Building Java Libraries - Gradle User Manual

# Building Java Libraries

Support for building Java libraries using the software model is currently incubating. Please be aware that the DSL, APIs and other configuration may change in later Gradle versions.

The Java software plugins are intended to replace the Java plugin, and leverage the Gradle software model to achieve the best performance, improved expressiveness and support for variant-aware dependency management.

## Features

The Java software plugins provide:

- Support for building Java libraries and other components that run on the JVM.

- Support for several source languages.

- Support for building different variants of the same software, for different Java versions, or for any purpose.

- Build time definition and enforcement of Java library API.

- Compile avoidance.

- Dependency management between Java software components.

## Java Software Model

The Java software plugins provide a software model that describes Java based software and how it should be built. This Java software model extends the base Gradle software model, to add support for building JVM libraries. A JVM library is a kind of library that is built for and runs on the JVM. It may be built from Java source, or from various other languages. All JVM libraries provide an API of some kind.

## Usage

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

Example 522. Using the Java software plugins

`build.gradle`

    plugins {
        id 'jvm-component'
        id 'java-lang'
    }

## Creating a library

A library is created by declaring a `JvmLibrarySpec` under the `components` element of the `model`:

Example 523. Creating a java library

`build.gradle`

    model {
        components {
            main(JvmLibrarySpec)
        }
    }

Output of `gradle build`

    > gradle build
    :compileMainJarMainJava
    :processMainJarMainResources
    :createMainJar
    :mainApiJar
    :mainJar
    :assemble
    :check UP-TO-DATE
    :build

    BUILD SUCCESSFUL in 0s
    4 actionable tasks: 4 executed

This example creates a library named `main`, which will implicitly create a `JavaSourceSet` named `java`. The conventions of the legacy Java plugin are observed, where Java sources are expected to be found in `src/main/java`, while resources are expected to be found in `src/main/resources`.

## Source Sets

Source sets represent logical groupings of source files in a library. A library can define multiple source sets and all sources will be compiled and included in the resulting binaries. When a library is added to a build, the following source sets are added by default.

Table 101. Java plugin - default source sets

Source Set | Type | Directory
java | `JavaSourceSet` | src/${library.name}/java
resources | `JvmResourceSet` | src/${library.name}/resources

It is possible to configure an existing source set through the `sources` container:

Example 524. Configuring a source set

`build.gradle`

    components {
        main {
            sources {
                java {
                    // configure the "java" source set
                }
            }
        }
    }

It is also possible to create an additional source set, using the `JavaSourceSet` type:

Example 525. Creating a new source set

`build.gradle`

    components {
        main {
            sources {
                mySourceSet(JavaSourceSet) {
                    // configure the "mySourceSet" source set
                }
            }
        }
    }

## Tasks

By default, when the plugins above are applied, no new tasks are added to the build. However, when libraries are defined, conventional tasks are added which build and package each binary of the library.

For each binary of a library, a single lifecycle task is created which executes all tasks associated with building the binary. To build all binaries, the standard `build` lifecycle task can be used.

Table 102. Java plugin - lifecycle tasks

Component Type | Binary Type | Lifecycle Task
`JvmLibrarySpec` | `JvmBinarySpec` | ${library.name}${binary.name}

For each source set added to a library, tasks are added to compile or process the source files for each binary.

Table 103. Java plugin - source set tasks

Source Set Type | Task name | Type | Description
`JavaSourceSet` | compile${library.name}${binary.name}${library.name}${sourceset.name} | `PlatformJavaCompile` | Compiles the sources of a given source set.
`JvmResourceSet` | process${library.name}${binary.name}${library.name}${sourceset.name} | `ProcessResources` | Copies the resources in the given source set to the classes output directory.

For each binary in a library, a packaging task is added to create the jar for that binary.

Table 104. Java plugin - packaging tasks

Binary Type | Task name | Depends on | Type | Description
`JvmBinarySpec` | create${library.name}${binary.name} | all `PlatformJavaCompile` and `ProcessResources` tasks associated with the binary | `Jar` | Packages the compiled classes and processed resources of the binary.

