Java client mixed TCP-and-UDP functionality

Question

I'm writing a client-server application in Java in a way that a client and a server could interact with each through a UDP socket, ping-pong through a TCP socket, and users (clients) could chat with each other through TCP sockets.

I have two separate TCP and UDP threads. I have the TCP and UDP functionality for a server fused together (simply starting the TCP and UDP threads). But how do I do this for a client?

I've gone through the following pages here:

1) This guy has TCP and UDP segregated for two separate clients: Java TCP and UDP echo in one server

2) The thread Can two applications listen to the same port? suggests that I can't use both TCP and UDP for the same port number at the same time for the same client. Which brings us to ...

3) ... this page: Listening for TCP and UDP requests on the same port. However, it does not suggest anything on how to implement the client. Plus, he has a thread, not a client (weird) receiving packets from and sending them to the server.

I've been cruising the Web (Google in particular) and couldn't come up with anything. I believe that very few, if any, have addressed this issue.

So, again, my question is: how do I fuse TCP and UDP functionality in the client? I want the server to be able to connect with the client through TCP and UDP sockets, and use TCP between clients. I don't know what to do next.

Any help is appreciated. Thanks in advance.

===========================================

Here's my client code in its current form:

        String hostName = args[0];
        int portNumber = Integer.parseInt(args[1]);

        MulticastSocket udpSocket = new MulticastSocket(4446);
        InetAddress address = InetAddress.getByName("230.0.0.1");
        udpSocket.joinGroup(address);

        DatagramPacket packet;

        // UDP: get a few quotes
        for (int i = 0; i < 5; i++) {

            byte[] buf = new byte[256];
            packet = new DatagramPacket(buf, buf.length);
            udpSocket.receive(packet);

            String received = new String(packet.getData(), 0, packet.getLength());
            System.out.println("Quote of the Moment: " + received);
        }

        udpSocket.leaveGroup(address);
        udpSocket.close();

        // TCP

        try (
            Socket tcpSocket = new Socket(hostName, portNumber);
            PrintWriter out = new PrintWriter(tcpSocket.getOutputStream(), true);
            BufferedReader in = new BufferedReader(
                new InputStreamReader(tcpSocket.getInputStream()));
        ) {
            BufferedReader stdIn =
                new BufferedReader(new InputStreamReader(System.in));
            String fromServer;
            String fromUser;

            // AS LONG AS server and client are interacting:
            while ((fromServer = in.readLine()) != null) {
                System.out.println(fromServer);
                if (fromServer.equals("Bye."))
                    break;

                fromUser = stdIn.readLine();
                if (fromUser != null) {
                    //System.out.println(fromUser);
                    out.println(fromUser);
                }
            }
        } catch (UnknownHostException e) {
            System.err.println("Unknown host: " + hostName);
            System.exit(1);
        } catch (IOException e) {
            System.err.println("Unable to find I/O for connection to " +
                hostName);
            System.exit(1);
        }

In this case the client receives the datagrams properly in UDP connection, then switches to the TCP functionality as expected. Two problems, though:

1) The server cannot get out of the listening loop once the proper input is typed in (I hardcoded in an "exit" command).

2) The server and the client will maintain a UDP connection only once before switching to TCP.

The thread you cite for (2) doesn't say you can't use the same port for TCP and UDP anywhere that I can see. It isn't even about that. Any source that does say that is wrong. You can. The rest of your question is too broad and there is too much code to be answered clearly. Please reduce it to a manageable size and an answerable question. — user207421, Nov 11 '13 at 22:50
@EJP : (2) clearly states that "For TCP, no. You can only have one application listening on a single port at one time." Will resize my post shortly. Also, if the question is too broad, what needs to be specified? — Andrei, Nov 12 '13 at 04:03
It clearly does *not* state that you can't have a TCP and a UDP socket bound to the same port at the same time. I assure you it is possible, as you will see for yourself when you actually try it. — user207421, Nov 12 '13 at 05:35
@EJP : Anyway, I've provided one chunk of code (client code) instead of four this time. Given that code, how can I have the server listen to both TCP and UDP requests from the same client? — Andrei, Nov 12 '13 at 05:56

user207421 · Accepted Answer · 2013-11-12T06:12:44.447

I'm having trouble identifying an actual question here, but:

2) The thread Can two applications listen to the same port? suggests that I can't use both TCP and UDP for the same port number at the same time for the same client.

It doesn't say any such thing. It says you can't have two TCP sockets bound to the same port. In any case the fact of the matter is that you can.

Which brings us to ...

3) ... this page: Listening for TCP and UDP requests on the same port.

And there is a counter-example that proves the point.

However, it does not suggest anything on how to implement the client.

Such as? You only need to create a TCP or UDP socket, and either connect the TCP socket to the remote IP:port or just use the UDP socket to send to that IP:port. I don't see what the problem is here.

Plus, he has a thread, not a client (weird) receiving packets from and sending them to the server.

So the thread is the client. Nothing weird about it. Just lift the code and put it into a program. Voila: a client program.

how can I have the server listen to both TCP and UDP requests from the same client?

Just create a new ServerSocket(port) for TCP and new DatagramSocket(port) for UDP and have a thread listening to each. The TCP thread should loop calling accept() and spawning a new thread per accepted Socket; the UDP thread can just loop on DatagramSocket.receive().

Thanks. I'll follow that last advice you gave here. Sorry, I'm new, so I'm learning. — Andrei, Nov 12 '13 at 06:07

Java client mixed TCP-and-UDP functionality

1 Answers1