Java-通过套接字进行通信

Question

Let's say, within a program, there are capabilities to connect to a socket and server(which would be my own computer - localhost) and has all the handling for communicating to the server and back (receiving and sending messages like a chat service). If i send this program as a .jar file to two other users their respective machines, as the users executes the program, it would connect to this socket and server. If all was set up correctly (correctly writes and reads from the socket inputstream and outputstreams) and displays the messages on a GUI chat box, would the two users be able to communicate with this chat service?

Answer 1

Yes, but the main barrier that would prevent users from doing this is a little something called Network Address Translation (NAT).

In short, the vast majority of computers connected to the Internet do not have their own, unique, public IPv4 address (and the vast majority do not have IPv6 addresses either, because ISPs are dragging their feet in rolling out IPv6, but that's another matter entirely).

Dedicated servers often do have (usually multiple) unique IPv4 addresses, but significant numbers of IP addresses are usually only available to hosting companies that operate their own Autonomous System -- and even then, purchasing IPs these days is very expensive, because they're a valuable and rare commodity.

Anyway, two random users on some sort of home Internet connection (like cable, DSL, or fiber) almost certainly have a router or modem of some sort that performs NAT, giving each user device (computer, tablet, etc.) their own private LAN IP, instead of giving each device a unique public IP.

This means that inbound connections from the public Internet are, by default, ignored by the NAT gateway. So if you try to open a socket to your friend's public IP, unless that gateway (modem/router) has been configured correctly, it will just silently ignore your connection attempt, and you'll get a timeout or "connection refused" type message.

There are a few techniques for getting around this, but most of them either require much more complicated application network code, or require the user to mess around in their router configuration pages (which casual users usually aren't willing to do).

• Port forwarding : Users must manually configure their router to "forward" public IP ports to a specific private IP, thus allowing the connection attempt on the public IP to pass through the Network Address Translation table to the private LAN. There is no general-purpose way of automating this without user intervention due to the differences between router/gateway devices.
• NAT Traversal , which requires one of a variety of NAT-T protocols to be implemented in your program, and requires the explicit support of the desired NAT-T protocol for the routers involved (yours and your friend's). There are loads of different NAT-T protocols available; it'd be up to you to determine which one to use, based on compatibility, availability, etc.
• Using an interposing server as an indirect way of routing packets between the two: In this case, you create a server on the public Internet that does have a unique IP (or has ports forwarded to it), and have each of your two client computers initiate an outbound connection to that server. The server would accept connections from both clients, then process the packets and pass them along, like talking to someone through a middleman. This doesn't require any special configuration on the clients, as long as they can make outbound connections on the desired ports (by default, most can, except in restricted enterprise environments where your ports may be limited to a few like 80 and 443.)

You'd need to adopt one of these techniques if your clients are using NAT. If one of your clients has a static IP directly connected to their computer and can accept incoming connections directly (without NAT), your program concept would work without modification, as long as the user behind the NAT initiates the connection to the one with a static IP.