` for filtering or a file collection used to support the transformation of the input artifact. To pass these parameters to the transform action, you must define a new type with the desired parameters. This type must implement the marker interface TransformParameters.

The parameters must be represented using managed properties and the parameter type must be a managed type. You can use an interface or abstract class to declare the getters, and Gradle will generate the implementation. All getters need to have proper input annotations, as described in the incremental build annotations table.

Here is the implementation of a `Minify` transform that makes JARs smaller by only keeping certain classes in them. The `Minify` transform requires the classes to keep as parameters:

build.gradle.kts

    abstract class Minify : TransformAction<Minify.Parameters> {   (1)
        interface Parameters : TransformParameters {               (2)
            @get:Input
            var keepClassesByArtifact: Map<String, Set<String>>

        }

        @get:PathSensitive(PathSensitivity.NAME_ONLY)
        @get:InputArtifact
        abstract val inputArtifact: Provider<FileSystemLocation>

        override
        fun transform(outputs: TransformOutputs) {
            val fileName = inputArtifact.get().asFile.name
            for (entry in parameters.keepClassesByArtifact) {      (3)
                if (fileName.startsWith(entry.key)) {
                    val nameWithoutExtension = fileName.substring(0, fileName.length - 4)
                    minify(inputArtifact.get().asFile, entry.value, outputs.file("${nameWithoutExtension}-min.jar"))
                    return
                }
            }
            println("Nothing to minify - using ${fileName} unchanged")
            outputs.file(inputArtifact)                            (4)
        }

        private fun minify(artifact: File, keepClasses: Set<String>, jarFile: File) {
            println("Minifying ${artifact.name}")
            // Implementation ...
        }
    }

build.gradle

    abstract class Minify implements TransformAction<Parameters> { (1)
        interface Parameters extends TransformParameters {         (2)
            @Input
            Map<String, Set<String>> getKeepClassesByArtifact()
            void setKeepClassesByArtifact(Map<String, Set<String>> keepClasses)
        }

        @PathSensitive(PathSensitivity.NAME_ONLY)
        @InputArtifact
        abstract Provider<FileSystemLocation> getInputArtifact()

        @Override
        void transform(TransformOutputs outputs) {
            def fileName = inputArtifact.get().asFile.name
            for (entry in parameters.keepClassesByArtifact) {      (3)
                if (fileName.startsWith(entry.key)) {
                    def nameWithoutExtension = fileName.substring(0, fileName.length() - 4)
                    minify(inputArtifact.get().asFile, entry.value, outputs.file("${nameWithoutExtension}-min.jar"))
                    return
                }
            }
            println "Nothing to minify - using ${fileName} unchanged"
            outputs.file(inputArtifact)                            (4)
        }

        private void minify(File artifact, Set<String> keepClasses, File jarFile) {
            println "Minifying ${artifact.name}"
            // Implementation ...
        }
    }

1 | Declare the parameter type
2 | Interface for the transform parameters
3 | Use the parameters
4 | Use the unchanged input artifact when no minification is required

Observe how you can obtain the parameters by `TransformAction.getParameters()` in the `transform()` method. The implementation of the `transform()` method requests a location for the minified JAR by using `TransformOutputs.file()` and then creates the minified JAR at this location.

Remember that the input artifact is a dependency, which may have its own dependencies. Suppose your artifact transform needs access to those transitive dependencies. In that case, it can declare an abstract getter returning a `FileCollection` and annotate it with @InputArtifactDependencies. When your transform runs, Gradle will inject the transitive dependencies into the `FileCollection` property by implementing the getter. Note that using input artifact dependencies in a transform has performance implications; only inject them when needed.

#### Artifact Transforms with Caching

Artifact Transforms can make use of the build cache for their outputs.

To enable the build cache for an artifact transform, add the `@CacheableTransform` annotation on the action class.

For cacheable transforms, you must annotate its @InputArtifact property — and any property marked with @InputArtifactDependencies — with normalization annotations such as @PathSensitive.

The following example demonstrates a more complex transform that relocates specific classes within a JAR to a different package. This process involves rewriting the bytecode of both the relocated classes and any classes that reference them (class relocation):

build.gradle.kts

    @CacheableTransform                                                          (1)
    abstract class ClassRelocator : TransformAction<ClassRelocator.Parameters> {
        interface Parameters : TransformParameters {                             (2)
            @get:CompileClasspath                                                (3)
            val externalClasspath: ConfigurableFileCollection
            @get:Input
            val excludedPackage: Property<String>
        }

        @get:Classpath                                                           (4)
        @get:InputArtifact
        abstract val primaryInput: Provider<FileSystemLocation>

        @get:CompileClasspath
        @get:InputArtifactDependencies                                           (5)
        abstract val dependencies: FileCollection

        override
        fun transform(outputs: TransformOutputs) {
            val primaryInputFile = primaryInput.get().asFile
            if (parameters.externalClasspath.contains(primaryInputFile)) {       (6)
                outputs.file(primaryInput)
            } else {
                val baseName = primaryInputFile.name.substring(0, primaryInputFile.name.length - 4)
                relocateJar(outputs.file("$baseName-relocated.jar"))
            }
        }

        private fun relocateJar(output: File) {
            // implementation...
            val relocatedPackages = (dependencies.flatMap { it.readPackages() } + primaryInput.get().asFile.readPackages()).toSet()
            val nonRelocatedPackages = parameters.externalClasspath.flatMap { it.readPackages() }
            val relocations = (relocatedPackages - nonRelocatedPackages).map { packageName ->
                val toPackage = "relocated.$packageName"
                println("$packageName -> $toPackage")
                Relocation(packageName, toPackage)
            }
            JarRelocator(primaryInput.get().asFile, output, relocations).run()
        }
    }

build.gradle

    @CacheableTransform                                                          (1)
    abstract class ClassRelocator implements TransformAction<Parameters> {
        interface Parameters extends TransformParameters {                       (2)
            @CompileClasspath                                                    (3)
            ConfigurableFileCollection getExternalClasspath()
            @Input
            Property<String> getExcludedPackage()
        }

        @Classpath                                                               (4)
        @InputArtifact
        abstract Provider<FileSystemLocation> getPrimaryInput()

        @CompileClasspath
        @InputArtifactDependencies                                               (5)
        abstract FileCollection getDependencies()

        @Override
        void transform(TransformOutputs outputs) {
            def primaryInputFile = primaryInput.get().asFile
            if (parameters.externalClasspath.contains(primaryInput)) {           (6)
                outputs.file(primaryInput)
            } else {
                def baseName = primaryInputFile.name.substring(0, primaryInputFile.name.length - 4)
                relocateJar(outputs.file("$baseName-relocated.jar"))
            }
        }

        private relocateJar(File output) {
            // implementation...
            def relocatedPackages = (dependencies.collectMany { readPackages(it) } + readPackages(primaryInput.get().asFile)) as Set
            def nonRelocatedPackages = parameters.externalClasspath.collectMany { readPackages(it) }
            def relocations = (relocatedPackages - nonRelocatedPackages).collect { packageName ->
                def toPackage = "relocated.$packageName"
                println("$packageName -> $toPackage")
                new Relocation(packageName, toPackage)
            }
            new JarRelocator(primaryInput.get().asFile, output, relocations).run()
        }
    }

1 | Declare the transform cacheable
2 | Interface for the transform parameters
3 | Declare input type for each parameter
4 | Declare a normalization for the input artifact
5 | Inject the input artifact dependencies
6 | Use the parameters

Note the classes to be relocated are determined by examining the packages of the input artifact and its dependencies. Additionally, the transform ensures that packages contained in JAR files on an external classpath are not relocated.

