The Java Plugin

version 8.14.3

Contents

The Java plugin adds Java compilation along with testing and bundling capabilities to a project.It serves as the basis for many of the other JVM language Gradle plugins.You can find a comprehensive introduction and overview to the Java Plugin in theBuilding Java Projects chapter.

As indicated above, this plugin adds basic building blocks for working with JVM projects.Its feature set has been superseded by other plugins, offering more features based on your project type.Instead of applying it directly to your project, you should look into thejava-library orapplication plugins or one of the supported alternative JVM language.

Usage

To use the Java plugin, include the following in your build script:

Example 1.Using the Java plugin

build.gradle.kts

plugins {    java}

build.gradle

plugins {    id 'java'}

Tasks

The Java plugin adds a number of tasks to your project, as shown below.

compileJava —JavaCompile: Depends on: All tasks which contribute to the compilation classpath, includingjar tasks from projects that are on the classpath via project dependencies
Compiles production Java source files using the JDK compiler.
processResources —ProcessResources: Copies production resources into the production resources directory.
classes: Depends on:compileJava,processResources
This is an aggregate task that just depends on other tasks. Other plugins may attach additional compilation tasks to it.
compileTestJava —JavaCompile: Depends on:classes, and all tasks that contribute to the test compilation classpath
Compiles test Java source files using the JDK compiler.
processTestResources —Copy: Copies test resources into the test resources directory.
testClasses: Depends on:compileTestJava,processTestResources
This is an aggregate task that just depends on other tasks. Other plugins may attach additional test compilation tasks to it.
jar —Jar: Depends on:classes
Assembles the production JAR file, based on the classes and resources attached to themain source set.
javadoc —Javadoc: Depends on:classes
Generates API documentation for the production Java source using Javadoc.
test —Test: Depends on:testClasses, and all tasks which produce the test runtime classpath
Runs the unit tests using JUnit or TestNG.
clean —Delete: Deletes the project build directory.
cleanTaskName —Delete: Deletes files created by the specified task. For example,cleanJar will delete the JAR file created by thejar task andcleanTest will delete the test results created by thetest task.

SourceSet Tasks

For each source set you add to the project, the Java plugin adds the following tasks:

compileSourceSetJava —JavaCompile: Depends on: All tasks which contribute to the source set’s compilation classpath
Compiles the given source set’s Java source files using the JDK compiler.
processSourceSetResources —Copy: Copies the given source set’s resources into the resources directory.
sourceSetClasses —Task: Depends on:compileSourceSetJava,processSourceSetResources
Prepares the given source set’s classes and resources for packaging and execution. Some plugins may add additional compilation tasks for the source set.

Lifecycle Tasks

The Java plugin attaches some of its tasks to the lifecycle tasks defined by theBase Plugin — which the Java Plugin applies automatically — and it also adds a few other lifecycle tasks:

assemble: Depends on:jar
Aggregate task that assembles all the archives in the project. This task is added by the Base Plugin.
check: Depends on:test
Aggregate task that performs verification tasks, such as running the tests. Some plugins add their own verification tasks tocheck. You should also attach any customTest tasks to this lifecycle task if you want them to execute for a full build. This task is added by the Base Plugin.
build: Depends on:check,assemble
Aggregate tasks that performs a full build of the project. This task is added by the Base Plugin.
buildNeeded: Depends on:build, andbuildNeeded tasks in all projects that are dependencies in thetestRuntimeClasspath configuration.
Performs a full build of the project and all projects it depends on.
buildDependents: Depends on:build, andbuildDependents tasks in all projects that have this project as a dependency in theirtestRuntimeClasspath configurations
Performs a full build of the project and all projects which depend upon it.
buildConfigName — task rule: Depends on: all tasks that generate the artifacts attached to the named — ConfigName — configuration
Assembles the artifacts for the specified configuration. This rule is added by the Base Plugin.

The following diagram shows the relationships between these tasks.

