Chapter 36. Maven Publishing (new)

Table of Contents

36.1. The “maven-publish” Plugin
36.2. Publications
36.3. Repositories
36.4. Performing a publish
36.5. Publishing to Maven Local
36.6. Generating the POM file without publishing

This chapter describes the new incubating Maven publishing support provided by the “maven-publish” plugin. Eventually this new publishing support will replace publishing via the Upload task.

Note: Signing the generated POM file generated by this plugin is currently not supported. Future versions of Gradle might add this functionality. Please use the Maven plugin for the purpose of publishing your artifacts to Maven Central.

If you are looking for documentation on the original Maven publishing support using the Upload task please see Chapter 32, Publishing artifacts.

This chapter describes how to publish build artifacts to an Apache Maven Repository. A module published to a Maven repository can be consumed by Maven, Gradle (see Chapter 25, Dependency Management) and other tools that understand the Maven repository format.

36.1. The “maven-publish” Plugin

The ability to publish in the Maven format is provided by the “maven-publish” plugin.

The “publishing” plugin creates an extension on the project named “publishing” of type PublishingExtension. This extension provides a container of named publications and a container of named repositories. The “maven-publish” plugin works with MavenPublication publications and MavenArtifactRepository repositories.

Example 36.1. Applying the 'maven-publish' plugin

build.gradle

apply plugin: 'maven-publish'

Applying the “maven-publish” plugin does the following:

36.2. Publications

If you are not familiar with project artifacts and configurations, you should read the Chapter 32, Publishing artifacts that introduces these concepts. This chapter also describes “publishing artifacts” using a different mechanism than what is described in this chapter. The publishing functionality described here will eventually supersede that functionality.

Publication objects describe the structure/configuration of a publication to be created. Publications are published to repositories via tasks, and the configuration of the publication object determines exactly what is published. All of the publications of a project are defined in the PublishingExtension.getPublications() container. Each publication has a unique name within the project.

For the “maven-publish” plugin to have any effect, a MavenPublication must be added to the set of publications. This publication determines which artifacts are actually published as well as the details included in the associated POM file. A publication can be configured by adding components, customizing artifacts, and by modifying the generated POM file directly.

36.2.1. Publishing a Software Component

The simplest way to publish a Gradle project to a Maven repository is to specify a SoftwareComponent to publish. The components presently available for publication are:

Table 36.1. Software Components

Name Provided By Artifacts Dependencies
java Chapter 47, The Java Plugin Generated jar file Dependencies from 'runtime' configuration
web Chapter 50, The War Plugin Generated war file No dependencies

In the following example, artifacts and runtime dependencies are taken from the `java` component, which is added by the Java Plugin.

Example 36.2. Adding a MavenPublication for a Java component

build.gradle

publishing {
    publications {
        mavenJava(MavenPublication) {
            from components.java
        }
    }
}

36.2.2. Publishing custom artifacts

It is also possible to explicitly configure artifacts to be included in the publication. Artifacts are commonly supplied as raw files, or as instances of AbstractArchiveTask (e.g. Jar, Zip).

For each custom artifact, it is possible to specify the extension and classifier values to use for publication. Note that only one of the published artifacts can have an empty classifier, and all other artifacts must have a unique classifier/extension combination.

Configure custom artifacts as follows:

Example 36.3. Adding additional artifact to a MavenPublication

build.gradle

task sourceJar(type: Jar) {
    from sourceSets.main.allJava
}

publishing {
    publications {
        mavenJava(MavenPublication) {
            from components.java

            artifact sourceJar {
                classifier "sources"
            }
        }
    }
}

See the MavenPublication class in the API documentation for more information about how artifacts can be customized.

36.2.3. Identity values in the generated POM

The attributes of the generated POM file will contain identity values derived from the following project properties:

Overriding the default identity values is easy: simply specify the groupId, artifactId or version attributes when configuring the MavenPublication.

Example 36.4. customizing the publication identity

build.gradle

publishing {
    publications {
        maven(MavenPublication) {
            groupId 'org.gradle.sample'
            artifactId 'project1-sample'
            version '1.1'

            from components.java
        }
    }
}

Certain repositories will not be able to handle all supported characters. For example, the ':' character cannot be used as an identifier when publishing to a filesystem-backed repository on Windows.

Maven restricts 'groupId' and 'artifactId' to a limited character set ([A-Za-z0-9_\\-.]+) and Gradle enforces this restriction. For 'version' (as well as artifact 'extension' and 'classifier'), Gradle will handle any valid Unicode character.

The only Unicode values that are explicitly prohibited are '\', '/' and any ISO control character. Supplied values are validated early in publication.

36.2.4. Modifying the generated POM

The generated POM file may need to be tweaked before publishing. The “maven-publish” plugin provides a hook to allow such modification.

Example 36.5. Modifying the POM file

build.gradle

publications {
    mavenCustom(MavenPublication) {
        pom.withXml {
            asNode().appendNode('description',
                                'A demonstration of maven POM customization')
        }
    }
}

In this example we are adding a 'description' element for the generated POM. With this hook, you can modify any aspect of the POM. For example, you could replace the version range for a dependency with the actual version used to produce the build.

See MavenPom.withXml(org.gradle.api.Action) in the API documentation for more information.

It is possible to modify virtually any aspect of the created POM. This means that it is also possible to modify the POM in such a way that it is no longer a valid Maven POM, so care must be taken when using this feature.