#### Incremental Artifact Transforms

Similar to incremental tasks, Artifact Transforms can avoid work by only processing changed files from the last execution. This is done by using the InputChanges interface.

For Artifact Transforms, only the input artifact is an incremental input; therefore, the transform can only query for changes there. To use InputChanges in the transform action, inject it into the action.

For more information on how to use InputChanges, see the corresponding documentation for incremental tasks.

Here is an example of an incremental transform that counts the lines of code in Java source files:

build.gradle.kts

    abstract class CountLoc : TransformAction<TransformParameters.None> {

        @get:Inject                                                         (1)
        abstract val inputChanges: InputChanges

        @get:PathSensitive(PathSensitivity.RELATIVE)
        @get:InputArtifact
        abstract val input: Provider<FileSystemLocation>

        override
        fun transform(outputs: TransformOutputs) {
            val outputDir = outputs.dir("${input.get().asFile.name}.loc")
            println("Running transform on ${input.get().asFile.name}, incremental: ${inputChanges.isIncremental}")
            inputChanges.getFileChanges(input).forEach { change ->          (2)
                val changedFile = change.file
                if (change.fileType != FileType.FILE) {
                    return@forEach
                }
                val outputLocation = outputDir.resolve("${change.normalizedPath}.loc")
                when (change.changeType) {
                    ChangeType.ADDED, ChangeType.MODIFIED -> {

                        println("Processing file ${changedFile.name}")
                        outputLocation.parentFile.mkdirs()

                        outputLocation.writeText(changedFile.readLines().size.toString())
                    }
                    ChangeType.REMOVED -> {
                        println("Removing leftover output file ${outputLocation.name}")
                        outputLocation.delete()
                    }
                }
            }
        }
    }

build.gradle

    abstract class CountLoc implements TransformAction<TransformParameters.None> {

        @Inject                                                             (1)
        abstract InputChanges getInputChanges()

        @PathSensitive(PathSensitivity.RELATIVE)
        @InputArtifact
        abstract Provider<FileSystemLocation> getInput()

        @Override
        void transform(TransformOutputs outputs) {
            def outputDir = outputs.dir("${input.get().asFile.name}.loc")
            println("Running transform on ${input.get().asFile.name}, incremental: ${inputChanges.incremental}")
            inputChanges.getFileChanges(input).forEach { change ->          (2)
                def changedFile = change.file
                if (change.fileType != FileType.FILE) {
                    return
                }
                def outputLocation = new File(outputDir, "${change.normalizedPath}.loc")
                switch (change.changeType) {
                    case ADDED:
                    case MODIFIED:
                        println("Processing file ${changedFile.name}")
                        outputLocation.parentFile.mkdirs()

                        outputLocation.text = changedFile.readLines().size()

                    case REMOVED:
                        println("Removing leftover output file ${outputLocation.name}")
                        outputLocation.delete()

                }
            }
        }
    }

1 | Inject `InputChanges`
2 | Query for changes in the input artifact

### Registering Artifact Transforms

You need to register the artifact transform actions, providing parameters if necessary so that they can be selected when resolving dependencies.

To register an artifact transform, you must use registerTransform() within the `dependencies {}` block.

There are a few points to consider when using `registerTransform()`:

- The `from` and `to` attributes are required.

- The transform action itself can have configuration options. You can configure them with the `parameters {}` block.

- You must register the transform on the project that has the configuration that will be resolved.

- You can supply any type implementing TransformAction to the `registerTransform()` method.

For example, imagine you want to unpack some dependencies and put the unpacked directories and files on the classpath. You can do so by registering an artifact transform action of type `Unzip`, as shown here:

build.gradle.kts

    val artifactType = Attribute.of("artifactType", String::class.java)

    dependencies {
        registerTransform(Unzip::class) {
            from.attribute(artifactType, "jar")
            to.attribute(artifactType, "java-classes-directory")
        }
    }

build.gradle

    def artifactType = Attribute.of('artifactType', String)

    dependencies {
        registerTransform(Unzip) {
            from.attribute(artifactType, 'jar')
            to.attribute(artifactType, 'java-classes-directory')
        }
    }

Another example is that you want to minify JARs by only keeping some `class` files from them. Note the use of the `parameters {}` block to provide the classes to keep in the minified JARs to the `Minify` transform:

build.gradle.kts

    val artifactType = Attribute.of("artifactType", String::class.java)
    val minified = Attribute.of("minified", Boolean::class.javaObjectType)
    val keepPatterns = mapOf(
        "guava" to setOf(
            "com.google.common.base.Optional",
            "com.google.common.base.AbstractIterator"
        )
    )

    dependencies {
        registerTransform(Minify::class) {
            from.attribute(minified, false).attribute(artifactType, "jar")
            to.attribute(minified, true).attribute(artifactType, "jar")

            parameters {
                keepClassesByArtifact = keepPatterns
            }
        }
    }

build.gradle

    def artifactType = Attribute.of('artifactType', String)
    def minified = Attribute.of('minified', Boolean)
    def keepPatterns = [
        "guava": [
            "com.google.common.base.Optional",
            "com.google.common.base.AbstractIterator"
        ] as Set
    ]

    dependencies {
        registerTransform(Minify) {
            from.attribute(minified, false).attribute(artifactType, "jar")
            to.attribute(minified, true).attribute(artifactType, "jar")

            parameters {
                keepClassesByArtifact = keepPatterns
            }
        }
    }

### Executing Artifact Transforms

On the command line, Gradle runs tasks; not Artifact Transforms: `./gradlew build.` So how and when does it run transforms?

There are two ways Gradle executes a transform:

1. Artifact Transforms execution for project dependencies can be discovered ahead of task execution and therefore can be scheduled before the task execution.

2. Artifact Transforms execution for external module dependencies cannot be discovered ahead of task execution and, therefore are scheduled inside the task execution.

In well-declared builds, project dependencies can be fully discovered during task configuration ahead of task execution scheduling. If the project dependency is badly declared (e.g., missing task input), the transform execution will happen inside the task.

It’s important to remember that Artifact Transforms:

- can be run in parallel

- are cacheable

- are reusable (if separate resolutions used by different tasks require the same transform to be executed on the same artifacts, the transform results will be cached and shared)

# Working in a Multi-repo Environment

## Composite Builds

A composite build is a build that includes other builds.

