CODEWITHSUNDEEP
javac++ceclipsedll
newbee7
newbee7

Reputation: 63

call c/c++ DLL from Java in Eclipse IDE

I want to call this minimal dummy C program (named "TEST.c"):

extern "C"
void Java_TEST_run() {}

from this Java code (named "Example.java"):

public class Example 
{
    public static void main(String args[]) 
    {
      System.out.println("START");   

      TEST test = new TEST(); 

      test.dll_call();

      System.out.println("ALL DONE!");      
    }   
}    

class TEST
{
    public void dll_call()
    {
      run();       
    }
    static {
        try {
            System.out.println("Load DLL = start ");

            System.load("/home/user/Desktop/TEST.dll");

            System.out.println("Load DLL = finish ");
        } catch (UnsatisfiedLinkError e) {
          System.err.println("Native code library failed to load.\n");
          System.exit(1);
        }
      }
     public native void run();    
}

I create the C dll file by the following commands:

g++ -c  TEST.c
g++ -shared  -o TEST.dll TEST.o

This works all fine within the console environment, esp. I got the successful Java program execution output:

START
Load DLL = start 
Load DLL = finish 
ALL DONE!

Now, if I try to run the Java program from the Eclipse IDE, I got the following error:

START
Load DLL = start 
Load DLL = finish 
Exception in thread "main" java.lang.UnsatisfiedLinkError: 'void Example.TEST.run()'
    at test1/Example.TEST.run(Native Method)
    at test1/Example.TEST.dll_call(Example.java:21)
    at test1/Example.Example.main(Example.java:11)

To my understanding, this means the Java program running from Eclipse does successfully find the c dll file, but when trying to enter the dll file, it fails by finding the dummy function Java_TEST_run().

As the c code "TEST.c" is already as minimal as possible, and as the execution works fine from the console, I do not understand, why it fails from Eclipse.

Can someone please advice me, how to make this minimal c code working from Java in Eclipse?

Operating system is Ubuntu 18.04 with openjdk 11.0.6 and Eclipse 4.14.0.

This is a follow up question from this question.

Any help much appreciated, thank you!

Upvotes: 1

Views: 841

Answers (2)

newbee7
newbee7

Reputation: 63

I was able to resolve the issue with Eclipse.

The C/C++ code needs to include the name of the Eclipse project, e.g. "Example", in the function name. In above code, this means:

extern "C" void Java_TEST_run() {}

needs to be changed to:

extern "C" void Java_Example_TEST_run() {}

Upvotes: 2

Dusan Todorovic
Dusan Todorovic

Reputation: 480

I'll give you quick answer First of all you will need JNI for c/c++ and it will generate header for your .java file. For example, if we have class from your example we will have header output like this

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

#ifndef _Included_TEST
#define _Included_TEST
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     TEST
 * Method:    run
 * Signature: ()V
 */
JNIEXPORT void JNICALL Java_TEST_run
  (JNIEnv *, jobject);

#ifdef __cplusplus
}
#endif
#endif

You can generate this type of header by typing javac -h . TEST.java for your given class

Last thing we have to do is create our .cpp file to add function body, you can just copy decleration from JNI generated header file

#include "TEST.h"
JNIEXPORT void JNICALL Java_TEST_run(JNIEnv * env, jobject obj){
    //my very special code
}

And that would be it. In order to add JNI to your Visual Studio (if you are using one) you will have to go to project properties and add additional include and additional library from jdk folder. If you are using g++, add include directory of JDK and library.

Upvotes: 0

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