Native methods in the JDK often provide an interface to functions in the operating system. Such functions, by their nature, cannot be reproduced by native
Java code. System.currentTimeMillis() is an example of that. A few native methods in the JDK are things that could be done by native Java code, but are done as native methods for speed. System.arraycopy() is one of these.
I think one may be able to get hold of the source code of Sun's JDK. Of course, this is not in the Java source code that comes with the JDK. But I think you can get the native (C, C++ ...?) code somewhere. It will vary between platforms, of course. I seem to remember having to sign up to something, to get some source code for something else in the past. Sorry to be vague.
That said, I'm confused why you want this source code? Can you explain what you want to do with it?