Question

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 the Java console, I get the following output:

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

On 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 following output in the Java console:

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

But on the 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 doPOST method is not called. What is the reason for this?

Answer 1

I fixed the problem by receiving message (a status message) after sending the message in the applet side. In the servlet side, in the doPost method, I sent the status message ("1"), after reading the message.

I don’t know how exactly this fixed the problem, but I guess that since I had setDoInput(true);, it was waiting for some message to read.

Anyway, the good news is that I have at least got the desired result of the above debugging process.

Also, it was necessary to use ObjectInputStream instead of DataInputStream in the getNewMessage method (since the message was sent by ObjectOutputStream). Now the chat server works smoothly.