Associating native calls with the calling object

Question

I have a problem where calls into a native library need to retain data for subsequent method calls. Imagine the following two native methods (just imagine that the signature is write for JNI methods):
void initialise(int value)
// Stores the value somehow
void dosomething()
  // Does something with the previously stored value
}If I was to store the value in a global variable, then it would be shared across all instance of the java class wouldnt it? So if object 1 called initialise, then object 2 called initialise, whichever of the objects then called dosomething, it would be based on the latest call to initialise.
How do I get around this sort of thing?
I currently work in C, though I would be quite happy to switch to C++ if necessary.
Mossop

Answer 1

I haven't used JNI_OnUnload, but may see why it isn't called. The doc says it runs in the finalizer for the classloader that loaded the library. Assuming this is the system (not custom) classloader, then its finalizer will never run because it gets gc'd only at shutdown, and finalizers don't run at shutdown by default. You could try calling Runtime.runFinalizersOnExit, but it's deprecated and sounds unsafe.
So my next idea would be to add a shutdown hook (Runtime.addShutdownHook) the first time one of your objects is constructed. The hook would call a native method to do the cleanup. Or since a shutdown hook is a Thread subclass, just make your overridden run() be the native cleanup function.
Of course this now requires keeping a native-side data structure to track the native objects, though I'd still vote for storing each native peer's pointer in a Java long field for fast dereference, and make this native data structure be a simple array or list (not map) for usage only by the cleanup logic.
Another idea is to clean up in a function called by the OS or C++ runtime when your library is unloaded, independent of Java. This could be DllMain (for Windows) or a C++ global object destructor.
-slj-