This may seem like a broad question, but bear with me. I'm using Java to design a 2d multiplayer side scroller type game. I hear that UDP sockets are preferred for realtime/action games, so that's what I assume I will be using. My question is, what's a good design for the logic of a network system in Java? For example, should the server be multithreaded (one thread per client) or not? How do we do NAT hole-punching (or do we even need to)? That sort of thing.

Thanks in advance for any advice.

If you control the server and can give it a public IP address, then you don't need NAT hole punching.

There is only a single UDP socket bound to the incoming port, so multi-threading would be entirely useless. You could have one thread that keeps reading from the UDP socket, or you could just poll/drain the socket each time through your main loop; your choice.

Thanks for the reply. What if I allow the players to host the games themselves? Do I make sure they know to use port forwarding, or is there something I can do to ensure that there is no NAT trouble?

Also, I kind of wanted to use the poll/drain method in my main loop, but I couldn't figure out how to read the socket until it's empty.