:gradle-issues: https://github.com/gradle/gradle/issues/

[[config_cache]]
= Configuration cache

// Run tests for snippets included in this chapter with:
// ./gradlew :docs:docsTest --tests="ExemplarExternalSamplesFunctionalTest.snippet-configuration-cache-*"
// ./gradlew :docs:docsTest --tests="ExemplarExternalSamplesFunctionalTest.snippet-value-providers-*"

[NOTE]
====
The configuration cache is an <<feature_lifecycle.adoc#feature_lifecycle,incubating>> feature, and the details described here may change.
====

[[config_cache:intro]]
== Introduction

The configuration cache is a feature that significantly improves build performance by caching the result of the <<build_lifecycle#build_lifecycle,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#build_cache,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 saves the output of the configuration phase, and the build cache saves the outputs of the execution phase.

[IMPORTANT]
====
This feature is currently *_incubating_* and not enabled by default. This feature has the following limitations:

- The configuration cache does not support all <<configuration_cache#config_cache:plugins:core, core Gradle plugins>> and <<configuration_cache#config_cache:not_yet_implemented, features>>. Full support is a work in progress.
- Your build and the plugins you depend on might require changes to fulfil the <<configuration_cache#config_cache:requirements, requirements>>.
- IDE imports and syncs do not yet use the configuration cache.
====

[[config_cache:intro:how_does_it_work]]
=== 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 a snapshot of the task graph to a new configuration cache entry, for later Gradle invocations.
Gradle then loads the task graph from the configuration cache, so that it can apply optimizations to the tasks, and then runs the execution phase as normal.
Generally, 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 configuration inputs", such as build scripts, for the entry have changed.
If a build configuration 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 configuration inputs include:

- Init scripts
- Settings scripts
- Build scripts
- System properties used during the configuration phase
- Gradle properties used during the configuration phase
- Environment variables used during the configuration phase
- Configuration files accessed using value suppliers such as providers
- `buildSrc` and plugin included build inputs, including build configuration inputs and source files.

Gradle uses its own optimized serialization mechanism and format to store the configuration cache entries.
It automatically serializes the state of arbitrary object graphs.
If your tasks hold references to objects with simple state or of supported types you don't have anything to do to support the serialization.

As a fallback and to provide some aid in migrating existing tasks, some semantics of <<configuration_cache#config_cache:not_yet_implemented:java_serialization, Java Serialization>> are supported. But it is not recommended relying on it, mostly for performance reasons.

[[config_cache:intro:performance_improvements]]
=== Performance improvements

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

- All tasks run in parallel by default, subject to dependency constraints.
- Dependency resolution is cached.
- Configuration state and dependency resolution state is discarded from heap after writing the task graph. This reduces the peak heap usage required for a given set of tasks.

[[config_cache:in_action]]
=== Configuration caching in action

image::configuration-cache/running-help.gif[]

[[config_cache:usage]]
== Using the configuration cache

It is recommended to get started with the simplest task invocation possible.
Running `help` with the configuration cache enabled is a good first step:

----
❯ gradle --configuration-cache help
include::{snippetsPath}/configurationCache/noProblem/tests/store.out[]
...
BUILD SUCCESSFUL in 4s
1 actionable task: 1 executed
Configuration cache entry stored.
----

Running this for the first time, the configuration phase executes, calculating the task graph.

Then, run the same command again.
This reuses the cached configuration:

----
❯ gradle --configuration-cache help
include::{snippetsPath}/configurationCache/noProblem/tests/load.out[]
...
BUILD SUCCESSFUL in 500ms
1 actionable task: 1 executed
Configuration cache entry reused.
----

If it succeeds on your build, congratulations, you can now try with more useful tasks.
You should target your development loop.
A good example is running tests after making incremental changes.

If any problem is found caching or reusing the configuration, an HTML report is generated to help you diagnose and fix the issues. The report also shows detected build configuration inputs like
system properties, environment variables and value suppliers read during the configuration phase.
See the <<configuration_cache#config_cache:troubleshooting>> section below for more information.

Keep reading to learn how to tweak the configuration cache, manually invalidate the state if something goes wrong and use the configuration cache from an IDE.

[[config_cache:usage:enable]]
=== Enabling the configuration cache

By default, Gradle does not use the configuration cache.
To enable the cache at build time, use the `configuration-cache` flag:

----
❯ gradle --configuration-cache
----

You can also enable the cache persistently in a `gradle.properties` file using the `org.gradle.unsafe.configuration-cache` property:

[source,properties]
----
org.gradle.unsafe.configuration-cache=true
----

If enabled in a `gradle.properties` file, you can override that setting and disable the cache at build time with the `no-configuration-cache` flag:

----
❯ gradle --no-configuration-cache
----

[[config_cache:usage:ignore_problems]]
=== Ignoring problems

By default, Gradle will fail the build if any configuration cache problems are encountered.
When gradually improving your plugin or build logic to support the configuration cache it can be useful to temporarily turn problems into warnings.

This can be done from the command line:

----
❯ gradle --configuration-cache-problems=warn
----

or in a `gradle.properties` file:

[source,properties]
----
org.gradle.unsafe.configuration-cache-problems=warn
----

[[config_cache:usage:max_problems]]
=== 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:

[source,properties]
----
org.gradle.unsafe.configuration-cache.max-problems=5
----

[[config_cache:usage:invalidate]]
=== Invalidating the cache

The configuration cache is automatically invalidated when inputs to the configuration phase change.
However, certain inputs are not tracked yet, so you may have to manually invalidate the configuration cache when untracked inputs to the configuration phase change.
This can happen if you <<configuration_cache#config_cache:usage:ignore_problems,ignored problems>>.
See the <<configuration_cache#config_cache:requirements>> and <<configuration_cache#config_cache: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
----

Configuration cache entries are checked periodically (at most every 24 hours) for whether they are still in use.
They are deleted if they haven't been used for 7 days.

[[config_cache:stable]]
== Stable configuration cache

Working towards the stabilization of configuration caching we implement some strictness behind a feature flag when it is too disruptive for early adopters.

You can enable that feature flag as follows:

====
include::sample[dir="snippets/configurationCache/stableFeatureFlag/kotlin",files="settings.gradle.kts[]"]
include::sample[dir="snippets/configurationCache/stableFeatureFlag/groovy",files="settings.gradle[]"]
====

The `STABLE_CONFIGURATION_CACHE` feature flag enables the following:

Undeclared shared build service usage::
When enabled, tasks using a <<build_services#build_services, shared build service>> without declaring the requirement via the `Task.usesService` method will emit a deprecation warning.

