• Maven Central Release Action
  • Deploying Maven Artifacts to Maven Central in One Action
  • How to Use Maven Central Release Action
    • Step 1 - Push Tag
    • Step 2 - Create GPG Key
      • Installing GnuPG
      • Generating a Key Pair
    • Step 3 - Distributing Public Key
    • Step 4 - Configuring Maven Central Credentials
    • Step 5 - Preparing POM File
      • A Complete Example POM
    • Step 6 - Defining Action File
      • Optional Parameters
  • Troubleshooting
    • "Invalid signature for file"
    • keyserver send/receive failed
  • License

Maven Central Release Action offers a convenient "one-click" approach for publishing artifacts to Maven Central

Manually create the first tag on master branch. For example, 1.0.0:

git tag -a 1.0.0 -m "1.0.0"
git push origin 1.0.0

We recommend all-number version such as "1.0.0" as opposed to "v1.0.0"

One of the requirements for publishing our artifacts to the Central Repository, is that they have been signed with PGP. GnuPG or GPG is a freely available implementation of the OpenPGP standard. GPG provides us with the capability to generate a signature, manage keys, and verify signatures.

Download the binary of GnuPG or install it with our favorite package manager and verify it by running a gpg command with the --version flag

$ gpg --version
gpg (GnuPG) 2.2.19
libgcrypt 1.8.5
Copyright (C) 2019 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <https://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Home: /home/mylocaluser/.gnupg
Supported algorithms:
Pubkey: RSA, ELG, DSA, ECDH, ECDSA, EDDSA
Cipher: IDEA, 3DES, CAST5, BLOWFISH, AES, AES192, AES256, TWOFISH,
        CAMELLIA128, CAMELLIA192, CAMELLIA256
Hash: SHA1, RIPEMD160, SHA256, SHA384, SHA512, SHA224
Compression: Uncompressed, ZIP, ZLIB, BZIP2

A key pair allows us to sign artifacts with GPG and users can subsequently validate that artifacts have been signed by us. We can generate a key with:

gpg --gen-key

Enter our name and email when asked for it and also, the time of validity for the key defaults to 2 years. Once they key is expired we can extend it, provided we own the key and therefore know the passphrase.

$ gpg --gen-key
gpg (GnuPG) 2.2.19; Copyright (C) 2019 Free Software Foundation, Inc.
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Note: Use "gpg --full-generate-key" for a full featured key generation dialog.

GnuPG needs to construct a user ID to identify your key.

Real name: my-gpg-keyname
Email address: [email protected]
You selected this USER-ID:
    "my-gpg-keyname <[email protected]>"

Change (N)ame, (E)mail, or (O)kay/(Q)uit? O
We need to generate a lot of random bytes. It is a good idea to perform
some other action (type on the keyboard, move the mouse, utilize the
disks) during the prime generation; this gives the random number
generator a better chance to gain enough entropy.
We need to generate a lot of random bytes. It is a good idea to perform
some other action (type on the keyboard, move the mouse, utilize the
disks) during the prime generation; this gives the random number
generator a better chance to gain enough entropy.
gpg: key 8190C4130ABA0F98 marked as ultimately trusted
gpg: revocation certificate stored as
'/home/mylocaluser/.gnupg/openpgp-revocs.d/CA925CD6C9E8D064FF05B4728190C4130ABA0F98.rev'
public and secret key created and signed.

pub   rsa3072 2021-06-23 [SC] [expires: 2023-06-23]
      CA925CD6C9E8D064FF05B4728190C4130ABA0F98
uid                      my-gpg-keyname <[email protected]>
sub   rsa3072 2021-06-23 [E] [expires: 2023-06-23]

We have to provide our name and email. These identifiers are essential as they will be seen by anyone downloading a software artifact and validating a signature. Finally, we must provide a passphrase to protect our secret key. It is essential that we choose a secure passphrase and that we do not divulge it to any one. This passphrase and gpg's private key are all that is needed to sign artifacts with our signature.

Please keep in deep mind that GPG_KEYNAME cannot contain white space!

