Maven Plugin Plugin – Using Plugin Tools Java5 Annotations

Using Plugin Tools Java5 Annotations

Since version 3.0 of the maven-plugin-plugin, you can use Java5 annotations to generate the plugin descriptor file.

NOTE With annotations, your Mojo super class does not any more require to be in the same project. Provided that the super class also uses annotations, it can now come from reactor projects or external dependencies. By default all dependencies are scanned, but this can be reduced with the mojoDependencies parameter. As javadoc doclet are still useful for @since, @deprecated and comments, the sources are still scanned. So if you use an external dependency, you must still provide an artifact with sources (sources classifier) to provide documentation (the tooling will skip error if this artifact sources is missing).

Annotations

Information for plugin descriptor generation is specified using 4 annotations:

2 class-level annotations:
- @Mojo: This annotation will mark your class as a Mojo,
- @Execute: Used if your Mojo needs to fork a lifecycle,
2 field-level annotations:
- @Parameter: Used to configure your Mojo parameters,
- @Component: Used to configure injection of Plexus components or Maven context components.

For more information on these annotations, see the corresponding documentation.

Notice that Plugin Tools Java 5 Annotations are named after Plugin Tools Javadoc Tags with following little differences:

Plugin Tools Javadoc Tags	Plugin Tools Java 5 Annotation
`@goal "goal-name"`	`@Mojo( name = "goal-name" )`
`@phase "<phase-name>"`	`@Mojo( defaultPhase = LifecyclePhase.<phase> )`

POM configuration

To be able to use these Java 5 annotations, you need to add some configuration to your plugin POM:

add maven-plugin-annotations dependency, preferably with provided scope,
override Maven core's default-descriptor execution phase to process-classes,

<project>
  ...
  <packaging>maven-plugin</packaging>
  ...
  <dependencies>
    <!-- dependencies to annotations -->
    <dependency>
      <groupId>org.apache.maven.plugin-tools</groupId>
      <artifactId>maven-plugin-annotations</artifactId>
      <version>3.5.2</version>
      <optional>true</optional> <!-- annotations are not used at runtime because @Retention(value=CLASS), they are needed only to build the plugin -->
    </dependency>
  </dependencies>
  ...
  <build>
    <plugins>
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-plugin-plugin</artifactId>
        <version>3.5.2</version>
        <executions>
          <execution>
            <id>default-descriptor</id>
            <phase>process-classes</phase>
          </execution>
          <!-- if you want to generate help goal -->
          <execution>
            <id>help-goal</id>
            <goals>
              <goal>helpmojo</goal>
            </goals>
          </execution>
        </executions>
      </plugin>
    </plugins>
  ...
  </build>
  ...
</project>

Notice: this configuration will not work with Maven 2, which does not support execution phase overriding. If you absolutely need to build with Maven 2, you should use skipErrorNoDescriptorsFound parameter to avoid failure on default execution phase and add another execution:

<project>
  ...
  <profiles>
    <profile>
      <id>maven-2</id>
      <activation>
        <file>
          <!--  This employs that the basedir expression is only recognized by Maven 3.x (see MNG-2363) -->
          <missing>${basedir}</missing>
        </file>
      </activation>
      <build>
        <plugins>
          <plugin>
            <groupId>org.apache.maven.plugins</groupId>
            <artifactId>maven-plugin-plugin</artifactId>
            <configuration>
              <!-- see https://issues.apache.org/jira/browse/MNG-5346 -->
              <skipErrorNoDescriptorsFound>true</skipErrorNoDescriptorsFound>
            </configuration>
            <executions>
              <execution>
                <id>mojo-descriptor</id>
                <goals>
                  <goal>descriptor</goal>
                </goals>
              </execution>
            </executions>
          </plugin>
        </plugins>
      </build>
    </profile>    
  </profiles>
</project>