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

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!