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Java Native Interface (JNI)

Article by:

Kanad Deshpande (4/15/2005)

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Summary:

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.

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Java Native Interface (JNI)

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 ?

You want some platform specific information and the standard Java class library may not support the platform-dependent features needed by your application.
You have some library application written in other language and you want to use it in your java application.
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.

Select All Code

<pre>
//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); 

        }
}
</pre>

Compile the above code using

prompt>javac firstJNI.java

Then create header File using

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 the real part comes - making the DLL!. First we see generating the DLL using the Borland compiler. Now we will write some simple C Code

Select All Code

<pre>
/*firstJNI.c*/
#include <jni.h>
#include "HelloWorld.h"
#include <stdio.h>


JNIEXPORT void JNICALL 
Java_firstJNI_displayHelloWorld(JNIEnv *env, jobject obj) 
{
    printf("Hello world! This DLL was generated using Borland Compiler \n");
    return;
}


JNIEXPORT void JNICALL 
Java_firstJNI_displayOther(JNIEnv *env, jobject obj) 
{

printf("Hello world! This is Other Function. This DLL was generated using Borland Compiler \n");
    
}

  
JNIEXPORT jstring JNICALL
Java_firstJNI_getLine(JNIEnv *env, jobject obj, jstring enter)
{
    char buf[128];
    const char *str = (*env)->GetStringUTFChars(env, enter, 0);
    printf("%s", str);
    (*env)->ReleaseStringUTFChars(env, enter, str);

    scanf("%s", buf);
    return (*env)->NewStringUTF(env, buf);

}
</pre>

In the above code, the first two methods only Display message, while the third one takes input from the user
The Syntax For Declaring Methods

--------------------

JNIEXPORT <return type> JNICALL

Java_<className>_<methodName>(JNIEnv *env, jobject obj)

{

   ...........

}

-------------------

To generate DLL
create bcc32.cfg file in %BORLAND_INSTALL%/BCC5/bin Folder
In the bcc32.cfg file add following code
-I"%BORLAND_INSTALL%\BCC55\include;%JAVA_HOME%\include;%JAVA_HOME%\include\win32"
-L"%BORLAND_INSTALL%\BCC55\lib;%BORLAND_INSTALL%\BCC55\Lib\PSDK;"
i.e the path for borland compiler and java.

now goto prompt and say

prompt>bcc32 -tWM -tWD firstJNI.c

This will create firstJNI.lib File
Now say

prompt>implib -c firstJNI.lib firstJNI.dll

That's it!!
It will generate DLL file.
Now you can run the firstJNI.class file by using java.
--------------------
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

Select All Code

<pre>
// 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);

}
</pre>

The syntax declaration is same as discussed above. In VC++ it may throw error for unable to include for jni.h or firstJNI.h where you have to give path for these header files while including.

The above code will also check for RAM size and print it. Borland compiler do not provide bios.h so it is not included in the code. Build all Application and you will get the DLL file. Go to prompt and run firstJNI.class using java. So here we can print RAM size!!! Now since this is the basic we can go further with this. If you don't set the library path correctly it will throw exception
java.lang.UnsatisfiedLinkError: no hello in shared library path
at java.lang.Runtime.loadLibrary(Runtime.java)
at java.lang.System.loadLibrary(System.java)
at
at java.lang.Thread.init(Thread.java)

so copy library(dll) in same folder.

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View highlighted Comments

User Comments on 'Java Native Interface (JNI)'

Posted by : sagiragu at 01:19 on Friday, March 10, 2006

Thanks Dude

Posted by : srodrigues at 14:35 on Tuesday, May 09, 2006

Important:

Exception in thread "main" java.lang.UnsatisfiedLinkError:
at com.wilshire.tucs.jni.example.HelloWorld.print
at com.wilshire.tucs.jni.example.HelloWorld.main

I searched till I dropped to figure this out and finally found it in a forum to get this resolved.

If you have the Java class declared in a package com.mycompany.jni.MyClass, please add it as follows.

javah -jni com.mycompany.jni.MyClass

You will get the following header file..

com_mycompany_jni_MyClass.h

with the following header signature inside this file

JNIEXPORT void JNICALL Java_com_mycompany_jni_MyClass_print
(JNIEnv *, jobject, jstring);

Include this header as is in the .c/.cpp file and after that create the DLL / .so file.

It took me 3 to 4 hours to figure this out, hope it takes you a few minutes :-)

Posted by : kanad at 08:42 on Tuesday, December 19, 2006

srodrigues, Thanks
Sorry for my late reply though.
Plus in above

I have written
----------------------------------
Then create header File using
prompt>javah javah -jni HelloWorld
----------------------------------
It should be
prompt>javah -jni firstJNI.

Posted by : ptobra at 01:04 on Monday, June 11, 2007

This topic is little outta JNI. I am new entry to code toad. Donno how to create a thread.

I want to create .dll for all platforms using one file and cross platform compilers. The platforms I want to include are windows, unix versions (linux, hpux, aix , solaris).
Any suggestions

Posted by : kanad at 08:55 on Wednesday, June 13, 2007

This book explains about threads.
http://java.sun.com/docs/books/jni/html/invoke.html#9726
Please go through the book
http://java.sun.com/docs/books/jni/
About compiler tips
http://weblogs.java.net/blog/kellyohair/archive/2006/01/compilation_of_1.html

Posted by : vikramrc at 03:07 on Sunday, July 01, 2007

Good article ! but I would like to point out that it is immensely hard to do this when you are talking to a technology like COM, keeping all their threading models , GC and interface pointer semantics.

Their exists a library in open source domain, which provides non native (JNI less) access to COM from Java. The best part is that by this virtue(lack of JNI) it can run from any platform supported by Java. More information can be found at http://j-interop.sourceforge.net

just my 2 cents,
Thanks,
best regards,
Vikram

Posted by : kanad at 04:03 on Thursday, July 26, 2007

vikramrc,
Thanks for your link. Looks a lot interesting.
The JACOB Project: A Java-COM Bridge
http://danadler.com/jacob/ is also there. It uses JNI to make "native" calls into the COM and Win32 libraries.

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