## Finding out more about your project

Gradle provides a report that you can run from the command-line that shows details about the components and binaries that your project produces. To use this report, just run `gradle components`. Below is an example of running this report for one of the sample projects:

Example 526. The components report

Output of `gradle components`

    > gradle components
    :components

    ------------------------------------------------------------
    Root project
    ------------------------------------------------------------

    JVM library 'main'
    ------------------

    Source sets
        Java source 'main:java'
            srcDir: src/main/java
        Java source 'main:mySourceSet'
            srcDir: src/main/mySourceSet
        JVM resources 'main:resources'
            srcDir: src/main/resources

    Binaries
        Jar 'main:jar'
            build using task: :mainJar
            target platform: java7
            tool chain: JDK 7 (1.7)
            classes dir: build/classes/main/jar
            resources dir: build/resources/main/jar
            API Jar file: build/jars/main/jar/api/main.jar
            Jar file: build/jars/main/jar/main.jar

    Note: currently not all plugins register their components, so some components may not be visible here.

    BUILD SUCCESSFUL in 0s
    1 actionable task: 1 executed

## Dependencies

A component in the Java software model can declare dependencies on other Java libraries. If component `main` depends on library `util`, this means that the API of `util` is required when compiling the sources of `main`, and the runtime of `util` is required when running or testing `main`. The terms 'API' and 'runtime' are examples of usages of a Java library.

### Library usage

The 'API' usage of a Java library consists of:

- Artifact(s): the Jar file(s) containing the public classes of that library

- Dependencies: the set of other libraries that are required to compile against that library

When library `main` is compiled with a dependency on `util`, the 'API' dependencies of 'util' are resolved transitively, resulting in the complete set of libraries required to compile. For each of these libraries (including 'util'), the 'API' artifacts will be included in the compile classpath.

Similarly, the 'runtime' usage of a Java library consists of artifacts and dependencies. When a Java component is tested or bundled into an application, the runtime usage of any runtime dependencies will be resolved transitively into the set of libraries required at runtime. The runtime artifacts of these libraries will then be included in the testing or runtime classpath.

### Dependency types

Two types of Java library dependencies can be declared:

- Dependencies on a library defined in a local Gradle project

- Dependencies on a library published to a Maven repository

Dependencies onto libraries published to an Ivy repository are not yet supported.

### Declaring dependencies

Dependencies may be declared for a specific `JavaSourceSet`, for an entire `JvmLibrarySpec` or as part of the `JvmApiSpec` of a component:

Example 527. Declaring a dependency onto a library

`build.gradle`

    model {
        components {
            server(JvmLibrarySpec) {
                sources {
                    java {
                        dependencies {
                            library 'core'
                        }
                    }
                }
            }

            core(JvmLibrarySpec) {
                dependencies {
                    library 'commons'
                }
            }

            commons(JvmLibrarySpec) {
                api {
                    dependencies {
                        library 'collections'
                    }
                }
            }

            collections(JvmLibrarySpec)
        }
    }

Output of `gradle serverJar`

    > gradle serverJar
    :compileCollectionsJarCollectionsJava
    :collectionsApiJar
    :compileCommonsJarCommonsJava
    :commonsApiJar
    :compileCoreJarCoreJava
    :processCoreJarCoreResources
    :coreApiJar
    :compileServerJarServerJava
    :createServerJar
    :serverApiJar
    :serverJar

    BUILD SUCCESSFUL in 0s
    10 actionable tasks: 10 executed

Dependencies declared for a source set will only be used for compiling that particular source set.

Dependencies declared for a component will be used when compiling all source sets for the component.

Dependencies declared for the component `api` are used for compiling all source sets for the component, and are also exported as part of the component’s API. See Enforcing API boundaries at compile time for more details.

The previous example declares a dependency for the `java` source set of the `server` library onto the `core` library of the same project. However, it is possible to create a dependency on a library in a different project as well:

Example 528. Declaring a dependency onto a project with an explicit library

`build.gradle`

    client(JvmLibrarySpec) {
        sources {
            java {
                dependencies {
                    project ':util' library 'main'
                }
            }
        }
    }

Output of `gradle clientJar`

    > gradle clientJar
    :util:compileMainJarMainJava
    :util:mainApiJar
    :compileClientJarClientJava
    :clientApiJar
    :createClientJar
    :clientJar

    BUILD SUCCESSFUL in 0s
    5 actionable tasks: 5 executed

When the target project defines a single library, the `library` selector can be omitted altogether:

