11.3.3. Plugin Dependencies  TOC  11.4.2. A Simple Java Mojo

11.4. Writing a Custom Plugin

When you write a custom plugin, you are going to be writing a series of Mojos (goals). Every Mojo is a single Java class which contains a series of annotations that tell Maven how to generate the Plugin descriptor described in the previous section. Before you can start writing Mojo classes, you will need to create Maven project with the appropriate packaging and POM.

11.4.1. Creating a Plugin Project

To create a plugin project, you should use the Maven Archetype plugin. The following command-line will create a plugin with a groupId of org.sonatype.mavenbook.plugins and the artifactId of first-maven-plugin: 

$ mvn archetype:create \
  -DgroupId=org.sonatype.mavenbook.plugins \
  -DartifactId=first-maven-plugin \
  -DarchetypeGroupId=org.apache.maven.archetypes \
  -DarchetypeArtifactId=maven-archetype-mojo

The Archetype plugin is going to create a directory named my-first-plugin which contains the following POM.

Example 11.2. A Plugin Project's POM

<?xml version="1.0" encoding="UTF-8"?><project>
  <modelVersion>4.0.0</modelVersion>
  <groupId>org.sonatype.mavenbook.plugins</groupId>
  <artifactId>first-maven-plugin</artifactId>
  <version>1.0-SNAPSHOT</version>
  <packaging>maven-plugin</packaging>
  <name>first-maven-plugin Maven Mojo</name>
  <url>http://maven.apache.org</url>
  <dependencies>
    <dependency>
      <groupId>org.apache.maven</groupId>
      <artifactId>maven-plugin-api</artifactId>
      <version>2.0</version>
    </dependency>
    <dependency>
      <groupId>junit</groupId>
      <artifactId>junit</artifactId>
      <version>3.8.1</version>
      <scope>test</scope>
    </dependency>
  </dependencies>
</project>

The most import element in a plugin project's POM is the packaging element which has a value of maven-plugin. This packaging element customizes the Maven lifecycle to include the necessary goals to create a plugin descriptor. The plugin lifecycle was introduced in Section 4.2.3, “Maven Plugin”, it is similar to the Jar lifecycle with three exceptions: plugin:descriptor is bound to the generate-resources phase, plugin:addPluginArtifactMetadata is added to the package phase, and plugin:updateRegistry is added to the install phase.

The other important piece of a plugin project's POM is the dependency on the Maven Plugin API. This project depends on version 2.0 of the maven-plugin-api and it also adds in JUnit as a test-scoped dependency.

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11.3.3. Plugin Dependencies  Sponsored by Sonatype  11.4.2. A Simple Java Mojo