Maven: A Developer's Notebook

Adding Dynamic Dependencies

Maven dependency handling supports only static dependencies defined in project.xml. However, there are times when a plug-in will need to add a dependency at runtime.

How do I do that?

Consider your favorite plug-in—Logifier. The way you wrote it in the previous lab has a severe limitation. Imagine that you're running it on a Maven project that has unit tests. As you know, running the jar goal will automatically execute the test:test goal and run the unit tests. As your logifier:logify goal depends on the jar goal, it'll end up running the unit tests on the "logified" code... Yikes! This means that the test will fail, with this error:

java.lang.NoClassDefFoundError: org/aspectj/lang/Signature

Not only is this error message unexpected to someone not familiar with the Logifier plug-in, but also you have a plug-in causing problems in an unrelated plug-in—test. In order to fix this, you need to rewrite the JAR plug-in so that it doesn't execute tests, or tell it to skip the tests. Or better yet, add the aspectjrt JAR to the classpath at runtime. This sounds like the best solution.

You can add a dependency dynamically by using the addPath tag provided by Maven:

  <goal name="logifier:init">
[...]
    <ant:path id="aspectjrt.classpath">
               <ant:pathelement 
               path="${plugin.getDependencyPath('aspectj:aspectjrt')}"/>
               </ant:path>
               <maven:addPath id="maven.dependency.classpath" 
        refid="aspectjrt.classpath"/>
   
  </goal>

When your project executes, the logifier:init goal will be called ...