If you need to call existing C++ code using JNI, take a look at NewJNI, the natural C++ interface to Java. NewJNI is included free with NewJ for C++.
http://www.pure-native.com/newj.html Things to consider:
- Full source code for NewJNI libraries is included, which can help you learn JNI.
- NewJNI is only implemented for Windows at this time, although the source code is overall very portable C++ and JNI.