Creating a Hard Link in java

you could try JNA in place of JNI (JNA has some clear advantages over JNI); yes, check the JSR 203

This is very easy with JNA:

public interface CLibrary extends Library {
    CLibrary INSTANCE = (CLibrary)
        Native.loadLibrary((Platform.isWindows() ? "msvcrt" : "c"),
                           CLibrary.class);
    int link(String fromFile, String toFile);
}

public static void main(String[] args) {
    CLibrary.INSTANCE.link(args[0], args[1]);
}

Compile and run!

It’s easy in Java 7 using createLink:

Files.createLink(Paths.get("newlink"), Paths.get("existing"));