We just built this doohickey for our game, and maybe other folks might like it too.
It’s basically an HTTP-based delta update facility for java applications. It doesn’t do any sort of access control or anything, so if you (try to) control copying of your game, it probably won’t help you without some additional work. But, our game is monetized on a service model, with a free-to-download client, so this is precisely what we needed.
Also, it isn’t going to play very nicely if you have all of your assets zipped up in one jar file. Change one texture, and it’ll download that whole file again. But, if you have your assets raw in a directory tree (like we do), then a la mode will download only what’s changed.
It isn’t super well tested, and I’m the only one who’s looked at the code… But, here’s the github repository.
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Hm sounds like a great idea for a public auto updater for several of the games here in the forum 
Gotta take a look