Multithreaded Java app calling C via JNI...need help!

I have a multithreaded Java app making a JNI call into some C code. It is a high speed imaging algorithm that I simply can't/don't want to do in Java. I am very new to JNI and I am hoping to get some expert advice on how to solve this issue. Basically, I need a seperate JNI session (if that is such a thing) per thread. In other words, I need to essentially keep state of each JNI call as it pertains to the current thread.
The 2 ideas I have thought about are:
1. Have the native code pass the data back into java, and have the java object hold it. Then, when subsequent native calls are made, you pass the data as a paramter of the native method.
(This can be painful as it a lot of complicated and/or a lot of data.)
2. Define some sort of a structure to hold each instance of the data on the C side, returning to java a lookup key. Subsequent native calls include the key as a paramter, and the data is looked up. ( a quick example of this would be nice).
I would appreciate any information/expereince/pains with this one.
Thanks!

2. Define some sort of a structure to hold each instance of the data on the C side, returning to java a lookup key. Subsequent native calls include the key as a paramter, and the data is looked up. ( a quick example of this would be nice).The lookup key, or a simple "pointer", can be returned to Java as a plain old "int" or "long" (I don't know what is the size of the pointers in your platform, if 32 or 64-bit). So you can return a "long", that can be treated by Java programs as an "opaque handle" (a pompous name for things that can't be handled adequately at the "client" side).
I assume that C/C++ allocates the memory and it remains "fixed" (untouched by garbage collection) between calls. So a real C pointer can be used in such case. If you have some smarter scheme of allocating things in your program, return another kind of lookup key (for instance, if you allocate things in fixed-size C arrays, you can return the index of the C object in the array instead of returning a plain pointer.

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