jWAM is a 100% Java Warren Abstract Machine implementation. Currently it is still under development though all the basic functionalities (compiling, resolution) are working. As far as I know this is the only open source Java based implementation out there that does not simplify Warren's design. 
I have done some comparison's with other Java based Prolog engine's and jWAM does seem to outperform them all quite well (tuProlog, JLog, JIP, jTrolog). However, if you need a Java based Prolog engine 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
jWAM
A 100% Java Warren Abstract Machine implementation.
Status: Pre-Alpha
                
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