# Configuration cache

version 6.5

| The configuration cache is an incubating feature, and the details described here may change.

## Introduction

The configuration cache is a feature that significantly improves build performance by caching the result of the configuration phase and reusing this for subsequent builds. Using the configuration cache, Gradle can skip the configuration phase entirely when nothing that affects the build configuration, such as build scripts, has changed. Gradle also applies some performance improvements to task execution as well.

The configuration cache is conceptually similar to the build cache, but caches different information. The build cache takes care of caching the outputs and intermediate files of the build, such as task outputs or artifact transform outputs. The configuration cache takes care of caching the build configuration for a particular set of tasks. In other words, the configuration cache caches the output of the configuration phase, and the build cache caches the outputs of the execution phase.

This feature is currently experimental and not enabled by default.

### How does it work?

When the configuration cache is enabled and you run Gradle for a particular set of tasks, for example by running `gradlew check`, Gradle checks whether a configuration cache entry is available for the requested set of tasks. If available, Gradle uses this entry instead of running the configuration phase. The cache entry contains information about the set of tasks to run, along with their configuration and dependency information.

The first time you run a particular set of tasks, there will be no entry in the configuration cache for these tasks and so Gradle will run the configuration phase as normal:

1. Run init scripts.

2. Run the settings script for the build, applying any requested settings plugins.

3. Configure and build the `buildSrc` project, if present.

4. Run the builds scripts for the build, applying any requested project plugins.

5. Calculate the task graph for the requested tasks, running any deferred configuration actions.

Following the configuration phase, Gradle writes the state of the task graph to the configuration cache, taking a snapshot for later Gradle invocations. The execution phase then runs as normal. This means you will not see any build performance improvement the first time you run a particular set of tasks.

When you subsequently run Gradle with this same set of tasks, for example by running `gradlew check` again, Gradle will load the tasks and their configuration directly from the configuration cache and skip the configuration phase entirely. Before using a configuration cache entry, Gradle checks that none of the "build inputs", such as build scripts, for the entry have changed. If a build input has changed, Gradle will not use the entry and will run the configuration phase again as above, saving the result for later reuse.

Build inputs include:

- Init scripts, settings scripts, build scripts.

- System properties, Gradle properties and configuration files used during the configuration phase, accessed using value suppliers (TODO - add docs for this).

- build inputs and source files for `buildSrc` projects.

### Performance improvements

Apart from skipping the configuration phase, the configuration cache provides some additional performance improvements:

- All tasks run in parallel by default.

- Dependency resolution is cached.

## Using the configuration cache

### Enabling the configuration cache

By default, the configuration cache is not enabled. It can be enabled from the command line:

    $ gradle --configuration-cache=on

It can also be enabled persistently in a `gradle.properties` file:

    org.gradle.unsafe.configuration-cache=on

If it is enabled in a `gradle.properties` file, it can be disabled on the command line for one build invocation:

    $ gradle --configuration-cache=off

### Ignoring problems

By default, Gradle will fail the build if any configuration cache problems are encountered. Configuration cache problems can be turned into warnings on the command line:

    $ gradle --configuration-cache=warn

or in a `gradle.properties` file:

    org.gradle.unsafe.configuration-cache=warn

### Allowing a maximum number of problems

When configuration cache problems are turned into warnings, Gradle will fail the build if `512` problems are found by default.

This can be adjusted by specifying an allowed maximum number of problems on the command line:

    $ gradle -Dorg.gradle.unsafe.configuration-cache.max-problems=5

or in a `gradle.properties` file:

    org.gradle.unsafe.configuration-cache.max-problems=5

### Invalidating the cache

The configuration cache is automatically invalidated when inputs to the configuration phase change. But, you may have to manually invalidate the configuration cache when untracked inputs to the configuration phase change. This can happen if you ignored problems, or when remote dependencies changed for example. See the Constraints and Not yet implemented sections below for more information.

