// Copyright (C) 2024 Gradle, Inc.
//
// Licensed under the Creative Commons Attribution-Noncommercial-ShareAlike 4.0 International License.;
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      https://creativecommons.org/licenses/by-nc-sa/4.0/
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

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

[[config_cache_requirements]]
= Configuration Cache Requirements for your Build Logic

[[config_cache:requirements]]
To capture and reload the task graph state using the Configuration Cache, Gradle enforces specific requirements on tasks and build logic.
Any violation of these requirements is reported as a Configuration Cache "problem," which causes the build to fail.

In most cases, these requirements expose undeclared inputs, making builds more strict, correct, and reliable.
Using the Configuration Cache is effectively an opt-in to these improvements.

The following sections describe each requirement and provide guidance on resolving issues in your build.

[[config_cache:requirements:disallowed_types]]
== Certain Types must not be Referenced by Tasks

Some types must not be referenced by task fields or in task actions (methods annotated `@TaskAction`, or the `doFirst {}` and `doLast {}` DSL blocks).

These types fall into the following categories:

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

These restrictions exist because these types cannot easily be stored or reconstructed by the Configuration Cache.

=== Live JVM State Types

Live JVM state types (e.g., `ClassLoader`, `Thread`, `OutputStream`, `Socket`) are disallowed, as they do not represent task inputs or outputs.

The only exceptions are standard streams (`System.in`, `System.out`, `System.err`), which can be used, for example, as parameters for link:{groovyDslPath}/org.gradle.api.tasks.Exec.html[`Exec`] and link:{groovyDslPath}/org.gradle.api.tasks.JavaExec.html[`JavaExec`] tasks.

==== Concurrency and Synchronization Primitives

Common synchronization primitives from the JDK must not be referenced by task fields or captured in task actions. These types represent live state that cannot be safely serialized.

Prohibited types include interfaces (and all their implementations, e.g. `ReentrantLock`) and classes from the `java.util.concurrent` and `java.util.concurrent.locks` packages, such as:

* `Lock`, `ReadWriteLock`
* `CountDownLatch`, `CyclicBarrier`, `Phaser`, `Semaphore`
* `Exchanger`, `SynchronousQueue`

Using these for cross-task synchronization will not work because the configuration cache isolates tasks: each task instance receives its own independent object, and no global synchronization occurs.
The isolation also applies to objects used in `synchronized` blocks, so they cannot be used for cross-task synchronization either. Unlike the primitives above, Gradle cannot detect this case automatically.

To share or coordinate state between tasks, use <<build_services.adoc#build_services, shared build services>>.

[[config_cache:requirements:gradle_model_types]]
=== Gradle Model Types

Gradle model types (e.g., `Gradle`, `Settings`, `Project`, `SourceSet`, `Configuration`) are often used to pass task inputs that should instead be explicitly declared.

For example, instead of referencing a Project to retrieve `project.version` at execution time, declare the project version as a `Property<String>` input.
Similarly, instead of referencing a `SourceSet` for source files or classpath resolution, declare these as a `FileCollection` input.

=== Dependency Management Types

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

Some dependency management types, such as `Configuration` and `SourceDirectorySet`, should not be used as task inputs because they contain unnecessary state and are not precise.
Use a less specific type that gives necessary features instead:

* If referencing a `Configuration` to get resolved files, declare a `FileCollection` input.
* If referencing a `SourceDirectorySet`, declare a `FileTree` input.

Additionally, referencing resolved dependency results is disallowed (e.g., `ArtifactResolutionQuery`, `ResolvedArtifact`, `ArtifactResult`).
Instead:

* Use a `Provider<ResolvedComponentResult>` from `ResolutionResult.getRootComponent()`.
* Use `ArtifactCollection.getResolvedArtifacts()`, which returns a `Provider<Set<ResolvedArtifactResult>>`.

Tasks should avoid referencing _resolved_ results and instead rely on lazy specifications to defer dependency resolution until execution time.

