If you want to have a look at a generic multi-threaded server, check out the Java networking tutorial.. there's a basic example server that responds to a "Knock knock, who's there?" conversation.. you can start off with it..
Of course it's not a game server but you could try sticking in a game engine to maintain the game world and change the user sessions to a proper game protocol.. e.g. sending key strokes or joystick movements. You could also get the server to send player and enemy positions etc...
Good luck!