Example 529. Declaring a dependency onto a project with an implicit library

`build.gradle`

    dependencies {
        project ':util'
    }

Dependencies onto libraries published to Maven repositories can be declared via `module identifiers` consisting of a `group name`, a `module name` plus an optional `version selector`:

Example 530. Declaring a dependency onto a library published to a Maven repository

`build.gradle`

    verifier(JvmLibrarySpec) {
        dependencies {
            module 'asm'          group 'org.ow2.asm' version '5.0.4'
            module 'asm-analysis' group 'org.ow2.asm'
        }
    }

Output of `gradle verifierJar`

    > gradle verifierJar
    :compileVerifierJarVerifierJava
    :createVerifierJar
    :verifierApiJar
    :verifierJar

    BUILD SUCCESSFUL in 0s
    3 actionable tasks: 3 executed

A shorthand notation for module identifiers can also be used:

Example 531. Declaring a module dependency using shorthand notation

`build.gradle`

    dependencies {
        module 'org.ow2.asm:asm:5.0.4'
        module 'org.ow2.asm:asm-analysis'
    }

Module dependencies will be resolved against the configured repositories as usual:

Example 532. Configuring repositories for dependency resolution

`build.gradle`

    repositories {
        mavenCentral()
    }

The `DependencySpecContainer` class provides a complete reference of the dependencies DSL.

## Defining a Library API

Every library has an API, which consists of artifacts and dependencies that are required to compile against the library. The library may be explicitly declared for a component, or may be implied based on other component metadata.

By default, all `public` types of a library are considered to be part of its API. In many cases this is not ideal; a library will contain many public types that intended for internal use within that library. By explicitly declaring an API for a Java library, Gradle can provide compile-time encapsulation of these internal-but-public types. The types to include in a library API are declared at the package level. Packages containing API types are considered to be exported.

By default, dependencies of a library are not considered to be part of its API. By explicitly declaring a dependency as part of the library API, this dependency will then be made available to consumers when compiling. Dependencies declared this way are considered to be exported, and are known as 'API dependencies'.

JDK 9 will introduce Jigsaw, the reference implementation of the Java Module System. Jigsaw will provide both compile-time and run-time enforcement of API encapsulation.

Gradle anticipates the arrival of JDK 9 and the Java Module System with an approach to specifying and enforcing API encapsulation at compile-time. This allows Gradle users to leverage the many benefits of strong encapsulation, and prepare their software projects for migration to JDK 9.

### Some terminology

- An API is a set of classes, interfaces, methods that are exposed to a consumer.

- An API specification is the specification of classes, interfaces or methods that belong to an API, together with the set of dependencies that are part of the API. It can be found in various forms, like `module-info.java` in Jigsaw, or the `api { …​ }` block that Gradle defines as part of those stories. Usually, we can simplify this to a list of packages, called exported packages.

- A runtime jar consists of API classes and non-API classes used at execution time. There can be multiple runtime jars depending on combinations of the variant dimensions: target platform, hardware infrastructure, target application server, …​

- API classes are classes of a variant which match the API specification

- Non-API classes are classes of a variant which do not match the API specification.

- A stubbed API class is an API class for which its implementation and non public members have been removed. It is meant to be used when a consumer is going to be compiled against an API.

- An API jar is a collection of API classes. There can be multiple API jars depending on the combinations of variant dimensions.

- A stubbed API jar is a collection of stubbed API classes. There can be multiple stubbed API jars depending on the combinations of variant dimensions.

- An ABI (application binary interface) corresponds to the public signature of an API, that is to say the set of stubbed API classes that it exposes (and their API visible members).

We avoid the use of the term implementation because it is too vague: both API classes and Non-API classes can have an implementation. For example, an API class can be an interface, but also a concrete class. Implementation is an overloaded term in the Java ecosystem, and often refers to a class implementing an interface. This is not the case here: a concrete class can be member of an API, but to compile against an API, you don’t need the implementation of the class: all you need is the signatures.