The identifier (groupId, artifactId, version) of the published module is an exception; these values cannot be modified in the POM using the `withXML` hook.

36.2.5. Publishing multiple modules

Sometimes it's useful to publish multiple modules from your Gradle build, without creating a separate Gradle subproject. An example is publishing a separate API and implementation jar for your library. With Gradle this is simple:

Example 36.6. Publishing multiple modules from a single project

build.gradle

task apiJar(type: Jar) {
    baseName "publishing-api"
    from sourceSets.main.output
    exclude '**/impl/**'
}

publishing {
    publications {
        impl(MavenPublication) {
            groupId 'org.gradle.sample.impl'
            artifactId 'project2-impl'
            version '2.3'

            from components.java
        }
        api(MavenPublication) {
            groupId 'org.gradle.sample'
            artifactId 'project2-api'
            version '2'

            artifact apiJar
        }
    }
}

If a project defines multiple publications then Gradle will publish each of these to the defined repositories. Each publication must be given a unique identity as described above.

36.3. Repositories

Publications are published to repositories. The repositories to publish to are defined by the PublishingExtension.getRepositories() container.

Example 36.7. Declaring repositories to publish to

build.gradle

publishing {
    repositories {
        maven {
            // change to point to your repo, e.g. http://my.org/repo
            url "$buildDir/repo"
        }
    }
}

The DSL used to declare repositories for publication is the same DSL that is used to declare repositories to consume dependencies from, RepositoryHandler. However, in the context of Maven publication only MavenArtifactRepository repositories can be used for publication.

36.4. Performing a publish

The “maven-publish” plugin automatically creates a PublishToMavenRepository task for each MavenPublication and MavenArtifactRepository combination in the publishing.publications and publishing.repositories containers respectively.

The created task is named “publish«PUBNAME»PublicationTo«REPONAME»Repository”.

Example 36.8. Publishing a project to a Maven repository

build.gradle

apply plugin: 'java'
apply plugin: 'maven-publish'

group = 'org.gradle.sample'
version = '1.0'

publishing {
    publications {
        mavenJava(MavenPublication) {
            from components.java
        }
    }
}
publishing {
    repositories {
        maven {
            // change to point to your repo, e.g. http://my.org/repo
            url "$buildDir/repo"
        }
    }
}

Output of gradle publish

> gradle publish
:generatePomFileForMavenJavaPublication
:compileJava
:processResources NO-SOURCE
:classes
:jar
:publishMavenJavaPublicationToMavenRepository
:publish

BUILD SUCCESSFUL in 0s
4 actionable tasks: 4 executed

In this example, a task named “publishMavenJavaPublicationToMavenRepository” is created, which is of type PublishToMavenRepository. This task is wired into the publish lifecycle task. Executing “gradle publish” builds the POM file and all of the artifacts to be published, and transfers them to the repository.

36.5. Publishing to Maven Local

For integration with a local Maven installation, it is sometimes useful to publish the module into the local .m2 repository. In Maven parlance, this is referred to as 'installing' the module. The “maven-publish” plugin makes this easy to do by automatically creating a PublishToMavenLocal task for each MavenPublication in the publishing.publications container. Each of these tasks is wired into the publishToMavenLocal lifecycle task. You do not need to have `mavenLocal` in your `publishing.repositories` section.

The created task is named “publish«PUBNAME»PublicationToMavenLocal”.

Example 36.9. Publish a project to the Maven local repository

Output of gradle publishToMavenLocal

> gradle publishToMavenLocal
:generatePomFileForMavenJavaPublication
:compileJava
:processResources NO-SOURCE
:classes
:jar
:publishMavenJavaPublicationToMavenLocal
:publishToMavenLocal

BUILD SUCCESSFUL in 0s
4 actionable tasks: 4 executed

The resulting task in this example is named “publishMavenJavaPublicationToMavenLocal”. This task is wired into the publishToMavenLocal lifecycle task. Executing “gradle publishToMavenLocal” builds the POM file and all of the artifacts to be published, and “installs” them into the local Maven repository.

36.6. Generating the POM file without publishing

At times it is useful to generate a Maven POM file for a module without actually publishing. Since POM generation is performed by a separate task, it is very easy to do so.

The task for generating the POM file is of type GenerateMavenPom, and it is given a name based on the name of the publication: “generatePomFileFor«PUBNAME»Publication”. So in the example below, where the publication is named “mavenCustom”, the task will be named “generatePomFileForMavenCustomPublication”.

Example 36.10. Generate a POM file without publishing

build.gradle

model {
    tasks.generatePomFileForMavenCustomPublication {
        destination = file("$buildDir/generated-pom.xml")
    }
}

Output of gradle generatePomFileForMavenCustomPublication

> gradle generatePomFileForMavenCustomPublication
:generatePomFileForMavenCustomPublication

BUILD SUCCESSFUL in 0s
1 actionable task: 1 executed

All details of the publishing model are still considered in POM generation, including components, custom artifacts, and any modifications made via pom.withXml.

The “maven-publish” plugin leverages some experimental support for late plugin configuration, and any GenerateMavenPom tasks will not be constructed until the publishing extension is configured. The simplest way to ensure that the publishing plugin is configured when you attempt to access the GenerateMavenPom task is to place the access inside a model block, as the example above demonstrates.

The same applies to any attempt to access publication-specific tasks like PublishToMavenRepository. These tasks should be referenced from within a model block.