Multithreading issue with Client/Server chat program

Question

In a nutshell, I just recently starting attempting to use Sockets and decided I wanted to give a go at a chat program. I have some experience with threads but I know little about how to find and fix multithreading issues. I think this is my problem right now since I am deadlocking while connecting and disconnecting client-side ... and updates about connection status of a client are not always displaying correctly server-side.
[ Code Snippet|http://snipplr.com/view/15206/clientserver-chat-program/]
Thanks for the help

Answer 1

NOTE: all catch clauses have been omitted for clarity. They all just perform System.err.println() with the msg embedded
Very valid point. I cut out the GUIs and just tried having the Server/Client communicate. I am still having concurrency issues. This is my first attempt at synchronized methods and locking objects so go easy on me if I did something(s) noob =D
public class MySocket
    public static final String QUIT = "~~QUIT~~";
    private ObjectOutputStream out;
    private ObjectInputStream in;
    private Socket conn;
    public MySocket(String ip)
        obsList = new ArrayList<IClientObs>();
        try
            conn = new Socket(ip, 5000);
            if (conn.isConnected())
                out = new ObjectOutputStream(conn.getOutputStream());
                in = new ObjectInputStream(conn.getInputStream());
    public synchronized String nextMsg()
        String msg = "";
        try
            synchronized(in)
                msg = (String)in.readObject();
                notify(msg);
        return(msg);
    public synchronized boolean sendMsg(String msg)
        boolean sentMsg = false;
        try
            synchronized(out)
                if (out != null)
                    out.writeObject(msg);
                    out.flush();
                    sentMsg = true;
        return(sentMsg);
    public synchronized void closeConn()
        try
            synchronized(this)
                sendMsg(QUIT);
                conn.close();
                out.close();
                in.close();
    public synchronized Socket getConn()
        return(conn);
    public synchronized ObjectOutputStream getOutStream()
        return(out);
    public synchronized ObjectInputStream getInStream()
        return(in);
   //Observer Pattern implemented below   
public class Server extends Thread
    public static final int MAX_CLIENTS = 2;
    public static final String QUIT = "~~QUIT~~";
    private ServerSocket server;
    private ArrayList<ConnClient> conns;
    private int connCount;
     * Constructor for objects of class Server
    public Server()
        conns = new ArrayList<ConnClient>();
        obsList = new ArrayList<IServerObs>();
        connCount = 0;
    public void startNow()
        this.start();
    public void run()
        runServer();
    public synchronized void runServer()
        try
            setup();
            while (true)
                waitForConn();
                processComms();
    private synchronized void setup() throws IOException
        server = new ServerSocket(5000);
        notify("Server initialized.\n");
    private synchronized void waitForConn() throws IOException
        if (connCount < MAX_CLIENTS)
            notify("Waiting for connection...\n");
            Socket conn = server.accept();
            if (conn.isConnected())
                conns.add(new ConnClient(conn));
                notify("Client connected @ '" + conns.get(connCount).getIP() + "'.\n");
                connCount++;
        else
            notify("Connection request rejected; max connections have been reached.\n");
    private synchronized void processComms() throws IOException, ClassNotFoundException
        //Receive any msgs sent by clients and forward msg to all clients
        for(ConnClient rcvClient : conns)
            String msg = rcvClient.nextMsg();
            //if client quit, then close connection and remove it from list
            if (msg.equals(QUIT))
                notify("Client disconnected @ '" + rcvClient.getIP() + "'.\n");
                rcvClient.closeConn();
                conns.remove(rcvClient);
                connCount--;
            else
                for(ConnClient sndClient : conns)
                    sndClient.sendMsg(msg);
    public synchronized void shutdown()
        try
            server.close();
            for(ConnClient client :conns)
                client.closeConn();
   //Observer Pattern implemented below
}I also found another issue that I haven't thought up a way to deal with yet. When the user starts the program the follow line is executed "conn = server.accept();" which halts execution
on that thread until a connection is established. What if the user wants to stop the server before a connection is made? The thread keeps running, waiting for a connection. How do I kill this thread?
On this last issue (I figured by adding the follow code to my action listener inside of my server gui I could stop the thread safely but it's no good so far)
public void actionPerformed(ActionEvent e)
        Object src = e.getSource();
        if (src == strBtn)
            if (server == null)
                strBtn.setEnabled(false);
                stpBtn.setEnabled(true);
                server = new Server();
                server.addObserver(this);
                server.start();
            else
                console.append("Console: Server is alread initiated.\n");
        else if (src == stpBtn)
            synchronized(server)
            strBtn.setEnabled(true);
            stpBtn.setEnabled(false);
            server.shutdown();
            server = null;
            console.append("Console: Server has been stopped.\n");
    }Edited by: mcox05 on May 21, 2009 10:05 AM
Edited by: mcox05 on May 21, 2009 10:17 AM
Edited by: mcox05 on May 21, 2009 10:58 AM
Edited by: mcox05 on May 21, 2009 11:01 AM
Edited by: mcox05 on May 21, 2009 11:03 AM
Edited by: mcox05 on May 21, 2009 11:03 AM