### Specifying API classes

Example 533. Specifying api packages

`build.gradle`

    model {
        components {
            main(JvmLibrarySpec) {
                api {
                    exports 'org.gradle'
                    exports 'org.gradle.utils'
                }
            }
        }
    }

### Specifying API dependencies

Example 534. Specifying api dependencies

`build.gradle`

    commons(JvmLibrarySpec) {
        api {
            dependencies {
                library 'collections'
            }
        }
    }

### Compile avoidance

When you define an API for your library, Gradle enforces the usage of that API at compile-time. This comes with 3 direct consequences:

- Trying to use a non-API class in a dependency will now result in a compilation error.

- Changing the implementation of an API class will not result in recompilation of consumers if the ABI doesn’t change (that is to say, all public methods have the same signature but not necessarily the same body).

- Changing the implementation of a non-API class will not result in recompilation of consumers. This means that changes to non-API classes will not trigger recompilation of downstream dependencies, because the ABI of the component doesn’t change.

Given a main component that exports `org.gradle`, `org.gradle.utils` and defines those classes:

Example 535. Main sources

`src/main/java/org/gradle/Person.java`

    package org.gradle;

    public class Person {
        private final String name;

        public Person(String name) {
            this.name = name;
        }

        public String getName() {
            return name;
        }
    }

`src/main/java/org/gradle/internal/PersonInternal.java`

    package org.gradle.internal;

    import org.gradle.Person;

    public class PersonInternal extends Person {
        public PersonInternal(String name) {
            super(name);
        }
    }

`src/main/java/org/gradle/utils/StringUtils.java`

    package org.gradle.utils;

    public abstract class StringUtils {

    }

Compiling a component client that declares a dependency onto main will succeed:

Example 536. Client component

`build.gradle`

    model {
        components {
            client(JvmLibrarySpec) {
                sources {
                    java {
                        dependencies {
                            library 'main'
                        }
                    }
                }
            }
        }
    }

`src/client/java/org/gradle/Client.java`

    package org.gradle;

    public class Client {
        private Person person;

        public void setPerson(Person p) { this.person = p; }
        public Person getPerson() { return person; }
    }

Output of `gradle :clientJar`

    > gradle :clientJar
    :compileMainJarMainJava
    :processMainJarMainResources
    :mainApiJar
    :compileClientJarClientJava
    :clientApiJar
    :createClientJar
    :clientJar

    BUILD SUCCESSFUL in 0s
    6 actionable tasks: 6 executed

But trying to compile a component brokenclient that declares a dependency onto main but uses an non-API class of main will result in a compile-time error:

Example 537. Broken client component

`src/brokenclient/java/org/gradle/Client.java`

    package org.gradle;

    import org.gradle.internal.PersonInternal;

    public class Client {
        private PersonInternal person;

        public void setPerson(PersonInternal p) { this.person = p; }
        public PersonInternal getPerson() { return person; }
    }

Output of `gradle :brokenclientJar`

    > gradle :brokenclientJar
    :compileMainJarMainJava
    :processMainJarMainResources
    :mainApiJar
    :compileBrokenclientJarBrokenclientJava FAILED
    4 actionable tasks: 4 executed

On the other hand, if Person.java in client is updated and its API hasn’t changed, client will not be recompiled.

Example 538. Making non-API implementation-only change

`src/main/java/org/gradle/Person.java`

    package org.gradle;

    public class Person {
        private final String name;

        public Person(String name) {
            // we updated the body if this method
            // but the signature doesn't change
            // so we will not recompile components
            // that depend on this class
            this.name = name.toUpperCase();
        }

        public String getName() {
            return name;
        }
    }

This is in particular important for incremental builds of large projects, where we can avoid the compilation of dependencies in chain, and then dramatically reduce build duration:

Example 539. Recompiling the client

Output of `gradle :clientJar`

    > gradle :clientJar
    :compileMainJarMainJava
    :processMainJarMainResources UP-TO-DATE
    :mainApiJar
    :compileClientJarClientJava UP-TO-DATE
    :clientApiJar UP-TO-DATE
    :createClientJar UP-TO-DATE
    :clientJar UP-TO-DATE

    BUILD SUCCESSFUL in 0s
    6 actionable tasks: 2 executed, 4 up-to-date

## Platform aware dependency management

### Specifying the target platform

The software model extracts the target platform as a core concept. In the Java world, this means that a library can be built, or resolved, against a specific version of Java. For example, if you compile a library for Java 5, we know that such a library can be consumed by a library built for Java 6, but the opposite is not true. Gradle lets you define which platforms a library targets, and will take care of:

