# Chapter 34. Maven Publishing (new)

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.

If you are looking for documentation on the original Maven publishing support using the `Upload` task please see Chapter 30, 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 23, Dependency Management) and other tools that understand the Maven repository format.

## 34.1. 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 34.1. Applying the 'maven-publish' plugin

`build.gradle`

    apply plugin: 'maven-publish'

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

- Applies the “`publishing`” plugin
- Establishes a rule to automatically create a `GenerateMavenPom` task for each `MavenPublication` added (see Section 34.2, “Publications”).
- Establishes a rule to automatically create a `PublishToMavenRepository` task for the combination of each `MavenPublication` added (see Section 34.2, “Publications”), with each `MavenArtifactRepository` added (see Section 34.3, “Repositories”).
- Establishes a rule to automatically create a `PublishToMavenLocal` task for each `MavenPublication` added (seeSection 34.2, “Publications”).

## 34.2. Publications

If you are not familiar with project artifacts and configurations, you should read the Chapter 30, 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.

### 34.2.1. 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 34.1. Software Components

Name | Provided By | Artifacts | Dependencies
`java` | Chapter 45, The Java Plugin | Generated jar file | Dependencies from 'runtime' configuration
`web` | Chapter 47, 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 34.2. Adding a MavenPublication for a Java component

`build.gradle`

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

### 34.2.2. 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 34.3. 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.

### 34.2.3. Identity values in the generated POM

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

- `groupId` - `Project.getGroup()`
- `artifactId` - `Project.getName()`
- `version` - `Project.getVersion()`

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

Example 34.4. 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.

### 34.2.4. 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 34.5. 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 should you need to. 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.

### 34.2.5. 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 34.6. 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.

## 34.3. Repositories

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

Example 34.7. 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.

## 34.4. 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 34.8. 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 UP-TO-DATE
    :classes
    :jar
    :publishMavenJavaPublicationToMavenRepository
    :publish

    BUILD SUCCESSFUL

    Total time: 1 secs

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.

## 34.5. 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 34.9. Publish a project to the Maven local repository

Output of `gradle publishToMavenLocal`

    > gradle publishToMavenLocal
    :generatePomFileForMavenJavaPublication
    :compileJava
    :processResources UP-TO-DATE
    :classes
    :jar
    :publishMavenJavaPublicationToMavenLocal
    :publishToMavenLocal

    BUILD SUCCESSFUL

    Total time: 1 secs

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.

## 34.6. 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 34.10. 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

    Total time: 1 secs

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.