A composite build is similar to a Gradle multi-project build, except that instead of including `subprojects`, entire `builds` are included.

Composite builds allow you to:

- Combine builds that are usually developed independently, for instance, when trying out a bug fix in a library that your application uses.

- Decompose a large multi-project build into smaller, more isolated chunks that can be worked on independently or together as needed.

A build that is included in a composite build is referred to as an included build. Included builds do not share any configuration with the composite build or the other included builds. Each included build is configured and executed in isolation.

### Defining a composite build

The following example demonstrates how two Gradle builds, normally developed separately, can be combined into a composite build.

    my-composite
    ├── gradle
    ├── gradlew
    ├── settings.gradle.kts
    ├── build.gradle.kts
    ├── my-app
    │   ├── settings.gradle.kts
    │   └── app
    │       ├── build.gradle.kts
    │       └── src/main/java/org/sample/my-app/Main.java
    └── my-utils
        ├── settings.gradle.kts
        ├── number-utils
        │   ├── build.gradle.kts
        │   └── src/main/java/org/sample/numberutils/Numbers.java
        └── string-utils
            ├── build.gradle.kts
            └── src/main/java/org/sample/stringutils/Strings.java

The `my-utils` multi-project build produces two Java libraries, `number-utils` and `string-utils`. The `my-app` build produces an executable using functions from those libraries.

The `my-app` build does not depend directly on `my-utils`. Instead, it declares binary dependencies on the libraries produced by `my-utils`:

my-app/app/build.gradle.kts

    plugins {
        id("application")
    }

    application {
        mainClass = "org.sample.myapp.Main"
    }

    dependencies {
        implementation("org.sample:number-utils:1.0")
        implementation("org.sample:string-utils:1.0")
    }

my-app/app/build.gradle

    plugins {
        id 'application'
    }

    application {
        mainClass = 'org.sample.myapp.Main'
    }

    dependencies {
        implementation 'org.sample:number-utils:1.0'
        implementation 'org.sample:string-utils:1.0'
    }

#### Defining a composite build via `--include-build`

The `--include-build` command-line argument turns the executed build into a composite, substituting dependencies from the included build into the executed build.

For example, the output of `./gradlew run --include-build ../my-utils` run from `my-app`:

    $ ./gradlew --include-build ../my-utils run
    > Task :app:processResources NO-SOURCE
    > Task :my-utils:string-utils:compileJava
    > Task :my-utils:string-utils:processResources NO-SOURCE
    > Task :my-utils:string-utils:classes
    > Task :my-utils:string-utils:jar
    > Task :my-utils:number-utils:compileJava
    > Task :my-utils:number-utils:processResources NO-SOURCE
    > Task :my-utils:number-utils:classes
    > Task :my-utils:number-utils:jar
    > Task :app:compileJava
    > Task :app:classes

    > Task :app:run
    The answer is 42

    BUILD SUCCESSFUL in 0s
    6 actionable tasks: 6 executed

#### Defining a composite build via the settings file

It’s possible to make the above arrangement persistent by using Settings.includeBuild(java.lang.Object) to declare the included build in the `settings.gradle(.kts)` file.

The settings file can be used to add subprojects and included builds simultaneously.

Included builds are added by location:

settings.gradle.kts

    includeBuild("my-utils")

In the example, the settings.gradle(.kts) file combines otherwise separate builds:

settings.gradle.kts

    rootProject.name = "my-composite"

    includeBuild("my-app")
    includeBuild("my-utils")

settings.gradle

    rootProject.name = 'my-composite'

    includeBuild 'my-app'
    includeBuild 'my-utils'

To execute the `run` task in the `my-app` build from `my-composite`, run `./gradlew my-app:app:run`.

You can optionally define a `run` task in `my-composite` that depends on `my-app:app:run` so that you can execute `./gradlew run`:

build.gradle.kts

    tasks.register("run") {
        dependsOn(gradle.includedBuild("my-app").task(":app:run"))
    }

build.gradle

    tasks.register('run') {
        dependsOn gradle.includedBuild('my-app').task(':app:run')
    }

#### Including builds that define Gradle plugins

A special case of included builds are builds that define Gradle plugins.

These builds should be included using the `includeBuild` statement inside the `pluginManagement {}` block of the settings file.

Using this mechanism, the included build may also contribute a settings plugin that can be applied in the settings file itself:

settings.gradle.kts

    pluginManagement {
        includeBuild("../url-verifier-plugin")
    }

settings.gradle

    pluginManagement {
        includeBuild '../url-verifier-plugin'
    }

### Restrictions on included builds

Most builds can be included in a composite, including other composite builds. There are some restrictions.

In a regular build, Gradle ensures that each project has a unique project path. It makes projects identifiable and addressable without conflicts.

In a composite build, Gradle adds additional qualification to each project from an included build to avoid project path conflicts. The full path to identify a project in a composite build is called a build-tree path. It consists of a build path of an included build and a project path of the project.

By default, build paths and project paths are derived from directory names and structure on disk. Since included builds can be located anywhere on disk, their build path is determined by the name of the containing directory. This can sometimes lead to conflicts.

To summarize, the included builds must fulfill these requirements:

- Each included build must have a unique build path.

- Each included build path must not conflict with any project path of the main build.

These conditions guarantee that each project can be uniquely identified even in a composite build.

If conflicts arise, the way to resolve them is by changing the build name of an included build:

settings.gradle.kts

    includeBuild("some-included-build") {
        name = "other-name"
    }

| When a composite build is included in another composite build, both builds have the same parent. In other words, the nested composite build structure is flattened.

### Interacting with a composite build

Interacting with a composite build is generally similar to a regular multi-project build. Tasks can be executed, tests can be run, and builds can be imported into the IDE.

#### Executing tasks

Tasks from an included build can be executed from the command-line or IDE in the same way as tasks from a regular multi-project build. Executing a task will result in task dependencies being executed, as well as those tasks required to build dependency artifacts from other included builds.

You can call a task in an included build using a fully qualified path, for example, `:included-build-name:project-name:taskName`. Project and task names can be abbreviated.

    $ ./gradlew :included-build:subproject-a:compileJava
    > Task :included-build:subproject-a:compileJava

    $ ./gradlew :i-b:sA:cJ
    > Task :included-build:subproject-a:compileJava

To exclude a task from the command line, you need to provide the fully qualified path to the task.

| Included build tasks are automatically executed to generate required dependency artifacts, or the including build can declare a dependency on a task from an included build.

#### Importing into the IDE

One of the most useful features of composite builds is IDE integration.

Importing a composite build permits sources from separate Gradle builds to be easily developed together. For every included build, each subproject is included as an IntelliJ IDEA Module or Eclipse Project. Source dependencies are configured, providing cross-build navigation and refactoring.

