Someone at work asked me about doing JNI. They want to make a java wrapper for a scene graph they have made. The question is - Can you have C code calling Java? IE - Everytime an event happens, the java code needs to run. It ‘seems’ like an inversion of the normal JNI stuff, where you call a java method and it in turn runs native code.
Can anyone speak to this?
Any sites where I would find good info?
Regards,
Dr. A>
Yeah, it’s not hard. The Java JNI reference book has it all in detail.
Cas 
Yup as Cas says, the hardest thing is working out the method “signiture” of the method you are calling so you can get a Method object from the VM to call through (and yes thisi so n the C side, you get a sturcture that represents the method from the VM.)
In fact JNI is totally bi-directional. You can even launch a new VM from inside of a C program using JNI.
A general JNI caution. Passing a lot of data between the Java heap and the C heap can be quite slow. If thats what you want to do you might want to investiagte Native ByteBuffers (which a modern JNI book should also talk about) which is a way to allow the Java side to interract directly with C-heap located data.
Oh, and another thing, which took me a while to figure out! - if the calling thread is NOT from the VM then you need to attach it before calling into the VM using AttachCurrentThread.