External processes used at configuration time::
When enabled, external processes launched at configuration time are reported as problems unless done in accordance with the <<configuration_cache#config_cache:requirements:external_processes>> requirement.

It is recommended to enable it as soon as possible in order to be ready for when we remove the flag and make the linked features the default.

[[config_cache:ide]]
== IDE support

If you enable and configure the configuration cache from your `gradle.properties` file, then the configuration cache will be enabled when your IDE delegates to Gradle.
There's nothing more to do.

`gradle.properties` is usually checked in to source control.
If you don't want to enable the configuration cache for your whole team yet you can also enable the configuration cache from your IDE only as explained below.

Note that syncing a build from an IDE doesn't benefit from the configuration cache, only running tasks does.

[[config_cache:ide:intellij]]
=== IntelliJ based IDEs

In IntelliJ IDEA or Android Studio this can be done in two ways, either globally or per run configuration.

To enable it for the whole build, go to `Run > Edit configurations...`.
This will open the IntelliJ IDEA or Android Studio dialog to configure Run/Debug configurations.
Select `Templates > Gradle` and add the necessary system properties to the `VM options` field.

For example to enable the configuration cache, turning problems into warnings, add the following:

----
-Dorg.gradle.unsafe.configuration-cache=true -Dorg.gradle.unsafe.configuration-cache-problems=warn
----

You can also choose to only enable it for a given run configuration.
In this case, leave the `Templates > Gradle` configuration untouched and edit each run configuration as you see fit.

Combining these two ways you can enable globally and disable for certain run configurations, or the opposite.

[TIP]
====
You can use the link:https://github.com/JetBrains/gradle-idea-ext-plugin[gradle-idea-ext-plugin] to configure IntelliJ run configurations from your build.
This is a good way to enable the configuration cache only for the IDE.
====

[[config_cache:ide:eclipse]]
=== Eclipse IDEs

In Eclipse IDEs you can enable and configure the configuration cache through Buildship in two ways, either globally or per run configuration.

To enable it globally, go to `Preferences > Gradle`.
You can use the properties described above as system properties.
For example to enable the configuration cache, turning problems into warnings, add the following JVM arguments:

* `-Dorg.gradle.unsafe.configuration-cache=true`
* `-Dorg.gradle.unsafe.configuration-cache-problems=warn`

To enable it for a given run configuration, go to `Run configurations...`, find the one you want to change, go to `Project Settings`, tick the `Override project settings` checkbox and add the same system properties as a `JVM argument`.

Combining these two ways you can enable globally and disable for certain run configurations, or the opposite.

[[config_cache:plugins]]
== Supported plugins

The configuration cache is brand new and introduces new requirements for plugin implementations.
As a result, both core Gradle plugins, and community plugins need to be adjusted.
This section provides information about the current support in <<configuration_cache#config_cache:plugins:core, core Gradle plugins>> and <<configuration_cache#config_cache:plugins:community, community plugins>>.

[[config_cache:plugins:core]]
=== Core Gradle plugins

Not all <<plugin_reference#plugin_reference, core Gradle plugins>> support configuration caching yet.

[cols=3*]
|===
h| JVM languages and frameworks
h| Native languages
h| Packaging and distribution

a|
[horizontal]
link:{gradle-issues}13457[[.green]#✓#]:: <<java_plugin.adoc#java_plugin,Java>>
link:{gradle-issues}13458[[.green]#✓#]:: <<java_library_plugin.adoc#java_library_plugin,Java Library>>
link:{gradle-issues}13459[[.green]#✓#]:: <<java_platform_plugin.adoc#java_platform_plugin,Java Platform>>
link:{gradle-issues}13460[[.green]#✓#]:: <<groovy_plugin.adoc#groovy_plugin,Groovy>>
link:{gradle-issues}13461[[.green]#✓#]:: <<scala_plugin.adoc#scala_plugin,Scala>>
link:{gradle-issues}13462[[.green]#✓#]:: <<antlr_plugin.adoc#antlr_plugin,ANTLR>>

a|
[horizontal]
link:{gradle-issues}13484[[.red]#✖#]:: <<cpp_application_plugin.adoc#cpp_application_plugin,C++ Application>>
link:{gradle-issues}13485[[.red]#✖#]:: <<cpp_library_plugin.adoc#cpp_library_plugin,C++ Library>>
link:{gradle-issues}13514[[.red]#✖#]:: <<cpp_unit_test_plugin.adoc#cpp_unit_test_plugin,C++ Unit Test>>
link:{gradle-issues}13515[[.red]#✖#]:: <<swift_application_plugin.adoc#swift_application_plugin,Swift Application>>
link:{gradle-issues}13487[[.red]#✖#]:: <<swift_library_plugin.adoc#swift_library_plugin,Swift Library>>
link:{gradle-issues}13488[[.red]#✖#]:: <<xctest_plugin.adoc#xctest_plugin,XCTest>>

a|
[horizontal]
link:{gradle-issues}13463[[.green]#✓#]:: <<application_plugin.adoc#application_plugin,Application>>
link:{gradle-issues}13466[[.green]#✓#]:: <<war_plugin.adoc#war_plugin,WAR>>
link:{gradle-issues}13467[[.red]#✖#]:: <<ear_plugin.adoc#ear_plugin,EAR>>
link:{gradle-issues}13468[[.green]#✓#]:: <<publishing_maven.adoc#publishing_maven,Maven Publish>>
link:{gradle-issues}13469[[.red]#✖#]:: <<publishing_ivy.adoc#publishing_ivy,Ivy Publish>>
link:{gradle-issues}13464[[.green]#✓#]:: <<distribution_plugin.adoc#distribution_plugin,Distribution>>
link:{gradle-issues}13465[[.green]#✓#]:: <<java_library_distribution_plugin.adoc#java_library_distribution_plugin,Java Library Distribution>>

h| Code analysis
h| IDE project files generation
h| Utility

a|
[horizontal]
link:{gradle-issues}13475[[.green]#✓#]:: <<checkstyle_plugin.adoc#checkstyle_plugin,Checkstyle>>
link:{gradle-issues}13478[[.green]#✓#]:: <<codenarc_plugin.adoc#codenarc_plugin,CodeNarc>>
link:{gradle-issues}13477[[.green]#✓#]:: <<jacoco_plugin.adoc#jacoco_plugin,JaCoCo>>
[.green]#✓#:: <<jacoco_report_aggregation_plugin#jacoco_report_aggregation_plugin,JaCoCo Report Aggregation>>
link:{gradle-issues}13476[[.green]#✓#]:: <<pmd_plugin.adoc#pmd_plugin,PMD>>
[.green]#✓#:: <<test_report_aggregation_plugin#test_report_aggregation_plugin,Test Report Aggregation>>

