Re: jni and threads

From: Gordon Beaton (not_at_for.email)
Date: 10/16/04

  • Next message: Mako Klaic: "java packages" 
    Date: 16 Oct 2004 10:06:00 +0200

    [ Followup set ]

    On Fri, 15 Oct 2004 15:16:09 -0400, Thomas Smith wrote:
    > I am building a binding to a runtime engine that was written in C++.
    > I have no problems setting up the mapping from object calls to the
    > native implementation, and storing reference to those objects. The
    > problem comes when I start a new thread... The new thread does not
    > have access to the same memory, and I end up with a pointer to dead
    > space.

    Please don't multipost.

    Two important things to keep in mind:

    - the JNIEnv* pointer is only valid in the thread for which it was
      created. Whenever you make normal calls from Java to C, you get a
      new, valid JNIEnv*. Don't attempt to store the pointer for use in
      any other context. If you have created threads in the native code
      and need to use JNI functions, use JNI_GetCreatedJavaVMs() and
      AttachCurrentThread() to get a valid JNIEnv* for each thread that
      needs one.

    - when you store object references that will live across multiple
      native calls, it is important to only store _global_ references
      created with NewGlobalRef(). The JVM doesn't know what your native
      variables contain, only what gets passed in and out of your native
      code. When you return from a native call, the JVM releases any local
      references you held during the call, so there is a risk that such
      objects will become eligible for garbage collection unless you
      create global references. Don't forget to delete any global
      references when you're done with them or they will never become
      eligible for garbage collection.

    /gordon 

    -- 
[  do not email me copies of your followups  ]
g o r d o n + n e w s @  b a l d e r 1 3 . s e
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