I am designing a chat server in java. The communication is Http based and not socket based. In the client side I have an applet. In the server side I have a servlet.

Applet: I create a new thread to listen for incoming messages(GET method). The main thread is used to send messages(POST messages).

The partial code is :

public void start() {
    System.out.println("Creating new thread");
    Thread thread = new Thread(this);
    thread.start();
}

private String getNewMessage() {
    System.out.println("Inside getNewMessage");
    String msg = null;
    try {
        while(msg == null) {
            System.out.println("Trying to listen to servlet");
            URL servlet = new URL(getCodeBase(), "NewServlet?mode=msg");
            URLConnection con = servlet.openConnection();

            con.setUseCaches(false);

            DataInputStream din = new DataInputStream(new BufferedInputStream(con.getInputStream()));
            msg = din.readUTF();
            System.out.println("message read :" + msg);
        }
    } catch (Exception e) {
        e.printStackTrace();
    }
    return msg + "\n";
}
public void run() {
    System.out.println("Inside new thread");
    while(true) {
        System.out.println("inside first while");
        String newMsg = getNewMessage();
        chatOutput.append(newMsg);
        System.out.println("Appended!!");
    }
}
private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {
    String message = chatInput.getText();
    chatInput.setText("");
    chatOutput.append(message + "\n");
    try {
        System.out.println("Trying to send msg :" + message);
        URL url = new URL(getCodeBase(), "NewServlet");
        URLConnection servletConnection = url.openConnection();

        servletConnection.setDoInput(true);
        servletConnection.setDoOutput(true);
        servletConnection.setUseCaches(false);
        servletConnection.setRequestProperty("Content-Type", "application/octet-stream");

        ObjectOutputStream out = new ObjectOutputStream(servletConnection.getOutputStream());
        out.writeObject(message);
        out.flush();
        out.close();

        System.out.println("Message sent!");
    } catch (Exception e) {
        e.printStackTrace();
    }

}

This next code is from the servlet side. it uses the Observable interface to identify and send messages to clients.

public class NewServlet extends HttpServlet {
// getNextMessage() returns the next new message.  // It blocks until there is one.
public String getNextMessage() {
// Create a message sink to wait for a new message from the
// message source.
  System.out.println("inside getNextMessage");
return new MessageSink().getNextMessage(source);}

@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
    System.out.println("Inside Doget");
    response.setContentType("text/plain");
    PrintWriter out = response.getWriter();

    out.println(getNextMessage());
} 

// broadcastMessage() informs all currently listening clients that there
// is a new message. Causes all calls to getNextMessage() to unblock.
public void broadcastMessage(String message) {
// Send the message to all the HTTP-connected clients by giving the
// message to the message source
source.sendMessage(message);  }
@Override
protected void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
    System.out.println("Inside DoPost");
    try {
    ObjectInputStream din= new ObjectInputStream(request.getInputStream());
    String message = (String)din.readObject();

        System.out.println("received msg");
    if (message != null) broadcastMessage(message);
        System.out.println("Called broadcast");
// Set the status code to indicate there will be no response
    response.setStatus(response.SC_NO_CONTENT);
    } catch (Exception e) {
        e.printStackTrace();
    }
}

/** 
 * Returns a short description of the servlet.
 * @return a String containing servlet description
 */
@Override
public String getServletInfo() {
    return "Short description";
}

MessageSource source = new MessageSource();}

class MessageSource extends Observable {
public void sendMessage(String message) {
  System.out.println("inside sendMsg");
setChanged();
notifyObservers(message);
}
}

class MessageSink implements Observer {
String message = null;  // set by update() and read by getNextMessage()
// Called by the message source when it gets a new message
synchronized public void update(Observable o, Object arg) {
// Get the new message
message = (String)arg;
// Wake up our waiting thread
notify();
}
// Gets the next message sent out from the message source
synchronized public String getNextMessage(MessageSource source) {
// Tell source we want to be told about new messages
source.addObserver(this);
  System.out.println("AddedObserver");
// Wait until our update() method receives a message
while (message == null) {
  try { wait(); } catch (Exception ignored) { }
}
// Tell source to stop telling us about new messages
source.deleteObserver(this);
// Now return the message we received
// But first set the message instance variable to null
// so update() and getNextMessage() can be called again.
String messageCopy = message;
message = null;
  System.out.println("Returning msg");
return messageCopy;
}
}

As you can see I have included System.out.println("Some message"); in some places. this was just for debugging purposes. In java console, i get the following output:

Creating new thread Inside new thread. inside first while. Inside getNewMessage. Trying to listen to servlet.

In the servlet side, i get the following output in the tomcat logs:

Inside Doget. inside getNextMessage. AddedObserver.

After i type a message in the applet, and send it, I get the foll output in java console:

Trying to send msg :you deR?? Message sent!

But in servlet side, I dont get anything in the logs. I used the O'Reily Java Servlet Programming as reference(The observer interface comes from there). But I am not getting any chat communication between two clients. As can be understood from the logs, the POST method is not called. Any reason for this?