- generating a binary for each target platform (eg, a Java 5 jar as well as a Java 6 jar)

- resolving dependencies against a matching platform

The `targetPlatform` DSL defines which platforms a library should be built against:

Example 540. Declaring target platforms

`core/build.gradle`

    model {
        components {
            main(JvmLibrarySpec) {
                targetPlatform 'java5'
                targetPlatform 'java6'
            }
       }
    }

Output of `gradle :core:build`

    > gradle :core:build
    :core:compileMainJava5JarMainJava
    :core:processMainJava5JarMainResources
    :core:createMainJava5Jar
    :core:mainJava5ApiJar
    :core:mainJava5Jar
    :core:compileMainJava6JarMainJava
    :core:compileMainJava6JarMainJava6JarJava
    :core:processMainJava6JarMainResources
    :core:createMainJava6Jar
    :core:mainJava6ApiJar
    :core:mainJava6Jar
    :core:assemble
    :core:check UP-TO-DATE
    :core:build

    BUILD SUCCESSFUL in 0s
    9 actionable tasks: 9 executed

When building the application, Gradle generates two binaries: `java5MainJar` and `java6MainJar` corresponding to the target versions of Java. These artifacts will participate in dependency resolution as described here.

### Binary specific source sets

For each `JvmLibrarySpec` it is possible to define additional source sets for each binary. A common use case for this is having specific dependencies for each variant and source sets that conform to those dependencies. The example below configures a `java6` source set on the `main.java6Jar` binary:

Example 541. Declaring binary specific sources

`core/build.gradle`

    main {
        binaries.java6Jar {
            sources {
                java(JavaSourceSet) {
                    source.srcDir 'src/main/java6'
                }
            }
        }
    }

Output of `gradle clean :core:mainJava6Jar`

    > gradle clean :core:mainJava6Jar
    :core:clean UP-TO-DATE
    :server:clean UP-TO-DATE
    :core:compileMainJava6JarMainJava
    :core:compileMainJava6JarMainJava6JarJava
    :core:processMainJava6JarMainResources
    :core:createMainJava6Jar
    :core:mainJava6ApiJar
    :core:mainJava6Jar

    BUILD SUCCESSFUL in 0s
    7 actionable tasks: 5 executed, 2 up-to-date

### Dependency resolution

When a library targets multiple versions of Java and depends on another library, Gradle will make its best effort to resolve the dependency to the most appropriate version of the dependency library. In practice, this means that Gradle chooses the highest compatible version:

- for a binary `B` built for Java `n`

- for a dependency binary `D` built for Java `m`

- `D` is compatible with `B` if `m<=n`

- for multiple compatible binaries `D(java 5), D(java 6), …​D(java m)`, choose the compatible D binary with the highest Java version

Example 542. Declaring target platforms

`server/build.gradle`

    model {
        components {
            main(JvmLibrarySpec) {
                targetPlatform 'java5'
                targetPlatform 'java6'
                sources {
                    java {
                        dependencies {
                            project ':core' library 'main'
                        }
                    }
                }
            }
        }
    }

Output of `gradle clean :server:build`

    > gradle clean :server:build
    :core:clean UP-TO-DATE
    :server:clean UP-TO-DATE
    :core:compileMainJava5JarMainJava
    :core:processMainJava5JarMainResources
    :core:mainJava5ApiJar
    :server:compileMainJava5JarMainJava
    :server:createMainJava5Jar
    :server:mainJava5ApiJar
    :server:mainJava5Jar
    :core:compileMainJava6JarMainJava
    :core:compileMainJava6JarMainJava6JarJava
    :core:processMainJava6JarMainResources
    :core:mainJava6ApiJar
    :server:compileMainJava6JarMainJava
    :server:createMainJava6Jar
    :server:mainJava6ApiJar
    :server:mainJava6Jar
    :server:assemble
    :server:check UP-TO-DATE
    :server:build

    BUILD SUCCESSFUL in 0s
    15 actionable tasks: 13 executed, 2 up-to-date

In the example above, Gradle automatically chooses the Java 6 variant of the dependency for the Java 6 variant of the `server` component, and chooses the Java 5 version of the dependency for the Java 5 variant of the `server` component.