Figure 1. Java plugin - tasks

Project layout

The Java plugin assumes the project layout shown below. None of these directories need to exist or have anything in them. The Java plugin will compile whatever it finds, and handles anything which is missing.

src/main/java: Production Java source.
src/main/resources: Production resources, such as XML and properties files.

src/test/java: Test Java source.
src/test/resources: Test resources.

src/sourceSet/java: Java source for the source set namedsourceSet.
src/sourceSet/resources: Resources for the source set namedsourceSet.

Changing the project layout

You configure the project layout by configuring the appropriate source set. This is discussed in more detail in the following sections. Here is a brief example which changes the main Java and resource source directories.

Example 2.Custom Java source layout

build.gradle.kts

sourceSets {    main {        java {            setSrcDirs(listOf("src/java"))        }        resources {            setSrcDirs(listOf("src/resources"))        }    }}

build.gradle

sourceSets {    main {        java {            srcDirs = ['src/java']        }        resources {            srcDirs = ['src/resources']        }    }}

Source sets

The plugin adds the followingsource sets:

main: Contains the production source code of the project, which is compiled and assembled into a JAR.
test: Contains your test source code, which is compiled and executed using JUnit or TestNG. These are typically unit tests, but you can include any test in this source set as long as they all share the same compilation and runtime classpaths.

Source set properties

The following table lists some of the important properties of a source set. You can find more details in the API documentation forSourceSet.

name — (read-only)String: The name of the source set, used to identify it.
output — (read-only)SourceSetOutput: The output files of the source set, containing its compiled classes and resources.
output.classesDirs — (read-only)FileCollection: Default value:layout.buildDirectory.dir("classes/java/$name"), e.g.build/classes/java/main
The directories to generate the classes of this source set into. May contain directories for other JVM languages, e.g.build/classes/kotlin/main.
output.resourcesDir — File: Default value:layout.buildDirectory.dir("resources/$name"), e.g.build/resources/main
The directory to generate the resources of this source set into.
compileClasspath — FileCollection: Default value:${name}CompileClasspath configuration
The classpath to use when compiling the source files of this source set.
annotationProcessorPath — FileCollection: Default value:${name}AnnotationProcessor configuration
The processor path to use when compiling the source files of this source set.
runtimeClasspath — FileCollection: Default value:$output,${name}RuntimeClasspath configuration
The classpath to use when executing the classes of this source set.
java — (read-only)SourceDirectorySet: The Java source files of this source set. Contains only.java files found in the Java source directories, and excludes all other files.
java.srcDirs — Set<File>: Default value:src/$name/java, e.g.src/main/java
The source directories containing the Java source files of this source set. You can set this to any value that is described inthis section.
java.destinationDirectory — DirectoryProperty: Default value:layout.buildDirectory.dir("classes/java/$name"), e.g.build/classes/java/main
The directory to generate compiled Java sources into. You can set this to any value that is described inthis section.
resources — (read-only)SourceDirectorySet: The resources of this source set. Contains only resources, and excludes any.java files found in the resource directories. Other plugins, such as theGroovy Plugin, exclude additional types of files from this collection.
resources.srcDirs — Set<File>: Default value:[src/$name/resources]
The directories containing the resources of this source set. You can set this to any type of value that is described inthis section.
allJava — (read-only)SourceDirectorySet: Default value: Same asjava property
All Java files of this source set. Some plugins, such as the Groovy Plugin, add additional Java source files to this collection.
allSource — (read-only)SourceDirectorySet: Default value: Sum of everything in theresources andjava properties
All source files of this source set of any language. This includes all resource files and all Java source files. Some plugins, such as the Groovy Plugin, add additional source files to this collection.

Defining new source sets

See theintegration test example in theTesting in Java & JVM projects chapter.

Some other simple source set examples

Adding a JAR containing the classes of a source set:

Example 3.Assembling a JAR for a source set

build.gradle.kts

tasks.register<Jar>("intTestJar") {    from(sourceSets["intTest"].output)}

build.gradle

tasks.register('intTestJar', Jar) {    from sourceSets.intTest.output}

Generating Javadoc for a source set:

Example 4.Generating the Javadoc for a source set

build.gradle.kts

tasks.register<Javadoc>("intTestJavadoc") {    source(sourceSets["intTest"].allJava)    classpath = sourceSets["intTest"].compileClasspath}

build.gradle

tasks.register('intTestJavadoc', Javadoc) {    source sourceSets.intTest.allJava    classpath = sourceSets.intTest.compileClasspath}