Some types, such as `Publication` or `Dependency`, are not serializable but could be made so in the future.
Gradle may allow them as task inputs if necessary.

The following task references a `SourceSet`, which is not allowed:

====
include::sample[dir="snippets/optimizing-builds/configuration-cache/disallowedTypes/kotlin",files="build.gradle.kts[tags=task-type]"]
include::sample[dir="snippets/optimizing-builds/configuration-cache/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 the fixed version:

====
include::sample[dir="snippets/optimizing-builds/configuration-cache/disallowedTypesFixed/kotlin",files="build.gradle.kts[tags=task-type]"]
include::sample[dir="snippets/optimizing-builds/configuration-cache/disallowedTypesFixed/groovy",files="build.gradle[tags=task-type]"]
====
<1> No more problems reported, the task now uses the supported type `ConfigurableFileCollection`

If an ad-hoc task in a script captures a disallowed reference in a `doLast {}` closure:

====
include::sample[dir="snippets/optimizing-builds/configuration-cache/disallowedTypes/kotlin",files="build.gradle.kts[tags=ad-hoc-task]"]
include::sample[dir="snippets/optimizing-builds/configuration-cache/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 fulfill the same requirement, that is, do not reference the disallowed type during task execution.

This is how the task declaration above can be fixed:

====
include::sample[dir="snippets/optimizing-builds/configuration-cache/disallowedTypesFixed/kotlin",files="build.gradle.kts[tags=ad-hoc-task]"]
include::sample[dir="snippets/optimizing-builds/configuration-cache/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

Sometimes, a disallowed type is indirectly referenced through another type.
For example, a task may reference an allowed type that, in turn, references a disallowed type.
The hierarchical view in the HTML problem report can help you trace such issues and identify the offending reference.

[[config_cache:requirements:use_project_during_execution]]
== Using the `Project` Object at Execution Time

Tasks must not use any `Project` objects during execution.
This includes calling `Task.getProject()` while a task is running.

Some cases can be resolved similarly to those described in <<configuration_cache_requirements.adoc#config_cache:requirements:disallowed_types, disallowed types>>.

Often, equivalent functionality is available on both `Project` and `Task`.
For example:

* If you need a `Logger`, use `Task.logger` instead of `Project.logger`.
* For file operations, use <<service_injection.adoc#service_injection, injected services>> rather than `Project` methods.

The following task incorrectly references the `Project` object at execution time:

====
include::sample[dir="snippets/optimizing-builds/configuration-cache/projectAtExecution/kotlin",files="build.gradle.kts[tags=task-type]"]
include::sample[dir="snippets/optimizing-builds/configuration-cache/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

Fixed version:

====
include::sample[dir="snippets/optimizing-builds/configuration-cache/projectAtExecutionFixed/kotlin",files="build.gradle.kts[tags=task-type]"]
include::sample[dir="snippets/optimizing-builds/configuration-cache/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 {}`

If the same problem occurs in an ad-hoc task in a script:

====
include::sample[dir="snippets/optimizing-builds/configuration-cache/projectAtExecution/kotlin",files="build.gradle.kts[tags=ad-hoc-task]"]
include::sample[dir="snippets/optimizing-builds/configuration-cache/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

Fixed version:

====
include::sample[dir="snippets/optimizing-builds/configuration-cache/projectAtExecutionFixed/kotlin",files="build.gradle.kts[tags=ad-hoc-task]"]
include::sample[dir="snippets/optimizing-builds/configuration-cache/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 {}`

Fixing ad-hoc tasks in scripts requires additional effort, making it a good opportunity to refactor them into proper task classes.

The table below lists recommended replacements for commonly used `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`
| `project.buildDir` has been deprecated. You should use `project.layout.buildDirectory`.
Use a task input or output property or a script variable to capture the result of using `project.layout.buildDirectory` 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:{groovyDslPath}/org.gradle.api.provider.ProviderFactory.html#org.gradle.api.provider.ProviderFactory:gradleProperty(java.lang.String)[Value providers for Gradle properties].

