registering java function as a callback in C function

Question

I am trying to implement some C code in Java by using SWIG 1.3. Now I have to rebuild some existing C into Java code and to provide a function pointer to a Java function to

Answer 1

I remember scratching my head over this reference in the SWIG manual too.

You can do this as follows without the esoteric features:

• You need a mechanism to dispatch the incoming C callback into Java. For that you need the object ID of the object that you are calling into, and the method ID of your handler. In your C registration helper, create Global References for those and cache them for use by the callback.

• You also need a class ID and constructor method ID for anything that you want to pass to the java callback as a parameter. You also want to cache Global References to those.

• In the C part of the callback, look up your method IDs, construct arguments and call into Java.

• The thread that the callback comes in on, needs to be attached to the Java VM (with the JNI function AttachCurrentThread()). This is where you get your JNIEnv pointer from. This pointer is only valid in the context of the thread that you invoked AttachCurrentThread() from! What this means is that if you have callbacks coming in on multiple threads, you need to cache the JNIEnv * in thread local storage.

• Make sure you check return values after returning from JNI functions

• Make sure to check ExceptionOccurred() after any and all calls back into Java. Not doing this really gets you in trouble in hard to debug ways.

• I found this relatively easy to debug with Eclipse and Visual Studio as follows: Start main Java program from Eclipse, attach Visual Studio Debugger to that process. You can set breakpoints on either side.