How to call this Comets application from a web page

StackOverflow https://stackoverflow.com/questions/1159819

Question

I have implemented the two classes shown at http://tomcat.apache.org/tomcat-6.0-doc/aio.html which gives a messenger application using Tomcat's comet implementation.

How do I connect this to a web interface and get something to display.

I am thinking these are the basic steps (I don't know the details).

  1. I should create some traditional event - a button click or AJAX event - that calls the ChatServlet and passes in a CometEvent (somehow) - perhaps BEGIN
  2. From then I have my code call the event method every time I want to send something to the client using the READ event as the input parameter.

I have copied the two classes below:

package controller;

import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.ArrayList;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import org.apache.catalina.CometEvent;
import org.apache.catalina.CometProcessor;

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);
                System.out.println("you have an error");

                return;
            }
        } while (is.available() > 0);
    }
}
}

package controller;

import java.io.IOException;
import java.io.PrintWriter;
import java.util.ArrayList;
import javax.servlet.http.HttpServletResponse;

public class MessageSender implements Runnable {

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

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) {
                    System.out.println("IOExeption sending message" + e);
                }
            }
        }

    }

}

}
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Solution

Here you have a complete example for Tomcat with source code to download at the bottom:Developing with Comet and Java

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