# Variant Selection and Attribute Matching

version 9.0.0

Gradle’s dependency management engine is variant aware.

In the previous section, Gradle built a graph of resolved dependencies. During graph resolution, Gradle selected the proper variants of each dependency based on the requirements of the build.

Variants represent different ways a component can be used, such as for Java compilation, native linking, or documentation. Each variant may have its own artifacts and dependencies.

Gradle uses attributes to determine which variant to choose. These attributes add context to each variant, describing when they should be used.

## Components

Let’s review our description of components in the previous section. A component:

- contains variants

  - which contain one or more artifacts

  - which contain zero or more dependencies

  - which is described by metadata

In the example above, `org.jetbrains.kotlinx:kotlinx-serialization-json:1.5.1` is the component. It has a module `org.jetbrains.kotlinx:kotlinx-serialization-json` and a version `1.5.1`:

## Variants and Attributes

Variants represent different versions or aspects of a component, like `api` vs `implementation` or `jar` vs `classes`.

In the example above, `org.jetbrains.kotlinx:kotlinx-serialization-json:1.5.1` has four variants: `jvm`, `android`, `js`, and `native`.

Attributes are type-safe key-value pairs used by both the consumer and the producer during variant selection: `attribute : value`.

In the example above, there are two attributes of importance for the variants of `org.jetbrains.kotlinx:kotlinx-serialization-json:1.5.1`:

Variant: | Attribute 1: | Attribute 2:
--- | --- | ---
JVM | `org.gradle.usage: java-runtime` | `org.jetbrains.kotlin.platform.type: jvm`
Android | `org.gradle.usage: java-runtime` | `org.jetbrains.kotlin.platform.type: androidJvm`
Javascript | `org.gradle.usage: java-runtime` | `org.jetbrains.kotlin.platform.type: js`
Native | `org.gradle.usage: native-link` | `org.jetbrains.kotlin.platform.type: native`

Note that the metadata of `org.jetbrains.kotlinx:kotlinx-serialization-json:1.5.1` showcases many more attributes to describe its variants such as `org.gradle.libraryelements` or `org.gradle.category`. However, Gradle may use any number of them during dependency resolution as some attributes are more relevant than others:

    "variants": [
    ...
      {
        "name": "jsLegacyRuntimeElements-published",
        "attributes": {
          "org.gradle.category": "library",
          "org.gradle.usage": "kotlin-runtime",
          "org.jetbrains.kotlin.js.compiler": "legacy",
          "org.jetbrains.kotlin.platform.type": "js"
        }
      },
      {
        "name": "jvmRuntimeElements-published",
        "attributes": {
          "org.gradle.category": "library",
          "org.gradle.libraryelements": "jar",
          "org.gradle.usage": "java-runtime",
          "org.jetbrains.kotlin.platform.type": "jvm"
        }
      },
    ...
    ]

There are two types of attributes Gradle uses to match the available variants to the one required for the build:

- Consumer attributes: Define the desired characteristics of a variant requested by a resolvable configuration.

- Producer attributes: Each variant has a set of attributes that describe its purpose.

## Variant Attribute Matching

There are no restrictions on how many variants a component can define.

A typical component will include at least an implementation variant but may also provide additional variants, such as test fixtures, documentation, or source code. Furthermore, a component can offer different variants for the same usage, depending on the consumer. For instance, during compilation, a component may provide different headers for Linux, Windows, and macOS.

Gradle performs variant-aware selection by matching the attributes specified by the consumer with those defined by the producer using an attribute matching algorithm.

| The variant name is primarily used for debugging and error messages. It does not play a role in variant matching; only the variant’s attributes are used in the matching process.

## Attribute Matching Algorithm

Gradle’s dependency resolution engine follows this attribute matching algorithm to determine the best candidate (or fail if no match is found):

### Step 1: Find Compatible Candidates

The requested attributes are matched against the candidate attributes.

- FOR each candidate:

  - FOR each candidate attribute:

    - IF the candidate attribute is missing, mark the candidate as missing.

    - ELSE IF the candidate attribute exactly matches the requested attribute, the candidate remains compatible.

    - ELSE IF the candidate attribute passes the compatibility test, the candidate remains compatible.

    - ELSE, the candidate is eliminated.

### Step 2: Most Matching Attributes

After filtering, Gradle checks if one candidate is a strict superset of all others.

- IF only one candidate remains, that candidate wins.

