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Thread: calling native c code from java using JNI

  1. #1
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    Default calling native c code from java using JNI

    hi guys im fed up with jni couldnt even execute a simple helloworld program.. here is what i did as per the instruction given in a JNI book..

    STEP 1:
    Creating HelloWorld.java
    class HelloWorld {
    	private native void print();
     
    	public static void main(String[] args) {
    		new HelloWorld().print();
    	}
     
    	static {
    		System.loadLibrary("HelloWorld.dll");
    	}
    }
    compile HelloWorld.java to get HelloWorld.class

    STEP 2
    Creating a jni style header

    command: javah -jni HelloWorld will generate a header file HelloWorld.h

    STEP 3:
    Creating dll and lib files
    #include <jni.h>
    #include <stdio.h>
    #include "HelloWorld.h"
     
    JNIEXPORT void JNICALL Java_HelloWorld_print(JNIEnv *env, jobject obj)
    {
    	printf("Hello World!\n");
    	return;
    }
     
    int main(int argc, char **argv)
    {
    	// dummy
    	return 0;
    }
    compile with file along with the header in Visual Studio in a dll project will generate dll files

    STEP 4

    now i ve created a java class file header file and the library HelloWorld.dll

    when i execute the java HelloWorld.class it throws the the error

    Exception in thread "main" java.lang.UnsatisfiedLinkError: no HelloWorld.dll in
    java.library.path
    at java.lang.ClassLoader.loadLibrary(Unknown Source)
    at java.lang.Runtime.loadLibrary0(Unknown Source)
    at java.lang.System.loadLibrary(Unknown Source)
    at HelloWorld.<clinit>(HelloWorld.java:9)
    Could not find the main class: HelloWorld. Program will exit.


    guys please help me i dont know how to fix and i followed lot of other tutorials and i get the same error
    Last edited by helloworld922; February 4th, 2011 at 02:32 AM.


  2. #2
    Super Moderator helloworld922's Avatar
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    Default Re: calling native c code from java using JNI

    Java's smart enough to figure out the extension of HelloWorld for you, and actually requires that you don't give it. The reason you don't want to put the extension on there is because different OS's use different extensions (for example, windows use .dll, linux use .so, etc.).

    static {
            System.loadLibrary("HelloWorld");
        }

  3. #3
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    Default Re: calling native c code from java using JNI

    Just on the off chance someone has this same issue under linux I would like to point out that your library needs to be lib[whatever].so and when you do the system.loadlibrary() you put in [whatever] instead of the loadlibrary function excluding the lib and .so. I ran into this issue not long ago and it drove me nuts until I figured it out.

  4. The Following User Says Thank You to FearTheCron For This Useful Post:

    helloworld922 (February 28th, 2011)

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    Default Re: calling native c code from java using JNI

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