To export the private key, list it along with any other keys installed:

$ gpg --list-keys
/home/mylocaluser/.gnupg/pubring.kbx
---------------------------------
pub   rsa3072 2021-06-23 [SC] [expires: 2023-06-23]
      CA925CD6C9E8D064FF05B4728190C4130ABA0F98
uid           [ultimate] my-gpg-keyname <[email protected]>
sub   rsa3072 2021-06-23 [E] [expires: 2023-06-23]

The output displays the path to the public keyring file. The line starting with pub shows the size (rsa3072), the keyid (CA925CD6C9E8D064FF05B4728190C4130ABA0F98), and the creation date (2023-06-23) of the public key. Some values may vary depending on our GnuPG version, but we will definitely see the keyid or part of it (called shortID, last 8 characters of the keyid, in this example 0ABA0F98, which we can ask gpg to output using gpg --list-keys --keyid-format short).

The next line shows the UID of the key, which is composed of a name, a comment, and an email.

Next we export an ascii armored version of the private key:

gpg --output private.pgp --armor --export-secret-key CA925CD6C9E8D064FF05B4728190C4130ABA0F98

The maven-central-release-action would need the public key to verify the files, we want to distribute GPG public key to a key server:

gpg --keyserver keyserver.ubuntu.com --send-keys CA925CD6C9E8D064FF05B4728190C4130ABA0F98

The --keyserver parameter identifies the target key server address. The --send-keys is the keyid of the key we want to distribute. We can get our keyid by listing the public keys.

Now the action can import our public key from the key server to CI/CD server:

gpg --keyserver keyserver.ubuntu.com --recv-keys CA925CD6C9E8D064FF05B4728190C4130ABA0F98

Releasing to Maven Central requires credentials by generating a user token via the Maven Central account page.

As part of the deployment, we are required to submit a POM file. This is the Project Object Model file used by Apache Maven to define our project and its build. When building with other tools we have to assemble it and ensure it contains the following information.

• Correct Coordinates: The project coordinates, also known as GAV, coordinates determine the location of your project in the repository. The values are

  • groupId: the top level namespace level for our project starting with the reverse domain name
  • artifactId: the unique name for our artifact
  • version: the version string for our artifact

  The version can be an arbitrary string and can not end in -SNAPSHOT, since this is the reserved string used to identify versions that are currently in development. We must use semantic versioning such as 1.0.0

  A valid example is

  <groupId>com.example.applications</groupId>
  <artifactId>example-application</artifactId>
  <version>1.4.7</version>

• Project Name, Description and URL: For some human-readable information about our project and a pointer to our project website for more, we need the presence of name, description, and url:

  <name>Example Application</name>
  <description>
      A application used as an example on how to set up pushing its components to the Central Repository.
  </description>
  <url>http://www.example.com/example-application</url>

• License Information - We need to declare the license(s) used for distributing our artifacts. E.g. if we use the Apache License we can use

  <licenses>
      <license>
          <name>The Apache Software License, Version 2.0</name>
          <url>http://www.apache.org/licenses/LICENSE-2.0.txt</url>
          <distribution>repo</distribution>
      </license>
  </licenses>

• Developer Information - In order to be able to associate the project it is required to add a developers section.

  <developers>
      <developer>
          <name>Paion Data Team</name>
          <url>https://github.com/paion-data</url>
      </developer>
  </developers>

• SCM Information

  <scm>
      <developerConnection>scm:git:ssh://[email protected]/github-username/repo-name.git</developerConnection>
      <url>https://github.com/github-username/repo-name.git</url>
      <tag>HEAD</tag>
  </scm>

  Please modify the github-username and repo-name above accordinly

<?xml version="1.0" encoding="UTF-8"?>

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>com.paiondata.athena</groupId>
    <artifactId>athena-parent-pom</artifactId>
    <version>1.0.0</version>
    <packaging>pom</packaging>

    <name>Athena: Parent POM</name>
    <url>https://github.com/paiondata/athena</url>
    <description>Athena: Parent POM</description>

    <developers>
        <developer>
            <name>Paion Data Team</name>
            <url>https://github.com/paion-data</url>
        </developer>
    </developers>

    <licenses>
        <license>
            <name>The Apache Software License, Version 2.0</name>
            <url>http://www.apache.org/licenses/LICENSE-2.0.txt</url>
            <distribution>repo</distribution>
        </license>
    </licenses>

    <scm>
        <developerConnection>scm:git:ssh://[email protected]/paion-data/athena.git</developerConnection>
        <url>https://github.com/paion-data/athena.git</url>
        <tag>HEAD</tag>
    </scm>

