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Clayton E. Cramer
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 Posted: Mon Jul 26, 2004 5:11 am Post subject: An example of a Java application calling C, and the C callin |
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I have been able to use the JNI to write a Java application that calls C
functions, but I am having a heck of a time getting the C functions to
reliably call Java methods. I believe that the problem has to do with
multiple threads and not having the correct value for the JNIEnv when the C
function decides to invoke a Java method. There must be some way for a C
function to retrieve the current JNIEnv that invoked this C function,
without passing it in.
Please?
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Roedy Green
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| Posted: Mon Jul 26, 2004 8:17 am Post subject: Re: An example of a Java application calling C, and the C ca |
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On Sun, 25 Jul 2004 23:11:11 -0600, "Clayton E. Cramer"
<clayton (AT) claytonDELETETHIScramer (DOT) com> wrote or quoted :
| Quote: | I have been able to use the JNI to write a Java application that calls C
functions, but I am having a heck of a time getting the C functions to
reliably call Java methods. I believe that the problem has to do with
multiple threads and not having the correct value for the JNIEnv when the C
function decides to invoke a Java method. There must be some way for a C
function to retrieve the current JNIEnv that invoked this C function,
without passing it in.
|
Methods you may find of interest
JNICreateJavaJVM -- fires up a brand new JVM inside a standalone C
Program.
which will give you an env.
You can start up a new thread in the JVM with AttachCurrenThread.
I suggest you look at http://mindprod.com/jgloss/jni.html
and follow the link to Liang's book, chapter 7.
--
Canadian Mind Products, Roedy Green.
Coaching, problem solving, economical contract programming.
See http://mindprod.com/jgloss/jgloss.html for The Java Glossary.
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Jim Sculley
Guest
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| Posted: Mon Jul 26, 2004 9:23 pm Post subject: Re: An example of a Java application calling C, and the C ca |
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Clayton E. Cramer wrote:
| Quote: | I have been able to use the JNI to write a Java application that calls C
functions, but I am having a heck of a time getting the C functions to
reliably call Java methods. I believe that the problem has to do with
multiple threads and not having the correct value for the JNIEnv when the C
function decides to invoke a Java method. There must be some way for a C
function to retrieve the current JNIEnv that invoked this C function,
without passing it in.
|
Er, every JNI invocation of a C function should already have the JNIEnv
pointer (for the current thread) passed in. Are you creating threads on
the native side? Then you might have a problem. If so, see the
AttachCurrentThread function:
http://java.sun.com/j2se/1.5.0/docs/guide/jni/spec/invocation.html#wp1060
====================
The JNI interface pointer (JNIEnv) is valid only in the current thread.
Should another thread need to access the Java VM, it must first call
AttachCurrentThread() to attach itself to the VM and obtain a JNI
interface pointer. Once attached to the VM, a native thread works just
like an ordinary Java thread running inside a native method. The native
thread remains attached to the VM until it calls DetachCurrentThread()
to detach itself.
====================
Jim S.
--
Remove my extraneous mandibular appendages to reply via email.
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Clayton E. Cramer
Guest
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| Posted: Mon Jul 26, 2004 11:38 pm Post subject: Re: An example of a Java application calling C, and the C ca |
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"Roedy Green" <look-on (AT) mindprod (DOT) com.invalid> wrote
| Quote: | On Sun, 25 Jul 2004 23:11:11 -0600, "Clayton E. Cramer"
[email]clayton (AT) claytonDELETETHIScramer (DOT) com[/email]> wrote or quoted :
I have been able to use the JNI to write a Java application that calls C
functions, but I am having a heck of a time getting the C functions to
reliably call Java methods. I believe that the problem has to do with
multiple threads and not having the correct value for the JNIEnv when the
C
function decides to invoke a Java method. There must be some way for a C
function to retrieve the current JNIEnv that invoked this C function,
without passing it in.
Methods you may find of interest
JNICreateJavaJVM -- fires up a brand new JVM inside a standalone C
Program.
which will give you an env.
|
This isn't a standalone C program. Java application calls C functions; the
C functions,
much later and deeper, need to call Java methods. I could change all the C
code to
pass the JNIEnv down to the C functions that do a callback to the Java
methods, but I was
hoping for some other way to do this. Apparently caching the JNIEnv from
the first Java
invocation of the C function doesn't work reliably for later invocations of
the C functions.
| Quote: | You can start up a new thread in the JVM with AttachCurrenThread.
I suggest you look at http://mindprod.com/jgloss/jni.html
and follow the link to Liang's book, chapter 7.
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Thanks for the pointer.
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