Why not use GNUStep?
GNUStep is awesome, but it’s an Objective-C framework. The Java bridge is great, but even Apple is backpedaling on that approach with their pure-Java version of WebObjects, due to performance problems and conceptual difficulties in development. Additionally, GNUStep may not be available or stable on your platform. Wotonomy has the same license (GNU Lesser Public License), and is written completely in Java.