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//
//
// NativeDemo
// Author: huang_jc 
// file name: NativeDemo.java
public class NativeDemo 
{  
         int i;
    int j;
    public static void main(String args[])
    {
             NativeDemo ob = new NativeDemo();
           ob.i = 10;
            ob.j = ob.test();
            System.out.println("this is ob.i:"+ob.i+"\n");
            System.out.println("this is ob.j:"+ob.j);
    } 
    public native int test();
    static 
    {
     System.loadLibrary("NativeDemo");    
    }
}

/* DO NOT EDIT THIS FILE - it is machine generated */
#include 
/* Header for class NativeDemo */

#ifndef _Included_NativeDemo
#define _Included_NativeDemo

#pragma pack(4)

typedef struct ClassNativeDemo {
long i;
long j;
} ClassNativeDemo;
HandleTo(NativeDemo);

#pragma pack()

#ifdef __cplusplus
extern "C" {
#endif
extern long NativeDemo_test(struct HNativeDemo *);
#ifdef __cplusplus
}
#endif
#endif
b : use javah -stubs NativeDemo to produce NativeDemo.c which seems as:
/* DO NOT EDIT THIS FILE - it is machine generated */
#include 

/* Stubs for class NativeDemo */
/* SYMBOL: "NativeDemo/test()I", Java_NativeDemo_test_stub */
__declspec(dllexport) stack_item *Java_NativeDemo_test_stub(stack_item *_P_,struct execenv *_EE_) {
    extern long NativeDemo_test(void *);
    _P_[0].i = NativeDemo_test(_P_[0].p);
    return _P_ + 1;
}

#include 
#include "nativedemo.h"
#include 

long NativeDemo_test(HNativeDemo *this)
{
    printf("hello:!\n");
    printf("this is in the native method!\n");
    return (unhand(this)->i)*2;
}