Adding a test suite to run the tests in a source set:

Example 5.Running tests in a source set

build.gradle.kts

tasks.register<Test>("intTest") {    testClassesDirs = sourceSets["intTest"].output.classesDirs    classpath = sourceSets["intTest"].runtimeClasspath}

build.gradle

tasks.register('intTest', Test) {    testClassesDirs = sourceSets.intTest.output.classesDirs    classpath = sourceSets.intTest.runtimeClasspath}

Dependency management

The Java plugin adds a number ofdependency configurations to your project, as shown below.Tasks such ascompileJava andtest then use one or more of those configurations to get the corresponding files and use them, for example by placing them on a compilation or runtime classpath.

Dependency configurations

For information on thedefault andarchives configurations, please consult theBase Plugin reference documentation.

For information on theapi orcompileOnlyApi configurations, please consult theJava Library Plugin reference documentation andDependency Management for Java Projects.

Dependency Declaration Configurations

implementation: Represents dependencies that are required both at compile time and runtime for the main source set of a project (i.e., implementation only dependencies).
compileOnly: Represents dependencies that are required only at compile time and are not included in the runtime classpath (i.e., compile time only dependencies, not used at runtime).
runtimeOnly: Represents dependencies that are required only at runtime and are not included in the compile classpath (i.e., dependencies required only at runtime).
testImplementation extendsimplementation: Represents dependencies that are required both at compile time and runtime for the test source set of a project (i.e., implementation only dependencies for tests).
testCompileOnly: Represents dependencies that are required only at compile time for the test source set of a project and are not included in the runtime classpath (i.e., additional dependencies only for compiling tests, not used at runtime).
testRuntimeOnly extendsruntimeOnly: Represents dependencies that are required only at runtime for the test source set of a project (i.e., runtime only dependencies for running tests).
annotationProcessor: Represents annotation processors used during the compilation of a project’s source code (i.e., annotation processors used during compilation).

Resolvable Configurations

compileClasspath extendscompileOnly, implementation: Represents the classpath used when compiling the main sources, which includes dependencies from bothcompileOnly andimplementation configurations. Used by taskcompileJava.
runtimeClasspath extendsruntimeOnly, implementation: Represents the classpath used to run the main sources, which includes dependencies from bothruntimeOnly andimplementation configurations. Used by taskrun.
testCompileClasspath extendstestCompileOnly, testImplementation: Represents the classpath used to compile the test sources, which includes dependencies from bothtestCompileOnly andtestImplementation configurations. Used by taskcompileTestJava.
testRuntimeClasspath extendstestRuntimeOnly, testImplementation: Represents the classpath used to run the test sources, which includes dependencies from bothtestRuntimeOnly andtestImplementation configurations. Used by tasktest.

The following diagrams show the dependency configurations for themain andtest source sets respectively. You can use this legend to interpret the colors:

blue background — you can declare dependencies against the configuration.
green background — the configuration is for consumption by tasks, not for you to declare dependencies.
grey background — a task.

Figure 2. Java plugin -main source set dependency configurations

Figure 3. Java plugin -test source set dependency configurations

For each source set you add to the project, the Java plugins adds the following dependency configurations:

SourceSet dependency configurations

sourceSetImplementation: Compile time dependencies for the given source set. Used bysourceSetCompileClasspath,sourceSetRuntimeClasspath.
sourceSetCompileOnly: Compile time only dependencies for the given source set, not used at runtime.
sourceSetCompileClasspath extendssourceSetCompileOnly,sourceSetImplementation: Compile classpath, used when compiling source. Used bycompileSourceSetJava.
sourceSetAnnotationProcessor: Annotation processors used during compilation of this source set.
sourceSetRuntimeOnly: Runtime only dependencies for the given source set.
sourceSetRuntimeClasspath extendssourceSetRuntimeOnly,sourceSetImplementation: Runtime classpath contains elements of the implementation, as well as runtime only elements.

Contributed extension

The Java plugin adds thejava extension to the project.This allows to configure a number of Java related properties inside a dedicated DSL block.