a|
[horizontal]
link:{gradle-issues}13479[[.red]#✖#]:: <<eclipse_plugin.adoc#eclipse_plugin,Eclipse>>
link:{gradle-issues}13480[[.red]#✖#]:: <<idea_plugin.adoc#idea_plugin,IntelliJ IDEA>>
link:{gradle-issues}13482[[.red]#✖#]:: <<visual_studio_plugin.adoc#visual_studio_plugin,Visual Studio>>
link:{gradle-issues}13483[[.red]#✖#]:: <<xcode_plugin.adoc#xcode_plugin,Xcode>>

a|
[horizontal]
link:{gradle-issues}13455[[.green]#✓#]:: <<base_plugin.adoc#base_plugin,Base>>
link:{gradle-issues}13456[[.green]#✓#]:: <<build_init_plugin.adoc#build_init_plugin,Build Init>>
link:{gradle-issues}13470[[.red]#✖#]:: <<signing_plugin.adoc#signing_plugin,Signing>>
link:{gradle-issues}13471[[.green]#✓#]:: <<java_gradle_plugin.adoc#java_gradle_plugin,Java Plugin Development>>
link:{gradle-issues}23029[[.green]#✓#]:: <<custom_plugins.adoc#sec:precompiled_plugins,Groovy DSL Plugin Development>>
link:{gradle-issues}13472[[.green]#✓#]:: <<kotlin_dsl.adoc#sec:kotlin-dsl_plugin,Kotlin DSL Plugin Development>>
link:{gradle-issues}13473[[.red]#✖#]:: <<project_report_plugin.adoc#project_report_plugin,Project Report Plugin>>

|===

[horizontal]
[.green]#✓#:: Supported plugin
[.yellow]#⚠#:: Partially supported plugin
[.red]#✖#:: Unsupported plugin

[[config_cache:plugins:community]]
=== Community plugins

Please refer to issue link:{gradle-issues}13490[gradle/gradle#13490] to learn about the status of community plugins.

[[config_cache:troubleshooting]]
== Troubleshooting

The following sections will go through some general guidelines on dealing with problems with the configuration cache.
This applies to both your build logic and to your Gradle plugins.

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.
This report is useful and allows you to drill down into problems, understand what is causing them.

Let's look at a simple example build script that contains a couple problems:

====
include::sample[dir="snippets/configurationCache/problemsKotlin/kotlin",files="build.gradle.kts[]"]
include::sample[dir="snippets/configurationCache/problemsGroovy/groovy",files="build.gradle[]"]
====
<1> <<config_cache:requirements:reading_sys_props_and_env_vars, A system property read at configuration time>>
<2> <<config_cache:requirements:use_project_during_execution, Using the `Project` object at execution time>>

Running that task fails and print the following in the console:

----
❯ gradle --configuration-cache someTask -DsomeDestination=dest
...
include::{snippetsPath}/configurationCache/problemsGroovy/tests/fail.out[]
Configuration cache entry discarded with 1 problem.
----

The configuration cache entry was discarded because of the found problem failing the build.

Details can be found in the linked HTML report:

image::configuration-cache/problems-report.png[]

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

The report also includes a list of detected build configuration inputs, such as environment variables, system properties and value suppliers that were read at configuration phase:

image::configuration-cache/inputs-report.png[]

[TIP]
====
Problems displayed in the report have links to the corresponding <<configuration_cache#config_cache:requirements,requirement>> where you can find guidance on how to fix the problem or to the corresponding <<configuration_cache#config_cache:not_yet_implemented,not yet implemented>> feature.

When changing your build or plugin to fix the problems you should consider <<configuration_cache#config_cache:testing, testing your build logic with TestKit>>.
====

At this stage, you can decide to either <<configuration_cache#config_cache:usage:ignore_problems, turn the problems into warnings>> and continue exploring how your build reacts to the configuration cache, or fix the problems at hand.

Let's ignore the reported problem, and run the same build again twice to see what happens when reusing the cached problematic configuration:

----
❯ gradle --configuration-cache --configuration-cache-problems=warn someTask -DsomeDestination=dest
include::{snippetsPath}/configurationCache/problemsGroovy/tests/store.out[]
Configuration cache entry stored with 1 problem.
❯ gradle --configuration-cache --configuration-cache-problems=warn someTask -DsomeDestination=dest
include::{snippetsPath}/configurationCache/problemsGroovy/tests/load.out[]
Configuration cache entry reused with 1 problem.
----

The two builds succeed reporting the observed problem, storing then reusing the configuration cache.

With the help of the links present in the console problem summary and in the HTML report we can fix our problems.
Here's a fixed version of the build script:

====
include::sample[dir="snippets/configurationCache/problemsFixed/kotlin",files="build.gradle.kts[tags=fixed]"]
include::sample[dir="snippets/configurationCache/problemsFixed/groovy",files="build.gradle[tags=fixed]"]
====
<1> We turned our ad-hoc task into a proper task class,
<2> with inputs and outputs declaration,
<3> and injected with the `FileSystemOperations` service, a supported replacement for <<configuration_cache#config_cache:requirements:use_project_during_execution, `project.copy {}`>>.

Running the task twice now succeeds without reporting any problem and reuses the configuration cache on the second run:

----
❯ gradle --configuration-cache someTask -DsomeDestination=dest
include::{snippetsPath}/configurationCache/problemsFixed/tests/store.out[]
Configuration cache entry stored.
❯ gradle --configuration-cache someTask -DsomeDestination=dest
include::{snippetsPath}/configurationCache/problemsFixed/tests/load.out[]
Configuration cache entry reused.
----

But, what if we change the value of the system property?

----
❯ gradle --configuration-cache someTask -DsomeDestination=another
include::{snippetsPath}/configurationCache/problemsFixed/tests/store-another.out[]
Configuration cache entry stored.
----

The previous configuration cache entry could not be reused, and the task graph had to be calculated and stored again.
This is because we read the system property at configuration time, hence requiring Gradle to run the configuration phase again when the value of that property changes.
Fixing that is as simple as obtaining the provider of the system property and wiring it to the task input, without reading it at configuration time.