### Declaring dependencies substituted by an included build

By default, Gradle will configure each included build to determine the dependencies it can provide. The algorithm for doing this is simple. Gradle will inspect the group and name for the projects in the included build and substitute project dependencies for any external dependency matching `${project.group}:${project.name}`.

| By default, substitutions are not registered for the main build. To make the (sub)projects of the main build addressable by `${project.group}:${project.name}`, you can tell Gradle to treat the main build like an included build by self-including it: `includeBuild(".")`.

There are cases when the default substitutions determined by Gradle are insufficient or must be corrected for a particular composite. For these cases, explicitly declaring the substitutions for an included build is possible.

For example, a single-project build called `anonymous-library`, produces a Java utility library but does not declare a value for the group attribute:

build.gradle.kts

    plugins {
        java
    }

build.gradle

    plugins {
        id 'java'
    }

When this build is included in a composite, it will attempt to substitute for the dependency module `undefined:anonymous-library` (`undefined` being the default value for `project.group`, and `anonymous-library` being the root project name). Clearly, this isn’t useful in a composite build.

To use the unpublished library in a composite build, you can explicitly declare the substitutions that it provides:

settings.gradle.kts

    includeBuild("anonymous-library") {
        dependencySubstitution {
            substitute(module("org.sample:number-utils")).using(project(":"))
        }
    }

settings.gradle

    includeBuild('anonymous-library') {
        dependencySubstitution {
            substitute module('org.sample:number-utils') using project(':')
        }
    }

With this configuration, the `my-app` composite build will substitute any dependency on `org.sample:number-utils` with a dependency on the root project of `anonymous-library`.

#### Deactivate included build substitutions for a configuration

If you need to resolve a published version of a module that is also available as part of an included build, you can deactivate the included build substitution rules on the ResolutionStrategy of the Configuration that is resolved. This is necessary because the rules are globally applied in the build, and Gradle does not consider published versions during resolution by default.

For example, we create a separate `publishedRuntimeClasspath` configuration that gets resolved to the published versions of modules that also exist in one of the local builds. This is done by deactivating global dependency substitution rules:

build.gradle.kts

    configurations.create("publishedRuntimeClasspath") {
        resolutionStrategy.useGlobalDependencySubstitutionRules = false

        extendsFrom(configurations.runtimeClasspath.get())
        isCanBeConsumed = false
        attributes.attribute(Usage.USAGE_ATTRIBUTE, objects.named(Usage.JAVA_RUNTIME))
    }

build.gradle

    configurations.create('publishedRuntimeClasspath') {
        resolutionStrategy.useGlobalDependencySubstitutionRules = false

        extendsFrom(configurations.runtimeClasspath)
        canBeConsumed = false
        attributes.attribute(Usage.USAGE_ATTRIBUTE, objects.named(Usage, Usage.JAVA_RUNTIME))
    }

A use-case would be to compare published and locally built JAR files.

#### Cases where included build substitutions must be declared

Many builds will function automatically as an included build, without declared substitutions. Here are some common cases where declared substitutions are required:

- When the `archivesBaseName` property is used to set the name of the published artifact.

- When a configuration other than `default` is published.

- When the `MavenPom.addFilter()` is used to publish artifacts that don’t match the project name.

- When the `maven-publish` or `ivy-publish` plugins are used for publishing and the publication coordinates don’t match `${project.group}:${project.name}`.

#### Cases where composite build substitutions won’t work

Some builds won’t function correctly when included in a composite, even when dependency substitutions are explicitly declared. This limitation is because a substituted project dependency will always point to the `default` configuration of the target project. Any time the artifacts and dependencies specified for the default configuration of a project don’t match what is published to a repository, the composite build may exhibit different behavior.

Here are some cases where the published module metadata may be different from the project default configuration:

- When a configuration other than `default` is published.

- When the `maven-publish` or `ivy-publish` plugins are used.

- When the `POM` or `ivy.xml` file is tweaked as part of publication.

Builds using these features function incorrectly when included in a composite build.

### Depending on tasks in an included build

While included builds are isolated from one another and cannot declare direct dependencies, a composite build can declare task dependencies on its included builds. The included builds are accessed using Gradle.getIncludedBuilds() or Gradle.includedBuild(java.lang.String), and a task reference is obtained via the IncludedBuild.task(java.lang.String) method.

Using these APIs, it is possible to declare a dependency on a task in a particular included build:

build.gradle.kts

    tasks.register("run") {
        dependsOn(gradle.includedBuild("my-app").task(":app:run"))
    }

build.gradle

    tasks.register('run') {
        dependsOn gradle.includedBuild('my-app').task(':app:run')
    }

Or you can declare a dependency on tasks with a certain path in some or all of the included builds:

build.gradle.kts

    tasks.register("publishDeps") {
        dependsOn(gradle.includedBuilds.map { it.task(":publishMavenPublicationToMavenRepository") })
    }

build.gradle

    tasks.register('publishDeps') {
        dependsOn gradle.includedBuilds*.task(':publishMavenPublicationToMavenRepository')
    }

### Limitations of composite builds

Limitations of the current implementation include:

- No support for included builds with publications that don’t mirror the project default configuration.
  See Cases where composite builds won’t work.

- Multiple composite builds may conflict when run in parallel if more than one includes the same build.
  Gradle does not share the project lock of a shared composite build between Gradle invocations to prevent concurrent execution.

# Publishing Libraries

## Publishing a project as module

The vast majority of software projects build something that aims to be consumed in some way. It could be a library that other software projects use or it could be an application for end users. Publishing is the process by which the thing being built is made available to consumers.

In Gradle, that process looks like this:

1. Define what to publish

2. Define where to publish it to

3. Do the publishing

Each of the these steps is dependent on the type of repository to which you want to publish artifacts. The two most common types are Maven-compatible and Ivy-compatible repositories, or Maven and Ivy repositories for short.

As of Gradle 6.0, the Gradle Module Metadata will always be published alongside the Ivy XML or Maven POM metadata file.

Gradle makes it easy to publish to these types of repository by providing some prepackaged infrastructure in the form of the Maven Publish Plugin and the Ivy Publish Plugin. These plugins allow you to configure what to publish and perform the publishing with a minimum of effort.

Figure 5. The publishing process

Let’s take a look at those steps in more detail:

What to publish

  Gradle needs to know what files and information to publish so that consumers can use your project. This is typically a combination of artifacts and metadata that Gradle calls a publication. Exactly what a publication contains depends on the type of repository it’s being published to.

  For example, a publication destined for a Maven repository includes:

  - One or more artifacts — typically built by the project,

  - The Gradle Module Metadata file which will describe the variants of the published component,

  - The Maven POM file will identify the primary artifact and its dependencies. The primary artifact is typically the project’s production JAR and secondary artifacts might consist of "-sources" and "-javadoc" JARs.

  In addition, Gradle will publish checksums for all of the above, and signatures when configured to do so. From Gradle 6.0 onwards, this includes `SHA256` and `SHA512` checksums.

Where to publish

  Gradle needs to know where to publish artifacts so that consumers can get hold of them. This is done via repositories, which store and make available all sorts of artifact. Gradle also needs to interact with the repository, which is why you must provide the type of the repository and its location.

