title: UDP socket chat application in java
body: <p>I'm trying to create a very simple chat application in Java using UDP sockets. There is one server and multiple clients.</p>
  
  <p>right now my code is :</p>
  
  <p>Server :</p>
  
  <pre><code>package chat;
  
  import java.io.*;
  import java.net.*;
  import java.util.HashMap;
  import java.util.Set;
  
  class User{
      InetAddress addr;
      int port;
  
      User(InetAddress a,int p){
          addr = a;
          port = p;
      }
  }
  
  
  
  public class Server {
      private static final int PORT = 27012;
      private static DatagramSocket sckt;
      private static DatagramPacket in,out;
      private static byte[] buffer;
      private static HashMap&lt;String,User&gt; users;
      public static void main(String[] args) {
          try{
              System.out.println("Opening port...");
              sckt = new DatagramSocket(PORT);
              users = new HashMap&lt;String,User&gt;();
  
          }
          catch(SocketException e){
              System.out.println("Port connection failed!");
              System.exit(1);
          }
  
          handleClient();
      }
          private static void sendMsg(InetAddress addr, int port, String msg){
              try{
              out = new DatagramPacket(msg.getBytes(),msg.length(),addr,port);
              sckt.send(out);
              }catch(IOException e){
                  e.printStackTrace();
              }
          }
      private static void handleClient(){
          try{
              String msgIn,msgOut="",senderNick;
              do{
              buffer = new byte[256];
              in = new DatagramPacket(buffer,buffer.length);
              sckt.receive(in);
              InetAddress clientAddress = in.getAddress();
              int clientPort = in.getPort();
              msgIn = new String(in.getData(),0,in.getLength());
              //print msgIn
              //System.out.println(msgIn);
                          senderNick = msgIn.substring(0,msgIn.indexOf(" "));
                          msgIn = msgIn.substring(msgIn.indexOf(" ")+1);
              if(msgIn.equals("/connect")){
                  //String nick = msgIn.substring(msgIn.indexOf(" ") + 1);
                  System.out.println(senderNick);
                  if(users.containsKey(senderNick)){
                      msgOut = "Nick already in use!";
                  }
                  else{
                      users.put(senderNick, new User(clientAddress,clientPort));
                      msgOut = "Connected!";
                  }
                                  sendMsg(clientAddress,clientPort,msgOut);
              }
                          else if(msgIn.equals("/list")){
                              Set userNames;
                              userNames = users.keySet();
                              msgOut = "Users : \n";
                              msgOut += userNames.toString();
                              sendMsg(clientAddress,clientPort,msgOut);
                          }
                          else if(msgIn.startsWith("/msg")){
                              String tmp = msgIn.substring(msgIn.indexOf(" ")+1);
                              String receiverName = tmp.substring(0,tmp.indexOf(" "));
                              String message = tmp.substring(tmp.indexOf(" ")+1);
  
                              if(!users.containsKey(receiverName)){
                                  msgOut = "User " + receiverName + " not found!";
                                  sendMsg(clientAddress,clientPort,msgOut);
                              }
                              else{
                                  User receiver = users.get(receiverName);
                                  msgOut = "Message from "+  senderNick  +" : "+message;
                                  sendMsg(clientAddress,clientPort,"Message Sent!");
                                  sendMsg(receiver.addr,receiver.port,msgOut);
                              }
  
                          }
                          else if(msgIn.startsWith("/nick")){
                              String newNick = msgIn.substring(msgIn.indexOf(" ")+1);
                              if(users.containsKey(newNick)){
                                  msgOut = "Nick already in use!";
                              }
                              else{
                                  users.put(newNick,users.get(senderNick));
                                  users.remove(senderNick);
                                  msgOut = "Nick changed!";
                              }
                              sendMsg(clientAddress,clientPort,msgOut);
                          }
                          else if(msgIn.equals("/disconnect")){
                              users.remove(senderNick);
                          }
  
              //out = new DatagramPacket(msgOut.getBytes(),msgOut.length(),clientAddress,clientPort);
              //sckt.send(out);
              }while(true);
          }
          catch(IOException e){
              e.printStackTrace();
          }
          finally{
              System.out.println("Closing connection.");
              sckt.close();
          }
      }
  
  }
  </code></pre>
  
  <p>Client :</p>
  
  <pre><code>package chat;
  
  import java.io.*;
  import java.net.*;
  import java.util.Scanner;
  
  public class Client {
  
      private static InetAddress host;
      private static final int PORT = 27012;
      private static DatagramSocket sckt;
      private static DatagramPacket in, out;
      private static byte[] buffer;
      private static String nick;
  
      static class MessageListener implements Runnable {
          //DatagramPacket in;
          //byte[] buffer;
          String reply;
          public void run() {
              do{
              try{
              buffer = new byte[256];
              in = new DatagramPacket(buffer,buffer.length);
              sckt.receive(in);
              reply = new String(in.getData(),0,in.getLength());
              System.out.println("SERVER&gt; "+reply);
              }
              catch(IOException e){
                  e.printStackTrace();
              }
              }while(true);
          }
      }
  
      public static void main(String[] args) {
          try {
              host = InetAddress.getLocalHost();
  
          } catch (UnknownHostException e) {
              System.out.println("Host not found!");
              System.exit(1);
          }
          connect();
  
      }
  
      private static void sendMsg(String msg) {
          try {
              out = new DatagramPacket(msg.getBytes(), msg.length(), host, PORT);
              sckt.send(out);
          } catch (IOException e) {
              e.printStackTrace();
          }
      }
  
      private static void connect() {
          Scanner sc = new Scanner(System.in);
  
          try {
              buffer = new byte[256];
              sckt = new DatagramSocket();
              String reply = "";
              do {
                  System.out.println("Name: ");
                  nick = sc.nextLine();
                  sendMsg(nick + " /connect");
                  in = new DatagramPacket(buffer, buffer.length);
                  sckt.receive(in);
                  reply = new String(in.getData(), 0, in.getLength());
                  System.out.println("SERVER&gt; " + reply);
              } while (!reply.equals("Connected!"));
              accessServer();
          } catch (IOException e) {
              e.printStackTrace();
          }
      }
  
      private static void accessServer() {
          try {
              sckt = new DatagramSocket();
              Scanner sc = new Scanner(System.in);
              String msg = "", reply = "";
              Thread myT = new Thread(new MessageListener());
              myT.start();
              do {
                  System.out.print("Enter Message: ");
                  msg = sc.nextLine();
                  if (!msg.equals("/quit")) {
                      buffer = new byte[256];
  
                      sendMsg(nick + " " + msg);
  
  
                  } else {
                      sendMsg(nick + " /disconnect");
                  }
              } while (!msg.equals("/quit"));
  
              sc.close();
          } catch (IOException e) {
              e.printStackTrace();
          } finally {
              System.out.println("Connection closing...");
              sckt.close();
          }
  
      }
  }
  </code></pre>
  
  <p>I want to be able to send a message to a single user or to all the users but the problem is that i can't receive the message on the client because as it stands now the client only receives a message after it sends one to the server. So my thought was to use a thread that continuosly listens for messages from the server but it doesn't work. What's the easiest way to go about this ?</p>