====
include::sample[dir="snippets/configurationCache/problemsFixedReuse/kotlin",files="build.gradle.kts[tags=fixed-reuse]"]
include::sample[dir="snippets/configurationCache/problemsFixedReuse/groovy",files="build.gradle[tags=fixed-reuse]"]
====
<1> We wired the system property provider directly, without reading it at configuration time.

With this simple change in place we can run the task any number of times, change the system property value, and reuse the configuration cache:

----
❯ gradle --configuration-cache someTask -DsomeDestination=dest
include::{snippetsPath}/configurationCache/problemsFixedReuse/tests/store.out[]
Configuration cache entry stored.
❯ gradle --configuration-cache someTask -DsomeDestination=another
include::{snippetsPath}/configurationCache/problemsFixedReuse/tests/load-another.out[]
Configuration cache entry reused.
----

We're now done with fixing the problems with this simple task.

Keep reading to learn how to adopt the configuration cache for your build or your plugins.

[[config_cache:task_opt_out]]
=== Declare a task incompatible with the configuration cache

It is possible to declare that a particular task is not compatible with the configuration cache
via the link:{javadocPath}/org/gradle/api/Task.html#notCompatibleWithConfigurationCache-java.lang.String-[Task.notCompatibleWithConfigurationCache()] method.

Configuration cache problems found in tasks marked incompatible will no longer cause the build to fail.

And, when an incompatible task is scheduled to run, Gradle discards the configuration state at the end of the build. You can use this to help with migration, by temporarily opting out certain tasks that are difficult to change to work with the configuration cache.

Check the method documentation for more details.

[[config_cache:adoption]]
== Adoption steps

An important prerequisite is to keep your Gradle and plugins versions up to date.
The following explores the recommended steps for a successful adoption.
It applies both to builds and plugins.
While going through these steps, keep in mind the HTML report and the solutions explained in the <<configuration_cache#config_cache:requirements, requirements>> chapter below.

Start with `:help`::
Always start by trying your build or plugin with the simplest task `:help`.
This will exercise the minimal configuration phase of your build or plugin.

Progressively target useful tasks::
Don't go with running `build` right away.
You can also use `--dry-run` to discover more configuration time problems first.
+
When working on a build, progressively target your development feedback loop.
For example, running tests after making some changes to the source code.
+
When working on a plugin, progressively target the contributed or configured tasks.

Explore by turning problems into warnings::
Don't stop at the first build failure and <<configuration_cache#config_cache:usage:ignore_problems, turn problems into warnings>> to discover how your build and plugins behave.
If a build fails, use the HTML report to reason about the reported problems related to the failure.
Continue running more useful tasks.
+
This will give you a good overview of the nature of the problems your build and plugins are facing.
Remember that when turning problems into warnings you might need to <<configuration_cache#config_cache:usage:invalidate, manually invalidate the cache>> in case of troubles.

Step back and fix problems iteratively::
When you feel you know enough about what needs to be fixed, take a step back and start iteratively fixing the most important problems.
Use the HTML report and this documentation to help you in this journey.
+
Start with problems reported when _storing_ the configuration cache.
Once fixed, you can rely on a valid cached configuration phase and move on to fixing problems reported when _loading_ the configuration cache if any.

Report encountered issues::
If you face a problem with a <<configuration_cache#config_cache:not_yet_implemented, Gradle feature>> or with a <<configuration_cache#config_cache:plugins:core, Gradle core plugin>> that is not covered by this documentation, please report an issue on link:https://github.com/gradle/gradle/issues/new/choose[`gradle/gradle`].
+
If you face a problem with a community Gradle plugin, see if it is already listed at link:{gradle-issues}13490[gradle/gradle#13490] and consider reporting the issue to the plugin's issue tracker.
+
A good way to report such issues is by providing information such as:
+
* a link to this very documentation,
* the plugin version you tried,
* the custom configuration of the plugin if any, or ideally a reproducer build,
* a description of what fails, for example problems with a given task
* a copy of the build failure,
* the self-contained `configuration-cache-report.html` file.

Test, test, test::
Consider adding tests for your build logic.
See the below section on <<configuration_cache#config_cache:testing, testing your build logic>> for the configuration cache.
This will help you while iterating on the required changes and prevent future regressions.

Roll it out to your team::
Once you have your developer workflow working, for example running tests from the IDE, you can consider enabling it for your team.
A faster turnaround when changing code and running tests could be worth it.
You'll probably want to do this as an opt-in first.
+
If needed, turn problems into warnings and set the maximum number of allowed problems in your build `gradle.properties` file.
Keep the configuration cache disabled by default.
Let your team know they can opt-in by, for example, enabling the configuration cache on their IDE run configurations for the supported workflow.
+
Later on, when more workflows are working, you can flip this around.
Enable the configuration cache by default, configure CI to disable it, and if required communicate the unsupported workflow(s) for which the configuration cache needs to be disabled.

[[config_cache:testing]]
== Testing your build logic

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 <<test_kit.adoc#test_kit,dedicated chapter>>.

To enable configuration caching in your tests, you can pass the `--configuration-cache` argument to link:{javadocPath}/org/gradle/testkit/runner/GradleRunner.html[GradleRunner] or use one of the other methods described in <<configuration_cache.adoc#config_cache:usage:enable,Enabling the configuration cache>>.

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

.Testing the configuration cache
====
include::sample[dir="snippets/configurationCache/testKit/kotlin",files="src/test/kotlin/org/example/BuildLogicFunctionalTest.kt[tags=functional-test-configuration-cache]"]
include::sample[dir="snippets/configurationCache/testKit/groovy",files="src/test/groovy/org/example/BuildLogicFunctionalTest.groovy[tags=functional-test-configuration-cache]"]
====
<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.

[TIP]
====
A good testing strategy for a Gradle plugin is to run its whole test suite with the configuration cache enabled.
This requires testing the plugin with a supported Gradle version.

If the plugin already supports a range of Gradle versions it might already have tests for multiple Gradle versions.
In that case we recommend enabling the configuration cache starting with the Gradle version that supports it.

If this can’t be done right away, using tests that run all tasks contributed by the plugin several times, for e.g. asserting the `UP_TO_DATE` and `FROM_CACHE` behavior, is also a good strategy.
====

[[config_cache:requirements]]
== Requirements

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 requirements to tasks and other build logic.
Each of these requirements is treated as a configuration cache "problem" and fails the build if violations are present.

For the most part these requirements are actually surfacing some undeclared inputs.
In other words, using the configuration cache is an opt-in to more strictness, correctness and reliability for all builds.

