Debugging Maven Plugins

When developing maven plugins things don’t always work the way you expect so you need to debug the Mojo to see what’s really going on. I had a weird case where my plugin worked when I ran it independently but when I ran it with mvn clean package it always failed. First thing you can do is run in debug mode which produces a lot more output and shows all the plugin execution configuration. You can enable it with -X argument like this:

mvn -X clean package

Now that didn’t quite help with my case so next thing I did was to run it with remote debugger. That way I could step through the code line by line and inspect all the variables. To do that you just modify the MAVEN_OPTS environment variable in the shell where you are executing you maven plugin and add java debugger agentlib config like this:

MAVEN_OPTS="-agentlib:jdwp=transport=dt_socket,address=8000,server=y,suspend=y"

I used suspend=y so that it would wait for my debugger to attach before continuing the execution. Then you just add some breakpoints in you IDE and remote debug it like any java application. That by the way solved my issue as I realized each of my Liferay maven plugins were initialing Liferay configuration but since they were all run after each other in the same context only the first one mattered.