如何在Java中设置客户端端口号？

Question

Can anyone please tell me how to set a client port numbers once the server decides to accept the connection of the client. I want the port number of client should be between 50000 to 60000.

Answer 1

If you are asking how to code the server in order to set the client 's port, then the answer is that you can't. The client chooses its own port, and that is already in use by the time the server is involved.

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If you are asking how to code the client to choose a local port for a connection to the server, then the documentation for Socket is the place to look.

If you don't specify a port, then the OS gives an arbitrary port number from its ephemeral port range .

If you do specify a local port, for example with the constructor public Socket(InetAddress address, int port, InetAddress localAddr, int localPort) then the client will attempt to bind to that local port.

However this will fail if the port is not available. The reason we typically ask the OS for an ephemeral port is to avoid "address in use" failures when two instances of the same client, or when a socket remains in TIME_WAIT status.

There is no API that lets you ask the OS for an ephemeral port in a given range.

The best you can do is to ask for an arbitrary port, and retry in a loop until you get one that's in your desired range:

 Socket socket;
 do {
    socket = new Socket();
    socket.bind(null); // null means any local address, ephemeral port
 } while (! isInRange(socket.getLocalPort());
 socket.connect(...);

The inefficiency of this is obvious; you could create and discard any number of sockets before stumbling on an acceptable one. It's a reasonable approach if your desired range is a good sized chunk of the OS's ephemeral range.

Alternatively you can loop through your acceptable range, catching bind errors and retrying:

    Socket socket = new Socket();
    for ( int port = MIN_PORT ; port <= MAX_PORT; port++ )
    {
        try {
            s.bind(new InetSocketAddress(port));
            break;
        } catch (IllegalArgumentException | IOException e) {
            // try the next port
        }
        // deal with not having found a suitable port
    }
    socket.connect(...);

This has different inefficiencies, which would become especially relevant if the client runs frequently and concurrently. You'd constantly be "fighting" over the ports in the lower end of the range. If you must use this approach, it would be worth amending it to at least start looking at a random position within the range.

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If your motivation to do this is to get around a firewall rule, try hard to get the (stupid, pointless) firewall rule fixed. However, there are clients in the wild that do use the approach I've described, because sometimes the firewall admin just won't give up.

Answer 2

The constructor you need is

public Socket(InetAddress address,
              int port,
              InetAddress localAddr,
              int localPort)

https://docs.oracle.com/javase/8/docs/api/java/net/Socket.html#Socket-java.net.InetAddress-int-java.net.InetAddress-int-

This method allows you to give a specific source port. You can chose one at random and retry if it's already taken.