Example 6.Using thejava extension to configure a toolchain

build.gradle.kts

java {    toolchain {        languageVersion = JavaLanguageVersion.of(17)    }}

build.gradle

java {    toolchain {        languageVersion = JavaLanguageVersion.of(17)    }}

Below is the list of properties and DSL functions with short explanations available inside thejava extension.

Toolchain and compatibility

toolchain: Java toolchain to be used by tasks using JVM tools, such as compilation and execution. Default value: build JVM toolchain.
JavaVersion sourceCompatibility: Java version compatibility to use when compiling Java source. Default value: language version of the toolchain from this extension.
Note that using atoolchain is preferred to using a compatibility setting for most cases.
JavaVersion targetCompatibility: Java version to generate classes for. Default value:sourceCompatibility.
Note that using atoolchain is preferred to using a compatibility setting for most cases.

Packaging

withJavadocJar(): Automatically packages Javadoc and creates a variantjavadocElements with an artifact-javadoc.jar, which will be part of the publication.
withSourcesJar(): Automatically packages source code and creates a variantsourceElements with an artifact-sources.jar, which will be part of the publication.

Directory properties

String reporting.baseDir: The name of the directory to generate reports into, relative to the build directory. Default value:reports
(read-only) File reportsDir: The directory to generate reports into. Default value:reporting.baseDirectory
String testResultsDirName: The name of the directory to generate test result .xml files into, relative to the build directory. Default value:test-results
(read-only) File testResultsDir: The directory to generate test result .xml files into. Default value:layout.buildDirectory.dir(testResultsDirName)
String testReportDirName: The name of the directory to generate the test report into, relative to the reports directory. Default value:tests
(read-only) File testReportDir: The directory to generate the test report into. Default value:reportsDir/testReportDirName
String libsDirName: The name of the directory to generate libraries into, relative to the build directory. Default value:libs
(read-only) File libsDir: The directory to generate libraries into. Default value:layout.buildDirectory.dir(libsDirName)
String distsDirName: The name of the directory to generate distributions into, relative to the build directory. Default value:distributions
(read-only) File distsDir: The directory to generate distributions into. Default value:layout.buildDirectory.dir(distsDirName)
String docsDirName: The name of the directory to generate documentation into, relative to the build directory. Default value:docs
(read-only) File docsDir: The directory to generate documentation into. Default value:layout.buildDirectory.dir(docsDirName)
String dependencyCacheDirName: The name of the directory to cache source dependency information relative to the build directory. Default value:dependency-cache.

Other properties

(read-only)SourceSetContainer sourceSets: Contains the project’s source sets. Default value: Not nullSourceSetContainer
String archivesBaseName: The basename to use for archives, such as JAR or ZIP files. Default value:projectName
Manifest manifest: The manifest to include in all JAR files. Default value: an empty manifest.

Convention properties (deprecated)

The Java Plugin adds a number of convention properties to the project. You can use these properties in your build script as though they were properties of the project object.These aredeprecated and superseded by the extension described above.See theJavaPluginConvention DSL documentation for information on them.

Testing

See theTesting in Java & JVM projects chapter for more details.

Publishing

components.java: ASoftwareComponent forpublishing the production JAR created by thejar task.This component includes the runtime dependency information for the JAR.

Incremental Java compilation

Gradle comes with a sophisticated incremental Java compiler that is active by default.

This gives you the following benefits

Incremental builds are much faster.
The smallest possible number of class files are changed.Classes that don’t need to be recompiled remain unchanged in the output directory.An example scenario when this is really useful is using JRebel — the fewer output classes are changed the quicker the JVM can use refreshed classes.

To help you understand how incremental compilation works, the following provides a high-level overview:

Gradle will recompile all classesaffected by a change.
A class isaffected if it has been changed or if it depends on another affected class.This works no matter if the other class is defined in the same project, another project or even an external library.
A class’s dependencies are determined from type references in its bytecode or symbol analysis via a compiler plugin.
Since source-retention annotations are not visible in bytecode, changes to a source-retention annotation will result in full recompilation.
You can improve incremental compilation performance by applying good software design principles like loose coupling.For instance, if you put an interface between a concrete class and its dependents, the dependent classes are only recompiled when the interface changes, but not when the implementation changes.
The class analysis is cached in the project directory, so the first build after a clean checkout can be slower. Consider turning off the incremental compiler on your build server.
The class analysis is also an output stored in the build cache, which means that if a compilation output is fetched from the build cache, then the incremental compilation analysis will be too and the next compilation will be incremental.

