ChatServer for SO question

package com.j256.ormlite;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.ServerSocket;
import java.net.Socket;
import java.util.Hashtable;

public class ChatServer {
    private static int port = 8001;

    public static void main(String[] args) {

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

        Socket client = null;
        while (true) {
            try {
                client = server.accept();
            } catch (IOException e) {
                System.err.println("Accept failed.");
                System.err.println(e);
                System.exit(1);
            }
            /* start a new thread to handle this client */
            Thread t = new Thread(new ClientConn(client));
            t.start();
        }
    }

    private static class ChatServerProtocol {
        private String nick;
        private ClientConn conn;

        private static Hashtable<String, ClientConn> nicks = new Hashtable<String, ClientConn>();

        private static final String msg_OK = "OK";
        private static final String msg_INVALID = "INVALID COMMAND";
        private static final String msg_SEND_FAILED = "FAILED TO SEND";

        private boolean add_nick(String nick, ClientConn c) {
            if (nicks.containsKey(nick)) {
                return false;
            } else {
                nicks.put(nick, c);
                return true;
            }
        }

        private boolean remove_nick() {
            if (nick == null || !(nicks.containsKey(nick))) {
                return false;
            } else {
                nicks.remove(nick);
                return true;
            }
        }

        public ChatServerProtocol(ClientConn c) {
            nick = null;
            conn = c;
        }

        private boolean sendMsg(String recipient, String msg) {
            if (nicks.containsKey(recipient)) {
                ClientConn c = nicks.get(recipient);
                c.sendMsg(nick + ": " + msg);
                return true;
            } else {
                return false;
            }
        }

        public String process(String msg) {

            String output = "";
            if (msg.startsWith("Autore: ")) {
                String tryauthor = msg.substring(8);
                if (add_nick(tryauthor, this.conn)) {
                    // log("Benvenuto autore " + tryauthor);
                    this.nick = tryauthor;
                    output = "Benvenuto autore " + tryauthor;
                } else {
                    output = "Errore: autore gia' collegato";
                }
            }

            else if (msg.startsWith("msg")) {
                String[] msg_parts = msg.split(":");
                for (int i = 0; i < msg_parts.length; i++) {
                    System.out.println(msg_parts[i]);
                }
                String msg_type = msg_parts[0];
                if (msg_type.equals("msg")) {
                    if (msg_parts.length < 3)
                        output = msg_INVALID;
                    if (sendMsg(msg_parts[1], msg_parts[2]))
                        output = msg_OK;
                    else
                        output = msg_SEND_FAILED;
                } else {
                    output = msg_INVALID;
                }
            }

            return output;
        }
    }

    private static class ClientConn implements Runnable {
        private Socket client;
        private BufferedReader in = null;
        private PrintWriter out = null;

        public ClientConn(Socket client) {
            this.client = client;
            try {
                /* obtain an input stream to this client ... */
                in = new BufferedReader(new InputStreamReader(client.getInputStream()));
                /* ... and an output stream to the same client */
                out = new PrintWriter(client.getOutputStream(), true);
            } catch (IOException e) {
                System.err.println(e);
                return;
            }
        }

        public void run() {
            try {
                ChatServerProtocol protocol = new ChatServerProtocol(this);
                /*
                 * loop reading lines from the client which are processed according to our protocol and the resulting
                 * response is sent back to the client
                 */
                while (true) {
                    String msg = in.readLine();
                    if (msg == null || msg.equals("quit")) {
                        break;
                    }
                    String response = protocol.process(msg);
                    out.println("SERVER: " + response);
                }
                this.close();
                protocol.remove_nick();
            } catch (IOException e) {
                System.err.println(e);
            }
        }

        public void sendMsg(String msg) {
            out.println(msg);
        }

        public void close() throws IOException {
            in.close();
            out.close();
            client.close();
        }
    }
}