The Meta-Maven-Plugin Maven Plugin

The meta-maven-plugin-maven-plugin is a Maven plugin for generating Maven meta-plugins.

A what for what? Maven meta-plugins are Maven plugins that execute other maven plugins with a pre-configured plugin configuration. Meta-plugins allow for sharing plugin configuration across multiple projects without the need for a shared parent POM. Additionally, meta-plugins allow for the execution of multiple plugins with a single plugin import, something not even a shared parent POM can do. If you find yourself copy/pasting the same hundred lines of plugin configuration XML between POMs, you would benefit from using a meta-plugin.

Authoring Maven meta-plugins has traditionally been a manual affair. You had to implement a Maven mojo class and explicitly invoke one or more maven plugins with code. The meta-maven-plugin-maven-plugin automates this process by generating that code for you instead using the native Maven plugin configuration XML already present in your POMs.

Quick Start

Execute the following maven archetype command to generate a Maven meta-plugin project:

mvn archetype:generate \
  -DarchetypeGroupId=io.github.rmichela \
  -DarchetypeArtifactId=meta-maven-plugin-maven-plugin-archetype \
  -DarchetypeVersion=1.0.2 \
  -DgroupId=com.example \
  -DartifactId=hello-world-meta-maven-plugin \
  -Dversion=0.1.0-SNAPSHOT \
  -Dpackage=com.example \
  -DgoalPrefix=helloworld

-DgroupId, -DartifactId, -Dversion - The GAV coordinates of your meta-plugin.
-Dpackage - Which java package to generate the mojo class in.
-DgoalPrefix - The Maven mojo goal prefix to use for your plugin, when calling it directly from the command line.

The above command will generate a Maven project with no code, but with a pom.xml file that contains the meta-maven-plugin-maven-plugin configuration necessary to generate a basic Hello World meta-plugin during the Maven generate-sources phase, and a basic documentation site during the Maven site phase.

Configuring the Meta-Maven-Plugin Maven Plugin

The meta-maven-plugin-maven-plugin is configured primarily using the <paramaters> and <plugins> configuration elements. Consider the following basic meta-plugin example which prints out “Hello World” during the Maven compile phase.

<plugin>
    <groupId>io.github.rmichela</groupId>
    <artifactId>meta-maven-plugin-maven-plugin</artifactId>
    <version>1.0.2</version>
    <executions>
        <execution>
            <goals>
                <goal>meta-meta-plugin</goal>
            </goals>
        </execution>
    </executions>
    <configuration>
        <parameters>
            <parameter>
                <name>greeting</name>
                <defaultValue>Hello World</defaultValue>
            </parameter>
        </parameters>
        <plugins>
            <plugin>
                <groupId>com.github.ekryd.echo-maven-plugin</groupId>
                <artifactId>echo-maven-plugin</artifactId>
                <version>2.0.0</version>
                <executions>
                    <execution>
                        <goals>
                            <goal>echo</goal>
                        </goals>
                        <phase>compile</phase>
                    </execution>
                </executions>
                <configuration>
                    <message>@{greeting}</message>
                </configuration>
            </plugin>
        </plugins>
    </configuration>
</plugin>

The <parameter> configuration is used to identify a Maven plugin parameter that can be configured by the user of the meta-plugin.
Within the <plugin> configuration, we reference the @{greeting} parameter from within the meta-plugin's embedded plugin configuration. Note the use of @{} to reference the parameter to trigger parameter interpolation during meta-plugin execution in its host POM instead of during meta-plugin generation.

Consuming the Generated Meta-Plugin

Once generated, the meta-plugin can be used like any other Maven plugin. The following POM example consumes the above meta-plugin and configures the greeting parameter to print out “Hello Developer!”. The meta-maven-plugin-maven-plugin generates a plugin goal for each build phase used by the embedded configuration.

<plugin>
    <groupId>my-group-id</groupId>
    <artifactId>hello-world</artifactId>
    <version>1.0.2</version>
    <executions>
        <execution>
            <goals>
                <goal>compile</goal>
            </goals>
        </execution>
    </executions>
    <configuration>
        <greeting>Hello Developer</greeting>
    </configuration>
</plugin>