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Publishing a Gradle Plugin to Maven Central

So you want to publish your Gradle plugin to Maven Central (vs the Gradle Plugin Portal). There are many steps to this process. Here they are.

Outline

  1. Create a Jira account and create a new project ticket. Read this guide. While waiting on that manual, human-driven process, get your project ready.

    1. If you are trying to publish to a domain you control, you will have to verify that you do in fact control it. I was able to do this by publishing a DNS TXT record that referenced the Jira ticket number on my domain.

  2. Create a GPG key for signing your plugin project.

  3. Publish this GPG key to a public keyserver.

  4. Add the signing plugin to your project and configure it. All published files must be signed.

  5. Configure publishing for your project with the maven-publish and java-gradle-plugin plugins.

  6. Ensure your POM meets OSS Sonatype standards.

  7. Ensure you are also publishing javadoc and sources.

  8. Publish to the Sonatype snapshots repository and verify you can apply your plugin to a real project.

  9. Publish to the staging repository and:

    1. "Close" the project (this triggers validation of your publication by Sonatype).

    2. Fix any validation errors that might occur (for example, maybe you didn’t sign all your artifacts, or maybe you forgot to publish your GPG key to a public keyserver).

    3. "Release" the project. It will automatically drop after successfully releasing.

    4. Comment on your original Jira ticket that you have promoted your first artifact. A human will review and then sync your project with Maven Central.

Create a Jira account and new project ticket

Read this guide.

Create a GPG key, publish to a public keyserver, and use it to sign your build artifacts

Note
 DISCLAIMER: I readily admit I know very little about GPG and much of the following I treat as magic invocations.

Create a GPG key

Github has an excellent guide on creating keys, and if you’re already planning to sign your commits (you should), you can use the same key for both purposes.

Publish to a public keyserver

Once you have generated a key, you need the public key ID so you can publish it to a keyserver.

$ gpg -K
/Users/trobalik/.gnupg/pubring.kbx
----------------------------------
sec   rsa4096 2020-01-01 [SC]
      W7WQ5NZWC8S339RVAAOCR0SCMV7T00FKDHG570SZ
uid           [ultimate] Tony Robalik <[email protected]>
ssb   rsa4096 2020-01-01 [E]

In this example, the public key id is DHG570SZ (the last 8 characters in the 40-character line above). Now publish that key:

$ gpg --send-keys --keyserver keyserver.ubuntu.com DHG570SZ

Verify the key has been published by executing

$ gpg --recv-keys --keyserver keyserver.ubuntu.com DHG570SZ
gpg: key MV7T00FKDHG570SZ: "Tony Robalik <[email protected]>" not changed
gpg: Total number processed: 1
gpg:              unchanged: 1

If it failed to publish, you will instead see

$ gpg --recv-keys --keyserver keyserver.ubuntu.com DHG570SZ
gpg: no valid OpenPGP data found.
gpg: Total number processed: 0

I struggled for a while to publish my key, and only succeeded after seeing this comment on a random Github issue tracker.

Like I said, magic.

Set up signing for your build artifacts

The following is largely distilled from the documentation on the Gradle signing plugin.

$ gpg --keyring secring.gpg --export-secret-keys > ~/.gnupg/secring.gpg

Open up ~/.gradle/gradle.properties (creating if it does not exist) and add the following:

signing.keyId=DHG570SZ
signing.password=secret
signing.secretKeyRingFile=/Users/me/.gnupg/secring.gpg

Where keyId is from the previous step, secret is the secret passphrase you used when creating your key, and secretKeyRingFile is the absolute path to the file, secring.gpg, you just created.

If you prefer not to store your information in ~/.gradle/gradle.properties, please see the signing plugin documentation for alternatives.

We will revisit the topic of signing in the publishing section which follows, since the final step requires two publications be available to sign.

Export your key to another computer

Now that you have a key for signing your artifacts, you’ll want to store it somewhere safe in the event you need to do work on another computer. I found a short guide on the subject, but here are the steps:

$ gpg --list-secret-keys
W7WQ5NZWC8S339RVAAOCR0SCMV7T00FKDHG570SZ
uid           [ultimate] Tony Robalik <[email protected]>
ssb   rsa4096 2020-01-01 [E]

Now export the key:

$ gpg --export-secret-keys DHG570SZ > my-private-key.asc

Copy the key somewhere (consider scp), and on the second machine, import with:

$ gpg --import my-private-key.asc

You will then need to follow the advice from Set up signing for your build artifacts to set up signing on the new computer.