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

| `project.provider {}`
| link:{groovyDslPath}/org.gradle.api.provider.ProviderFactory.html#org.gradle.api.provider.ProviderFactory: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 {}`
| link:{javadocPath}/org/gradle/api/file/FileSystemOperations.html#copySpec--[FileSystemOperations.copySpec {}]

| `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 must not directly access the state of another task instance.
Instead, they should be connected using <<lazy_configuration#working_with_task_dependencies_in_lazy_properties, input and output relationships>>.

This requirement ensures that tasks remain isolated and correctly cacheable.
As a result, it is unsupported to write tasks that configure other tasks at execution time.

[[config_cache:requirements:shared_objects]]
== Sharing Mutable Objects

When storing a task in the Configuration Cache, all objects referenced through the task’s fields are serialized.

In most cases, deserialization preserves reference equality—if two fields `a` and `b` reference the same instance at configuration time, they will still reference the same instance after deserialization (`a == b`, or `a === b` in Groovy/Kotlin syntax).

However, for performance reasons, certain classes—such as `java.lang.String`, `java.io.File`, and many `java.util.Collection` implementations—are serialized without preserving reference equality.
After deserialization, fields that referred to these objects may reference different but equal instances.

Consider a task that stores a user-defined object and an `ArrayList` as task fields:

====
include::sample[dir="snippets/optimizing-builds/configuration-cache/sharedObjects/kotlin",files="build.gradle.kts[]"]
include::sample[dir="snippets/optimizing-builds/configuration-cache/sharedObjects/groovy",files="build.gradle[]"]
====
<1> `doLast` action captures the references from the enclosing scope. These captured references are also serialized to the Configuration Cache.
<2> Compare the reference to an object of user-defined class stored in the task field and the reference captured in the `doLast` action.
<3> Compare the reference to `ArrayList` instance stored in the task field and the reference captured in the `doLast` action.
<4> Check the equality of stored and captured lists.

Without Configuration Cache, reference equality is preserved in both cases:

[source,bash]
----
$ ./gradlew --no-configuration-cache checkEquality
----
[source,text]
----
include::{snippetsPath}/optimizing-builds/configuration-cache/sharedObjects/tests/sharingObjectsWithoutConfigurationCache.out[]
----

With Configuration Cache enabled, only user-defined object references remain identical.
List references are different, although the lists themselves remain equal:

[source,bash]
----
$ ./gradlew --configuration-cache checkEquality
----
[source,text]
----
include::{snippetsPath}/optimizing-builds/configuration-cache/sharedObjects/tests/sharingObjectsWithConfigurationCache.out[]
----

Best Practices:

* Avoid sharing mutable objects between configuration and execution phases.
* If sharing state is necessary, wrap it in a user-defined class.
* Do not rely on reference equality for standard Java, Groovy, Kotlin, or Gradle-defined types.

Reference equality is never preserved between tasks—each task is an isolated "realm."
To share objects across tasks, use a <<build_services#build_services, Build Service>> to wrap the shared state.

[[config_cache:requirements:task_extensions]]
== Accessing Task Extensions or Conventions

Tasks must **not** access conventions, extensions, or extra properties at execution time.

Instead, any value relevant to task execution should be explicitly modeled as a task property to ensure proper caching and reproducibility.

[[config_cache:requirements:build_listeners]]
== Using Build Listeners

Plugins and build scripts must **not** register build listeners that are created at configuration time and triggered at execution time.
This includes listeners such as `BuildListener` or `TaskExecutionListener`.

Recommended Alternatives:

- Use <<build_services#build_services, Build Services>> to handle execution-time logic.
- Register a Build Service to receive <<build_services#operation_listener, task execution notifications>>.
- Replace `buildFinished` listeners with <<dataflow_actions.adoc#dataflow_action, dataflow actions>> to manage build results.