How to publish

  Gradle automatically generates publishing tasks for all possible combinations of publication and repository, allowing you to publish any artifact to any repository. If you’re publishing to a Maven repository, the tasks are of type PublishToMavenRepository, while for Ivy repositories the tasks are of type PublishToIvyRepository.

What follows is a practical example that demonstrates the entire publishing process.

### Setting up basic publishing

The first step in publishing, irrespective of your project type, is to apply the appropriate publishing plugin. As mentioned in the introduction, Gradle supports both Maven and Ivy repositories via the following plugins:

- Maven Publish Plugin

- Ivy Publish Plugin

These provide the specific publication and repository classes needed to configure publishing for the corresponding repository type. Since Maven repositories are the most commonly used ones, they will be the basis for this example and for the other samples in the chapter. Don’t worry, we will explain how to adjust individual samples for Ivy repositories.

Let’s assume we’re working with a simple Java library project, so only the following plugins are applied:

Example 112. Applying the necessary plugins
build.gradle.kts

    plugins {
        `java-library`
        `maven-publish`
    }

build.gradle

    plugins {
        id 'java-library'
        id 'maven-publish'
    }

Once the appropriate plugin has been applied, you can configure the publications and repositories. For this example, we want to publish the project’s production JAR file — the one produced by the `jar` task — to a custom Maven repository. We do that with the following `publishing {}` block, which is backed by PublishingExtension:

Example 113. Configuring a Java library for publishing
build.gradle.kts

    group = "org.example"
    version = "1.0"

    publishing {
        publications {
            create<MavenPublication>("myLibrary") {
                from(components["java"])
            }
        }

        repositories {
            maven {
                name = "myRepo"
                url = uri(layout.buildDirectory.dir("repo"))
            }
        }
    }

build.gradle

    group = 'org.example'
    version = '1.0'

    publishing {
        publications {
            myLibrary(MavenPublication) {
                from components.java
            }
        }

        repositories {
            maven {
                name = 'myRepo'
                url = layout.buildDirectory.dir("repo")
            }
        }
    }

This defines a publication called "myLibrary" that can be published to a Maven repository by virtue of its type: MavenPublication. This publication consists of just the production JAR artifact and its metadata, which combined are represented by the `java` component of the project.

| Components are the standard way of defining a publication. They are provided by plugins, usually of the language or platform variety. For example, the Java Plugin defines the `components.java` SoftwareComponent, while the War Plugin defines `components.web`.

The example also defines a file-based Maven repository with the name "myRepo". Such a file-based repository is convenient for a sample, but real-world builds typically work with HTTPS-based repository servers, such as Maven Central or an internal company server.

| You may define one, and only one, repository without a name. This translates to an implicit name of "Maven" for Maven repositories and "Ivy" for Ivy repositories. All other repository definitions must be given an explicit name.

In combination with the project’s `group` and `version`, the publication and repository definitions provide everything that Gradle needs to publish the project’s production JAR. Gradle will then create a dedicated `publishMyLibraryPublicationToMyRepoRepository` task that does just that. Its name is based on the template `publishPubNamePublicationToRepoNameRepository`. See the appropriate publishing plugin’s documentation for more details on the nature of this task and any other tasks that may be available to you.

You can either execute the individual publishing tasks directly, or you can execute `publish`, which will run all the available publishing tasks. In this example, `publish` will just run `publishMyLibraryPublicationToMavenRepository`.

| Basic publishing to an Ivy repository is very similar: you simply use the Ivy Publish Plugin, replace `MavenPublication` with IvyPublication, and use `ivy` instead of `maven` in the repository definition. There are differences between the two types of repository, particularly around the extra metadata that each support — for example, Maven repositories require a POM file while Ivy ones have their own metadata format — so see the plugin chapters for comprehensive information on how to configure both publications and repositories for whichever repository type you’re working with.

That’s everything for the basic use case. However, many projects need more control over what gets published, so we look at several common scenarios in the following sections.

### Suppressing validation errors

Gradle performs validation of generated module metadata. In some cases, validation can fail, indicating that you most likely have an error to fix, but you may have done something intentionally. If this is the case, Gradle will indicate the name of the validation error you can disable on the `GenerateModuleMetadata` tasks:

Example 114. Disabling some validation errors
build.gradle.kts

    tasks.withType<GenerateModuleMetadata> {
        // The value 'enforced-platform' is provided in the validation
        // error message you got
        suppressedValidationErrors.add("enforced-platform")
    }

build.gradle

    tasks.withType(GenerateModuleMetadata).configureEach {
        // The value 'enforced-platform' is provided in the validation
        // error message you got
        suppressedValidationErrors.add('enforced-platform')
    }

## Understanding Gradle Module Metadata

Gradle Module Metadata is a format used to serialize the Gradle component model. It is similar to Apache Maven™'s POM file or Apache Ivy™ ivy.xml files. The goal of metadata files is to provide to consumers a reasonable model of what is published on a repository.

Gradle Module Metadata is a unique format aimed at improving dependency resolution by making it multi-platform and variant-aware.

In particular, Gradle Module Metadata supports:

- rich version constraints

- dependency constraints

- component capabilities

- variant-aware resolution

Publication of Gradle Module Metadata will enable better dependency management for your consumers:

- early discovery of problems by detecting incompatible modules

- consistent selection of platform-specific dependencies

- native dependency version alignment

- automatically getting dependencies for specific features of your library

Gradle Module Metadata is automatically published when using the Maven Publish plugin or the Ivy Publish plugin.

The specification for Gradle Module Metadata can be found here.

### Mapping with other formats

Gradle Module Metadata is automatically published on Maven or Ivy repositories. However, it doesn’t replace the pom.xml or ivy.xml files: it is published alongside those files. This is done to maximize compatibility with third-party build tools.

Gradle does its best to map Gradle-specific concepts to Maven or Ivy. When a build file uses features that can only be represented in Gradle Module Metadata, Gradle will warn you at publication time. The table below summarizes how some Gradle specific features are mapped to Maven and Ivy:

Table 10. Mapping of Gradle specific concepts to Maven and Ivy
Gradle | Maven | Ivy | Description
--- | --- | --- | ---
dependency constraints | `<dependencyManagement>` dependencies | Not published | Gradle dependency constraints are transitive, while Maven’s dependency management block isn’t
rich version constraints | Publishes the requires version | Published the requires version |
component capabilities | Not published | Not published | Component capabilities are unique to Gradle
Feature variants | Variant artifacts are uploaded, dependencies are published as optional dependencies | Variant artifacts are uploaded, dependencies are not published | Feature variants are a good replacement for optional dependencies
Custom component types | Artifacts are uploaded, dependencies are those described by the mapping | Artifacts are uploaded, dependencies are ignored | Custom component types are probably not consumable from Maven or Ivy in any case. They usually exist in the context of a custom ecosystem.

