How to call a c/c++ - function in a DLL from within Java
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How to use the JNI in an example:

This example is based on the JNI-interface from the Java-JDK.

Let's show an example to call a C-function from a java-program. For more informations about this, please have a look on the official site at SUN.

In this example, we want to call a function f with the following signature:

int f(int i, char *s).

So, we first write a java-wrapper with this function:
//
// Java-example function wrapper
//

class j
{
   native int f(int i, String s);

   static
   {
      System.loadLibrary("c_dll");
   }
};
 In the loadLibrary is the name of the DLL we will create.
Having this wrapper, we have to compile the class (javac) and then generate a c-header file with javah:
javah -classpath . j This will result in the file j.h:
/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class j */

#ifndef _Included_j
#define _Included_j
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class: j
 * Method: f
 * Signature: (ILjava/lang/String;)I
 */
JNIEXPORT jint JNICALL Java_j_f
  (JNIEnv *, jobject, jint, jstring);

#ifdef __cplusplus
}
#endif
#endif
Now lets implement a simple f function in C:
// This is the C-part of the JNI-example

#include "j.h"

// This is the function called by the java-program

JNIEXPORT jint JNICALL Java_j_f
  (jEnv, jObj, i, s)
  JNIEnv *jEnv;
  jobject jObj;
  jint i;
  jstring s;
{
  /* Obtain a C-copy of the Java string */
  const char *str = (*jEnv)->GetStringUTFChars(jEnv, s, 0);
  /* process the string */
  printf("The string got by parameter from the java-program is :%s:\n", str);
  printf("The int got by parameter from the java-program is :%d:\n", i);

  /* Now we are done with str */
  (*jEnv)->ReleaseStringUTFChars(jEnv, s, str);
  return i*i; // Return to java
}
Compile this function to a DLL under Windows (all this would also work on SUN, LINUX and so on with some changes in the compile-line)
cl c_dll.c -I %JDKPATH%\include -I %JDKPATH%\include\win32 /link /DLL /out:c_dll.dll
No a little java-main program to call the function f in the class j:
import j;

class jmain
{
  public static void main(String args[])
  {
    j ccall = new j();

    System.out.println("Starting java-program");
    System.out.println("Return from C-program: " + ccall.f(9, "Hello"));
  }
};
If you put all this together and run the java-program, you will get the following output:
Starting java-program
The string got by parameter from the java-program is :Hello:
The int got by parameter from the java-program is :9:
Return from C-program: 81
The first step is done. Now, perhaps you need other types like double, other java-classes and so on. This is described at SUN, please have a look on these pages.
For you, here the code-example to download: j2c.zip.
If you find errors in this code or have comments on it, please tell me at source @ stueben.com
Comments are welcome |