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Implementing Maven plugins has never been Groovier!

Table of Contents

Groovy Maven plugins are very similar to Java Maven plugins. Actually a Groovy Maven plugin is compiled into Java byte-code, and once built, Maven can not tell the difference between a plugin which has been implemented in Java or Groovy.

For the most part, the existing guide for Developing Maven plugins with Java will also apply to developing plugins with Groovy. There are some differences though, which are covered here which can make Maven plugin development with Groovy better, faster, stronger (smile)

Be sure to read over the documentation for Building Groovy Projects. A Groovy Maven plugin is a Groovy project, so most of the information there is relevant here as well.

Groovy Mojos

Just like Java-based Mojo's at its simplest, a Groovy Mojo consists of a single class. For more complicated plugins you are free to use as many classes as needed of course, just like Java... and you can even write Java classes too if you need too.

Groovy Mojo implementations are denoted by the @goal Javadoc annotation on the class (er, just like Java). Actually, all of the Javadoc annotations which are supported by Java plugins are also supported by Groovy plugins. This is because, when building a Groovy project, the sub-generator spits out Java sources with Javadocs intact, which the Maven Plugin Plugin then parses for annotations (wink)

A Simple Groovy Mojo

Here is our simple Mojo class which has no parameters and spits out a relatively meaningless string via logging:

Code Block
package sample.plugin

import org.codehaus.gmaven.mojo.GroovyMojo

 * Says "Hi" to the user... er well not really :-P.
 * @goal sayhi
public class GreetingMojo
    extends GroovyMojo
    void execute() {'Groovy baby!')

There are a few minor points to note here. First, we are referencing the plugins logger via, where in Java one would need to getLog().info().

Second, there is no need to mark the execute() method as throwing any exceptions, though you can still throw any exceptions you need to. You can of course declare what your methods are throwing if you want to.

Lastly, this example mojo extends from org.codehaus.groovy.maven.mojo.GroovyMojo, which is recommended for most Groovy mojos. Of course you can always use org.apache.maven.plugin.AbstractMojo at the cost of some additional bits of happiness which help make your Mojo's groovier.

Building Plugins

Project Definition

Groovy plugins use the same Java plugin descriptor extractor. The Java extractor uses the generated stubs to build the descriptor, so be sure to invoke the generateStubs goal.

Instead of depending on maven-plugin-api Groovy plugins depend on gmaven-mojo, which picks up the required Maven dependencies and the default Groovy runtime provider.

And of course, we need to hook up the gmaven-plugin to compile the Groovy sources into class files (as well as a few others).

Below is a POM for the simple sample groovy mojo:

Code Block
    <name>Sample Maven Plugin</name>


titleDependency and Plugin Versions

To use the above example you must configure the <version> element for the dependencies and plugins.

Once you have your pom setup then you can build the plugin in the normal way via:

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mvn install

Mojo Parameters

Mojo parameters work exactly the same as they do for Java plugins. Simply define a field in your Mojo implementation and annotate the field with a @parameter Javadoc tag.

Code Block
 * The greeting to display.
 * @parameter default-value="Groovy baby!"
private String greeting

Configuration of Mojo parameters is... as you might expect, the same as with Java Mojos:

Code Block
        <greeting>what say, you, we go out on the down and swing, baby? Yea</greeting>

The main thing to note about Groovy Mojos and parameters, is that fields should be typed so that Maven (or well, Plexus) can inject objects of the proper type. Using the def keyword is the same as typing the field as and Object which Maven will probably still inject just fine, but it won't perform any conversion.

Putting More Groove into your Mojo

Using ant

Using ant allows your Mojo access to any Ant tasks. Groovy Mojo's which extend from GroovyMojo have access to an AntBuilder instance bound to the ant field.
This field is lazily initialized when you first reference it.

Below are some small examples of using ant to perform common tasks, but really the sky is the limit on what you can do. See the Ant User Manual for more tasks you can execute (wink)


The following examples assume that your Mojo implementation has already defined a project property as in:

Code Block
 * @parameter expression="${project}"
 * @required
 * @readonly
org.apache.maven.project.MavenProject project

Touching a File

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ant.touch(file: "${}/stamp")

Copying Files

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def dir = "${}/backup"
ant.mkdir(dir: dir)
ant.copy(todir: dir) {
    fileset(dir: "${}") {
        include(name: '**/*')

Execute Something

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def propname = 'lsout'
ant.exec(executable: '/bin/ls', outputproperty: propname)
def value =[propname]

Using fail()

Most mojos need to report back some failure status, which is normally done by throwing a MojoExecutionException. Groovy Mojos can simply invoke the fail() method, which handles the details of throwing for you.

Failing with a simple string:

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fail("That ain't no woman! It's a man, man!")

Failing with an exception detail:

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try {
catch (Exception e) {

Failing with an exception detail and a message:

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try {
catch (Exception e) {
    fail("She's the village bicycle! Everybody's had a ride.", e)

gmaven-archetype-mojo Archetype

To help get Groovy plugins started faster, you can use the gmaven-archetype-mojo. This will create a new project with the basic POM configuration and an example Groovy-based Mojo class to get you started quickly:

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mvn archetype:generate -DarchetypeGroupId=org.codehaus.gmaven.archetypes -DarchetypeArtifactId=gmaven-archetype-mojo -DarchetypeVersion=1.0-rc-2

To use a specific version of an archetype specify -DarchetypeVersion=<VERSION>.

The Maven Archetype Plugin will ask a few questions about your new project:

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[INFO] [archetype:generate]
Define value for groupId: : org.mycompany.myproject
Define value for artifactId: : example-maven-plugin
Define value for version: : 1.0-SNAPSHOT
Define value for package: : org.mycompany.myproject.example
Confirm properties configuration:
name: Example Maven Plugin
groupId: org.mycompany.myproject
artifactId: example-maven-plugin
version: 1.0-SNAPSHOT
package: org.mycompany.myproject.example
 Y: : y
[INFO] ------------------------------------------------------------------------
[INFO] ------------------------------------------------------------------------

Please ignore any ReferenceException warnings about ${...}, they are harmless.

The above example would have created the following project structure:

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gmaven-plugin Packaging




Mojo Development

Groovy Development