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above are taken care of. Notice the projectArtifact element above with the implementation="" attribute. This is how you will inject the StubArtifact implementation into that projectArtifact private variable on the mojo. Additional elements can be inserted as child elements and they will in turn be assigned to the variables inside that object. An example would be setting the groupId variable on the StubMavenProject. Note: these internal variable might also need implementation="" attributes.

useful things to know

There are a couple of utility methods on the AbstractMojoTestCase that should make the asserts easier to work with.

   protected Object getVariableValueFromObject( Object mojo, String variable ) throws IllegalAccessException

   protected Map getVariablesAndValuesFromObject( Object mojo ) throws IllegalAccessException

These two methods wrap functionality in the ReflectionUtils class in plexus utils that will let you retrieve the value of variable on the mojo. Since most of the mojo's variables are private scope and most mojo's don't make use of getters and setters this will let you assert things about the internal state of the mojo. Both methods return objects that need to be casted to what you want to look at, and as testers of the mojo in question it should be perfectly fine to just cast away based on your knowledge of the mojo internals.

For example if I needed to check the value of the groupId in the project instance in a given mojo this is what I would do.

   // get the project variable
   MavenProject project = (MavenProject)getVariableValueFromObject( mojo, "project" );

   // get the groupId from inside the project
   String groupId = (String)getVariableFromObject( project, "groupId" );

   // assert 
   assertEquals("foo", groupId);

things to watch out for

I have noticed that since the declaration of the patchs are not directly in the same location as the verification (one being with the plugin configuration and the other in the assert clauses) that it is very easy to get them mixed up. And if you are validating that the file doesn't exist at the end of the operation as with the clean plugin, if you are not testing the absolute right thing then your assert will be working when the test itself might have failed. So make sure you check these conditions, perhaps even asserting something is there before the mojo execution and then asserting it isn't there at the end.