How to implement a callback to Java from C using JNI?

Question

I use JNI in my Java code to call native C code. This works fine and I'm doing some processing in the C code. The C code registers an event listener so that I'm notified every time a new result is ready in the C part.

Now I want to return the results to the Java code like that:

void notifyGazeEvent()
{
    jclass cls = (*env)->GetObjectClass(env, obj);
    jmethodID mid = (*env)->GetMethodID(env, cls, "newGazeEvent", "(I)V");
    if (mid == 0)
    {
        return; 
    }
    printf("In C, about to enter Java");
    (*env)->CallVoidMethod(env, obj, mid, 1);
}

The problem now is: I don't know where to get the JNIEnv object and the jObject object from. This is passed when I first call the C code:

JNIEXPORT void JNICALL
Java_a_b_C_doCalculation(JNIEnv *env, jobject obj)
{
// here I register all the handlers etc, 
}

But I don't know how I can pass the references until my method that does the callback to Java.

You could check out JavaCPP - Creating Callbacks Functions for that. It would save you a lot of trouble. — Samuel Audet
– Samuel Audet, Commented Aug 30, 2014 at 7:45

IgorGanapolsky · Accepted Answer · 2016-06-03 13:32:41Z

1

To keep the jobject obj outside the Java_a_b_C_doCalculation function, you have to create a global reference, since all objects (JNIEnv also) are no longer valid after the function returns. A global reference can easily be created with NewGlobalRef.

The JNIEnv pointer can't be saved that way, also if the thread that calls notifyGazeEvent() was created in C you have to attach the thread to the JVM first. Save the JavaVM *vm pointer from JNI_OnLoad, and either optain the JNIEnv with GetEnv or attach a newly created thread with AttachCurrentThread (AsDaemon).

edited Jun 3, 2016 at 13:32

IgorGanapolsky

26.9k23 gold badges121 silver badges151 bronze badges

answered Jul 22, 2014 at 19:13

AleX

1094 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

How to implement a callback to Java from C using JNI?

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related