    <build>
        <plugins>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-javadoc-plugin</artifactId>
                <version>3.5.0</version>
                <configuration>
                    <doclint>none</doclint>  <!-- Turnoff all checks -->
                </configuration>
                <executions>
                    <execution>
                        <id>attach-javadocs</id>
                        <goals>
                            <goal>jar</goal>
                        </goals>
                    </execution>
                </executions>
            </plugin>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-source-plugin</artifactId>
                <version>3.1.0</version>
                <executions>
                    <execution>
                        <id>attach-sources</id>
                        <phase>verify</phase>
                        <goals>
                            <goal>jar-no-fork</goal>
                        </goals>
                    </execution>
                </executions>
            </plugin>
        </plugins>
    </build>

    <profiles>
        <profile>
            <id>release</id>
            <build>
                <plugins>
                    <plugin>
                        <groupId>org.sonatype.central</groupId>
                        <artifactId>central-publishing-maven-plugin</artifactId>
                        <version>0.4.0</version>
                        <extensions>true</extensions>
                        <configuration>
                            <tokenAuth>true</tokenAuth>
                            <autoPublish>true</autoPublish>
                            <publishingServerId>${gpg.keyname}</publishingServerId>
                        </configuration>
                    </plugin>
                    <plugin>
                        <groupId>org.apache.maven.plugins</groupId>
                        <artifactId>maven-gpg-plugin</artifactId>
                        <version>3.1.0</version>
                        <executions>
                            <execution>
                                <id>sign-artifacts</id>
                                <phase>verify</phase>
                                <goals>
                                    <goal>sign</goal>
                                </goals>
                                <configuration>
                                    <gpgArguments>
                                        <arg>--pinentry-mode</arg>
                                        <arg>loopback</arg>
                                    </gpgArguments>
                                    <keyname>${gpg.keyname}</keyname>
                                    <passphraseServerId>${gpg.keyname}</passphraseServerId>
                                </configuration>
                            </execution>
                        </executions>
                    </plugin>
                </plugins>
            </build>
        </profile>
    </profiles>
</project>

Under regular .github/workflows directory, create a .yml file with a preferred name with the following example contents:

---
name: My App CI/CD

"on":
  pull_request:
  push:
    branches:
      - master

jobs:
  release:
    name: Release to Maven Central
    if: github.ref == 'refs/heads/master'
    runs-on: ubuntu-latest
    steps:
      - name: Release
        uses: paion-data/maven-central-release-action@master
        with:
          user: QubitPi
          email: [email protected]
          gpg-keyname: ${{ secrets.GPG_KEYNAME }}
          gpg-private-key: ${{ secrets.GPG_PRIVATE_KEY }}
          gpg-passphrase: ${{ secrets.GPG_PASSPHRASE }}
          server-username: ${{ secrets.MAVEN_CENTRAL_USERNAME }}
          server-password: ${{ secrets.MAVEN_CENTRAL_TOKEN }}

• version-properties: The names of all properties (separated by comma, not white space between them) that contains the artifact version. For example, we might have a POM property called

  <properties>
      <myproject.version>1.0-SNAPSHOT</myproject.version>
  </properties>

  then we will need to pass myproject.version via this option in order to fully set the release properties. Internally the action utilizes Versions Maven Plugin to update all specified properties to the lastly tagged version

- Invalid signature for file: ***-1.0.0-javadoc.jar
- Invalid signature for file: ***-1.0.0-sources.jar
- Invalid signature for file: ***-1.0.0-tests.jar
- Invalid signature for file: ***-1.0.0.jar
- Invalid signature for file: ***-1.0.0.pom

There could be multiple causes for this error. Most likely one or more of the following GPG parameters are not specified correctly:

• gpg-keyname:
• gpg-private-key
• gpg-passphrase

Please note that the gpg-keyname in the following example is my-gpg-keyname:

$ gpg --list-keys
/home/mylocaluser/.gnupg/pubring.kbx
---------------------------------
pub   rsa3072 2021-06-23 [SC] [expires: 2023-06-23]
      CA925CD6C9E8D064FF05B4728190C4130ABA0F98
uid           [ultimate] my-gpg-keyname <[email protected]>
sub   rsa3072 2021-06-23 [E] [expires: 2023-06-23]

If everything seems correct, we could try deleting and re-generating a new GPG key

gpg --list-keys

gpg --list-secret-keys

gpg --delete-secret-key CA925CD6C9E8D064FF05B4728190C4130ABA0F98
gpg --delete-key CA925CD6C9E8D064FF05B4728190C4130ABA0F98

Try

sudo pkill dirmngr

or send GPG key online (assuming keyserver.ubuntu.com keyserver)

The use and distribution terms for maven-central-release-action are covered by the Apache License, Version 2.0.