How to compile dynamic library for a JNI application on linux?

Question

I\'m using Ubuntu 10.10

So that\'s what I did.

Hello.java:

class Hello {
        public native void sayHello();

           


        

Answer 1

This complains about the C++ symbols not being available. I seem to remember, when I use to do JNI stuff all of the time that there were problems linking in C++ libraries and we always stuck to plain old C

If you change your code so that it's standard C (and rename the file):

#include <jni.h>
#include "Hello.h"
#include <stdio.h>

JNIEXPORT void JNICALL Java_Hello_sayHello (JNIEnv *env, jobject obj) {
        printf("Hello World");
        return;
}

And compile it

gcc -I/usr/lib/jvm/java-6-openjdk/include  -o libhellolib.so -shared Hello.c

It works

java -Djava.library.path=`pwd` Hello
Hello World

Answer 2

Finally my code works. This is hello.java

public class hello {
  public native void sayHello(int length) ;
  public static void main (String args[]) {
    String str = "I am a good boy" ;
    hello h = new hello () ;
    h.sayHello (str.length() ) ;
  }
  static {
    System.loadLibrary ( "hello" ) ;
  }
}

You should compile it as :

$ javac hello.java 

To create .h file you should run this command:

$ javah -jni hello

This is hello.h:

JNIEXPORT void JNICALL Java_hello_sayHello
(JNIEnv *, jobject, jint);

Here is hello.c:

#include<stdio.h>
#include<jni.h>
#include "hello.h" 

JNIEXPORT void JNICALL Java_hello_sayHello
  (JNIEnv *env, jobject object, jint len) {
  printf ( "\nLength is %d", len ); }

To compile this and to create a shared library we have to run this command :

$ gcc -I/usr/lib/jvm/java-6-openjdk/include -o libhello.so -shared hello.c

Then finally run this one :

$ java -Djava.library.path=. hello

Answer 3

Native library can be loaded by loadLibrary with a valid name. By example, libXXXX.so for linux family, your hellolib.so should rename to libhello.so. By the way, I develop java with jni, I will separate the implementation and native interface (.c or .cpp).

static {
    System.loadLibrary("hello"); // will load libhello.so
}

The implementation header(HelloImpl.h):

#ifndef _HELLO_IMPL_H
#define _HELLO_IMPL_H

#ifdef __cplusplus
        extern "C" {
#endif

        void sayHello ();

#ifdef __cplusplus
        }
#endif

#endif

HelloImpl.cpp:

#include "HelloImpl.h"
#include  <iostream>

using namespace std;

void sayHello () {
    cout << "Hello World!" << endl;
    return;
}

Hello.c (I prefer to compile jni in c):

#include <jni.h>
#include "Hello.h"
#include "HelloImpl.h"

JNIEXPORT void JNICALL Java_Hello_sayHello (JNIEnv *env, jobject obj) {
    sayHello();
    return;
}

Finally, we can compile them in some steps:

1. compile obj (generate HelloImpl.o)

g++ -c -I"/opt/java/include" -I"/opt/java/include/linux" HelloImpl.cpp

2. compile jni with .o

g++ -I"/opt/java/include" -I"/opt/java/include/linux" -o libhello.so -shared -Wl,-soname,hello.so Hello.c HelloImpl.o -static -lc

in step 2, we use g++ to compile it. This is very important. yor can see How to mix C and C++

After compilation, you can check the function naming with nm:

$ nm libhello.so |grep say
00000708 T Java_Hello_sayHello
00000784 t _GLOBAL__I_sayHello
00000718 T sayHello

There is a Java_Hello_sayHello marked T. It should extactly equal to your native method name. If everything is ok. you can run it:

$ java -Djava.library.path=. Hello
Hello World!