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dll inside java

Posted on 1998-05-18
Medium Priority
Last Modified: 2012-06-21
I have a dll called mydll.dll.
This dll have a function which header is:
double *func(double *data).

Both pointers points to arrays of six double.
How can I call 'func' inside a java class?
Please, give me a detailed sample code.

Question by:jjescri
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Accepted Solution

wex earned 400 total points
ID: 1221159
This is a JNI problem.  The solution is split into two halves.  First, you need some class to load the dll.  That's easy.  One class (foo) with one routine that does a System.loadLibrary("yourDLL");

Then to use the routine inside the DLL, you make another class which creates an instance of foo.  You can then call instance.func() with the appropriate args.

Now the Q is what are appropriate args.  I'd suggest using Vectors.  So something like:
      Vector results = instance.func(rgs)
where rgs is also a Vector.

Then you can pull the elements out by iterating over the Vector and casting each member to be a Double.  If you really need the double type, you can get Double.doubleVal()

That make sense?

Author Comment

ID: 1221160
Hi wex,

I have a DLL  called 'p' with a function 'add2':

int add2(int v){return (v+2);}

and when I compile the next code javac return an error.

class pjava
      public pjava()


class testp
      public static void main(String args[])
            pjava e=new pjava();
            int r;
            int s=2;


error: 'add2' is not a member of class 'pjava'.

Expert Comment

ID: 1221161
OK, change your pjava class to the following:

public class pjava
   public native int add2 (int v);

   static {

Also, you need to make sure that java can actually find the DLL for runtime.  On UNIX this
would be done by changing the LD_LIBRARY_PATH variable.  On PCs I use Symantec Cafe, which has an option to set the library path on a per-project basis.  I suspect you'll need to do something similar in VJ++
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Author Comment

ID: 1221162
Hi wex,
when I do 'java testp' an error is returned.

java.lang.UnsatisfiedLinkError: add2
       at testp.main<>


Expert Comment

ID: 1221163

I think you both are missing something fundamental here.
The solution will be

Write a wrapper java class with a native method as follows

public class DLLCallingClass {
public static native double[] func(double[] inpArra);
static {

Now, coming to how to generate nativedll.

compile java program


Then, generate jni header file

javah -jni DLLCallingClass

This creates a file DLLCallingClass.h

now write a c program with name

extern double *func(double *);

#include "DLLCallingClass.h"

JNIEXPORT jdoubleArray JNICALL Java_DLLCallingClass_func
  (JNIEnv *env, jclass myclass, jdoubleArray myarray)
       jsize length;
            jboolean isCopy;
            jdouble *localArray;
            jint i;

            length = (*env)->GetArrayLength(env, myarray);
            localArray = (*env)->GetDoubleArrayElements(env, myarray, &isCopy);

/* now here invoke your dll function since you have the
   double[] array in localArray */
            double *result = func(localArray);

            (*env)->ReleaseDoubleArrayElements(env, myarray, localArray, 0);      
/* convert back the result to java double array !!! */
/* how you are going to get this len parameter below is upto you.
you might know how many elements the func(double *) will return.
      jdoubleArray RetArray = env->NewDoubleArray(len);

       jdouble *localArray = (*env)->GetDoubleArrayElements (RetArray, NULL);

       for ( x = 0; x < len; x++)

               localArray[x] = result[i];

                             (*env)->ReleaseDoubleArrayElements(RetArray, localArray, 0);

                             return RetArray;

Please see that all the declarations of the variables is at the beginning of the program in above segment. (I am using C++ so i neednt).

To invoke the method from the dll, you must have the .lib file which you must link with this file to generate the final new dll
nativedll.dll (which is used in system.loadlibrary call)

Now you might have got a overall picture of what is going on.

Your java class invokes a c/c++ method by name
Java_DLLCallingClass_func but not just func.
This method inturn converts java double array to c/c++ double array, invokes the actual dll function,  packs the result into again the java double array and returns.

For more info

Author Comment

ID: 1221164
Thanks evijay

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