import java.io.*;
import java.net.*;
import java.util.*;
import javax.servlet.*;
import javax.servlet.http.*; 

public class ChatServlet extends HttpServlet {
    MessageSource source = new MessageSource();
    public void doGet (HttpServletRequest  req, 
                       HttpServletResponse res) 
                throws ServletException,
                       IOException 
    { res.setContentType("text/plain"); 
      PrintWriter out = res.getWriter();
      out.println(getNextMessage());
    }

    public void doPost (HttpServletRequest  req, 
                        HttpServletResponse res) 
                throws ServletException,
                       IOException 
    {
        String message = req.getParameter("message");
	if (message != null) broadcastMessage(message);
	res.setStatus(res.SC_NO_CONTENT);;
    }

    public String getNextMessage() 
    {
	return new MessageSink().getNextMessage(source);
    }

    public void broadcastMessage(String message) 
    {
	source.sendMessage(message);
    }
}

class MessageSource extends Observable 
{
    public void sendMessage(String message)
    {
	setChanged();
	notifyObservers(message);
    }
}

class MessageSink implements Observer 
{
    String message = null;

    synchronized public void update(Observable o, Object arg)
    {
	message = (String)arg;
	notify();
    }
         
    synchronized public String getNextMessage(MessageSource source)
    {
	source.addObserver(this);

	while (message == null) {
	    try {
		wait();
	    } catch (Exception ignored) { }
	}

	source.deleteObserver(this);
	String messageCopy = message;
	message = null;
	return messageCopy;
    }
}