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  JNI Callbacks  (Read 1130 times)
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Offline dranonymous

Junior Devvie

Hoping to become a Java Titan someday!

« Posted 2004-11-30 13:35:03 »

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?

Dr. A>
Offline princec

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« Reply #1 - Posted 2004-11-30 13:38:13 »

Yeah, it's not hard. The Java JNI reference book has it all in detail.

Cas Smiley

Offline Jeff

JGO Coder

Got any cats?

« Reply #2 - Posted 2004-12-06 04:39:18 »

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.

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Offline Matzon

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« Reply #3 - Posted 2004-12-06 07:07:30 »

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<AsDaemon>.

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