The source code is not publicly available yet, but all since the beginning we've planned to make it open source.
This is great news! But, what are you waiting for? As a java developer I'd be very interested in contributing to this project.
Edit after reading the article Peter linked:
As the whole team is a big fan of open-source software, we really decided to make this game run on multiple platforms right away. I personally still remember those early platform wars, and they seem to continue to this very day.

This quote confuses me.
You do know what open source means right? It's got absolutely nothing to do with cross-platform capablilty.