JNI, run Java methodes from C++

Question

I'm trying to understand the JNI for a while, but can't realy figure out how to run a java methode from a C++ code.
(I've managed to do the oposit; run a C++ methode from Java, but that is not what I need.)
The thing is that I have a C++ program wich outputs a jpg picture. And I have a Java program wich is using this picture. (Picture size is from 2 - 4KB)
My temparary (not good) solution for making the picture available for the java code is to have the C program save it to a file, and then make the java program read the file once in a while.
What I would like is to have the C++ to call the methode updatePicture(byte[] newJPEG) in my javacode. How do I do this?
Parts of the C++ sourcecode:
void CExCameraView::OnTimer(UINT nIDEvent){
static int timercount=0;
     CExCameraDoc* pDoc = GetDocument();
     timercount+=pDoc->mycameraconfig->RefreshInterval ;
     if (timercount<pDoc->mycameraconfig->LoadOverHeads )
          return;
     else
          timercount=pDoc->mycameraconfig->LoadOverHeads;
     ASSERT_VALID(pDoc);
//grab one image and save it to a jpeg file
// here I'd like to call the java updatePicture methode (not save as file)
     pDoc->GrabSaveNext(pDoc->myconfig->cmsTimeOut);
And parts of the java code:
class ImageHandler {
JpegImage jpegImage; // class JpegImage extends Image
public ImageHandler() {
jpegImage = new jpegImage();
setParameters();
public updatePicture(byte[] newJPEG) {
setImage(newJPEG);
// lots of other methods
Pete
"If we knew what it was we were doing, it would
not be called research, would it? [Einstein]"

Answer 1

Well, you seem to be defining a C program with some imbedded java, or a java program with some native C methods. So which is it?
o If it is a C program, then you have two things to do (with respect to java):
- Start a java JVM.
- During your processing, periodically call into your JVM to process data made available by your C program.
o If it is a java program, then what you want to do -- I guess - is call a native method to start up an asynchronous (C) process, then have that asynchronous
process periodically call back into java to process results.
Overview of JNI (which you need to flesh out with some further study):
o There is the socalled invocation API, which is used to get java started.
o There is the definition of java native methods, which are then implemented in a C library (DLL on Windows), which library is loaded up at runtime by the java program, allowing the native methods to be called.
o There are JNI methods available from C that allow you to look up objects and methods, and invoke those java methods.
Two references:
o Essential JNI by Rob Gordon.
o www.swig.org