Java：如何从一个端口上的客户端套接字连接到不同的服务器？

Question

A TCP connection is identified by four elements(maybe protocol included): client port, client address, server port, server address. So it's possible for a client with one port to connect to multiple different servers.Because they're different TCP connections.

Here's my demo: a local client on port 9999 conncecting to two local servers on port 12345 and port 12346. But the code isn't correct.

Can anybody help me? Tell me how to correct it, please.(not use SO_REUSEPORT or fork)

import java.io.IOException;
import java.net.InetSocketAddress;
import java.net.ServerSocket;
import java.net.Socket;
import java.net.UnknownHostException;

public class SocketTest {

    public static void main(String[] args) throws IOException {


        startServer(12345);
        startServer(12346);

        Socket socket = new Socket();
        socket.bind(new InetSocketAddress(9999));
        System.out.println("client: " + socket.getLocalSocketAddress().toString());

        startClient(socket, 12345);
        startClient(socket, 12346);
    }

    public static void startClient(Socket socket, int port) {
        (new Thread() {
            @Override
            public void run() {
                try {
                    // Problem: connect() can be called only once
                    socket.connect(new InetSocketAddress(port));
                } catch (UnknownHostException e) {
                    e.printStackTrace();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }).start();
    }

    public static void startServer(int port) {
        new Thread() {
            @Override
            public void run() {
                ServerSocket ss;
                try {
                    ss = new ServerSocket(port);
                    System.out.println("listen on: " + ss.getLocalSocketAddress());
                    while (true) {
                        Socket s = ss.accept();
                        System.out.println("accept from: " + s.getRemoteSocketAddress());
                    }
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }.start();
    }
}

Answer 1

You can bind 2 different sockets to the same local port (that is what SO_REUSEADDR is made for), but one single socket can only be connected to one single destination.

If you want to send one single message, and you want that message to be received by two servers, then you need multicast. multicast can only use UDP (not TCP) and one of the special multicast addresses (224.0.0.0 to 239.255.255.255) that isn't listed as reserved by IANA.