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

[[config_cache:requirements:disallowed_types]]
=== Certain types must not be referenced by tasks

There are a number of types that task instances must not reference from their fields.
The same applies to task actions as closures such as `doFirst {}` or `doLast {}`.

These types fall into some categories as follows:

* Live JVM state types
* Gradle model types
* Dependency management types

In all cases the reason these types are disallowed is that their state cannot easily be stored or recreated by the configuration cache.

Live JVM state types (e.g. `ClassLoader`, `Thread`, `OutputStream`, `Socket` etc...) are simply disallowed.
These types almost never represent a task input or output.

Gradle model types (e.g. `Gradle`, `Settings`, `Project`, `SourceSet`, `Configuration` etc...) are usually used to carry some task input that should be explicitly and precisely declared instead.

For example, if you reference a `Project` in order to get the `project.version` at execution time, you should instead directly declare the _project version_ as an input to your task using a `Property<String>`.
Another example would be to reference a `SourceSet` to later get the source files, the compilation classpath or the outputs of the source set.
You should instead declare these as a `FileCollection` input and reference just that.

The same requirement applies to dependency management types with some nuances.

Some types, such as `Configuration` or `SourceDirectorySet`, don't make good task input parameters, as they hold a lot of irrelevant state, and it is better to model these inputs as something more precise.
We don't intend to make these types serializable at all.
For example, if you reference a `Configuration` to later get the resolved files, you should instead declare a `FileCollection` as an input to your task.
In the same vein, if you reference a `SourceDirectorySet` you should instead declare a `FileTree` as an input to your task.

Referencing dependency resolution results is also disallowed (e.g. `ArtifactResolutionQuery`, `ResolvedArtifact`, `ArtifactResult` etc...).
For example, if you reference some `ResolvedComponentResult` instances, you should instead declare a `Provider<ResolvedComponentResult>` as an input to your task.
Such a provider can be obtained by invoking `ResolutionResult.getRootComponent()`.
In the same vein, if you reference some `ResolvedArtifactResult` instances, you should instead use `ArtifactCollection.getResolvedArtifacts()` that returns a `Provider<Set<ResolvedArtifactResult>>` that can be mapped as an input to your task.
The rule of thumb is that tasks must not reference _resolved_ results, but lazy specifications instead, in order to do the dependency resolution at execution time.

Some types, such as `Publication` or `Dependency` are not serializable, but could be.
We may, if necessary, allow these to be used as task inputs directly.

Here's an example of a problematic task type referencing a `SourceSet`:

====
include::sample[dir="snippets/configurationCache/disallowedTypes/kotlin",files="build.gradle.kts[tags=task-type]"]
include::sample[dir="snippets/configurationCache/disallowedTypes/groovy",files="build.gradle[tags=task-type]"]
====
<1> this will be reported as a problem because referencing `SourceSet` is not allowed

The following is how it should be done instead:

====
include::sample[dir="snippets/configurationCache/disallowedTypesFixed/kotlin",files="build.gradle.kts[tags=task-type]"]
include::sample[dir="snippets/configurationCache/disallowedTypesFixed/groovy",files="build.gradle[tags=task-type]"]
====
<1> no more problems reported, we now reference the supported type `FileCollection`

In the same vein, if you encounter the same problem with an ad-hoc task declared in a script as follows:

====
include::sample[dir="snippets/configurationCache/disallowedTypes/kotlin",files="build.gradle.kts[tags=ad-hoc-task]"]
include::sample[dir="snippets/configurationCache/disallowedTypes/groovy",files="build.gradle[tags=ad-hoc-task]"]
====
<1> this will be reported as a problem because the `doLast {}` closure is capturing a reference to the `SourceSet`

You still need to fulfil the same requirement, that is not referencing a disallowed type.
Here's how the task declaration above can be fixed:

====
include::sample[dir="snippets/configurationCache/disallowedTypesFixed/kotlin",files="build.gradle.kts[tags=ad-hoc-task]"]
include::sample[dir="snippets/configurationCache/disallowedTypesFixed/groovy",files="build.gradle[tags=ad-hoc-task]"]
====
<1> no more problems reported, the `doLast {}` closure now only captures `classpath` which is of the supported `FileCollection` type

Note that sometimes the disallowed type is indirectly referenced.
For example, you could have a task reference some type from a plugin that is allowed.
That type could reference another allowed type that in turn references a disallowed type.
The hierarchical view of the object graph provided in the HTML reports for problems should help you pinpoint the offender.

[[config_cache:requirements:use_project_during_execution]]
=== 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 for <<configuration_cache#config_cache:requirements:disallowed_types, disallowed types>>.

Often, similar things are available on both `Project` and `Task`.
For example if you need a `Logger` in your task actions you should use `Task.logger` instead of `Project.logger`.

Otherwise, you can use <<custom_gradle_types#service_injection,injected services>> instead of the methods of `Project`.

Here's an example of a problematic task type using the `Project` object at execution time:

====
include::sample[dir="snippets/configurationCache/projectAtExecution/kotlin",files="build.gradle.kts[tags=task-type]"]
include::sample[dir="snippets/configurationCache/projectAtExecution/groovy",files="build.gradle[tags=task-type]"]
====
<1> this will be reported as a problem because the task action is using the `Project` object at execution time

The following is how it should be done instead:

====
include::sample[dir="snippets/configurationCache/projectAtExecutionFixed/kotlin",files="build.gradle.kts[tags=task-type]"]
include::sample[dir="snippets/configurationCache/projectAtExecutionFixed/groovy",files="build.gradle[tags=task-type]"]
====
<1> no more problem reported, the injected `FileSystemOperations` service is supported as a replacement for `project.copy {}`

In the same vein, if you encounter the same problem with an ad-hoc task declared in a script as follows:

====
include::sample[dir="snippets/configurationCache/projectAtExecution/kotlin",files="build.gradle.kts[tags=ad-hoc-task]"]
include::sample[dir="snippets/configurationCache/projectAtExecution/groovy",files="build.gradle[tags=ad-hoc-task]"]
====
<1> this will be reported as a problem because the task action is using the `Project` object at execution time

Here's how the task declaration above can be fixed:

====
include::sample[dir="snippets/configurationCache/projectAtExecutionFixed/kotlin",files="build.gradle.kts[tags=ad-hoc-task]"]
include::sample[dir="snippets/configurationCache/projectAtExecutionFixed/groovy",files="build.gradle[tags=ad-hoc-task]"]
====
<1> services can't be injected directly in scripts, we need an extra type to convey the injection point
<2> create an instance of the extra type using `project.object` outside the task action
<3> no more problem reported, the task action references `injected` that provides the `FileSystemOperations` service, supported as a replacement for `project.copy {}`

As you can see above, fixing ad-hoc tasks declared in scripts requires quite a bit of ceremony.
It is a good time to think about extracting your task declaration as a proper task class as shown previously.

