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();

        static { System.loadLibrary("hellolib"); }

        public static void main(String[] args){
                Hello h = new Hello();
                h.sayHello();
        }
}

Then I ran the follwing commands:

dierre@cox:~/Scrivania/provajni$ javac Hello.java

dierre@cox:~/Scrivania/provajni$ javah -jni Hello 

I've obtained Hello.class and Hello.h.

Hello.h:

/* DO NOT EDIT THIS FILE - it is machine generated */
#include <jni.h>
/* Header for class Hello */

#ifndef _Included_Hello
#define _Included_Hello
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     Hello
 * Method:    sayHello
 * Signature: ()V
 */
JNIEXPORT void JNICALL Java_Hello_sayHello
  (JNIEnv *, jobject);

#ifdef __cplusplus
}
#endif
#endif

Then I created Hello.cpp:

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

using namespace std;

JNIEXPORT void JNICALL Java_Hello_sayHello (JNIEnv *env, jobject obj) {
        cout << "Hello World!" << endl;
        return;
}

And now the part where I think I screwed up. I was inspired by this guide (Compile the Dynamic or Shared Object Library section):

dierre@cox:~/Scrivania/provajni$ gcc -I"/usr/lib/jvm/java-6-sun/include" -I"/usr/lib/jvm/java-6-sun/include/linux" -o hellolib.so -shared -Wl,-soname,hello.so Hello.cpp -static -lc

that generates the file hellolib.so

But when I try to run it with java Hello I have this error:

Exception in thread "main" java.lang.UnsatisfiedLinkError: no hellolib in java.library.path
 at java.lang.ClassLoader.loadLibrary(ClassLoader.java:1734)
 at java.lang.Runtime.loadLibrary0(Runtime.java:823)
 at java.lang.System.loadLibrary(System.java:1028)
 at Hello.<clinit>(Hello.java:4)
Could not find the main class: Hello.  Program will exit.

I even tried this:

  LD_LIBRARY_PATH=`pwd`
  export LD_LIBRARY_PATH

with no results.

I know I'm doing something extremely stupid but I can't figure out what it is. The dynamic lib is generated with the -shared option, isn't it?

Update #1

I tried static { System.load("/home/dierre/Scrivania/provajni/hellolib.so"); } to see if that worked but now:

Exception in thread "main" java.lang.UnsatisfiedLinkError: /home/dierre/Scrivania/provajni/hello.so: /home/dierre/Scrivania/provajni/hello.so: undefined symbol: _ZSt4cout
    at java.lang.ClassLoader$NativeLibrary.load(Native Method)
    at java.lang.ClassLoader.loadLibrary0(ClassLoader.java:1803)
    at java.lang.ClassLoader.loadLibrary(ClassLoader.java:1699)
    at java.lang.Runtime.load0(Runtime.java:770)
    at java.lang.System.load(System.java:1003)
    at Hello.<clinit>(Hello.java:4)

Update #2 Ok, to solve the Update #1 problem I had to use g++ insted of gcc, obviously. Still having trouble to use the load method though. I can't seem to tell it the right path.

Answer 1

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!