JNI在C调用不同的VS C ++？ [英] JNI Calls different in C vs C++?

问题描述

所以我必须用C以下code，它利用Java本地接口但是我想将其转换为C ++，但我不知道如何。

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的程序：

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");
     }
 }

什么是其中JNI相互作用的差异与C和C ++？任何帮助是极大的AP preciated。

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

谢谢，皮特

推荐答案

我曾经有书基本JNI 。虽然它有点过时，其中大部分是今天仍然有效。

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

如果我没有记错，在C，Java的结构是简单的指针。因此，在你的code，（* ENV） - GT; 被取消引用指针给你访问底层方法

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.

对于C ++， ENV ，实际上是一个对象 - 一个不同的实体C指针。 （和JNI实际上可以为你的C提供实物++ code来操作，因为C ++实际上支持的对象。）因此， env-＆GT; 具有不同的含义C ++，它意味着调用包含在该对象的方法指向 ENV

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".

另外一个区别，我相信是很多的C-JNI函数需要你的参数之一是的JNIEnv * ENV 。因此，在c您可能不得不说（* ENV） - GT;富（ENV，吧）。用c ++，第二次提到的 ENV 是没有必要的，这样你就可以代替说 env-＆GT;富（巴）

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)"

不幸的是，我没有上面的书在我面前，所以我不能完全证实了这一点！但我认为，调查这两件事情（专找他们对谷歌或其他JNI code）将让你pretty远。

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.

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