Custom Maven 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. Before you can start writing Mojo classes, you will need to create Maven project with the appropriate packaging and POM.

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.tat.api and the artifactId of webapp-optimizer:

mvn archetype:generate -DgroupId=org.tat.api -DartifactId=webapp-optimizer -DarchetypeGroupId=org.apache.maven.archetypes -DarchetypeArtifactId=maven-archetype-mojo

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pom.xml

	<?xml version="1.0"?>
	<project
	xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"
	xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
	<modelVersion>4.0.0</modelVersion>
	<parent>
	<groupId>org.tat.api</groupId>
	<artifactId>tools</artifactId>
	<version>1.0-SNAPSHOT</version>
	</parent>
	<groupId>org.tat.api</groupId>
	<artifactId>webapp-optimizer</artifactId>
	<version>1.0-SNAPSHOT</version>
	<packaging>maven-plugin</packaging>
	<name>webapp-optimizer 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>

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The most important element in a plugin project’s POM is the packaging element which has a value of maven-plugin. 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.

The Mojo implementation shown in the following example implements the Mojo interface by extending the org.apache.maven.plugin.AbstractMojo class. Before we dive into the code for this Mojo, let’s take some time to explore the methods on the Mojo interface. Mojo provides the following methods:

void setLog( org.apache.maven.monitor.logging.Log log )

When Maven loads and executes a Mojo, it is going to call the setLog() method and supply the Mojo instance with a suitable logging destination to be used in your custom plugin.

protected Log getLog()

Maven is going to call setLog() before your Mojo is executed, and your Mojo can retrieve the logging object by calling getLog().

void execute() throws org.apache.maven.plugin.MojoExecutionException

This method is called by Maven when it is time to execute your goal.

	package org.tat.api;
	/*
	* Copyright 2001-2005 The Apache Software Foundation.
	*
	* Licensed under the Apache License, Version 2.0 (the "License");
	* you may not use this file except in compliance with the License.
	* You may obtain a copy of the License at
	*
	* http://www.apache.org/licenses/LICENSE-2.0
	*
	* Unless required by applicable law or agreed to in writing, software
	* distributed under the License is distributed on an "AS IS" BASIS,
	* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	* See the License for the specific language governing permissions and
	* limitations under the License.
	*/
	import org.apache.maven.plugin.AbstractMojo;
	import org.apache.maven.plugin.MojoExecutionException;
	import org.apache.maven.plugin.MojoFailureException;
	import java.io.File;
	import java.io.FileWriter;
	import java.io.IOException;
	/**
	* Echos an object string to the output screen.
	* @goal echo
	* @requiresProject false
	*/
	public class MyMojo
	extends AbstractMojo
	{
	/**
	* Any Object to print out.
	* @parameter expression="${echo.message}" default-value="Hello World..."
	*/
	private Object message;
	public void execute()
	throws MojoExecutionException, MojoFailureException
	{
	getLog().info( message.toString() );
	}
	}

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You can run the plug-in using the following command. The commands is as per the groupId:artifactId:version:goal format.

$mvn org.sonatype.mavenbook.plugins:first-maven-plugin:1.0-SNAPSHOT:echo

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 When you run this command-line you should see output that contains the output of the echo goal with the default message: "Hello Maven World…". If you want to customize the message, you can pass the value of the message parameter with the following command-line:

$ mvn org.tat.api:webapp-optimizer:1.0-SNAPSHOT:echo -Decho.message="Test is successful"

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