Jruby 1.7 simply doesn't work in GAE, and maybe it's not worth making it work there, which I would understand. I opened an issue a while ago (https://github.com/jruby/jruby/issues/2304) and I spent about a week trying to fix it. Each time I fixed one issue another one popped up and I gave up after hacking up JRuby beyond my comfort level.
Edit: I just want to stress, the JRuby team is amazing, and I've always been impressed with the responsiveness and openness of the team. This comment isn't meant as a critique in anyway, just a statement of fact with my experience trying to run JRuby on app engine.
Edit: I just want to stress, the JRuby team is amazing, and I've always been impressed with the responsiveness and openness of the team. This comment isn't meant as a critique in anyway, just a statement of fact with my experience trying to run JRuby on app engine.