The following table shows what APIs or injected service should be used as a replacement for each of the `Project` methods.

[cols="a,a",options="header"]
|===
| Instead of:
| Use:

| `project.rootDir`
| A task input or output property or a script variable to capture the result of using `project.rootDir` to calculate the actual parameter.

| `project.projectDir`
| A task input or output property or a script variable to capture the result of using `project.projectDir` to calculate the actual parameter.

| `project.buildDir`
| A task input or output property or a script variable to capture the result of using `project.buildDir` to calculate the actual parameter.

| `project.name`
| A task input or output property or a script variable to capture the result of using `project.name` to calculate the actual parameter.

| `project.description`
| A task input or output property or a script variable to capture the result of using `project.description` to calculate the actual parameter.

| `project.group`
| A task input or output property or a script variable to capture the result of using `project.group` to calculate the actual parameter.

| `project.version`
| A task input or output property or a script variable to capture the result of using `project.version` to calculate the actual parameter.

| `project.properties`, `project.property(name)`, `project.hasProperty(name)`, `project.getProperty(name)` or `project.findProperty(name)`
| link:{javadocPath}/org/gradle/api/provider/ProviderFactory.html#gradleProperty-java.lang.String-[Value providers for Gradle properties].

| `project.logger`
| link:{javadocPath}/org/gradle/api/Task.html#getLogger--[Task.logger]

| `project.provider {}`
| link:{javadocPath}/org/gradle/api/provider/ProviderFactory.html#provider-java.util.concurrent.Callable-[ProviderFactory.provider {}]

| `project.file(path)`
| A task input or output property or a script variable to capture the result of using `project.file(file)` to calculate the actual parameter.

| `project.uri(path)`
| A task input or output property or a script variable to capture the result of using `project.uri(path)` to calculate the actual parameter. Otherwise, `File.toURI()` or some other JVM API can be used.

| `project.relativePath(path)`
| link:{javadocPath}/org/gradle/api/file/ProjectLayout.html#getProjectDirectory--[ProjectLayout.projectDirectory].link:{javadocPath}/org/gradle/api/file/Directory.html#file-java.lang.String-[file(path)]

| `project.files(paths)`
| link:{javadocPath}/org/gradle/api/model/ObjectFactory.html#fileCollection--[ObjectFactory.fileCollection()].link:{javadocPath}/org/gradle/api/file/ConfigurableFileCollection.html#from-java.lang.Object...-[from(paths)]

| `project.fileTree(paths)`
| link:{javadocPath}/org/gradle/api/model/ObjectFactory.html#fileTree--[ObjectFactory.fileTree()].link:{javadocPath}/org/gradle/api/file/ConfigurableFileTree.html#from-java.lang.Object-[from(dir)]

| `project.zipTree(path)`
| link:{javadocPath}/org/gradle/api/file/ArchiveOperations.html#zipTree-java.lang.Object-[ArchiveOperations.zipTree(path)]

| `project.tarTree(path)`
| link:{javadocPath}/org/gradle/api/file/ArchiveOperations.html#tarTree-java.lang.Object-[ArchiveOperations.tarTree(path)]

| `project.resources`
| A task input or output property or a script variable to capture the result of using `project.resource` to calculate the actual parameter.

| `project.copySpec {}`
| A task input or output property or a script variable to capture the result of using `project.copySpec {}` to calculate the actual parameter.

| `project.copy {}`
| link:{javadocPath}/org/gradle/api/file/FileSystemOperations.html#copy-org.gradle.api.Action-[FileSystemOperations.copy {}]

| `project.sync {}`
| link:{javadocPath}/org/gradle/api/file/FileSystemOperations.html#sync-org.gradle.api.Action-[FileSystemOperations.sync {}]

| `project.delete {}`
| link:{javadocPath}/org/gradle/api/file/FileSystemOperations.html#delete-org.gradle.api.Action-[FileSystemOperations.delete {}]

| `project.mkdir(path)`
| The Kotlin, Groovy or Java API available to your build logic.

| `project.exec {}`
| link:{javadocPath}/org/gradle/process/ExecOperations.html#exec-org.gradle.api.Action-[ExecOperations.exec {}]

| `project.javaexec {}`
| link:{javadocPath}/org/gradle/process/ExecOperations.html#javaexec-org.gradle.api.Action-[ExecOperations.javaexec {}]

| `project.ant {}`
| link:{javadocPath}/org/gradle/api/Task.html#getAnt--[Task.ant]

| `project.createAntBuilder()`
| link:{javadocPath}/org/gradle/api/Task.html#getAnt--[Task.ant]

|===

[[config_cache:requirements:task_access]]
=== Accessing a task instance from another instance

Tasks should not directly access the state of another task instance.
Instead, tasks should be connected using <<lazy_configuration#working_with_task_dependencies_in_lazy_properties, inputs and outputs relationships>>.

Note that this requirement makes it unsupported to write tasks that configure other tasks at execution time.

[[config_cache:requirements:build_listeners]]
=== Using build listeners

Plugins and build scripts must not register any build listeners.
That is listeners registered at configuration time that get notified at execution time.
For example a `BuildListener` or a `TaskExecutionListener`.

These should be replaced by <<build_services#build_services,build services>>, registered to receive information about <<build_services#operation_listener, task execution>> if needed.

[[config_cache:requirements:external_processes]]
=== Running external processes

Plugin and build scripts should avoid running external processes at configuration time.
In general, it is preferred to run external processes in tasks with properly declared inputs and outputs to avoid unnecessary work when the task is up-to-date.
If necessary, only configuration-cache-compatible APIs should be used instead of Java and Groovy standard APIs or existing `ExecOperations`,
`Project.exec`, `Project.javaexec`, and their likes in settings and init scripts.
For simpler cases, when grabbing the output of the process is enough,
link:{javadocPath}/org/gradle/api/provider/ProviderFactory.html#exec-org.gradle.api.Action-[providers.exec()] and
link:{javadocPath}/org/gradle/api/provider/ProviderFactory.html#javaexec-org.gradle.api.Action-[providers.javaexec()] can be used:

====
include::sample[dir="snippets/valueProviders/externalProcessProvider/kotlin",files="build.gradle.kts[]"]
include::sample[dir="snippets/valueProviders/externalProcessProvider/groovy",files="build.gradle[]"]
====

For more complex cases a custom link:{javadocPath}/org/gradle/api/provider/ValueSource.html[ValueSource] implementation with injected `ExecOperations` can be used.
This `ExecOperations` instance can be used at configuration time without restrictions.

