JNI Calls different in C vs C++?

Question

Hey Folks,

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

The first code a correct C++, isn't it? So, you're done!

Seriously, what do you want to change in that code? Why?

Answer 2

Have you tried wrapping your C code in an extern "C". See C++ Faq Lite for more information, and for other possible mechanisms to let you use your C code with C++.

Answer 3

It's been a while since I touched standard C++ but I'm gonna try it anyway.

"(*env)->" looks weird to me. Shouldn't that simply be "env->" ?

Maybe I'm wrong and it is supposed to work, but why make things more complicated anyways?

Answer 4

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.