Creating a maven plugin with java 5 annotations rather than javadoc tags

Actually, creating a plugin with the Maven Plugin Tools, is something, thanks to the plugins themselves, which is relatively trivial; however, because it took me a long time to get my plugin working properly with Java 5 annotations rather than Javadoc Tags, I decided to write it down.

I must confess that, sadly enough, Maven has never been a strength of mine so that it was no surprise that my plugin didn't immediately function properly. The problem I faced was in the passing of the parameters to my Mojo during execution of my newly created plugin.

When converted to Java 5 annotation, the class "outputDirectory" attribute, in the default created Mojo,  was always null even though I had the proper configuration in my host project's pom.xml

<configuration>                     
    <outputDirectory>C:\Users\wnpr\Downloads</outputDirectory>         
 </configuration> 

To make sure that we all have the same starting point let me begin, by showing how I used the Maven Integration for Eclipse plugin, M2E, to create a new Maven project as shown below.

Then I selected the "maven-archetype-mojo" as a  temple for my new project,

  and finally gave my plugin a groupId and artefactId.

What follows is a view of the Maven project from within eclipse. As you can see, the project already contains a Mojo class, called MyMojo, which was created from me

Every plugin needs a plugin.xml file, which is generated by the maven-plugin-plugin and which can be found the the META-INF\maven folder of the generated jar file. By default, maven-plugin-plugin generates the plugin.xml file from the Mojo Javadoc Tags found in the Java code. These tags look like this:

/** 

 * Goal which touches a timestamp file. * 

 * @goal touch 

 * 

 * @phase process-sources 

 */

If you, as I did, would rather use Java 5 annotations, such as:

@Mojo( name = "touch" )

public class MyMojo

    extends AbstractMojo {

   @Parameter(defaultValue = "c:\\", required=true)

    public File outputDirectory;

   @Parameter(defaultValue = "Hello World!!!!", required=true)

    public String greeting;

.
.
}

for you plugin.xml generation, then you will have to rely on another plugin; namely: maven-plugin-annotations.

To do this you will have to add an additional dependency, to your plugin project's pom.xml.

<!-- dependencies to annotations -->
<dependency>
   <groupId>org.apache.maven.plugin-tools</groupId>
   <artifactId>maven-plugin-annotations</artifactId>
   <version>3.4</version>
    <scope>provided</scope>
</dependency>    


and override Maven core's default-descriptor execution phase to process-classes as follows:

  <build>
    <plugins>
      <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-plugin-plugin</artifactId>
        <version>3.4</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>

It wasn't until this last change that the @Parameter annotated attributes of my Mojo were finally set properly during execution of my plugin.  Both of these additions are well described here; however, when I read them for the first time, I wasn't smart enough to understand them; as often is the case, and needed to waste a day before I finally came to my senses.

If you get a message like this:

Error extracting plugin descriptor: 'Goal: touch already exists in the plugin descriptor for prefix: junk
[ERROR] Existing implementation is: org.wmmnpr.junk_maven_plugin.MyMojo

then be sure to delete the old Javadoc Tags.

Good luck!