JNI（Java和C ++）在Ubuntu 11.10上使用Eclipse

Question

Hi I'm new to JNI and am just trying to get a simple JNI example to work using Eclipse. I followed the steps in the following tutorial: http://codeandme.blogspot.com/2011/09/jni-made-easy.html My Java code was as follows:

public class Main 
{
    private native int getDouble(int n);
    public static void main(String[] args)
    {
        System.load("/home/ryan/Desktop/libJNILibrary3.so");
        Main m = new Main();
        System.out.println(m.getDouble(3));
    }
 }

I then used the javah tool contained in jdk1.7.0_11/bin to generate a header file.

/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class Main */

#ifndef _Included_Main
#define _Included_Main
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     Main
 * Method:    getDouble
 * Signature: (I)I
 */
JNIEXPORT jint JNICALL Java_Main_getDouble
  (JNIEnv *, jobject, jint);

#ifdef __cplusplus
}
#endif
#endif

I then created a C++ project in eclipse and created a Main.cpp source file. I placed the Main.h file in the same folder as Main.cpp. The code for Main.cpp was as follows:

#include "Main.h"

JNIEXPORT jint JNICALL Java_Main_getDouble
  (JNIEnv *env, jobject obj, jint n)
{
    return n*2;
}

I then followed the instructions in the tutorial for configuring the project build settings. When I built the project, it created the ".so" file "libJNILibrary3.so". I then ran my Java class "Main" and received the following error:

Exception in thread "main" java.lang.UnsatisfiedLinkError: Main.getDouble(I)I
    at Main.getDouble(Native Method)
    at Main.main(Main.java:9)

The strange thing is, if I redo this example, and make it so the getDouble method takes no arguments but merely returns the value 2, then everything works how it should. Making the method take arguments is somehow messing everything up. I have spent over ten hours on this error and have searched many forums, but have not found a solution to this problem. Any help would be greatly appreciated. Thanks!

Answer 1

You placed the body of your function in a cpp file and the JNI callback has to follow C style name mangling.

try this in your cpp file:

extern "C"
{

JNIEXPORT jint JNICALL Java_Main_getDouble
  (JNIEnv *env, jobject obj, jint n)
{
    return n*2;
}

}