JNI - passing and returning parameters by value

[java_to_c]

EDIT - this should be in the advanced forum - i dunno how i ended up in this section - SORRY! can this be moved to the appropriate place?

Hi all. I am by nature a C++ programmer and have dabbled in Java a little bit so i hope this is ok being in the advanced forum, especially since i cannot get anything definitive online about that.

i am writing a .java application(web service or otherwise) that needs to have access to a C++ function through JNI. i know that primitives are passed always by value but how would i go about changing the value of the primitive inside the C++ function. can anyone give me some code to show me? anyway as a sidenote i am using g++ on Linux(Ubuntu).

i already have the .java file compiling. this code is:

Code:

class HelloWorld
{
 public native void displayMessage(Double x);
 static
 {
   System.loadLibrary("HelloWorldImp");
 }
        public static void main(String[] args)
        {
                Double x = 0.0;
                HelloWorld hello = new HelloWorld();
                hello.displayMessage(x);
                System.out.println(x);
        }

}

this produces my .class file. i then do:

$javah - jni HelloWorld.

this produces the C header needed for java and C++ to communicate with each other.

the .h file:

Code:

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

#ifndef _Included_HelloWorld
#define _Included_HelloWorld
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     HelloWorld
 * Method:    displayMessage
 * Signature: (Ljava/lang/Double;)V
 */
JNIEXPORT void JNICALL Java_HelloWorld_displayMessage
  (JNIEnv *, jobject, jobject);

#ifdef __cplusplus
}
#endif
#endif

thus my c++ code should mimic this and i do this hereHelloWorld.cpp)

Code:

#include <iostream>
#include "HelloWorld.h"

using namespace std;

JNIEXPORT void JNICALL Java_HelloWorld_displayMessage(JNIEnv *env, jobject obj, jobject x)

{
  //i need to change the value of x, thus changing what i passed in from the java code. 

  cout << "hello world!, this is a test" << endl;
  //return (jfloat)3.0;
}

as you can see from my comments i have already successfully returned a single value with no problem and i can print out statements just fine. Now i need to be able to change the value of the 'jobject x'. i know i could make a single double array or that i could just simply return one value but what if the function passes in 2 arrays and 3 primitives?

As i understand it the object of x is passed in not the value which is hard to think about coming from C++ but i think i have that right. so how do i use jobject to change the value inside the object itself. what function member or members must i use? i would appreciate any and all help. thanks!!!

p.s. i know this is a whopper for my first post.