Test rich app (loaded from jar) with cucumber

Hello guys!

I'm just playing around with cucumber and thought that it would be nice if I could get references to the classes to test from a started app started from a jar like if I programmatically start it like:
Runtime runtime = Runtime.getRuntime();
Process process = runtime.exec( new String[] { "java", "-jar", fileName } );
I just thought that this would be a more realistic test.
But I do not find a way to get a reference to the loaded class Main for eg.
Do you think this is possible or should I just stop trying to do it this way?
Best regards
Fredrik