How to use UDP with multiple Clients in Java

Question

I have the following Situation.

I have a Server class. I have a Client class. I have a MultiServerThread class.

When a Client connects to a Server, the Server creates a new MultiServerThread, which is processing the Input from the Client. That way I can have multiple Clients. So far so good. The connection goes via TCP.

A short example:

Server class:

...
public static void main(String[] args) throws IOException {     
    ServerSocket serverSocket = null;
    boolean listening = true;


    try {
        serverSocket = new ServerSocket(9999);
    } catch (IOException e) {
      System.err.println("Could not listen on port: " +  serverSocket.getLocalPort() + ".");
        System.exit(-1);
    }

    while (listening) {
        new MultiServerThread(serverSocket.accept()).start();
    }
    serverSocket.close();
}
...

Client class:

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

    socket = new Socket(hostname, port);
    out = new PrintWriter(socket.getOutputStream(), true);
    in = new BufferedReader(new InputStreamReader(socket.getInputStream()));

    stdIn = new BufferedReader(new InputStreamReader(System.in));
    String fromServer;
    String fromUser;

    while ((fromServer = in.readLine()) != null) {
        System.out.println("Server: " + fromServer);
        if (fromServer.equals("Bye.")) {
            break;
        }

        fromUser = stdIn.readLine();
        if (fromUser != null) {
            System.out.println("Client: " + fromUser);
            out.println(fromUser);
        }
    }

    out.close();
    in.close();
    stdIn.close();
    socket.close();
}
...

MultiServerThread class:

...
public MultiServerThread(Socket socket) throws SocketException {
    super("MultiServerThread");
    this.socket = socket;
    //   dSocket = new DatagramSocket(4445);
}

public void run() {
    try {
        PrintWriter myOutput = new PrintWriter(socket.getOutputStream(), true);
        BufferedReader myInput = new BufferedReader(new InputStreamReader(socket.getInputStream()));
        myOutput.println("Connected to client and ready to accept commands.");

        while ((clientInput = myInput.readLine()) != null) {

            //A SIMPLE LOGIN A USER
            if (clientInput.contains("!login")) {

            //save some string given by client into loggedUser
            String loggedUser = clientInput.substring(7);

            }

        myOutput.close();
        myInput.close();
        socket.close();

    } catch (IOException e) {
        e.printStackTrace();
    }
}

What I need is:

I need to implement a notification that comes from a Server when for example the Username is "Bob". If the username is "Bob", the server should give a notification to the Client "Bob is here again!". In my project/homework this should be done with datagrams in Java.

So if the clientinput is "!login bob" then a datagram packet with the message ("Bob is here again!") should be sent to the client.

Question: Where exactly should I put the code of the Datagram request in? Can I put the datagram packet request into the MultiServerThread or into the Client?

It would be easier in the MultiServerThread because it already handles the !login. Here:

     if (clientInput.contains("!login")) {
            //save some string given by client into loggedUser
            String loggedUser = clientInput.substring(7);

            //send datagram request to Server???

            }

But this is going against the principle of networking?

Answer 1

you need to send the UDP port number to your client through the initial TCP connection. Then you start listening for UDP datagrams on your client on that port number. All other communications from server -> client will be on this udp socket. This is what your assignment suggests

Answer 2

I got it working ;-)

I definied a udp port in the thread and client class... the client class got his port with arguments... it gave the udp Port to the thread... so both had the udp ports ;)