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;
}

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;
}