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! 

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