Known issues

If you are using an annotation processor that reads resources (e.g. a configuration file), you need to declare those resources as an input of the compile task.
If a resource file is changed, Gradle will trigger a full recompilation.
Using a customexecutable orjavaHome deactivates some optimizations.The compile task does not use incremental build immediately after a compile error or if a Java constant changes.Usetoolchains instead if possible.
Having a source structure that does not match the package names, while legal for compilation, might end up causing trouble in the toolchain.Even more if annotation processing andcaching are involved.

Incremental annotation processing

Starting with Gradle 4.7, the incremental compiler also supports incremental annotation processing.All annotation processors need to opt in to this feature, otherwise they will trigger a full recompilation.

As a user you can see which annotation processors are triggering full recompilations in the--info log.Incremental annotation processing will be deactivated if a customexecutable orjavaHome is configured on the compile task.

Making an annotation processor incremental

Please first have a look atincremental Java compilation, as incremental annotation processing builds on top of it.

Gradle supports incremental compilation for two common categories of annotation processors: "isolating" and "aggregating".Please consult the information below to decide which category fits your processor.

You can then register your processor for incremental compilation using a file in the processor’s META-INF directory.The format is one line per processor, with the fully qualified name of the processor class and its case-insensitive category separated by a comma.

Example: Registering incremental annotation processors

processor/src/main/resources/META-INF/gradle/incremental.annotation.processors

org.gradle.EntityProcessor,isolatingorg.gradle.ServiceRegistryProcessor,dynamic

If your processor can only decide at runtime whether it is incremental or not, you can declare it as "dynamic" in the META-INF descriptorand return its true type at runtime using theProcessor#getSupportedOptions() method.

Example: Registering incremental annotation processors dynamically

processor/src/main/java/org/gradle/ServiceRegistryProcessor.java

@Overridepublic Set<String> getSupportedOptions() {    return Collections.singleton("org.gradle.annotation.processing.aggregating");}

Both categories have the following limitations:

They can only readCLASS orRUNTIME retention annotations.
They can only read parameter names if the user passes the-parameters compiler argument.
They must generate their files using theFiler API.Writing files any other way will result in silent failures later on, as these files won’t be cleaned up correctly.If your processor does this, it cannot be incremental.
They must not depend on compiler-specific APIs likecom.sun.source.util.Trees.Gradle wraps the processing APIs, so attempts to cast to compiler-specific types will fail.If your processor does this, it cannot be incremental, unless you have some fallback mechanism.
If they useFiler#createResource,thelocation argument must be one of these values fromStandardLocation:CLASS_OUTPUT,SOURCE_OUTPUT, orNATIVE_HEADER_OUTPUT.Any other argument will disable incremental processing.

"Isolating" annotation processors

The fastest category, these look at each annotated element in isolation, creating generated files or validation messages for it.For instance anEntityProcessor could create a<TypeName>Repository for each type annotated with@Entity.

Example: An isolated annotation processor

processor/src/main/java/org/gradle/EntityProcessor.java

Set<? extends Element> entities = roundEnv.getElementsAnnotatedWith(entityAnnotation);for (Element entity : entities) {    createRepository((TypeElement) entity);}

"Isolating" processors have the following additional limitations:

They must make all decisions (code generation, validation messages) for an annotated type based on information reachable from its AST.This means you can analyze the types' super-class, method return types, annotations etc., even transitively.But you cannot make decisions based on unrelated elements in the RoundEnvironment.Doing so will result in silent failures because too few files will be recompiled later.If your processor needs to make decisions based on a combination of otherwise unrelated elements, mark it as "aggregating" instead.
They must provide exactly one originating element for each file generated with theFiler API.If zero or many originating elements are provided, Gradle will recompile all source files.

