JNI Calls different in C vs C++?

Question

So i have the following code in C that utilizes Java Native Interface however i would like to convert this to C++ but am not sure how.

 #include <jni.h>  #include <stdio.h>  #include "InstanceMethodCall.h"   JNIEXPORT void JNICALL   Java_InstanceMethodCall_nativeMethod(JNIEnv *env, jobject obj)  {      jclass cls = (*env)->GetObjectClass(env, obj);      jmethodID mid = (*env)->GetMethodID(env, cls, "callback", "()V");      if (mid == NULL) {          return; /* method not found */      }      printf("In C\n");      (*env)->CallVoidMethod(env, obj, mid);  } 

Java Program:

 class InstanceMethodCall {      private native void nativeMethod();      private void callback() {          System.out.println("In Java");      }      public static void main(String args[]) {          InstanceMethodCall c = new InstanceMethodCall();          c.nativeMethod();      }      static {          System.loadLibrary("InstanceMethodCall");      }  } 

What are the differences in which JNI interacts with C and C++? Any help is greatly appreciated.

Thanks, Pete

Answer 1

I used to have the book Essential JNI. And while it is kinda dated, much of it still works today.

If I recall correctly, in C, Java constructs are simply pointers. Thus, in your code, "(*env)->" is dereferencing pointers to give you access to the underlying methods.

For C++, "env" is actually an object - a different entity than a C pointer. (And JNI can actually provide real objects for your C++ code to manipulate, since C++ actually supports objects.) So "env->" has a different meaning in C++, it means "call the method that is contained in the object pointed to by "env".

The other difference, I believe, is that many of the C-JNI functions require that one of your parameters be the "JNIEnv *env". So in C you might have to say (*env)->foo(env, bar). With c++, the second reference to "env" is not necessary, so you can instead say "env->foo(bar)"

Unfortunately, I don't have the above book in front of me, so I can't quite confirm this! But I think investigating those two things (specifically looking for them on google or in other JNI code) will get you pretty far.