I've been working on making my own GameUpdater libraries and I started with C# because it looked easier to me. I just finished and am now going to make one in Java (mainly because C# apparently isn't easily multi-platform, if it wasn't I would just use the C# one). The source may help you if you'd like to make your own. I'll update this post with my Java one when I finish that, but for now I will just have my C# one posted.
https://github.com/Sammidysam/GameUpdaterEDIT: I finally finished the Java version. I stayed up way later than I normally do because I forgot to add 1 to Calendar's month lol. I'll add a README to the repository tomorrow because I'm too tired right now. This is a library, by the way, so if you want to use this as your library, go ahead. Do you think it should go into tools or something? I would be amazed if this was good enough for that (though it is incredibly basic). It requires Java 7 as Java 7 added really helpful file functions. That shouldn't be too much of a problem however. If you use it as a library, be sure to use it in a separate JAR than your game JAR, so that it can update your game JAR. It can't update itself (I assume that would cause an error but I never tried that). It wouldn't be too troubling to have (for an example I'm using my game PixelZombies as the title) PixelZombiesLauncher.JAR and PixelZombies.JAR. Likely you could put PixelZombies.JAR in a folder so that noobs don't see it and click on it (though that wouldn't be the end of the world if they did). I'll say most of this in the README I make tomorrow but it will be in more depth, so stay tuned for that.
https://github.com/Sammidysam/GamePatcher