Implement callback function in JNI using Interface

Question

I need to implement callback function in Java using “interface”. I have wrote the application part as MyJavaFunction(int size, m_GetSizeInterface);

m_GetSizeInterface is an Interface which contains the callback function GetSize. This GetSize method is override in the application. In JNI I need to call a CPP function having prototype int MyCPPFunction(int size, int (*callback)(int* ID));

How can I pass this GetSize as parameter to MyCPPFunction in JNI? Please help

public int GetSize (m_SizeClass arg0)
{
    g_size = arg0.size;
        return 0;
}

Iapetus

Answer 1

The complication here is that you want to invoke native C++ code which you, in turn, want to invoke a java method. This is actually a bit tricky.

You need to create a JNI C++ function for java to call, and a C++ function matching the MyCPPFunction callback signature. The latter will act as a wrapper to call the java method.

Because the wrapper will need information about the JNI environment, which cannot be provided by parameters (lest we ruin the signature) you create a few global variables to hold it:

jobject g_getSizeIface;
jmethodID g_method;
JNIEnv *g_env;

The C++ function which java will call is the following:

JNIEXPORT void JNICALL Java_ClassName_MyCPPFunction
     (JNIEnv *env, jint size, jobject getSizeInterface)
{
      jclass objclass = env->GetObjectClass(getSizeInterface);
      jmethodID method = env->GetMethodID(objclass, "GetSize", "(m_SizeClass)I");
      if(methodID == 0){
          std::cout << "could not get method id!" << std::endl;
          return;
      }
      g_method = method;
      g_getSizeIface = getSizeInterface;
      g_env = env
      MyCPPFunction(size, WrapperFunc);
}

And the wrapper function is thus:

int WrapperFunc(int *id)
{
      jint retval;
      //marshalling an int* to a m_SizeClass boogy-woogy.
      ...
      g_env->ExceptionClear();
      retval = g_env->CallIntMethod(g_getSizeIface, g_method, 
                                    /*marshalled m_SizeClass*/);
      if(g_env->ExceptionOccurred()){
          //panic! Light fires! The British are coming!!!
          ...
          g_env->ExceptionClear();
      }     
      return rvalue;
}

Pentstemon

can we have full functional sample code..what will be declaration at Java layer? – Canale

what if one needs to register the callback with native code multiple times (passing different interface implementations), so that each WrapperFunc registration has an associated interface object/method to invoke – Pinna

How do you call the Java_ClassName_MyCPPFunction function from your java code? – Haunt

Answer 2

#include <functional>
#include <cstdlib>
#include <map>

class SimpleQueueEvent {

public:

    SimpleQueueEvent(){};
    ~SimpleQueueEvent(){};

    //for C++ code call
    int queueEvent(std::function<void(void)> func){
        int curTime = time(0) + rand() % 10000;
        eventMaps.insert(std::map<int, std::function<void(void)>>::value_type(curTime, func));
        return curTime;

        //Call Java method to invoke method, such as
        //env->FindClass("....");
        //jmethodID method = env->FindMethod("onPostQueueEvent"...);
        //env->InvokeVoidMethod();

        //Java code like this..
        // private void onPostQueueEvent(final int eventId){
        // listener.PostQueueEvent(new Runnable() {
        //    public void run() {
        //        nativeEventFunc(eventId);
        //    }
        // });
        // private static native void nativeEventFunc(int eventId);

    }

    void nativeEventFunc(int eventId){
        if(eventMaps.find(eventId) != eventMaps.end()){
            std::function<void(void)> func = eventMaps.at(eventId);
            func();
        }
    }



private:
    std::map<int, std::function<void(void)>> eventMaps;


};

//and test code is:

 SimpleQueueEvent queueEvent;
    std::function<void(void)> func = [](){
        printf("native runnable..\n");
    };

    int evenId = queueEvent.queueEvent(func);
    queueEvent.nativeEventFunc(evenId);

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