When a source file is recompiled, Gradle will recompile all files generated from it.When a source file is deleted, the files generated from it are deleted.

"Aggregating" annotation processors

These can aggregate several source files into one or more output files or validation messages.For instance, aServiceRegistryProcessor could create a singleServiceRegistry with one method for each type annotated with@Service.

Example: An aggregating annotation processor

processor/src/main/java/org/gradle/ServiceRegistryProcessor.java

JavaFileObject serviceRegistry = filer.createSourceFile("ServiceRegistry");Writer writer = serviceRegistry.openWriter();writer.write("public class ServiceRegistry {");for (Element service : roundEnv.getElementsAnnotatedWith(serviceAnnotation)) {    addServiceCreationMethod(writer, (TypeElement) service);}writer.write("}");writer.close();

Gradle will always reprocess (but not recompile) all annotated files that the processor was registered for.Gradle will always recompile any files the processor generates.

State of support in popular annotation processors

Many popular annotation processors support incremental annotation processing (see the table below).Check with the annotation processor project directly for the most up-to-date information and documentation.

Annotation Processor	Supported since	Details
Auto Value	1.6.3	N/A
Auto Service	1.6.3	N/A
Auto Value extensions	Partly supported.	Details in issue
Butterknife	10.2.0	N/A
Lombok	1.16.22	N/A
DataBinding	AGP 3.5.0-alpha5	Hidden behind a feature toggle
Dagger	2.18	2.18 Feature toggle support, 2.24 Enabled by default
kapt	1.3.30	Hidden behind a feature toggle
Toothpick	2.0	N/A
Glide	4.9.0	N/A
Android-State	1.3.0	N/A
Parceler	1.1.11	N/A
Dart and Henson	3.1.0	N/A
MapStruct	1.4.0.Beta1	N/A
Assisted Inject	0.5.0	N/A
Realm	5.11.0	N/A
Requery	Open issue	N/A
EventBus	3.2.0	N/A
EclipseLink	Open issue	N/A
PermissionsDispatcher	4.2.0	N/A
Immutables	Open issue	N/A
Room	2.2.0	2.2.0 Feature toggle support, 2.3.0-alpha02 Enabled by default
Lifecycle	2.2.0-alpha02	N/A
AndroidAnnotations	4.7.0	N/A
DBFlow	Open issue	N/A
AndServer	Open issue	N/A
Litho	0.25.0	N/A
Moxy	2.0	N/A
Epoxy	4.0.0-beta1	N/A
JPA Static Metamodel Generator	5.4.11	N/A
DeepLinkDispatch	5.0.0-beta01	Hidden behind a feature toggle
Shortbread	1.1.0	N/A

Compilation avoidance

If a dependent project has changed in anABI-compatible way (only its private API has changed), then Java compilation tasks will be up-to-date. This means that if projectA depends on projectB and a class inB is changed in an ABI-compatible way (typically, changing only the body of a method), then Gradle won’t recompileA.

Some of the types of changes that do not affect the public API and are ignored:

Changing a method body
Changing a comment
Adding, removing or changing private methods, fields, or inner classes
Adding, removing or changing a resource
Changing the name of jars or directories in the classpath
Renaming a parameter

Since implementation details matter for annotation processors, they must be declared separately on the annotation processor path.Gradle ignores annotation processors on the compile classpath.

Example 7.Declaring annotation processors

build.gradle.kts

dependencies {    // The dagger compiler and its transitive dependencies will only be found on annotation processing classpath    annotationProcessor("com.google.dagger:dagger-compiler:2.44")    // And we still need the Dagger library on the compile classpath itself    implementation("com.google.dagger:dagger:2.44")}

build.gradle

dependencies {    // The dagger compiler and its transitive dependencies will only be found on annotation processing classpath    annotationProcessor 'com.google.dagger:dagger-compiler:2.44'    // And we still need the Dagger library on the compile classpath itself    implementation 'com.google.dagger:dagger:2.44'}

Variant aware selection

The whole set of JVM plugins leveragevariant aware resolution for the dependencies used.They also install a set of attributes compatibility and disambiguation rules toconfigure the Gradle attributes for the specifics of the JVM ecosystem.

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The Java Plugin