- IF multiple candidates remain, check if one:

  - Matches more attributes than any other.

  - Has no missing attributes compared to another candidate.

- IF such a strict superset exists, that candidate wins.

### Step 3: Disambiguation Using Requested Attributes

If multiple candidates remain, Gradle attempts disambiguation:

- Requested attributes are sorted by precedence.

- FOR each requested attribute in precedence order:

  - IF a disambiguation rule exists, apply it.

  - IF a requested attribute has the same value across all remaining candidates, ignore it.

- IF only one candidate remains, that candidate wins.

If multiple candidates still remain, Gradle processes unordered attributes (those not in the precedence list):

- FOR each requested attribute without precedence order:

  - IF a disambiguation rule exists, apply it.

  - IF a requested attribute has the same value across all remaining candidates, ignore it.

- IF only one candidate remains, that candidate wins.

- IF no candidates remain, Gradle falls back to the most compatible candidates (step 2).

### Step 4: Disambiguation Using Extra Attributes

If multiple candidates still remain, Gradle considers extra attributes:

- These are attributes that exist only in some candidates.

- Candidates that lack extra attributes are preferred.

- The attribute precedence order is considered.

First, Gradle processes prioritized extra attributes:

- FOR each extra attribute in precedence order:

  - IF some candidates have the attribute and others don’t:

    - Candidates that lack the extra attribute are preferred.

    - Candidates with the extra attribute are eliminated.

- IF only one candidate remains, that candidate wins.

- IF no candidates remain, Gradle falls back to the original compatible candidates (step 1).

Then, Gradle processes unordered extra attributes:

- FOR each remaining extra attribute without defined precedence:

  - IF the attribute is present in only some candidates:

    - Candidates without the attribute are preferred.

    - Candidates with the extra attribute are eliminated.

- IF only one candidate remains, that candidate wins.

- IF no candidates remain, Gradle falls back to the original compatible candidates (step 1).

### Step 5: Disambiguation Using Extra Attribute Presence

If candidates still remain, Gradle eliminates candidates based on requested attributes:

- FOR each extra attribute:

  - IF all candidates provide the same value, ignore it.

  - ELSE, remove candidates that do provide it.

- IF only one candidate remains, that candidate wins.

### Step 6: Failure Condition

Unless otherwise indicated, if at any point no candidates remain, resolution fails.

Additionally, Gradle outputs a list of all compatible candidates from step 1 to help with debugging attribute matching failures.

Plugins and ecosystems can influence the selection algorithm by implementing compatibility rules, disambiguation rules, and defining the precedence of attributes. Attributes with a higher precedence are used to eliminate candidates 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`.

| There are two exceptions to the variant-aware resolution process: When a producer has no variants, a default artifact is selected. When a consumer explicitly selects a configuration by name, the artifacts associated with that configuration are used.

## A Simple Example

Let’s walk through an example where a consumer is trying to use a library for compilation.

First, the consumer details how it’s going to use the result of dependency resolution. This is achieved by setting attributes on the consumer’s resolvable configuration.

In this case, the consumer wants to resolve a variant that matches `org.gradle.usage=java-api`.

Next, the producer exposes different variants of its component:

- API variant (named `apiElements`) with the attribute `org.gradle.usage=java-api`

- Runtime variant (named `runtimeElements`) with the attribute `org.gradle.usage=java-runtime`

Finally, Gradle evaluates the variants and selects the correct one:

- The consumer requests a variant with attributes `org.gradle.usage=java-api`

- The producer’s `apiElements` variant matches this request.

- The producer’s `runtimeElements` variant does not match.

Consumer Requested Attributes | Producer Available Attributes | Producer Variant | Match?
--- | --- | --- | ---
`org.gradle.usage=java-api` | `org.gradle.usage=java-api` | `apiElements` | ✅ Yes
`org.gradle.usage=java-api` | `org.gradle.usage=java-runtime` | `runtimeElements` | ❌ No

As a result, Gradle selects the `apiElements` variant and provides its artifacts and dependencies to the consumer.

## A Complicated Example

In real-world scenarios, both consumers and producers often work with multiple attributes.

