Maven: The Complete Reference
11.3. Plugin Descriptor

A Maven plugin contains a road-map for Maven that tells Maven about the various Mojos and plugin configuration. This plugin descriptor is present in the plugin JAR file in META-INF/maven/plugin.xml. When Maven loads a plugin, it reads this XML file, instantiates and configures plugin objects to make the Mojos contained in a plugin available to Maven.

When you are writing custom Maven plugins, you will almost never need to think about writing a plugin descriptor. In Chapter 4, The Build Lifecycle, the lifecycle goals bound to the maven-plugin packaging type show that the plugin:descriptor goal is bound to the generate-resources phase. This goal generates a plugin descriptor off of the annotations present in a plugin’s source code. Later in this chapter, you will see how Mojos are annotated, and you will also see how the values in these annotations end up in the META-INF/maven/plugin.xml file.

Plugin Descriptor shows a plugin descriptor for the Maven Zip Plugin. This plugin is a contrived plugin that simply zips up the output directory and produces an archive. Normally, you wouldn’t need to write a custom plugin to create an archive from Maven, you could simply use the Maven Assembly Plugin which is capable of producing a distribution archive in multiple formats. Read through the following plugin descriptor to get an idea of the content it contains.

Plugin Descriptor. 

<plugin>
    <description></description>
    <groupId>com.training.plugins</groupId>
    <artifactId>maven-zip-plugin</artifactId>
    <version>1-SNAPSHOT</version>
    <goalPrefix>zip</goalPrefix>
    <isolatedRealm>false</isolatedRealm>
    <inheritedByDefault>true</inheritedByDefault>
    <mojos>
        <mojo>
            <goal>zip</goal>
            <description>Zips up the output directory.</description>
            <requiresDirectInvocation>false</requiresDirectInvocation>
            <requiresProject>true</requiresProject>
            <requiresReports>false</requiresReports>
            <aggregator>false</aggregator>
            <requiresOnline>false</requiresOnline>
            <inheritedByDefault>true</inheritedByDefault>
            <phase>package</phase>
            <implementation>com.training.plugins.ZipMojo</implementation>
            <language>java</language>
            <instantiationStrategy>per-lookup</instantiationStrategy>
            <executionStrategy>once-per-session</executionStrategy>
            <parameters>
                <parameter>
                    <name>baseDirectory</name>
                    <type>java.io.File</type>
                    <required>false</required>
                    <editable>true</editable>
                    <description>Base directory of the project.</description>
                </parameter>
                <parameter>
                    <name>buildDirectory</name>
                    <type>java.io.File</type>
                    <required>false</required>
                    <editable>true</editable>
                    <description>Directory containing the build files.</description>
                </parameter>
            </parameters>
            <configuration>
                <buildDirectory implementation="java.io.File">
                    ${project.build.directory}</buildDirectory>
                <baseDirectory implementation="java.io.File">
                    ${basedir}</baseDirectory>
            </configuration>
            <requirements>
                <requirement>
                    <role>org.codehaus.plexus.archiver.Archiver</role>
                    <role-hint>zip</role-hint>
                    <field-name>zipArchiver</field-name>
                </requirement>
            </requirements>
        </mojo>
    </mojos>
    <dependencies>
        <groupId>org.apache.commons</groupId>
        <artifactId>commons-io</artifactId>
        <version>1.3.2</version>
    </dependencies>
</plugin>

There are three parts to a plugin descriptor: the top-level configuration of the plugin which contains elements like groupId and artifactId, the declaration of mojos, and the declaration of dependencies. Let’s examine each of these sections in more detail.