#### Disabling metadata compatibility publication warnings

If you want to suppress warnings, you can use the following APIs to do so:

- For Maven, see the `suppress*` methods in MavenPublication

- For Ivy, see the `suppress*` methods in IvyPublication

Example 115. Disabling publication warnings
build.gradle.kts

    publications {
        register<MavenPublication>("maven") {
            from(components["java"])
            suppressPomMetadataWarningsFor("runtimeElements")
        }
    }

build.gradle

    publications {
        maven(MavenPublication) {
            from components.java
            suppressPomMetadataWarningsFor('runtimeElements')
        }
    }

### Interactions with other build tools

Because Gradle Module Metadata is not widely spread and because it aims at maximizing compatibility with other tools, Gradle does a couple of things:

- Gradle Module Metadata is systematically published alongside the normal descriptor for a given repository (Maven or Ivy)

- the `pom.xml` or `ivy.xml` file will contain a marker comment which tells Gradle that Gradle Module Metadata exists for this module

The goal of the marker is not for other tools to parse module metadata: it’s for Gradle users only. It explains to Gradle that a better module metadata file exists and that it should use it instead. It doesn’t mean that consumption from Maven or Ivy would be broken either, only that it works in degraded mode.

| This must be seen as a performance optimization: instead of having to do 2 network requests, one to get Gradle Module Metadata, then one to get the POM/Ivy file in case of a miss, Gradle will first look at the file which is most likely to be present, then only perform a 2nd request if the module was actually published with Gradle Module Metadata.

If you know that the modules you depend on are always published with Gradle Module Metadata, you can optimize the network calls by configuring the metadata sources for a repository:

Example 116. Resolving Gradle Module Metadata only
build.gradle.kts

    repositories {
        maven {
            setUrl("http://repo.mycompany.com/repo")
            metadataSources {
                gradleMetadata()
            }
        }
    }

build.gradle

    repositories {
        maven {
            url "http://repo.mycompany.com/repo"
            metadataSources {
                gradleMetadata()
            }
        }
    }

### Gradle Module Metadata validation

Gradle Module Metadata is validated before being published.

The following rules are enforced:

- Variant names must be unique,

- Each variant must have at least one attribute,

- Two variants cannot have the exact same attributes and capabilities,

- If there are dependencies, at least one, across all variants, must carry version information.

These rules ensure the quality of the metadata produced, and help confirm that consumption will not be problematic.

### Gradle Module Metadata reproducibility

The task generating the module metadata files is currently never marked `UP-TO-DATE` by Gradle due to the way it is implemented. However, if neither build inputs nor build scripts changed, the task result is effectively up-to-date: it always produces the same output.

If users desire to have a unique `module` file per build invocation, it is possible to link an identifier in the produced metadata to the build that created it. Users can choose to enable this unique identifier in their `publication`:

Example 117. Configuring the build identifier of a publication
build.gradle.kts

    publishing {
        publications {
            create<MavenPublication>("myLibrary") {
                from(components["java"])
                withBuildIdentifier()
            }
        }
    }

build.gradle

    publishing {
        publications {
            myLibrary(MavenPublication) {
                from components.java
                withBuildIdentifier()
            }
        }
    }

With the changes above, the generated Gradle Module Metadata file will always be different, forcing downstream tasks to consider it out-of-date.

### Disabling Gradle Module Metadata publication

There are situations where you might want to disable publication of Gradle Module Metadata:

- the repository you are uploading to rejects the metadata file (unknown format)

- you are using Maven or Ivy specific concepts which are not properly mapped to Gradle Module Metadata

In this case, disabling the publication of Gradle Module Metadata is done simply by disabling the task which generates the metadata file:

Example 118. Disabling publication of Gradle Module Metadata
build.gradle.kts

    tasks.withType<GenerateModuleMetadata> {
        enabled = false
    }

build.gradle

    tasks.withType(GenerateModuleMetadata) {
        enabled = false
    }

## Signing artifacts

The Signing Plugin can be used to sign all artifacts and metadata files that make up a publication, including Maven POM files and Ivy module descriptors. In order to use it:

1. Apply the Signing Plugin

2. Configure the signatory credentials — follow the link to see how

3. Specify the publications you want signed

Here’s an example that configures the plugin to sign the `mavenJava` publication:

Example 119. Signing a publication
build.gradle.kts

    signing {
        sign(publishing.publications["mavenJava"])
    }

build.gradle

    signing {
        sign publishing.publications.mavenJava
    }

This will create a `Sign` task for each publication you specify and wire all `publishPubNamePublicationToRepoNameRepository` tasks to depend on it. Thus, publishing any publication will automatically create and publish the signatures for its artifacts and metadata, as you can see from this output:

### Example: Sign and publish a project

Output of `gradle publish`

    > gradle publish
    > Task :compileJava
    > Task :processResources
    > Task :classes
    > Task :jar
    > Task :javadoc
    > Task :javadocJar
    > Task :sourcesJar
    > Task :generateMetadataFileForMavenJavaPublication
    > Task :generatePomFileForMavenJavaPublication
    > Task :signMavenJavaPublication
    > Task :publishMavenJavaPublicationToMavenRepository
    > Task :publish

    BUILD SUCCESSFUL in 0s
    10 actionable tasks: 10 executed

## Customizing publishing

### Modifying and adding variants to existing components for publishing

Gradle’s publication model is based on the notion of components, which are defined by plugins. For example, the Java Library plugin defines a `java` component which corresponds to a library, but the Java Platform plugin defines another kind of component, named `javaPlatform`, which is effectively a different kind of software component (a platform).

Sometimes we want to add more variants to or modify existing variants of an existing component. For example, if you added a variant of a Java library for a different platform, you may just want to declare this additional variant on the `java` component itself. In general, declaring additional variants is often the best solution to publish additional artifacts.

To perform such additions or modifications, the `AdhocComponentWithVariants` interface declares two methods called `addVariantsFromConfiguration` and `withVariantsFromConfiguration` which accept two parameters:

- the outgoing configuration that is used as a variant source

- a customization action which allows you to filter which variants are going to be published

To utilise these methods, you must make sure that the `SoftwareComponent` you work with is itself an `AdhocComponentWithVariants`, which is the case for the components created by the Java plugins (Java, Java Library, Java Platform). Adding a variant is then very simple:

Example 120. Adding a variant to an existing software component
InstrumentedJarsPlugin.kt

    val javaComponent = components.findByName("java") as AdhocComponentWithVariants
    javaComponent.addVariantsFromConfiguration(outgoing) {
        // dependencies for this variant are considered runtime dependencies
        mapToMavenScope("runtime")
        // and also optional dependencies, because we don't want them to leak
        mapToOptional()
    }

InstrumentedJarsPlugin.groovy

    AdhocComponentWithVariants javaComponent = (AdhocComponentWithVariants) project.components.findByName("java")
    javaComponent.addVariantsFromConfiguration(outgoing) {
        // dependencies for this variant are considered runtime dependencies
        it.mapToMavenScope("runtime")
        // and also optional dependencies, because we don't want them to leak
        it.mapToOptional()
    }