For instance, a Java Library project in Gradle will involve several attributes:

Attribute | Description
--- | ---
`org.gradle.usage` | Describes how the variant is used.
`org.gradle.dependency.bundling` | Describes how the variant handles dependencies (e.g., shadow jar, fat jar, regular jar).
`org.gradle.libraryelements` | Describes the packaging of the variant (e.g., classes or jar).
`org.gradle.jvm.version` | Describes the minimal version of Java the variant targets.
`org.gradle.jvm.environment` | Describes the type of JVM the variant targets.

Let’s consider a scenario where the consumer wants to run tests using a library on Java 8, and the producer supports two versions: Java 8 and Java 11.

Step 1: Consumer specifies the requirements.

The consumer wants to resolve a variant that:

- Can be used at runtime (`org.gradle.usage=java-runtime`).

- Can run on at least Java 8 (`org.gradle.jvm.version=8`).

Step 2: Producer exposes multiple variants.

The producer offers variants for both Java 8 and Java 11 for both API and runtime usage:

Variant Name | Attributes
--- | ---
`apiJava8Elements` | `org.gradle.usage=java-api`, `org.gradle.jvm.version=8`
`runtime8Elements` | `org.gradle.usage=java-runtime`, `org.gradle.jvm.version=8`
`apiJava11Elements` | `org.gradle.usage=java-api`, `org.gradle.jvm.version=11`
`runtime11Elements` | `org.gradle.usage=java-runtime`, `org.gradle.jvm.version=11`

Step 3: Gradle matches the attributes.

Gradle compares the consumer's requested attributes with the producer's variants:

- The consumer requests a variant with `org.gradle.usage=java-runtime` and `org.gradle.jvm.version=8`.

- Both `runtime8Elements` and `runtime11Elements` match the `org.gradle.usage=java-runtime` attribute.

- The API variants (`apiJava8Elements` and `apiJava11Elements`) are discarded as they don’t match `org.gradle.usage=java-runtime`.

- The variant `runtime8Elements` is selected because it is compatible with Java 8.

- The variant `runtime11Elements` is incompatible because it requires Java 11.

Consumer Requested Attributes | Producer Available Attributes | Producer Variant | Match?
--- | --- | --- | ---
`org.gradle.usage=java-runtime`, `org.gradle.jvm.version=8` | `org.gradle.usage=java-runtime`, `org.gradle.jvm.version=8` | `runtime8Elements` | ✅ Selected
`org.gradle.usage=java-runtime`, `org.gradle.jvm.version=8` | `org.gradle.usage=java-runtime`, `org.gradle.jvm.version=11` | `runtime11Elements` | ❌ Incompatible
`org.gradle.usage=java-runtime`, `org.gradle.jvm.version=8` | `org.gradle.usage=java-api`, `org.gradle.jvm.version=8` | `apiJava8Elements` | ❌ Discarded
`org.gradle.usage=java-runtime`, `org.gradle.jvm.version=8` | `org.gradle.usage=java-api`, `org.gradle.jvm.version=11` | `apiJava11Elements` | ❌ Discarded

Gradle selects `runtime8Elements` and provides its artifacts and dependencies to the consumer.

But what happens if the consumer sets `org.gradle.jvm.version=7`?

In this case, dependency resolution would fail, with an error explaining there is no suitable variant. Gradle knows the consumer requires a Java 7-compatible library, but the producer's minimum version is 8.

If the consumer requested `org.gradle.jvm.version=15`, Gradle could choose either the Java 8 or Java 11 variant. Gradle would then select the highest compatible version—Java 11.

## Visualizing Variant Information

Gradle offers built-in tasks to visualize the variant selection process and display the producer and consumer attributes involved.

### 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)

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:

- An optional parameter:

  - `--configuration <configurationName>` → Selects a single configuration to display.

- Flags for including or excluding legacy and deprecated configurations:

  - `--all` → Includes information about legacy and deprecated configurations.

  - `--no-all` → Excludes this information.

- Flags for controlling transitive extensions in the extended configurations section:

  - `--recursive` → Lists configurations that are extended transitively rather than directly.

  - `--no-recursive` → Excludes this information.

Here is the output of the `resolvableConfigurations` task on a freshly generated `java-library` project:

    > Task :resolvableConfigurations
    --------------------------------------------------
    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

    [...]

From this you can see the two main configurations used to resolve dependencies, `compileClasspath` and `runtimeClasspath`, as well as their corresponding test configurations (truncated).

Now that we understand variant selection and attribute matching, let’s move on to the artifact resolution phase of dependency resolution. This phase is also variant aware.

Next Step: Learn about Artifact Resolution >>