[[config_cache:requirements:value_source]]
== Using External Information Sources

Examples of external sources include client environment variables, system properties, configuration files, shell commands, network services, among others.
To represent such sources in a Gradle build in a Configuration Cache friendly way we use link:{javadocPath}/org/gradle/api/provider/ValueSource.html[`ValueSource`].
They make it possible for Gradle to transparently manage the Configuration Cache as values obtained from those sources change.
For example, a build might run a different set of tasks depending on whether the `CI` environment variable is set or not.

A `ValueSource` implementation is *exempt from the automatic detection of configuration cache inputs*.
For example, if the `obtain()` method reads a system property, an environment variable, or a file, those reads do not individually become configuration cache inputs.
Instead, only the _value returned_ by `obtain()` is tracked.
If that value changes between builds, the Configuration Cache is invalidated.

This makes `ValueSource` the recommended escape hatch when built-in providers like `providers.systemProperty()`, `providers.environmentVariable()`, or `providers.fileContents()` are too limited for your use case.

=== Creating a `ValueSource`

To integrate a new type of value source:

1. Create an abstract class implementing `ValueSource<T, P>`.
2. Implement the `obtain()` method to compute and return the value. The returned value must be effectively immutable (e.g., a `string`, `int`, `boolean`, an unmodifiable collection, or a serializable data class).
3. Use link:{javadocPath}/org/gradle/api/provider/ProviderFactory.html#of-java.lang.Class-org.gradle.api.Action-[`providers.of(Class, Action)`] to get a `Provider` backed by your source.

The returned `Provider` can be passed to task properties or queried during the configuration phase.
If the value is queried at configuration time, the source is automatically considered a build configuration input.

NOTE: Do not implement `getParameters()` in your class—Gradle provides the implementation automatically.

TIP: Your `ValueSource` implementation does not need to be thread-safe, Gradle synchronizes calls to `obtain()`.

=== Parameterizing a `ValueSource`

A value source implementation will most likely take parameters.
To do this, create a subtype of link:{javadocPath}/org/gradle/api/provider/ValueSourceParameters.html[`ValueSourceParameters`] and declare it as the type parameter `P` of your `ValueSource` implementation.
The parameters are configured in the `Action` passed to `providers.of()`.
If no parameters are needed, use `ValueSourceParameters.None`.

=== Injecting Services

Gradle <<service_injection.adoc#service_injection, services can be injected>> into a `ValueSource` implementation by adding a parameter to the constructor and annotating it with `@Inject`.
Currently, the following service is supported:

* link:{javadocPath}/org/gradle/process/ExecOperations.html[`ExecOperations`] — provides means to execute external processes.
This service can be used even at configuration time.
Because `obtain()` is called on every build (see <<#sec:configuration_cache_behavior,Configuration Cache Behavior>> below), only fast-running commands should be used.

You can also use standard Java/Kotlin/Groovy process APIs such as `java.lang.ProcessBuilder` inside `obtain()`.


[[sec:configuration_cache_behavior]]
=== Configuration Cache Behavior

When a provider backed by a `ValueSource` is queried at configuration time, the value becomes a build configuration input.
The `obtain()` method is then executed on every subsequent build to determine whether the Configuration Cache entry is still `UP-TO-DATE`:

* If the value has *not* changed, the cached configuration is reused.
* If the value *has* changed, the cache is invalidated and the configuration phase runs again.

Because `obtain()` runs on every build, it is recommended to keep the implementation fast.

`ValueSource` is the recommended approach for the following scenarios:

* <<config_cache:requirements:external_processes, Running external processes at configuration time>> — such as calling git or a CLI tool to compute a version string or check a precondition.
* <<config_cache:requirements:reading_sys_props_and_env_vars, Complex system property or environment variable access patterns>> — when you need to combine, transform, or conditionally read multiple properties.
* <<config_cache:requirements:undeclared_file_read, Reading files at configuration time>>  — when `providers.fileContents()` is insufficient, such as parsing a file in a custom format.

[[config_cache:requirements:external_processes]]
== Running External Processes

TIP: Plugin and build scripts should avoid running external processes at configuration time.

