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

c++, compilation, java, java-native-interface, linux

Solution

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:

- compile obj (generate HelloImpl.o)

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

- 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!

Problem

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.

Original source

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