How to call back to C via JNI in Java started from C?

Question

Hi!
This problem might seem outlandish, but I have not been able to find any other method to reach my goal. The situation is the following.
There is a C++ program I intend to interface with a piece of Java code, called the manager. The C++ code invokes a VM, starts a Java glue code, that connects to the manager via RMI. This works fine. The glue code, however, can be called from the manager, via RMI as well. The problem is, that these calls should in turn call functions in the C++ code, that originally started the Java glue code. JNI can only load a library, but this is not what I want to do now. I want to somehow connect back to the C code that started the Java code. Is this possible at all?
Thanks for your help,
Ambrus

Answer 1

What you want to do is not too tough, but there will be some details to be worked through. In particular, you have to figure out how to make your "callback" get from your "glue" code back into your C++.
1. Calling back out is from java to C is pretty easy. There is a JNI function for registering a native method with the JVM. Here is an example of the registration code:
// See if the service interface class is known.
javaClass = javaEnv->FindClass("JavaInterfaceObject");
if (javaClass != 0) {
//Register a native method to place java server messages in the service log.
// Define the service logger native method.
JNINativeMethod methods[] = {
{"addToMessageLog", "(Ljava/lang/String;)V", Java_addToMessageLog}
// Register the method with the jvm.
javaEnv->RegisterNatives(javaClass, methods, 1);
     javaEnv->ExceptionClear();     // Just in case not found.
     return TRUE;
2. You have to define the native method:
* Native method - callback to place java server messages in the service log.
JNIEXPORT void JNICALL Server::Java_addToMessageLog(JNIEnv * javaEnv, jclass javaClass, jstring javaMsg) {
     jboolean     isCopy;
     const char* msg = javaEnv->GetStringUTFChars(javaMsg, &isCopy);
     Server::theServer->logger->addToMessageLog((char*)msg);
     if (isCopy)
          javaEnv->ReleaseStringUTFChars(javaMsg, msg);
3. The real headache is that this is C code, not C++. In other words, if you really need to call into C++, then you need to seed your callback so that it has a pointer to the appropriate C++ object.