jWAM
A 100% Java Warren Abstract Machine implementation.
... for your project I recommend tuProlog and jTrolog as they are at the moment way more mature than jWAM.
Most notable to-do's:
- Built-in predicates.
- Debugging tools.
- Testing.
- Documentation and commenting.
- Android port. (I have the engine working with a test app but I still need to build a proper app).
- Alpha release