Question

Java Native Interface (JNI)

Java Native Interface (JNI) is one of
the intersting interface by java By
using Java Native Interface (JNI) you
can operate with other applications
and libraries.

JNI is the native programming interface for java that is part of JDK. Using JNI you can operate with other applications and libraries written in other language such as C,C++. But the basic question arises when should I use the JNI ?

1. You want some platform specific information and the standard Java class library may not support the platform-dependent features needed by your application.
2. You have some library application written in other language and you want to use it in your java application.
3. You want Java should interact with some low level programming language.

---

Below is given Simple Example; See that methods have ‘native’ KeyWord:

public native void displayHelloWorld();
public native void displayOther();
private native String getLine(String prompt);

The DLL we are going to use is firstJNI.DLL This DLL can be generated by VC++ or borland. Which we will discuss later.

//firstJNI.java

class firstJNI
{
    public native void displayHelloWorld();
    public native void displayOther();
    private native String getLine(String prompt);

    static {
     System.loadLibrary("firstJNI");//This is firstJNI.DLL
     /*if generated by borland
     System.loadLibrary("firstjni");//This is firstjni.dll
     */
    }

     public static void main(String[] args) 
     {
        firstJNI JN=new firstJNI();
        JN.displayHelloWorld();
        JN.displayOther();
        
        String input = JN.getLine("Enter Some Thing "); 
        System.out.println("You Entered " + input); 
     }
}

Compile the above code using (What does this mean ?)

prompt>javac firstJNI.java

Then create header File using (What does this mean ?)

prompt>javah javah -jni HelloWorld

This will create firstJNI.h file. In the header File you will see

-------------------------------------
JNIEXPORT void JNICALL Java_firstJNI_displayHelloWorld
(JNIEnv *, jobject);

/*
 * Class:     firstJNI
 * Method:    displayOther
 * Signature: ()V
 */
JNIEXPORT void JNICALL Java_firstJNI_displayOther
  (JNIEnv *, jobject);

/*
 * Class:     firstJNI
 * Method:    getLine
 * Signature: (Ljava/lang/String;)Ljava/lang/String;
 */
JNIEXPORT jstring JNICALL Java_firstJNI_getLine
  (JNIEnv *, jobject, jstring);
----------------------------------------------

Don’t edit header File

Now let see how to generate DLL using VC++, Click: File->New->Win32Dynamic-Link Library
Give name and Select
A simple DLL project
You will have
firstJNI.CPP file
Below is given the firstJNI.cpp file

// MYVCDLL.cpp : Defines the entry point for the DLL application.
//

#include "stdafx.h"
#include "D:\Kanad\Study\codeToad Articles\firstJNI.h"
#include "jni.h" //can copy or give full path
#include <math.h>

BOOL APIENTRY DllMain( HANDLE hModule, 
                       DWORD  ul_reason_for_call, 
                       LPVOID lpReserved
                     )
{
    return TRUE;
}

extern "C" __declspec(dllexport) int getMemorySize();
//And your function definition should look like this: 
extern "C" __declspec(dllexport) int getMemorySize()
{     //do something 

MEMORYSTATUS memoryStatus;  
int MB=1024*1024 ;
double memSize;  
memoryStatus.dwLength=sizeof(MEMORYSTATUS);  

GlobalMemoryStatus(&memoryStatus);  

__int64 size= memoryStatus.dwTotalPhys;  

memSize=(double)size/MB;  

printf("\nTotal Memory %.0lf MB",ceil(memSize));

 return 0;
}

JNIEXPORT void JNICALL 
Java_firstJNI_displayHelloWorld(JNIEnv *env, jobject obj) 
{
    printf("Hello world! This is using VC++ DLL\n");

}

JNIEXPORT void JNICALL 
Java_firstJNI_displayOther(JNIEnv *env, jobject obj) 
{
    
    printf("Hello world! This is using VC++ DLL Other Function \n");
    getMemorySize();
    
}

JNIEXPORT jstring JNICALL
Java_firstJNI_getLine(JNIEnv *env, jobject obj, jstring enter)
{

    char buf[128];
    const char *str = env->GetStringUTFChars(enter, 0);
    printf("\n%s", str);
    env->ReleaseStringUTFChars(enter, str);
    scanf("%s", buf);
    return env->NewStringUTF(buf);

}

Now I have questions about how can I use .dll file written in C++/C in my java application. I am developing application for android using Eclipse and I have some dll files and I haven’t their source … How can I use them in my project ???

Answer 1

First a disclaimer – I’m a bit sketchy on this, it’s been a while since I’ve used JNI.

Many JNI examples assume you own the code for the library you want to call, which in my experience is rarely the case. In the example you sight the javah util has been used to generate a header file, against which cpp implementation has been written – this is why you can see the jni header file and various Java keywords in the cpp file.

In order to use a 3rd party dll, you first need the documentation for that dll, without that you’re dead in the water. The reason you need the documentation is that you’re going to provide a wrapper dll that simply delegates to the 3rd party dll – you need to know how to call it and how to perform any type mappings. Obviously it’s this wrapper that will contain all the JNI stuff to allow Java to make the call to that wrapper, which in turn calls the 3rd party dll.

There’s various ways to do this but the easiest way I know is to use SWIG, which will generate all the C++ code required for the wrapper dll. It also helps to have someone that knows C++ on hand – they’ll be invaluable writing interface files (.i or .swg files) that SWIG uses to generate the wrapper code.