I write numerical application in java and I use one C++ library. I need just one function from that library, so I made my own JNI interface to that function and everything works fine (not everything, I have problems when allocating more memory in C++ code, see my own forum message in JNI forum...)
What is your problem? If you need working tutorial and the one I linked is not satisfactory, you may try googling for another one. I have Bruce Eckel's Thinking in Java and that was my start...
With regards,
Miro