Chapter 71. Building Java Libraries

Table of Contents

71.1. Features
71.2. Java Software Model
71.3. Usage
71.4. Creating a library
71.5. Source Sets
71.6. Tasks
71.7. Finding out more about your project
71.8. Dependencies
71.9. Defining a Library API
71.10. Platform aware dependency management
71.11. Custom variant resolution
71.12. Testing Java libraries
71.13. Declaring Java toolchains

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.

71.1. 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.

71.2. 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.

71.3. Usage

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

Example 71.1. Using the Java software plugins

build.gradle

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

71.4. Creating a library

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

Example 71.2. 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.

71.5. 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 71.1. 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 71.3. 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 71.4. Creating a new source set

build.gradle

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

71.6. 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 71.2. 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 71.3. 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 71.4. 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.

71.7. 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 71.5. 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

71.8. 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.

71.8.1. 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.

71.8.2. 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.

71.8.3. Declaring dependencies

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

Example 71.6. 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 71.7. 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 71.8. 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 71.9. 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 71.10. 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 71.11. Configuring repositories for dependency resolution

build.gradle

repositories {
    mavenCentral()
}

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

71.9. 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.

71.9.1. 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.

71.9.2. Specifying API classes

Example 71.12. Specifying api packages

build.gradle

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

71.9.3. Specifying API dependencies

Example 71.13. Specifying api dependencies

build.gradle

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

71.9.4. 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 71.14. 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 71.15. 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 71.16. 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. 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 71.17. Recompiling the client

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;
    }
}

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

71.10. Platform aware dependency management

71.10.1. 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 71.18. 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.

71.10.2. 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 71.19. 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

71.10.3. 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 71.20. 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.

71.11. 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.

71.12. Testing Java libraries

71.12.1. 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 71.21. 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 71.22. 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 71.23. Executing the test suite

build.gradle

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

71.12.2. 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 71.24. 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).

71.13. 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 71.25. 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'
        }
    }
}