V. Srikanth
Posts: 9
Nickname: iamnobody
Registered: Dec, 2002
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Re: How to do that ?Java application communicates with C application?
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Posted: Dec 18, 2002 4:26 AM
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Ever wonder on how to integrate JAVA and C programs ??
Ever had a question that "I have built a C library with my own reusable funtions and how do I use them in my Java application ??",
Following SIX STEPS demonstrate how to do that through a traditional Hello World program..
Have fun integrating your applications.
-Srikanth.
Step 1: (Write the JAVA code )
######################################################
cat Main.java
class Main {
public static void main(String[] args) {
new HelloWorld().displayHelloWorld();
}
}
class HelloWorld {
public native void displayHelloWorld();
static {
System.loadLibrary("hello");
}
}
##########################################################
* Note the keyword "native" used for the method displayHelloWorld(). This indicate it's method defined in a native library and not in any other Java class.
* Note the call to "System.loadlibrary" , this is the one which will load the "native library" in which displayHelloWorld is defined. (in case Unix system, System.loadLibrary("hello") will look for "libhello.so"
Step 2: (Compile the java code)
Using the command "javac Main.java"
This will generate two class files "Main.class" and "HelloWorld.class"
Step 3: (Create the .h file)
Using the command "javah HelloWorld"
This will generate header file HelloWorld.h which contain the Prototype of function to be put in the C file that we are going to write.
####################################################################### ############
cat HelloWorld.h
/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class HelloWorld */
#ifndef _Included_HelloWorld
#define _Included_HelloWorld
#ifdef __cplusplus
extern "C" {
#endif
/*
* Class: HelloWorld
* Method: displayHelloWorld
* Signature: ()V
*/
JNIEXPORT void JNICALL Java_HelloWorld_displayHelloWorld
(JNIEnv *, jobject);
#ifdef __cplusplus
}
#endif
#endif
cat HelloWorld.h
/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class HelloWorld */
#ifndef _Included_HelloWorld
#define _Included_HelloWorld
#ifdef __cplusplus
extern "C" {
#endif
/*
* Class: HelloWorld
* Method: displayHelloWorld
* Signature: ()V
*/
JNIEXPORT void JNICALL Java_HelloWorld_displayHelloWorld
(JNIEnv *, jobject);
#ifdef __cplusplus
}
#endif
#endif
################################################## #####################
Step 4: (Write the native method implementation)
Write the C program which will implementation for JNICALL Java_HelloWorld_displayHelloWorld() function.
#################################################################### ################
cat HelloWorldImp.c
#include "HelloWorld.h"
#include <stdio.h>
JNIEXPORT void JNICALL Java_HelloWorld_displayHelloWorld(JNIEnv *myjni, jobject myobject) {
printf("Hello World!\n");
return;
}
#################################################################### #################
Step 5: (Create a shared library).
Standard way of creating a ".so" file in Unix.
Using "cc -o libhello.so -G HelloWorldImp.c"
Step 6: (Run the program)
Now run the program as using "java Main"
You will be delighed to see the output "Hello World" on the screen !!!
Will this help Alex???
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