====
include::sample[dir="snippets/valueProviders/externalProcessValueSource/kotlin",files="build.gradle.kts[tags=value-source]"]
include::sample[dir="snippets/valueProviders/externalProcessValueSource/groovy",files="build.gradle[tags=value-source]"]
====

The `ValueSource` implementation can then be used to create a provider with link:{javadocPath}/org/gradle/api/provider/ProviderFactory.html#of-java.lang.Class-org.gradle.api.Action-[providers.of]:

====
include::sample[dir="snippets/valueProviders/externalProcessValueSource/kotlin",files="build.gradle.kts[tags=create-provider]"]
include::sample[dir="snippets/valueProviders/externalProcessValueSource/groovy",files="build.gradle[tags=create-provider]"]
====

In both approaches, if the value of the provider is used at configuration time then it will become a build configuration input.
The external process will be executed for every build to determine if the configuration cache is up-to-date, so it is recommended to only call fast-running processes at configuration time.
If the value changes then the cache is invalidated and the process will be run again during this build as part of the configuration phase.

[[config_cache:requirements:reading_sys_props_and_env_vars]]
=== Reading system properties and environment variables

Plugins and build scripts may read system properties and environment variables directly at configuration time with standard Java, Groovy, or Kotlin APIs or with the value supplier APIs.
Doing so makes such variable or property a build configuration input, so changing the value invalidates the configuration cache.
The configuration cache report includes a list of these build configuration inputs to help track them.

In general, you should avoid reading the value of system properties and environment variables at configuration time, to avoid cache misses when value changes.
Instead, you can connect the `Provider` returned by link:{javadocPath}/org/gradle/api/provider/ProviderFactory.html#systemProperty-java.lang.String-[providers.systemProperty()] or
link:{javadocPath}/org/gradle/api/provider/ProviderFactory.html#environmentVariable-java.lang.String-[providers.environmentVariable()] to task properties.

Some access patterns that potentially enumerate all environment variables or system properties (for example, calling `System.getenv().forEach()` or using the iterator of its `keySet()`) are
discouraged.
In this case, Gradle cannot find out what properties are actual build configuration inputs, so every available property becomes one.
Even adding a new property will invalidate the cache if this pattern is used.

Using a custom predicate to filter environment variables is an example of this discouraged pattern:

====
include::sample[dir="snippets/valueProviders/envVarsSysPropsDont/kotlin",files="build.gradle.kts[]"]
include::sample[dir="snippets/valueProviders/envVarsSysPropsDont/groovy",files="build.gradle[]"]
====

The logic in the predicate is opaque to the configuration cache, so all environment variables are considered inputs.
One way to reduce the number of inputs is to always use methods that query a concrete variable name, such as `getenv(String)`, or `getenv().get()`:

====
include::sample[dir="snippets/valueProviders/envVarsSysPropsDo/kotlin",files="build.gradle.kts[]"]
include::sample[dir="snippets/valueProviders/envVarsSysPropsDo/groovy",files="build.gradle[]"]
====

The fixed code above, however, is not exactly equivalent to the original as only an explicit list of variables is supported.
Prefix-based filtering is a common scenario, so there are provider-based APIs to access
link:{javadocPath}/org/gradle/api/provider/ProviderFactory.html#systemPropertiesPrefixedBy-java.lang.String-[system properties] and
link:{javadocPath}/org/gradle/api/provider/ProviderFactory.html#environmentVariablesPrefixedBy-java.lang.String-[environment variables]:

====
include::sample[dir="snippets/valueProviders/envVarsSysPropsDoPrefix/kotlin",files="build.gradle.kts[]"]
include::sample[dir="snippets/valueProviders/envVarsSysPropsDoPrefix/groovy",files="build.gradle[]"]
====
Note that the configuration cache would be invalidated not only when the value of the variable changes or the variable is removed but also
when another variable with the matching prefix is added to the environment.

For more complex use cases a custom link:{javadocPath}/org/gradle/api/provider/ValueSource.html[ValueSource] implementation can be used.
System properties and environment variables referenced in the code of the `ValueSource` do not become build configuration inputs, so any processing can be applied.
Instead, the value of the `ValueSource` is recomputed each time the build runs and only if the value changes the configuration cache is invalidated.
For example, a `ValueSource` can be used to get all environment variables with names containing the substring `JDK`:

====
include::sample[dir="snippets/valueProviders/envVarsSysPropsDoValueSource/kotlin",files="build.gradle.kts[tags=value-source,create-provider]"]
include::sample[dir="snippets/valueProviders/envVarsSysPropsDoValueSource/groovy",files="build.gradle[tags=value-source,create-provider]"]
====

[[config_cache:requirements:undeclared_file_read]]
=== 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 configuration inputs using the value supplier APIs.

This problem is caused by build logic similar to this:

====
include::sample[dir="snippets/valueProviders/fileContentsDont/kotlin",files="build.gradle.kts[]"]
include::sample[dir="snippets/valueProviders/fileContentsDont/groovy",files="build.gradle[]"]
====

To fix this problem, read files using link:{javadocPath}/org/gradle/api/provider/ProviderFactory.html#fileContents-org.gradle.api.file.RegularFile-[providers.fileContents()] instead:

====
include::sample[dir="snippets/valueProviders/fileContentsDo/kotlin",files="build.gradle.kts[]"]
include::sample[dir="snippets/valueProviders/fileContentsDo/groovy",files="build.gradle[]"]
====

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 link:{javadocPath}/org/gradle/api/provider/ProviderFactory.html#fileContents-org.gradle.api.file.RegularFile-[providers.fileContents()] to task properties.

[[config_cache:not_yet_implemented]]
== 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.

[[config_cache:not_yet_implemented:secrets]]
=== Handling of credentials and secrets

The configuration cache has currently no option to hide secrets that are used as inputs.
It means that they end up in the serialized configuration cache entry.

