The only good sources of info I have found on the matter are targeted towards Flash, which are a handful of MMO books. However, most of them use commercial server software, I assume, because of the complexity of making a solid game server.
I guess without a solid tutorial or book on the subject, you would want to get a firm grip on academic Java networking practices, maybe from something like
"Java Network Programming", and then move on to basically whatever you can to get a handle on game server programming. Maybe even something like
"MUD Game Programming" could help out. Although it is for MUDs, and is in C++, it is the only book I know of that goes through the entire process of creating a socket-based game server from scratch, and definitely the only one I know of centered on RPGs. You will have to do a lot of interpolation, though. There are a handful of "Multiplayer game programming" books, but most are not what they claim to be, as I have read pretty much all of them at some point or another, and they typically either talk about
design instead of programming, or work through such toy programs that they are basically useless.
Basically, the topic is so vast that there will be few comprehensive tutorials out there for free, so books will likely be the only option.
