客户端如何知道连接到哪个套接字？

Question

I am using Java to do the socket programming as below.

Client program is as below:

 Socket MyClient;
    try {
           MyClient = new Socket("Machine name", PortNumber);
    }
    catch (IOException e) {
        System.out.println(e);
    }

Server program is as below:

ServerSocket MyService;
    try {
       MyServerice = new ServerSocket(PortNumber);
        }
        catch (IOException e) {
           System.out.println(e);
        }

Socket clientSocket = null;
    try {
       clientSocket = MyService.accept();
        }
    catch (IOException e) {
       System.out.println(e);
    }

Now my question is if I run more than one thread to open several sockets in one port (as the server code above), how my client program know which socket it is connecting to?

Answer 1

Your client connects to the Servers port. So all clients will be having the same code MyClient = new Socket("Machine name", <port where server is listening>);
The port opened at client side is not important. The client will get a free port available in his OS.

Answer 2

how my client program know which socket it is connecting to?

The question doesn't make sense. It doesn't 'connect to a socket' at all, it connects to a listening port, and there is only one of those. Your server only accepts one client, so the second and subsequent threads will get an undefined behaviour ranging from a ConnectException to a ConnectionException to nothing, most probably the latter.

Answer 3

Your application knows it because you set it up with a specific port. There is no "auto discovery" built into TCP/IP, it's up to you to pick a server-port and make sure you set your clients up to connect to that port. Either you hard-code this into your client application or, better yet, have it in some configuration file you include with the client.

This is why you have a bunch of "known ports", like http is port 80. This means that a browser will always connect to port 80 on a web-server, unless you explicitly indicate another port in the URL.