Creating publications

At minimum, you should have the following plugins applied to your plugin project:

build.gradle.kts
plugins {
  `java-gradle-plugin` // for authoring Gradle plugins
  `maven-publish`      // for publishing Gradle artifacts
  signing              // for signing Gradle publications
}
Note
 nb. I also have the kotlin-dsl plugin applied, so the exact code samples that follow may require some adjustment if are using the Groovy DSL.

Tell Gradle about your plugin

Please see the official documentation for the Gradle Plugin Development Plugin for a thorough explanation of its usage.

build.gradle.kts
gradlePlugin {
  plugins {
    create("myPlugin") {
      id = "com.domain.my-plugin"
      implementationClass = "com.domain.MyPlugin"
    }
  }
}

Here we create a PluginDeclaration named "myPlugin" and add it to the plugins container. We say that the id is "com.domain.my-plugin" and the class that implements our plugin is "com.domain.MyPlugin". The authoring of Gradle plugins is outside the scope of this document, so please refer to the guide linked above if you would like to know more.

Important
 There is one point which must be mentioned, however, because the documentation is nearly silent on this. In the presence of the maven-publish plugin, the java-gradle-plugin will add a publishing task to your project; this task publishes the so-called "plugin marker artifact". When publishing your plugin jar, you must also publish this marker artifact, or else it will be very difficult to use the plugins {} syntax for applying your plugin.

Add publications

Open your ~/.gradle/gradle.properties file again, if you closed it, and add the following:

sonatypeUsername=my-user-name
sonatypePassword=my-password

These values will be used below.

The following is a very large code block; a complete example of adding and configuring your publications for publishing your Gradle plugin.

build.gradle.kts
// These two values are used by your publications below.
version = "0.1.0"
group = "com.domain"

java {
  // You may already have these first two
  sourceCompatibility = JavaVersion.VERSION_1_8
  targetCompatibility = JavaVersion.VERSION_1_8

  // OSS libraries published to Maven Central must have sources and javadoc attached.
  // these two methods are available since Gradle 6.
  withJavadocJar()
  withSourcesJar()
}

publishing {
  publications {
    // This needs to go into an afterEvaluate block because this publication is automatically added
    // by the java-gradle-plugin (well, there are other ways, but I haven't tested them yet).
    afterEvaluate {
      // Note the name is based on what you supplied above
      named<MavenPublication>("myPluginPluginMarkerMaven") {
        // This POM conforms to OSS Sonatype's requirements (and a bit more)
        pom {
          name.set("My Gradle Plugin")
          description.set("My plugin for doing things")
          url.set("https://github.com/me/my-gradle-plugin")
          licenses {
            license {
              name.set("The Apache License, Version 2.0")
              url.set("http://www.apache.org/licenses/LICENSE-2.0.txt")
            }
          }
          developers {
            developer {
              id.set("me")
              name.set("My Name")
            }
          }
          scm {
            connection.set("scm:git:git://github.com/me/my-gradle-plugin.git")
            developerConnection.set("scm:git:ssh://github.com/me/my-gradle-plugin.git")
            url.set("https://github.com/me/my-gradle-plugin")
          }
        }
      }
    }

    // This publication is for the plugin jar itself
    create<MavenPublication>("plugin") {
      from(components["java"])
      versionMapping {
        usage("java-api") {
          fromResolutionOf("runtimeClasspath")
        }
        usage("java-runtime") {
          fromResolutionResult()
        }
      }
      pom {
        // as above
      }
    }

    repositories {
      // If you think you might have outside contributors, be kind to them and don't make their builds
      // fail because they are missing credentials
      val sonatypeUsername = project.properties["sonatypeUsername"]?.toString()
      val sonatypePassword = project.properties["sonatypePassword"]?.toString()
      if (sonatypeUsername != null && sonatypePassword != null) {
        maven {
          name = "sonatype"
          val releasesRepoUrl = "https://oss.sonatype.org/service/local/staging/deploy/maven2"
          val snapshotsRepoUrl = "https://oss.sonatype.org/content/repositories/snapshots"
          url = uri(if (version.toString().endsWith("SNAPSHOT")) snapshotsRepoUrl else releasesRepoUrl)
          credentials {
            username = sonatypeUsername
            password = sonatypePassword
          }
        }
      }
    }
  }
}
Warning
 While it is possible to customize your artifact ID in your publication declarations above, I would not recommend it. By default, Gradle will use the name of your project as the artifact ID. It is simplest to just name your project well.

