Objective C calling Java methods using JNI

Question

Can someone show me how to call a Java method from Objective C.

In more detail this is actually what I would like to do

1) Have first call from the java si

Answer 1

The following seems to work. It is based on the above comment on following the C examples and this link.

http://urlgrey.net/?p=121

As the link says do not create a global variable for the env instead create a global variable of the jvm, also create a global reference to your class.

Here is how I implement the step 1 : " 1) Have first call from the java side to the objective C. during this call I would like to get a reference to the java object. "

First declare global variable in the header file for

1) the jvm:

JavaVM *jvm;

2) The Java class:

jclass smartCallbackClass;

3) The java Object:

jobject smartCallbackObject;

Next in the call that comes from the Java side to the Objective C set the values for these variables

1) For the JVM:

(*env)->GetJavaVM(env, &jvm);

2) For the Object:

smartCallbackObject = (*env)->NewGlobalRef(env, obj);

3) For the Class:

if (smartCallbackClass == NULL) {
    jclass localRefCls = (*env)->FindClass(env,"com/studyisland/nativelibs/smart/responsesdk/interfaces/thin/SMARTResponseThinClient"); 
    if (localRefCls == NULL) {
        NSLog(@"Unable to create a JNI Java Class reference \n");
    }
    //Create a global reference for JNI Java class
    smartCallbackClass = (*env)->NewGlobalRef(env,localRefCls);

    //Delete the local reference as it is no longer needed
    (*env)->DeleteLocalRef(env, localRefCls);

    //Is the global reference created successfully?
    if (smartCallbackClass == NULL) {
        NSLog(@"Unable to create JNI Java class reference \n");
        return 0;
    }       
}

Here is the link from where I got the code for the class

http://java.sun.com/docs/books/jni/html/refs.html

Now the second step

" 2) Later down the line I would like Objective C to use the reference got in the previous step to call Java methods "

To call from Objective C back to Java you need to make sure that the call is done on the same thread by which Java called Objective C, so here is the code.

-(void)classFailedToStop:(SMARTResponseCallBackEventArg*)anArg{
    JNIEnv *env;
    int attach = (*jvm)->AttachCurrentThread(jvm, (void**)&env, NULL);
    if(attach == 0){
        (*jvm)->GetEnv(jvm, (void**)&env, JNI_VERSION_1_4);
        jmethodID method = (*env)->GetMethodID(env, smartCallbackClass, "callback_onStopClassFailed", "()V");
        (*env)->CallVoidMethod(env, smartCallbackObject, method);
    }
    (*jvm)->DetachCurrentThread(jvm);

}

Hope this helps.