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Posted 10 Sep 2002

JNI Basics - 1

, 10 Sep 2002
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Calling native functions which are written in C or C++ from Java, using JNI.


In this tutorial, I will try to explain how to call native functions which are written in C or C++ from Java, using JNI.

What is JNI

JNI is an acronym of Java Native Interface. Using JNI, we can call functions which are written in other languages from Java. Advantages and disadvantages of this method are as follows:


  1. You can use your existing library which was previously written in another language.
  2. You can call Windows API functions.
  3. For the sake of execution speed.
  4. You can call API functions of some server product which is in C or C++, from Java client.


  1. You can’t say "write once run anywhere".
  2. Difficult to debug runtime error in native code.
  3. Potential security risk.
  4. You can’t call it from Applets.

Here we will try to call the MFC function AfxMessageBox() from Java. Our function in C++ will take no arguments and will not return any value.

class Test1
    public static void main(String ar[])
        System.out.println("Hello world from Java");
        Test1 t=new Test1();
    public native void inDll();

In static block, we are loading a DLL named TestDll.dll which should be placed in <WINDIR>/SYSTEM32 directory. I am not saying you cannot put this DLL in other directories, you can, but then you will have to register that path in JVM. Here you can call another method as well, instead of loadLibrary().


I think load() method is better as far as usage is concerned because you can easily place your DLL in any directory.

public native void inDll();

As you can guess, this native keyword specifies that inDll() is a native function which is implemented in the library. Also noticeable, there is no implementation. When we call this function in our code, JVM will look for the function in the loaded DLL (due to static block, DLL will be loaded immediately after the loading of the class) and then call that function. Otherwise it will throw java.lang.UnsatisfiedLinkError exception.

Compile this class normally. Now we will need to generate header file of this class.

You can use javah.exe utility which is included with JDK.

Javah -jni Test1

Header file is generated in current directory with the name Test1.h. Keep this header file safe as we will use this soon.

Now we are going to write a DLL which will contain the implementation of our native function. I have chosen MFC AppWizard (DLL) in project and named my project: TestDll.

Include your class header file which was generated previously using javah.exe in TestDll.h.

Copy following line from Test1.h and paste it at the end of TestDll.h.

JNIEXPORT void JNICALL Java_Test1_inDll(JNIEnv *, jobject);

Change this line to:

JNIEXPORT void JNICALL Java_Test1_inDll(JNIEnv *env, jobject obj)
    AfxMessageBox("Hello World from dll");

Compile this DLL, you will get an error from compiler because compiler was unable to find jni.h file which is included in Test1.h. So include the following two directories in IDE settings.

  • <Drive>:\<JDK Directory>\include
  • <Drive>:\<JDK Directory>\include\win32

Go to Project/Settings, on C/C++ tab, go to Project option, insert following lines at the end:

/I "<Drive>:\<JDK Directory>\include "
/I "<Drive>:\<JDK Directory>\include\win32"

Now compile and generate your DLL, put newly created DLL in system32 directory and then run your class file. It should work properly. If you receive an error like UnsatisfiedLinkError, then recheck your procedure. If you are using any IDE, then try using command prompt. Hope it will work.

In this tutorial, I have tried to explain the basics of JNI. I will write a second tutorial of the series soon in which I will explain how to pass parameters to native functions and return something, along with a practical example.

Waiting for your comments and questions.


This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

A list of licenses authors might use can be found here


About the Author

Irfan Dawood
Web Developer
Pakistan Pakistan
Irfan Dawood is the junior year student at Department of Computer Science, University of Karachi. His interest includes programming in C++ and Java.

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Comments and Discussions

Generalwithout JNI Pin
heinrichrohwer7-Jul-05 4:22
memberheinrichrohwer7-Jul-05 4:22 
Questioncan java access VB.NET .dlls? Pin
Abhishek Tiwari8-Apr-05 22:47
memberAbhishek Tiwari8-Apr-05 22:47 
AnswerRe: can java access VB.NET .dlls? Pin
Anonymous24-May-05 3:44
memberAnonymous24-May-05 3:44 
GeneralRe: can java access VB.NET .dlls? Pin
Abhishek Tiwari24-May-05 21:33
memberAbhishek Tiwari24-May-05 21:33 
GeneralBufferedImage to vtkSource Pin
Pythonian29-Mar-05 21:08
memberPythonian29-Mar-05 21:08 
GeneralJava code executed before nattive Pin
christophek22-Feb-05 0:22
memberchristophek22-Feb-05 0:22 
GeneralRe: Java code executed before nattive Pin
Anonymous14-Sep-05 13:47
memberAnonymous14-Sep-05 13:47 
GeneralRe: Java code executed before nattive Pin
pickson20-Mar-07 4:08
memberpickson20-Mar-07 4:08 
GeneralError in javah Pin
Lucas Ferst29-Oct-04 7:01
sussLucas Ferst29-Oct-04 7:01 
GeneralRe: Error in javah Pin
jan larsen8-Nov-04 21:50
memberjan larsen8-Nov-04 21:50 
GeneralExcecution an EXE file Pin
hashmi12-Apr-04 23:01
memberhashmi12-Apr-04 23:01 
GeneralJava with OPenGl Pin
prabu_univ2-Mar-04 20:55
memberprabu_univ2-Mar-04 20:55 
GeneralRe: Java with OPenGl Pin
Anonymous24-May-04 23:36
memberAnonymous24-May-04 23:36 
Generalcodeproject for Java Pin
Edelweiss18-Dec-03 6:46
memberEdelweiss18-Dec-03 6:46 
GeneralRe: codeproject for Java Pin
Anonymous13-Jan-04 3:08
memberAnonymous13-Jan-04 3:08 
GeneralCalling Java from cpp Pin
Anonymous16-Dec-03 20:17
memberAnonymous16-Dec-03 20:17 
GeneralRe: Calling Java from cpp Pin
Anonymous29-Dec-03 22:34
memberAnonymous29-Dec-03 22:34 
GeneralRe: Calling Java from cpp Pin
Anonymous21-Mar-04 15:06
memberAnonymous21-Mar-04 15:06 
GeneralRe: Calling Java from cpp Pin
maycol18-Aug-04 10:37
membermaycol18-Aug-04 10:37 
GeneralIt does not work for me Pin
Wilfried_Garnier29-May-03 7:50
memberWilfried_Garnier29-May-03 7:50 
GeneralRe: It does not work for me Pin
jan larsen8-Nov-04 21:58
memberjan larsen8-Nov-04 21:58 
GeneralRe: It does not work for me Pin
Anonymous17-Nov-04 8:05
memberAnonymous17-Nov-04 8:05 
GeneralRe: It does not work for me Pin
jan larsen17-Nov-04 19:33
memberjan larsen17-Nov-04 19:33 
QuestionWhat about the other side of the native interface? Pin
Peter Kiss22-Sep-02 6:56
memberPeter Kiss22-Sep-02 6:56 
AnswerRe: What about the other side of the native interface? Pin
Irfan Dawood22-Sep-02 10:05
memberIrfan Dawood22-Sep-02 10:05 

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