This means that you should:

* Either secure access to configuration cache entries that may contain secrets
* Or leverage `<<directory_layout.adoc#dir:gradle_user_home,GRADLE_USER_HOME>>/gradle.properties` for storing secrets.
The content of that file is not part of the configuration cache, only its fingerprint.
If you store secrets in that file, care must be taken to protect access to the file content.

See link:{gradle-issues}22618[gradle/gradle#22618].

[[config_cache:not_yet_implemented:sharing]]
=== Sharing the configuration cache

The configuration cache is currently stored locally only.
It can be reused by hot or cold local Gradle daemons.
But it can't be shared between developers or CI machines.

See link:{gradle-issues}13510[gradle/gradle#13510].

[[config_cache:not_implemented:store_parallel_execution]]
=== Parallel task execution on cache misses

When there's no configuration cache available and the work graph needs to be calculated, tasks are not executed in parallel by default.
Only builds that reuse the configuration cache currently execute all tasks in parallel by default.

See link:{gradle-issues}14624[gradle/gradle#14624].

[[config_cache:not_yet_implemented:source_dependencies]]
=== Source dependencies

Support for link:https://blog.gradle.org/introducing-source-dependencies[source dependencies] is not yet implemented.
With the configuration cache enabled, no problem will be reported and the build will fail.

See link:{gradle-issues}13506[gradle/gradle#13506].

[[config_cache:not_yet_implemented:testkit_build_with_java_agent]]
=== Using a Java agent with builds run using TestKit

When running builds using <<test_kit#test_kit, TestKit>>, the configuration cache can interfere with Java agents, such as the Jacoco agent, that are applied to these builds.

[[config_cache:not_yet_implemented:fine_grained_tracking_of_gradle_properties]]
=== Fine-grained tracking of Gradle properties as build configuration inputs

Currently, all external sources of Gradle properties (`gradle.properties` in project directories and in the `<<directory_layout.adoc#dir:gradle_user_home,GRADLE_USER_HOME>>`, environment variables and system properties that set properties, and properties specified with
command-line flags) are considered build configuration inputs regardless of what properties are actually used at configuration time. These sources, however, are not included in the configuration cache report.

[[config_cache:not_yet_implemented:java_serialization]]
=== Java Object Serialization

Gradle allows objects that support the link:https://docs.oracle.com/javase/8/docs/platform/serialization/spec/serialTOC.html[Java Object Serialization] protocol to be stored in the configuration cache.

The implementation is currently limited to serializable classes that implement the `java.io.Serializable` interface and define one of the following combination of methods:

- a `writeObject` method combined with a `readObject` method to control exactly which information to store;
- a `writeObject` method with no corresponding `readObject`; `writeObject` must eventually call `ObjectOutputStream.defaultWriteObject`;
- a `readObject` method with no corresponding `writeObject`; `readObject` must eventually call `ObjectInputStream.defaultReadObject`;
- a `writeReplace` method to allow the class to nominate a replacement to be written;
- a `readResolve` method to allow the class to nominate a replacement for the object just read;

The following _Java Object Serialization_ features are **not** supported:

- serializable classes implementing the `java.io.Externalizable` interface; objects of such classes are discarded by the configuration cache during serialization and reported as problems;
- the `serialPersistentFields` member to explicitly declare which fields are serializable; the member, if present, is ignored; the configuration cache considers all but `transient` fields serializable;
- the following methods of `ObjectOutputStream` are not supported and will throw `UnsupportedOperationException`:
** `reset()`, `writeFields()`, `putFields()`, `writeChars(String)`, `writeBytes(String)` and `writeUnshared(Any?)`.

- the following methods of `ObjectInputStream` are not supported and will throw `UnsupportedOperationException`:
** `readLine()`, `readFully(ByteArray)`, `readFully(ByteArray, Int, Int)`, `readUnshared()`, `readFields()`, `transferTo(OutputStream)` and `readAllBytes()`.
- validations registered via `ObjectInputStream.registerValidation` are simply ignored;
- the `readObjectNoData` method, if present, is never invoked;

See link:{gradle-issues}13588[gradle/gradle#13588].

[[config_cache:not_yet_implemented:storing_lambdas]]
=== Objects Storing Lambdas

Currently, Gradle cannot restore objects from the configuration cache that store user-provided lambdas.
For example, if an object provides a method that accepts a Single Abstract Method (SAM) interface argument (e.g. a `Function` argument), it is natural to implement this argument as a lambda.
If the lambda is then stored in the object for later invocation, the object will not be compatible with the configuration cache and will cause a failure when restored.
Instead of a lambda, the argument should be provided as a typed object implementing the SAM interface.

Alternatively, for Kotlin implementations, the Kotlin compiler can be configured to generate classes instead of lambdas during SAM conversion.

====
include::sample[dir="snippets/configurationCache/samConversion/kotlin",files="build.gradle.kts[tags=configure]"]
include::sample[dir="snippets/configurationCache/samConversion/groovy",files="build.gradle[tags=configure]"]
====

[[config_cache:not_yet_implemented:accessing_top_level_at_execution]]
=== Accessing top-level methods and variables of a build script at execution time

A common approach to reuse logic and data in a build script is to extract repeating bits into top-level methods and variables.
However, calling such methods at execution time is not currently supported if the configuration cache is enabled.

For builds scripts written in Groovy, the task fails because the method cannot be found.
The following snippet uses a top-level method in the `listFiles` task:
====
include::sample[dir="snippets/configurationCache/topLevel/groovy",files="build.gradle[tags=not-supported]"]
====

Running the task with the configuration cache enabled produces the following error:
----
include::{snippetsPath}/configurationCache/topLevel/tests-groovy/notSupported.out[]
----

To prevent the task from failing, convert the referenced top-level method to a static method within a class:
====
include::sample[dir="snippets/configurationCache/topLevel/groovy",files="build.gradle[tags=workaround]"]
====

Build scripts written in Kotlin cannot store tasks that reference top-level methods or variables at execution time in the configuration cache at all.
This limitation exists because the captured script object references cannot be serialized.
The first run of the Kotlin version of the `listFiles` task fails with the configuration cache problem.

====
include::sample[dir="snippets/configurationCache/topLevel/kotlin",files="build.gradle.kts[tags=not-supported]"]
====

To make the Kotlin version of this task compatible with the configuration cache, make the following changes:
====
include::sample[dir="snippets/configurationCache/topLevel/kotlin",files="build.gradle.kts[tags=workaround]"]
====
<1> Define the method inside an object.
<2> Define the variable in a smaller scope.

See link:{gradle-issues}22879[gradle/gradle#22879].