## Custom variant resolution

The Java plugin, in addition to the target platform resolution, supports resolution of custom variants. Custom variants can be defined on custom binary types, as long as they extend `JarBinarySpec`. Users interested in testing this incubating feature can check out the documentation of the `Variant` annotation.

## Testing Java libraries

### Standalone JUnit test suites

The Java software model supports defining standalone JUnit test suites as components of the model. Standalone test suite are components that are self contained, in the sense that there is no component under test: everything being tested must belong to the test suite sources.

A test suite is declared by creating a component of type `JUnitTestSuiteSpec`, which is available when you apply the `junit-test-suite` plugin:

Example 543. Using the JUnit plugin

`build.gradle`

    plugins {
        id 'jvm-component'
        id 'java-lang'
        id 'junit-test-suite'
    }

    model {
        testSuites {
            test(JUnitTestSuiteSpec) {
                jUnitVersion '4.12'
            }
        }
    }

In the example above, `test` is the name of our test suite. By convention, Gradle will create two source sets for the test suite, based on the name of the component: one for Java sources, and the other for resources: `src/test/java` and `src/test/resources`. If the component was named `integTest`, then sources and resources would have been found respectively in `src/integTest/java` and `src/integTest/resources`.

Once the component is created, the test suite can be executed running the `<<test suite name>>BinaryTest` task:

Example 544. Executing the test suite

`src/test/java/org/gradle/MyTest.java`

    package org.gradle;

    import org.junit.Test;

    import static org.junit.Assert.*;

    public class MyTest {
        @Test
        public void myTestMethod() {
            assertEquals(4, "test".length());
        }
    }

Output of `gradle testBinaryTest`

    > gradle testBinaryTest
    :compileTestBinaryTestJava
    :processTestBinaryTestResources
    :testBinaryTest

    BUILD SUCCESSFUL in 0s
    3 actionable tasks: 3 executed

It is possible to configure source sets in a similar way as libraries.

A test suite being a component can also declare dependencies onto other components.

A test suite can also contain resources, in which case it is possible to configure the resource processing task:

Example 545. Executing the test suite

`build.gradle`

    model {
        tasks.processTestBinaryTestResources {
            // uncomment lines
            filter { String line ->
                line.replaceAll('<!-- (.+?) -->', '$1')
            }
        }
    }

### Testing JVM libraries with JUnit

It is likely that you will want to test another JVM component. The Java software model supports it exactly like standalone test suites, by just declaring an additional component under test:

Example 546. Declaring a component under test

`build.gradle`

    model {
        components {
            main(JvmLibrarySpec)
        }
        testSuites {
            test(JUnitTestSuiteSpec) {
                jUnitVersion '4.12'
                testing $.components.main
            }
        }
    }

Output of `gradle testMainJarBinaryTest`

    > gradle testMainJarBinaryTest
    :compileMainJarMainJava
    :processMainJarMainResources
    :compileTestMainJarBinaryTestJava
    :testMainJarBinaryTest

    BUILD SUCCESSFUL in 0s
    4 actionable tasks: 4 executed

Note that the syntax to choose the component under test is a reference (`$.`). You can select any `JvmComponentSpec` as the component under test. It’s also worth noting that when you declare a component under test, a test suite is created for each binary of the component under test (for example, if the component under test has a Java 7 and Java 8 version, 2 different test suite binaries would be automatically created).

## Declaring Java toolchains

You can declare the list of local JVM installations using the `javaInstallations` model block. Gradle will use this information to locate your JVMs and probe their versions. Please note that this information is not yet used by Gradle to select the appropriate JDK or JRE when compiling your Java sources, or when executing Java applications. A local Java installation can be declared using the `LocalJava` type, independently of the fact they are a JDK or a JRE:

Example 547. Declaring local Java installations

`build.gradle`

    model {
        javaInstallations {
            openJdk6(LocalJava) {
                path '/usr/lib/jvm/jdk1.6.0-amd64'
            }
            oracleJre7(LocalJava) {
                path '/usr/lib/jvm/jre1.7.0'
            }
            ibmJdk8(LocalJava) {
                path '/usr/lib/jvm/jdk1.8.0'
            }
        }
    }