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August 2000



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Answer Java and DLL ( interesting enough )

Posted by Kishori Sharan on August 03, 2000 at 11:14 PM


Here is the procedure to call a C++ dll functions from java. I have
used this procedure and it works fine.
1. Create a class as follows declaring the method of C++ as native.

class HelloCPlus {
public native static void Hello ( );

/* Let us make sure dll is loaded when
we call the Hello () static method.
you can declare Hello () as non-static too.
But in that case you have to make sure that
you call loadLibrary ( ) before calling the
Hello() method. You may call it in one of the
constructors +/

static {
System.loadLibrary ( "HelloCPlus" ) ;

//. end of

2. Now compile this using
on command as you compile any other java program.

3. Now we will create the header file. Use
javah HelloCPlus
on command line which will create a file HellocPlus.h as follows
// HelloCPlus.h
/* DO NOT EDIT THIS FILE - it is machine generated */
/* Header for class HelloCPlus */

#ifndef _Included_HelloCPlus
#define _Included_HelloCPlus
#ifdef __cplusplus
extern "C" {
* Class: HelloCPlus
* Method: Hello
* Signature: ()V
JNIEXPORT void JNICALL Java_HelloCPlus_Hello
(JNIEnv *, jclass);

#ifdef __cplusplus

// end of HelloCPlus.h

4. Now you have to create the actual function Hello ( ) in c++. But the function
declaration won't be as simple as void Hello ( ). You can take the function declaration
in c++ function from HellocPlus.h. I am writing your actual C++ function

// HelloCPlus.cpp
#include "HelloCPlus.h"

extern "C" /* You must say extern "C" if you are writing C++ function.
However, you have to exclude it for c function Note from
HelloCPlus.h we took the function declaration which
prefix the actual c++ Hello () function name with java_HelloCPlus_. Also
note that we have changed the JNIEnv * to JNIEnv* env and jclass to jclass c1*/
JNIEXPORT void JNICALL Java_HelloCPlus_Hello (JNIEnv* env, jclass c1)
cout << "Hello Java \n --- From c++ \n" ;


5. Now you have to make a dll out of .cpp file . If you have MS VC++ then go to DOS prompt
and run the following .bat file
C:>vcvars32.bat which is located in your_vcplus_home\vc\bin directory.

When you run this .bat file you may get error like "out of environment space" as
I was getting initially. If you get this error then open vcvars32,bat using
c:> edit vcvars32.bat and try to change the PATH variable to something shorter.
You may choose to remove the last %PATH%. This hekped me getting rid of that error.

Once the above .bat file runs successfully you can use following command to create the
c:>cl -Ic:\java_home\include -Ic:\java_home\include\win32 -LD HelloCPlus.cpp -FeHelloCPlus.dll
Here: cl ( it is CL and C1(one ) ), it is located in vc_home\vc\bin\cl.exe
c:\java_home : By java_home I mean the directory where you have jdk installed
on your computer. If your jdk directory is c:\jdk1.3 then replace the java_home by jdk1.3
-I, -LD, -Fe are switches and you have to type as mentioned.

The above command will create ( if you are lucky enough to get it work first time ) a dll
named HelloCPlus.dll.

6. Now your C++ function is ready to be called. Create a java program as follows
class CallCPlus {
public static void main ( String[] args ) {
HelloCPlus.Hello ( ) ;

// end of

Now compile using
and then run it
c:\java CallcPlus
and it will print
Hello Java
---From C++



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