You should avoid using these APIs for running processes during configuration:

- *Java/Kotlin*: `ProcessBuilder`, `Runtime.exec(...)`, etc...
- *Groovy*: `*.execute()`, etc...
- *Gradle*: `ExecOperations.exec`, `ExecOperations.javaexec`, etc...

The flexibility of these methods prevent Gradle from determining how the calls impact the build configuration, making it difficult to ensure that the Configuration Cache entry can be safely reused.

However, if running processes is required at configuration time, you can use the configuration-cache-compatible APIs detailed below.

For simpler cases, when grabbing the output of the process is enough,
link:{groovyDslPath}/org.gradle.api.provider.ProviderFactory.html#org.gradle.api.provider.ProviderFactory:exec(org.gradle.api.Action)[`providers.exec()`] and
link:{groovyDslPath}/org.gradle.api.provider.ProviderFactory.html#org.gradle.api.provider.ProviderFactory:javaexec(org.gradle.api.Action)[`providers.javaexec()`] can be used:

====
include::sample[dir="snippets/optimizing-builds/configuration-cache/externalProcessProvider/kotlin",files="build.gradle.kts[]"]
include::sample[dir="snippets/optimizing-builds/configuration-cache/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 (see <<config_cache:requirements:value_source>> for a full overview of `ValueSource`).
This `ExecOperations` instance can be used at configuration time without restrictions:

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

You can also use standard Java/Kotlin/Groovy process APIs like `java.lang.ProcessBuilder` in the `ValueSource`.

The `ValueSource` implementation can then be used to create a provider with link:{groovyDslPath}/org.gradle.api.provider.ProviderFactory.html#org.gradle.api.provider.ProviderFactory:of(java.lang.Class,%20org.gradle.api.Action)[`providers.of`]:

====
include::sample[dir="snippets/optimizing-builds/configuration-cache/externalProcessValueSource/kotlin",files="build.gradle.kts[tags=create-provider]"]
include::sample[dir="snippets/optimizing-builds/configuration-cache/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 variables or properties a build configuration input.
Therefore, changing their 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 these values change.
Instead, you can connect the `Provider` returned by link:{groovyDslPath}/org.gradle.api.provider.ProviderFactory.html#org.gradle.api.provider.ProviderFactory:systemProperty(java.lang.String)[`providers.systemProperty()`] or
link:{groovyDslPath}/org.gradle.api.provider.ProviderFactory.html#org.gradle.api.provider.ProviderFactory: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/optimizing-builds/configuration-cache/envVarsSysPropsDont/kotlin",files="build.gradle.kts[]"]
include::sample[dir="snippets/optimizing-builds/configuration-cache/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/optimizing-builds/configuration-cache/envVarsSysPropsDo/kotlin",files="build.gradle.kts[]"]
include::sample[dir="snippets/optimizing-builds/configuration-cache/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:{groovyDslPath}/org.gradle.api.provider.ProviderFactory.html#org.gradle.api.provider.ProviderFactory:systemPropertiesPrefixedBy(java.lang.String)[system properties] and
link:{groovyDslPath}/org.gradle.api.provider.ProviderFactory.html#org.gradle.api.provider.ProviderFactory:environmentVariablesPrefixedBy(java.lang.String)[environment variables]:

====
include::sample[dir="snippets/optimizing-builds/configuration-cache/envVarsSysPropsDoPrefix/kotlin",files="build.gradle.kts[]"]
include::sample[dir="snippets/optimizing-builds/configuration-cache/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 (see <<config_cache:requirements:value_source>> for a full overview of `ValueSource`).
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/optimizing-builds/configuration-cache/envVarsSysPropsDoValueSource/kotlin",files="build.gradle.kts[tags=value-source,create-provider]"]
include::sample[dir="snippets/optimizing-builds/configuration-cache/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/optimizing-builds/configuration-cache/fileContentsDont/kotlin",files="build.gradle.kts[]"]
include::sample[dir="snippets/optimizing-builds/configuration-cache/fileContentsDont/groovy",files="build.gradle[]"]
====

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

====
include::sample[dir="snippets/optimizing-builds/configuration-cache/fileContentsDo/kotlin",files="build.gradle.kts[]"]
include::sample[dir="snippets/optimizing-builds/configuration-cache/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:{groovyDslPath}/org.gradle.api.provider.ProviderFactory.html#org.gradle.api.provider.ProviderFactory:fileContents(org.gradle.api.file.RegularFile)[`providers.fileContents()`] to task properties.

