Just a quick note. Sorry if you've already noticed this, but shouldn't the error message in:

 public void end(Transition transition) {
 if (this.end!=null){
 throw new IllegalStateException("task instance '"+id+"'is already started");
 }

read "task instance "+id+" has already ended"? I called end() and got this error message and I went:"Huh? I know it's already started, that's why I want to end it." : )