Maven Plugin Inheritance

Writing Plugins for Maven is not hard, and Maven: The Complete Reference has an entire chapter on this subject. Of course, nothing is ever complete in real life or in software development, and one of the topics not covered in the book is plugin inheritance.

The scenario:

• There is a (fictitious) maven-tea-plugin which takes a number of configuration parameters and has a tea goal. It is implemented by a TeaMojo.
• You are writing a maven-greentea-plugin which has almost but not quite the same behaviour as the maven-tea-plugin. It takes the same parameters and has the same goals.
• The natural idea is to derive a GreenTeaMojo from the TeaMojo and to override one or two methods of the base class, reusing all the parameters.

Unfortunately, this does not work, at least not out of the box. The problem is:

• Maven uses poor man's Javadoc annotations to inject configuration parameters into Mojo classes.
• This is still true in Maven 3.0, even though the traditional Plexus container has now been replaced by Guice.
• The maven-plugin-plugin (i.e. the plugin responsible for plugin building) needs to scan the Mojo sources for Javadoc annotations, but the maven-tea-plugin sources are not visible during the build of the maven-greentea-plugin.

Now the maven-inherit-plugin from OPS4J fills the gap by looking up the plugin descriptor from the maven-tea-plugin and merging it with any additional parameters defined in the maven-greentea-plugin. To use it, just follow the documentation.

There is just one point to note: You have to add an @extendsPlugin annotation to your GreenTeaMojo to indicate the plugin you want to extend, but the value of this annotation is not quite obvious. It has to be either the plugin artifact id, or the plugin short name, tea in our case, but this requires the artifact id to be maven-tea-plugin or tea-maven-plugin.