How to implement a chat server using HTTP Keepalive headers

StackOverflow https://stackoverflow.com/questions/4882739

  •  28-10-2019
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Question

I want to know about implementing a "push" chat server, which uses HTTP keepalive headers. I just came to know two technologies such chat servers use - long polling, and push - but I could not get any concepts regarding what happens behind the scenes.

Can anyone please help me ? Any client side / server side code would be appreciated a lot !!

Was it helpful?

Solution

Here is some comet code from the Tomcat Docs (hhttp://tomcat.apache.org/tomcat-7.0-doc/aio.html)

public class ChatServlet
extends HttpServlet implements CometProcessor {

protected ArrayList<HttpServletResponse> connections = 
    new ArrayList<HttpServletResponse>();
protected MessageSender messageSender = null;

public void init() throws ServletException {
    messageSender = new MessageSender();
    Thread messageSenderThread = 
        new Thread(messageSender, "MessageSender[" + getServletContext().getContextPath() + "]");
    messageSenderThread.setDaemon(true);
    messageSenderThread.start();
}

public void destroy() {
    connections.clear();
    messageSender.stop();
    messageSender = null;
}

/**
 * Process the given Comet event.
 * 
 * @param event The Comet event that will be processed
 * @throws IOException
 * @throws ServletException
 */
public void event(CometEvent event)
    throws IOException, ServletException {
    HttpServletRequest request = event.getHttpServletRequest();
    HttpServletResponse response = event.getHttpServletResponse();
    if (event.getEventType() == CometEvent.EventType.BEGIN) {
        log("Begin for session: " + request.getSession(true).getId());
        PrintWriter writer = response.getWriter();
        writer.println("<!doctype html public \"-//w3c//dtd html 4.0 transitional//en\">");
        writer.println("<head><title>JSP Chat</title></head><body bgcolor=\"#FFFFFF\">");
        writer.flush();
        synchronized(connections) {
            connections.add(response);
        }
    } else if (event.getEventType() == CometEvent.EventType.ERROR) {
        log("Error for session: " + request.getSession(true).getId());
        synchronized(connections) {
            connections.remove(response);
        }
        event.close();
    } else if (event.getEventType() == CometEvent.EventType.END) {
        log("End for session: " + request.getSession(true).getId());
        synchronized(connections) {
            connections.remove(response);
        }
        PrintWriter writer = response.getWriter();
        writer.println("</body></html>");
        event.close();
    } else if (event.getEventType() == CometEvent.EventType.READ) {
        InputStream is = request.getInputStream();
        byte[] buf = new byte[512];
        do {
            int n = is.read(buf); //can throw an IOException
            if (n > 0) {
                log("Read " + n + " bytes: " + new String(buf, 0, n) 
                        + " for session: " + request.getSession(true).getId());
            } else if (n < 0) {
                error(event, request, response);
                return;
            }
        } while (is.available() > 0);
    }
}

public class MessageSender implements Runnable {

    protected boolean running = true;
    protected ArrayList<String> messages = new ArrayList<String>();

    public MessageSender() {
    }

    public void stop() {
        running = false;
    }

    /**
     * Add message for sending.
     */
    public void send(String user, String message) {
        synchronized (messages) {
            messages.add("[" + user + "]: " + message);
            messages.notify();
        }
    }

    public void run() {

        while (running) {

            if (messages.size() == 0) {
                try {
                    synchronized (messages) {
                        messages.wait();
                    }
                } catch (InterruptedException e) {
                    // Ignore
                }
            }

            synchronized (connections) {
                String[] pendingMessages = null;
                synchronized (messages) {
                    pendingMessages = messages.toArray(new String[0]);
                    messages.clear();
                }
                // Send any pending message on all the open connections
                for (int i = 0; i < connections.size(); i++) {
                    try {
                        PrintWriter writer = connections.get(i).getWriter();
                        for (int j = 0; j < pendingMessages.length; j++) {
                            writer.println(pendingMessages[j] + "<br>");
                        }
                        writer.flush();
                    } catch (IOException e) {
                        log("IOExeption sending message", e);
                    }
                }
            }

        }

    }

}

}

OTHER TIPS

There are many specific techniques, generally now known under the name Comet. The Wikipedia article linked there is a pretty good intro.

Also see The Long Polling Technique.

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