Sign your publications

So, you have two publications you need to publish. Your actual compiled jar, and the plugin marker artifact. The former you have to add manually, while the latter is automatically added by the java-gradle-plugin (but which must nevertheless be configured by you).

Because you have to publish two publications (or artifacts), you must also sign both publications.

build.gradle.kts
afterEvaluate {
  signing {
    sign(publishing.publications["plugin"], publishing.publications["myPluginPluginMarkerMaven"])
  }
}

Publishing to OSS Sonatype

First, publish a snapshot

You should start by publishing a snapshot, and then verifying your snapshot can be applied to a real project.

The first step, then, is to change your version from

version = "0.1.0"

to

version = "0.1.0-SNAPSHOT"

If you then execute

$ ./gradlew tasks

in your project root, you will probably see very many Publishing tasks. My project had 12 available; YMMV. Which to use? If you followed the code sample above, you’ll want the following:

$ ./gradlew publishMyPluginPluginMarkerMavenPublicationToSonatypeRepository publishPluginPublicationToSonatypeRepository

This will, of course, publish both your marker artifact and your plugin jar itself to the snapshots repo.

Consuming from the snapshots repo

In your consumer project, open the appropriate build script and apply your plugin

build.gradle[.kts]
plugins {
  id("com.domain.my-plugin") version "0.1.0-SNAPSHOT"
}

If you try to execute any task in your project right now, your build will almost certainly fail during the initialization phase. Since the plugins {} only works (by default) with plugins sourced from the Gradle Plugin Portal, you must tell Gradle about other repositories. Do that by opening your settings script:

settings.gradle
pluginsManagement {
  repositories {
    maven {
      url "https://oss.sonatype.org/content/repositories/snapshots/"
    }
    gradlePluginPortal() // there by default
  }
}

Now try to sync or build your project again, and it should work. If you also use build scans, you can check the scan and verify that your plugin came from the snapshots repository (go to the Build dependencies tab).

Publish to Maven Central

Actually, first publish to the staging repository

Now that you’ve successfully published your snapshot and verified that it works, it’s time to publish to the staging repository and then promote your first release.

First, remove the -SNAPSHOT suffix from your version:

version = "0.1.0"

Now, execute the same tasks as before

$ ./gradlew publishMyPluginPluginMarkerMavenPublicationToSonatypeRepository publishPluginPublicationToSonatypeRepository

Recall that we defined our repositories in such a way as to make the URL depend on the version name. So this will now publish to the staging repository.

Promote your staging repository to release

Note
 It is possible to automate this process, but I haven’t attempted it, yet. When I do, I will update this documentation.

Go to https://oss.sonatype.org/ and login with your Jira credentials. Click on Staging Repositories on the left. You should see one repo, which you just staged with that Gradle command. Click on it. Now verify that all the files are there as expected. In addition to all of the normal files, you should see a duplicate of each with the extension .asc, indicating they have been signed. Every file must be signed or Maven Central will reject your repository.

Once you have verified the correctness of your publication, click the Close button near the top. This triggers a validation phase from Sonatype. It takes less than a minute, but the UI is old and crusty and will not self-update, so I recommend refreshing repeatedly until it finishes. If you followed all the steps above and the gods are on your side, this step will succeed. In my case, I experienced multiple failures relating to trouble publishing my GPG key, with not signing all my publications, and with not publishing the correct publications (recall I have 12 publishing tasks available and only need 2!).

Once closing is successful, you must then click the Release button. And then you must comment on your Jira ticket that you have promoted your first release. This triggers what I believe is the final bit of real-human intervention, as someone at Maven Central verifies everything is ok, and then sets up your open source repos to automatically sync with Maven Central. You should be able to find your plugin at https://repo1.maven.org/maven2/ almost immediately, and at search.maven.org within two hours.

Take a moment to breathe and congratulate yourself. Publishing is hard, and you’ve now done it! Next time will be much easier.

Consuming from maven central

Of course there is one final step, which is to verify you can consume your non-snapshot plugin. Go back to your settings script and update it

settings.gradle
pluginsManagement {
  repositories {
    mavenCentral() // woo-hoo!
    gradlePluginPortal() // there by default
  }
}