The configuration cache state is stored on disk in a directory named `.gradle/configuration-cache` in the root directory of the Gradle build in use. If you need to invalidate the cache, simply delete that directory:

    $ rm -rf .gradle/configuration-cache

## Constraints

In order to capture the state of the task graph to the configuration cache and reload it again in a later build, Gradle applies certain constraints to tasks and other build logic. Each of these constraints is treated as a configuration cache "problem" and fail the build if violations are present.

The following sections describe each of the constraints and how to change your build to fix the problems.

### Certain types must not be referenced by tasks

There are a number of types that task instances must not reference from their fields. Usually these types are used to carry some task input that should be explicitly declared instead.

To fix, you can often replace such a field with a `Property` or `Provider` typed field that exposes the exact information that the task will need at execution time.

### Using the `Project` object

A task must not use any `Project` objects at execution time. This includes calling `Task.getProject()` while the task is running.

Some cases can be fixed in the same way as the previous constraint.

Otherwise, to fix, use injected services instead of the methods of `Project`:

- `project.file(path)` → `ProjectLayout.projectDirectory.file(path)` or `ProjectLayout.buildDirectory.file(path)`

- `project.files(paths)` → `ObjectFactory.fileCollection().from(paths)`

- `project.fileTree(dir)` → `ObjectFactory.fileTree().from(dir)`

- `project.exec {}` → `ExecOperations.exec {}`

- `project.javaexec {}` → `ExecOperations.javaexec {}`

- `project.copy {}` → `FileSystemOperations.copy {}`

- `project.sync {}` → `FileSystemOperations.sync {}`

- `project.delete {}` → `FileSystemOperations.delete {}`

### Accessing a task instance from another instance

Tasks should not directly access the state of another instance. Instead, tasks should be connected using inputs and outputs.

To fix, connect tasks using input and output relationships. `Property` and `Provider` types can be useful for this.

### Using build listeners

Plugins and build scripts must not register any build listeners.

To fix, use a build service.

### Undeclared reading of system properties

Plugins and build scripts should not read system properties directly using the Java APIs at configuration time. Instead, these system properties must be declared as a potential build input by using the value supplier APIs.

This problem is caused by build logic similar to this:

build.gradle

    def enabled = System.getProperty("some-property")

build.gradle.kts

    val enabled = System.getProperty("some-property")

To fix this problem, read system properties using providers.systemProperty() instead:

build.gradle

    def enabled = providers.systemProperty("some-property").forUseAtConfigurationTime().present

build.gradle.kts

    val enabled = providers.systemProperty("some-property").forUseAtConfigurationTime().isPresent

In general, you should avoid reading the value of system properties at configuration time, to avoid invalidating configuration cache entries when the system property value changes. Instead, you can connect the `Provider` returned by providers.systemProperty() to task properties.

### Undeclared reading of environment variables

Plugins and build scripts should not read environment variables directly using the Java APIs at configuration time. Instead, declare environment variables as potential build inputs using the value supplier APIs.

This problem is caused by build logic similar to this:

build.gradle

    def enabled = System.getenv("SOME_ENV_VAR")

build.gradle.kts

    val enabled = System.getenv("SOME_ENV_VAR")

To fix this problem, read environment variables using providers.environmentVariable() instead:

build.gradle

    def enabled = providers.environmentVariable("SOME_ENV_VAR").forUseAtConfigurationTime().present

build.gradle.kts

    val enabled = providers.environmentVariable("SOME_ENV_VAR").forUseAtConfigurationTime().isPresent

In general, you should avoid reading the value of environment variables at configuration time, to avoid invalidating configuration cache entries when the environment variable value changes. Instead, you can connect the `Provider` returned by providers.environmentVariable() to task properties.

### Undeclared reading of files

Plugins and build scripts should not read files directly using the Java, Groovy or Kotlin APIs at configuration time. Instead, declare files as potential build inputs using the value supplier APIs.

This problem is caused by build logic similar to this:

build.gradle

    def config = file("some.conf").text

build.gradle.kts

    val config = file("some.conf").readText()

To fix this problem, read files using providers.fileContents() instead:

build.gradle

    def config = providers.fileContents("some.conf").forUseAtConfigurationTime().asText

build.gradle.kts

    val config = providers.fileContents("some.conf").forUseAtConfigurationTime().asText

In general, you should avoid reading files at configuration time, to avoid invalidating configuration cache entries when the file content changes. Instead, you can connect the `Provider` returned by providers.fileContents() to task properties.

