JNI Hello World on Linux Mint

I wrote similar tutorial for www.linux.com about four years ago. The motivation is the same as then, in then Sun and today Oracle documentation only Solaris and Windows are treated, Linux is omitted. Also there are slight differences in compilation process on 32 bit Ubuntu 9.04 then and 64 bit Linux Mint 13 today. Otherwise there is increase in popularity of JNI thanks to Android NDK. To do native Android development one should be familiar with C, C++, JNI and Java. As in that old article I will use C++ to create shared library.
We start with Java class:

class YAHelloWorld
{
    public native void sayHi(String name);
    static
    {
        System.loadLibrary("YAHelloWorld");
    }
    public static void main(String[] args)
    {
        YAHelloWorld h = new YAHelloWorld();
        h.sayHi("World");
    }
}

Beside normal Java stuff we have request to load native library and forward declaration of native method. We save it as YAHelloWorld.java and compile it

javac YAHelloWorld.java

After that we invoke javah to create native header:

/usr/lib/jvm/java-7-oracle/bin/javah -jni YAHelloWorld

I was lazy to export path, so that is a reason for full path to javah. If you try without full path you may get following suggestion:

The program 'javah' can be found in the following packages: * gcj-4.6-jdk * openjdk-6-jdk * gcj-4.5-jdk * openjdk-7-jdk Try: sudo apt-get install <selected package>

what is not really necessary.  Content of generated YAHelloWorld.h is:

/* DO NOT EDIT THIS FILE - it is machine generated */ #include <jni.h> /* Header for class YAHelloWorld */ #ifndef _Included_YAHelloWorld #define _Included_YAHelloWorld #ifdef __cplusplus extern "C" { #endif /* * Class: YAHelloWorld * Method: sayHi * Signature: (Ljava/lang/String;)V */ JNIEXPORT void JNICALL Java_YAHelloWorld_sayHi (JNIEnv *, jobject, jstring); #ifdef __cplusplus } #endif #endif

The first parameter, JNIEnv * is pointer to array of pointers of JNI functions, jobject is sort of this pointer and jstring is Java String. Now when we know all that we can write C++ implementation:

#include <iostream> #include "YAHelloWorld.h" using namespace std; JNIEXPORT void JNICALL Java_YAHelloWorld_sayHi (JNIEnv *env, jobject o, jstring s) { const char *str = env->GetStringUTFChars(s, 0); cout << "Hello " << str << "!" << endl; env->ReleaseStringUTFChars(s, str); }

Beside normal C++ stuff we convert Java String to C++ string at the beginning and at the end we release both of them. Compilation looks like this:

g++ -shared -fPIC -I/usr/lib/jvm/java-7-oracle/include -I/usr/lib/jvm/java-7-oracle/include/linux YAHelloWorld.cpp -o libYAHelloWorld.so

and finally we execute our Hello World example like this:

java -Djava.library.path=. YAHelloWorld

That should produce familiar Hello World output. Not too difficult.