Java updater code

I’ve looked around these forums and haven’t found anything that could help me with this question:

Could someone give me a starting point to figuring out how the Java updaters work? I’m programming an app in VB.NET that will let players add custom cards to my VASSAL module, and I’d like to allow them to make update files, too.

Any suggestions? Even a just a link would be great.

Thanks,
Dan