Writing a Maven Plugin in Groovy

Groovy is a dynamic language based on the Java Virtual Machine which compiles to Java bytecode. Groovy is a project in the Codehaus community. If you are fluent in Java, Groovy will seem like a natural choice for a scripting language. Groovy takes the features of Java, pares down the syntax a bit, and adds features like closures, duck-typing, and regular expressions. For more information about Groovy, please see the Groovy web site at http://groovy.codehaus.org.

To create a Maven Plugin using Groovy, you only need two files: a pom.xml and a single Mojo implemented in Groovy. To get started, create a project directory named firstgroovy-maven-plugin. Place the following pom.xml in this directory.

<?xml version="1.0" encoding="UTF-8"?>
<project>
  <modelVersion>4.0.0</modelVersion>
  <groupId>org.sonatype.mavenbook.plugins</groupId>
  <artifactId>firstgroovy-maven-plugin</artifactId>
  <name>Example Groovy Mojo - firstgroovy-maven-plugin</name>
  <packaging>maven-plugin</packaging>
  <version>1.0-SNAPSHOT</version>
  <dependencies>
    <dependency>
      <groupId>org.codehaus.mojo.groovy</groupId>
      <artifactId>groovy-mojo-support</artifactId>
      <version>1.0-beta-3</version>
    </dependency>
  </dependencies>
  <build>
    <plugins>
      <plugin>
        <artifactId>maven-plugin-plugin</artifactId>
        <version>2.4</version>
      </plugin>
      <plugin>
        <groupId>org.codehaus.mojo.groovy</groupId>
        <artifactId>groovy-maven-plugin</artifactId>
        <version>1.0-beta-3</version>
        <extensions>true</extensions>
        <executions>
          <execution>
            <goals>
              <goal>generateStubs</goal>
              <goal>compile</goal>
              <goal>generateTestStubs</goal>
              <goal>testCompile</goal>
            </goals>
          </execution>
        </executions>
      </plugin>
    </plugins>
  </build>
</project>

What's going on in this POM? First, notice that the packaging of the POM is maven-plugin because we are creating a project that will package a Maven plugin. Next, note that the project depends on the groovy-mojo-support artifact in the org.codehaus.mojo.groovy group.

Then under src/main/groovy in a directory org/sonatype/mavenbook/plugins, create a file named EchoMojo.groovy which contains the EchoMojo class.

package org.sonatype.mavenbook.plugins
import org.codehaus.mojo.groovy.GroovyMojo
/**
 * Example goal which echos a message
 *
 * @goal echo
 */
class EchoMojo extends GroovyMojo {
    /**
     * Message to print
     *
     * @parameter expression="${echo.message}"
     *            default-value="Hello Maven World"
     */
    String message
    void execute() {
      log.info( message )
    }
}