In other cases, you might want to modify a variant that was added by one of the Java plugins already. For example, if you activate publishing of Javadoc and sources, these become additional variants of the `java` component. If you only want to publish one of them, e.g. only Javadoc but no sources, you can modify the `sources` variant to not being published:

Example 121. Publish a java library with Javadoc but without sources
build.gradle.kts

    java {
        withJavadocJar()
        withSourcesJar()
    }

    val javaComponent = components["java"] as AdhocComponentWithVariants
    javaComponent.withVariantsFromConfiguration(configurations["sourcesElements"]) {
        skip()
    }

    publishing {
        publications {
            create<MavenPublication>("mavenJava") {
                from(components["java"])
            }
        }
    }

build.gradle

    java {
        withJavadocJar()
        withSourcesJar()
    }

    components.java.withVariantsFromConfiguration(configurations.sourcesElements) {
        skip()
    }

    publishing {
        publications {
            mavenJava(MavenPublication) {
                from components.java
            }
        }
    }

### Creating and publishing custom components

In the previous example, we have demonstrated how to extend or modify an existing component, like the components provided by the Java plugins. But Gradle also allows you to build a custom component (not a Java Library, not a Java Platform, not something supported natively by Gradle).

To create a custom component, you first need to create an empty adhoc component. At the moment, this is only possible via a plugin because you need to get a handle on the SoftwareComponentFactory :

Example 122. Injecting the software component factory
InstrumentedJarsPlugin.kt

    class InstrumentedJarsPlugin @Inject constructor(
        private val softwareComponentFactory: SoftwareComponentFactory) : Plugin<Project> {

InstrumentedJarsPlugin.groovy

    private final SoftwareComponentFactory softwareComponentFactory

    @Inject
    InstrumentedJarsPlugin(SoftwareComponentFactory softwareComponentFactory) {
        this.softwareComponentFactory = softwareComponentFactory
    }

Declaring what a custom component publishes is still done via the AdhocComponentWithVariants API. For a custom component, the first step is to create custom outgoing variants, following the instructions in this chapter. At this stage, what you should have is variants which can be used in cross-project dependencies, but that we are now going to publish to external repositories.

Example 123. Creating a custom, adhoc component
InstrumentedJarsPlugin.kt

    // create an adhoc component
    val adhocComponent = softwareComponentFactory.adhoc("myAdhocComponent")
    // add it to the list of components that this project declares
    components.add(adhocComponent)
    // and register a variant for publication
    adhocComponent.addVariantsFromConfiguration(outgoing) {
        mapToMavenScope("runtime")
    }

InstrumentedJarsPlugin.groovy

    // create an adhoc component
    def adhocComponent = softwareComponentFactory.adhoc("myAdhocComponent")
    // add it to the list of components that this project declares
    project.components.add(adhocComponent)
    // and register a variant for publication
    adhocComponent.addVariantsFromConfiguration(outgoing) {
        it.mapToMavenScope("runtime")
    }

First we use the factory to create a new adhoc component. Then we add a variant through the `addVariantsFromConfiguration` method, which is described in more detail in the previous section.

In simple cases, there’s a one-to-one mapping between a `Configuration` and a variant, in which case you can publish all variants issued from a single `Configuration` because they are effectively the same thing. However, there are cases where a `Configuration` is associated with additional configuration publications that we also call secondary variants. Such configurations make sense in the cross-project publications use case, but not when publishing externally. This is for example the case when between projects you share a directory of files, but there’s no way you can publish a directory directly on a Maven repository (only packaged things like jars or zips). Look at the ConfigurationVariantDetails class for details about how to skip publication of a particular variant. If `addVariantsFromConfiguration` has already been called for a configuration, further modification of the resulting variants can be performed using `withVariantsFromConfiguration`.

When publishing an adhoc component like this:

- Gradle Module Metadata will exactly represent the published variants. In particular, all outgoing variants will inherit dependencies, artifacts and attributes of the published configuration.

- Maven and Ivy metadata files will be generated, but you need to declare how the dependencies are mapped to Maven scopes via the ConfigurationVariantDetails class.

In practice, it means that components created this way can be consumed by Gradle the same way as if they were "local components".

### Adding custom artifacts to a publication

Instead of thinking in terms of artifacts, you should embrace the variant aware model of Gradle. It is expected that a single module may need multiple artifacts. However this rarely stops there, if the additional artifacts represent an optional feature, they might also have different dependencies and more.

Gradle, via Gradle Module Metadata, supports the publication of additional variants which make those artifacts known to the dependency resolution engine. Please refer to the variant-aware sharing section of the documentation to see how to declare such variants and check out how to publish custom components.

If you attach extra artifacts to a publication directly, they are published "out of context". That means, they are not referenced in the metadata at all and can then only be addressed directly through a classifier on a dependency. In contrast to Gradle Module Metadata, Maven pom metadata will not contain information on additional artifacts regardless of whether they are added through a variant or directly, as variants cannot be represented in the pom format.

The following section describes how you publish artifacts directly if you are sure that metadata, for example Gradle or POM metadata, is irrelevant for your use case. For example, if your project doesn’t need to be consumed by other projects and the only thing required as result of the publishing are the artifacts themselves.

In general, there are two options:

- Create a publication only with artifacts

- Add artifacts to a publication based on a component with metadata (not recommended, instead adjust a component or use a adhoc component publication which will both also produce metadata fitting your artifacts)

To create a publication based on artifacts, start by defining a custom artifact and attaching it to a Gradle configuration of your choice. The following sample defines an RPM artifact that is produced by an `rpm` task (not shown) and attaches that artifact to the `conf` configuration:

Example 124. Defining a custom artifact for a configuration
build.gradle.kts

    configurations {
        create("conf")
    }
    val rpmFile = layout.buildDirectory.file("rpms/my-package.rpm")
    val rpmArtifact = artifacts.add("conf", rpmFile.get().asFile) {
        type = "rpm"
        builtBy("rpm")
    }

build.gradle

    configurations {
        conf
    }
    def rpmFile = layout.buildDirectory.file('rpms/my-package.rpm')
    def rpmArtifact = artifacts.add('conf', rpmFile.get().asFile) {
        type 'rpm'
        builtBy 'rpm'
    }

The `artifacts.add()` method — from ArtifactHandler — returns an artifact object of type PublishArtifact that can then be used in defining a publication, as shown in the following sample:

Example 125. Attaching a custom PublishArtifact to a publication
build.gradle.kts

    publishing {
        publications {
            create<MavenPublication>("maven") {
                artifact(rpmArtifact)
            }
        }
    }

build.gradle

    publishing {
        publications {
            maven(MavenPublication) {
                artifact rpmArtifact
            }
        }
    }

- The `artifact()` method accepts publish artifacts as argument — like `rpmArtifact` in the sample — as well as any type of argument accepted by Project.file(java.lang.Object), such as a `File` instance, a string file path or a archive task.

- Publishing plugins support different artifact configuration properties, so always check the plugin documentation for more details. The `classifier` and `extension` properties are supported by both the Maven Publish Plugin and the Ivy Publish Plugin.