[[config_cache:requirements:bytecode_modifications_and_isolation]]
== Bytecode Modifications and Java Agent

To detect the configuration inputs, Gradle modifies the bytecode of classes on the build script classpath, like plugins and their dependencies.
Gradle uses a Java agent to modify the bytecode.
Integrity self-checks of some libraries may fail because of the changed bytecode or the agent's presence.

To work around this, you can use the <<worker_api.adoc#tasks_parallel_worker, Worker API>> with classloader or process isolation to encapsulate the library code.
The bytecode of the worker's classpath is not modified, so the self-checks should pass.
When process isolation is used, the worker action is executed in a separate worker process that doesn't have the Gradle Java agent installed.

In simple cases, when the libraries also provide command-line entry points (`public static void main()` method), you can also use the link:{javadocPath}/org/gradle/api/tasks/JavaExec.html[`JavaExec`] task to isolate the library.

[[config_cache:secrets]]
== Handling of Credentials and Secrets

Currently, the Configuration Cache does not have a built-in mechanism to prevent storing secrets used as inputs.
As a result, secrets may end up in the serialized Configuration Cache entry, which, by default, is stored under `.gradle/configuration-cache` in your project directory.

To mitigate the risk of accidental exposure, Gradle encrypts the Configuration Cache.
When required, Gradle transparently generates a machine-specific secret key, caches it under the
`<<directory_layout.adoc#dir:gradle_user_home,GRADLE_USER_HOME>>` directory, and uses it to encrypt data in the project-specific caches.

To further enhance security, follow these recommendations:

- Secure access to Configuration Cache entries.
- Use `<<directory_layout.adoc#dir:gradle_user_home,GRADLE_USER_HOME>>/gradle.properties` to store secrets.
The content of this file is **not** included in the Configuration Cache—only its fingerprint is.
If storing secrets in this file, ensure access is **properly restricted**.

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

[[config_cache:secrets:configuring_encryption_key]]
=== Providing an Encryption Key with the `GRADLE_ENCRYPTION_KEY` Environment Variable

By default, Gradle automatically generates and manages the encryption key as a Java keystore, stored under the `<<directory_layout.adoc#dir:gradle_user_home,GRADLE_USER_HOME>>` directory.

For environments where this behavior is undesirable—such as when the `<<directory_layout.adoc#dir:gradle_user_home,GRADLE_USER_HOME>>` directory is shared across multiple machines—you can explicitly provide an encryption key using the `GRADLE_ENCRYPTION_KEY` environment variable.

[IMPORTANT]
====
The same encryption key **must be consistently provided** across multiple Gradle runs; otherwise, Gradle will be unable to reuse existing cached configurations.
====

[[config_cache:secrets:generating_encryption_key]]
=== Generating an Encryption Key compatible with `GRADLE_ENCRYPTION_KEY`

To encrypt the Configuration Cache using a user-specified encryption key, Gradle requires the `GRADLE_ENCRYPTION_KEY` environment variable to be set with a valid AES key, encoded as a Base64 string.

You can generate a Base64-encoded AES-compatible key using the following command:

[source,bash]
----
$ openssl rand -base64 16
----

This command works on Linux and macOS, and on Windows if using a tool like Cygwin.

Once generated, set the Base64-encoded key as the value of the `GRADLE_ENCRYPTION_KEY` environment variable:

[source,bash]
----
$ export GRADLE_ENCRYPTION_KEY="your-generated-key-here"
----