## Not yet implemented

Support for using configuration caching with certain Gradle features is not yet implemented. Support for these features will be added in later Gradle releases.

### Build scan support

When it comes to build scans, only the `--scan` command line option is supported for now.

The build scan plugin is currently ignored when applied via an init script or build script, as is any configuration done via the build scan extension (server location, accept license, tags, etc). This means that scans will be published to the public `scans.gradle.com` instance by default.

### Composite builds

When using the configuration cache on a composite build a problem will be reported. If you ignore problems then the included builds will be skipped when reusing the configuration cache.

### Source dependencies

Support for source dependencies is not yet implemented. With the configuration cache enabled, no problem will be reported and the build will fail.

### Dependency locking

Locking dependency versions isn’t supported yet. With the configuration cache enabled, no problem will be reported and dependency locks will be ignored.

### Dynamic or changing dependencies

Dependencies with versions which change over time aren’t supported yet. With the configuration cache enabled, no problem will be reported and changes to dependencies won’t be detected.

### Filesystem repositories

Repositories on local file systems are not supported yet. With the configuration cache enabled, no problem will be reported and changes to the filesystem in the repositories won’t be detected.

This includes:

- Maven or Ivy repositories with `file://` URLs,

- `mavenLocal()`,

- and flat directory repositories.

## Testing Build Logic with TestKit

The Gradle TestKit (a.k.a. just TestKit) is a library that aids in testing Gradle plugins and build logic generally. For general guidance on how to use TestKit see the dedicated chapter.

To enable configuration caching in your tests, you can pass the `--configuration-cache=on` argument to GradleRunner or use one of the other methods described in Enabling the configuration cache.

You need to run your tasks twice. Once to prime the configuration cache. Once to reuse the configuration cache.

Example 1. Testing the configuration cache
src/test/groovy/org/example/BuildLogicFunctionalTest.groovy

        def "my task can be loaded from the configuration cache"() {
            given:
            buildFile << """
                plugins {
                    id 'org.example.my-plugin'
                }
            """

            when:
            runner()
                .withArguments('--configuration-cache=on', 'myTask')    (1)
                .build()

            and:
            def result = runner()
                .withArguments('--configuration-cache=on', 'myTask')    (2)
                .build()

            then:
            result.output.contains('Reusing configuration cache.')      (3)
            // ... more assertions on your task behavior
        }

src/test/kotlin/org/example/BuildLogicFunctionalTest.kt

        @Test
        fun `my task can be loaded from the configuration cache`() {

            buildFile.writeText("""
                plugins {
                    id 'org.example.my-plugin'
                }
            """)

            runner()
                .withArguments("--configuration-cache=on", "myTask")        (1)
                .build()

            val result = runner()
                .withArguments("--configuration-cache=on", "myTask")        (2)
                .build()

            require(result.output.contains("Reusing configuration cache.")) (3)
            // ... more assertions on your task behavior
        }

1 | First run primes the configuration cache.
2 | Second run reuses the configuration cache.
3 | Assert that the configuration cache gets reused.

If problems with the configuration cache are found then Gradle will fail the build reporting the problems, and the test will fail.

When gradually improving your plugin or build logic to support the configuration cache it can be useful to temporarily ignore problems or allow a given number of problems.

## Troubleshooting

Upon failure to serialize the state required to run the tasks, an HTML report of detected problems is generated. The Gradle failure output includes a clickable link to the report.

The report displays the set of problems twice. First grouped by error message, then grouped by task. The former allows to quickly see what classes of problems your build is facing. The latter allows to quickly see which tasks are problematic. In both cases you can expand the tree in order to discover where in the object graph is the culprit.

Problems displayed in the report have links to the corresponding constraint where you can find guidance on how to fix the problem or to the corresponding not yet implemented feature.