Re: JNI and Java Threads

From: nos (nos_at_nospam.com)
Date: 01/20/04

  • Next message: Jon A. Cruz: "Re: JNI and Java Threads" 
    Date: Mon, 19 Jan 2004 23:51:35 GMT

    "Rob" <.@.com> wrote in message
    news:400c3928$0$2439$cc9e4d1f@news.dial.pipex.com...
    > Hi there,
    >
    > Firstly, apologies for x-post but I'm hoping the post is relevant to both
    > groups...
    >
    > I have written a native implementation in C, and a java wrapper class.
    This
    > all works fine and correctly while I am calling the wrapper methods from a
    > single thread, however, my application requires calling the wrapper
    methods
    > for the native code from several different (java) threads.
    >
    > And this is where I get stuck! - I am getting garbage values back from
    > native unless I use the same thread each time.
    >
    > I have implemented something whereby I start a new thread, and then
    > wait/notify as each of the other requesting threads contact this single
    > thread, and then that single threads toodles off to native, but that
    causes
    > problems with waiting for return values and also makes the code far more
    > complicated that I believe it should be.
    >
    > I have seen mentions of the functions 'JNI_GetCreatedJavaVMs' and
    > 'AttachCurrentThread()', but as far as I can see, these are mainly for
    > contacting the JVM from the C and not the other way around. (I may well
    be
    > (and hope I am!) wrong).
    >
    > This is a passage from a book I have which seems to answer my question,
    > kinda:
    >
    > "Since a native method may be called from different threads in a Java
    > application, each different invocation of that native method will have a
    > different JNIEnv pointer passed to it. Passing this information to
    another
    > thread will result in unpredictable behaviour.
    > On the other hand, for a given method, from a given thread, the JVM
    > guarantees that the same JNIEnv pointer is passed on each call to that
    > method."
    >
    > This seems to say that what I want is not possible, but regardless its
    what
    > I need lol so one way or another I need to figure it out. (I'm calling
    > native methods from the GUI and from event register calls etc.) Can
    anyone
    > offer any relatively clean work arounds or suggestions to how this can be
    > done? (other than the messy way I've tested as suggested above?)
    >
    > Thanks for any anyone can offer, I've been on this for days!!! I'm hoping
    > I've been way off base and that the answer is simple.... but hey, I also
    > understand that this is computing...
    >
    > Rob.
    >
    >
    Does the 'c' program have to be re-entrant?

  • Next message: Jon A. Cruz: "Re: JNI and Java Threads"

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