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  JNI Callbacks  (Read 857 times)
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Offline dranonymous

Junior Member




Hoping to become a Java Titan someday!


« Posted 2004-11-30 14:35:03 »

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?

Regards,
Dr. A>
Offline princec

JGO Kernel


Medals: 339
Projects: 3
Exp: 16 years


Eh? Who? What? ... Me?


« Reply #1 - Posted 2004-11-30 14:38:13 »

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 05:39:18 »

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.

Got a question about Java and game programming?  Just new to the Java Game Development Community?  Try my FAQ.  Its likely you'll learn something!

http://wiki.java.net/bin/view/Games/JeffFAQ
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Offline Matzon

JGO Knight


Medals: 19
Projects: 1


I'm gonna wring your pants!


« Reply #3 - Posted 2004-12-06 08:07:30 »

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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