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JNI Invocation interface - Callback from java to C++

Posted on 2009-07-07
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Last Modified: 2013-12-14
Hi,
I am trying to use JNI invocation interface to start a java program from a c++ dll and then receive call back from the java program (it happens to be a socket server , so i need to know when someone is connecting to the socket server(java) )  . The problem is i can not find any documentation of call back in JNI invocation interface. I put my code here and can someone please tell me what I am doing wrong or point me in the right direction of some proper documentation on this matter (callback from java in JNI invocation interface.)

I have some clue about it though. I could pass a pointer to a function ( that is declared in my cpp ) to  the java program and then from the java program i could convert that pointer to a method and call it. But i do not know how to do it. So if anyone can provide me a sample code to do it , it will also be accepted as a solution.

Regards,
Shrouk Khan
// java code
public class Prog 
{
	public native void callcpp(); // trying to use this as a call back to cpp dll
	static{
		System.loadLibrary("CPPDll");
	}
    public static void main(String[] args) {
	
         System.out.println("Java program started, now going to do call back from java:  " + args[0]); //
       new Prog().callcpp(); // call a method in cpp
		 
    }
   
}
 
 
// dll code for starting the java program and receiving call back:
 
Callback  Connected=NULL;
int WINEXPORT start_java() //this is where i start the java program by calling its main metho
{
	
	printf("Inside \n");
	JNIEnv *env;
	JavaVM *jvm;
	jint res;
	jclass cls;
	jmethodID mid;
	jstring jstr;
	jclass stringClass;
	jobjectArray args;
 
	JavaVMInitArgs vm_args;
	JavaVMOption options[1];
	options[0].optionString =
		"-Djava.class.path=" USER_CLASSPATH;
	vm_args.version = 0x00010002;
	vm_args.options = options;
	vm_args.nOptions = 1;
	vm_args.ignoreUnrecognized = JNI_TRUE;
	/* Create the Java VM */
	res =JNI_CreateJavaVM(&jvm, (void **)&env, &vm_args);
	printf("JVM Created \n");
	if (res < 0) {
		printf("cant create \n");
	}
 
	cls = env->FindClass("Prog");
	printf("class found  \n");
	if (cls == 0) {
		goto destroy;
	}
 
	mid = env->GetStaticMethodID(cls, "main",
		"([Ljava/lang/String;)V");
	if (mid == 0) {
		goto destroy;
	}
	jstr = env->NewStringUTF(" from C!");
	if (jstr == 0) {
		goto destroy;
	}
	stringClass = env->FindClass("java/lang/String");
	args = env->NewObjectArray( 1, stringClass, jstr);
	if (args == 0) {
		goto destroy;
	}
	env->CallStaticVoidMethod(cls, mid, args);
 
destroy:
	if (env->ExceptionOccurred()) 
	{
		env->ExceptionDescribe();
	}
	jvm->DestroyJavaVM();
	return 1;
}
 
Callback  Connected=NULL;
 
JNIEXPORT void JNICALL 
Java_Prog_callcpp(JNIEnv *env, jobject obj)
{
	Connected();
	printf("Call back received from java !\n");
	return;
}

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Question by:Softverkpro
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Accepted Solution

by:
Softverkpro earned 0 total points
ID: 24810394
Hi, okay after 5 days of obsessing over this problem i figured out the solution. The problem
arises because I am using the DLL to load the java program (the DLL is actually loaded from c# ,
so the whole flow of control is like this : C# GUI --> C++ Dll (custom JNI wrapper) --> Java
Server ) . But the following snippet in my java code:

static
{
       System.loadLibrary("CPPDll");
}

instructs the java to load the cpp dll (the caller) even before cpp dll is able to load the java program. So it instantly crashes! The solution is to move that instruction to some place which will be called well after the java has started running.

So, this following Snippet solved the problem.

(i only needed to change my java code, cpp code was fine.)
public class Prog 
{
	public native void callcpp();
	
		
	
    public static void main(String[] args) {
		 System.loadLibrary("CPPDll");
         System.out.println("Hello world " + args[0]);
         System.out.println("Trying to callback");
		 callback();
		 System.out.println("Finished");
    }
    public static void callback() {
  		System.out.println("Inside callback");
         new Prog().callcpp();
         System.out.println("done calling callback");
    }
}

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Author Comment

by:Softverkpro
ID: 24810399
actually it is possible (i think!) to just use some super expensive third party library's like the ones from TeamDev but then again I or anyone else would not learn anything and keep hitting into problems like this. So i decided to write the library on my own. :)
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