- Custom artifacts need to be distinct within a publication, typically via a unique combination of `classifier` and `extension`. See the documentation for the plugin you’re using for the precise requirements.

- If you use `artifact()` with an archive task, Gradle automatically populates the artifact’s metadata with the `classifier` and `extension` properties from that task.

Now you can publish the RPM.

If you really want to add an artifact to a publication based on a component, instead of adjusting the component itself, you can combine the `from components.someComponent` and `artifact someArtifact` notations.

### Restricting publications to specific repositories

When you have defined multiple publications or repositories, you often want to control which publications are published to which repositories. For instance, consider the following sample that defines two publications — one that consists of just a binary and another that contains the binary and associated sources — and two repositories — one for internal use and one for external consumers:

Example 126. Adding multiple publications and repositories
build.gradle.kts

    publishing {
        publications {
            create<MavenPublication>("binary") {
                from(components["java"])
            }
            create<MavenPublication>("binaryAndSources") {
                from(components["java"])
                artifact(tasks["sourcesJar"])
            }
        }
        repositories {
            // change URLs to point to your repos, e.g. http://my.org/repo
            maven {
                name = "external"
                url = uri(layout.buildDirectory.dir("repos/external"))
            }
            maven {
                name = "internal"
                url = uri(layout.buildDirectory.dir("repos/internal"))
            }
        }
    }

build.gradle

    publishing {
        publications {
            binary(MavenPublication) {
                from components.java
            }
            binaryAndSources(MavenPublication) {
                from components.java
                artifact sourcesJar
            }
        }
        repositories {
            // change URLs to point to your repos, e.g. http://my.org/repo
            maven {
                name = 'external'
                url = layout.buildDirectory.dir('repos/external')
            }
            maven {
                name = 'internal'
                url = layout.buildDirectory.dir('repos/internal')
            }
        }
    }

The publishing plugins will create tasks that allow you to publish either of the publications to either repository. They also attach those tasks to the `publish` aggregate task. But let’s say you want to restrict the binary-only publication to the external repository and the binary-with-sources publication to the internal one. To do that, you need to make the publishing conditional.

Gradle allows you to skip any task you want based on a condition via the Task.onlyIf(String, org.gradle.api.specs.Spec) method. The following sample demonstrates how to implement the constraints we just mentioned:

Example 127. Configuring which artifacts should be published to which repositories
build.gradle.kts

    tasks.withType<PublishToMavenRepository>().configureEach {
        val predicate = provider {
            (repository == publishing.repositories["external"] &&
                publication == publishing.publications["binary"]) ||
            (repository == publishing.repositories["internal"] &&
                publication == publishing.publications["binaryAndSources"])
        }
        onlyIf("publishing binary to the external repository, or binary and sources to the internal one") {
            predicate.get()
        }
    }
    tasks.withType<PublishToMavenLocal>().configureEach {
        val predicate = provider {
            publication == publishing.publications["binaryAndSources"]
        }
        onlyIf("publishing binary and sources") {
            predicate.get()
        }
    }

build.gradle

    tasks.withType(PublishToMavenRepository) {
        def predicate = provider {
            (repository == publishing.repositories.external &&
                publication == publishing.publications.binary) ||
            (repository == publishing.repositories.internal &&
                publication == publishing.publications.binaryAndSources)
        }
        onlyIf("publishing binary to the external repository, or binary and sources to the internal one") {
            predicate.get()
        }
    }
    tasks.withType(PublishToMavenLocal) {
        def predicate = provider {
            publication == publishing.publications.binaryAndSources
        }
        onlyIf("publishing binary and sources") {
            predicate.get()
        }
    }

Output of `gradle publish`

    > gradle publish
    > Task :compileJava
    > Task :processResources
    > Task :classes
    > Task :jar
    > Task :generateMetadataFileForBinaryAndSourcesPublication
    > Task :generatePomFileForBinaryAndSourcesPublication
    > Task :sourcesJar
    > Task :publishBinaryAndSourcesPublicationToExternalRepository SKIPPED
    > Task :publishBinaryAndSourcesPublicationToInternalRepository
    > Task :generateMetadataFileForBinaryPublication
    > Task :generatePomFileForBinaryPublication
    > Task :publishBinaryPublicationToExternalRepository
    > Task :publishBinaryPublicationToInternalRepository SKIPPED
    > Task :publish

    BUILD SUCCESSFUL in 0s
    10 actionable tasks: 10 executed

You may also want to define your own aggregate tasks to help with your workflow. For example, imagine that you have several publications that should be published to the external repository. It could be very useful to publish all of them in one go without publishing the internal ones.

The following sample demonstrates how you can do this by defining an aggregate task — `publishToExternalRepository` — that depends on all the relevant publish tasks:

Example 128. Defining your own shorthand tasks for publishing
build.gradle.kts

    tasks.register("publishToExternalRepository") {
        group = "publishing"
        description = "Publishes all Maven publications to the external Maven repository."
        dependsOn(tasks.withType<PublishToMavenRepository>().matching {
            it.repository == publishing.repositories["external"]
        })
    }

build.gradle

    tasks.register('publishToExternalRepository') {
        group = 'publishing'
        description = 'Publishes all Maven publications to the external Maven repository.'
        dependsOn tasks.withType(PublishToMavenRepository).matching {
            it.repository == publishing.repositories.external
        }
    }

This particular sample automatically handles the introduction or removal of the relevant publishing tasks by using TaskCollection.withType(java.lang.Class) with the PublishToMavenRepository task type. You can do the same with PublishToIvyRepository if you’re publishing to Ivy-compatible repositories.

### Configuring publishing tasks

The publishing plugins create their non-aggregate tasks after the project has been evaluated, which means you cannot directly reference them from your build script. If you would like to configure any of these tasks, you should use deferred task configuration. This can be done in a number of ways via the project’s `tasks` collection.

For example, imagine you want to change where the `generatePomFileForPubNamePublication` tasks write their POM files. You can do this by using the TaskCollection.withType(java.lang.Class) method, as demonstrated by this sample:

Example 129. Configuring a dynamically named task created by the publishing plugins
build.gradle.kts

    tasks.withType<GenerateMavenPom>().configureEach {
        val matcher = Regex("""generatePomFileFor(\w+)Publication""").matchEntire(name)
        val publicationName = matcher?.let { it.groupValues[1] }
        destination = layout.buildDirectory.file("poms/${publicationName}-pom.xml").get().asFile
    }

build.gradle

    tasks.withType(GenerateMavenPom).all {
        def matcher = name =~ /generatePomFileFor(\w+)Publication/
        def publicationName = matcher[0][1]
        destination = layout.buildDirectory.file("poms/${publicationName}-pom.xml").get().asFile
    }

The above sample uses a regular expression to extract the name of the publication from the name of the task. This is so that there is no conflict between the file paths of all the POM files that might be generated. If you only have one publication, then you don’t have to worry about such conflicts since there will only be one POM file.