IBM的一个Comet实现例子的问题

源地址链接： http://www.ibm.com/developerworks/cn/web/wa-cometjava/

 

import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.net.URL;
import java.util.ArrayList;
import java.util.List;

import javax.servlet.ServletException;
import javax.servlet.ServletResponse;
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;

import com.sun.syndication.feed.synd.SyndEntry;
import com.sun.syndication.feed.synd.SyndFeed;
import com.sun.syndication.io.SyndFeedInput;

public class TomcatWeatherServlet extends HttpServlet implements CometProcessor {

    private MessageSender messageSender = null;
    private static final Integer TIMEOUT = 60 * 1000;
    @Override
    public void destroy() {
    	log("destroy()!!!");
        messageSender.stop();
        messageSender = null;

    }

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

    }

    public void event(final CometEvent event) throws IOException, ServletException {
        HttpServletRequest request = event.getHttpServletRequest();
        HttpServletResponse response = event.getHttpServletResponse();
        if (event.getEventType() == CometEvent.EventType.BEGIN) {
            request.setAttribute("org.apache.tomcat.comet.timeout", TIMEOUT);
            log("Begin for session: " + request.getSession(true).getId());
            messageSender.setConnection(response);
            Weatherman weatherman = new Weatherman(95118, 32408);
            weatherman.start();
        } else if (event.getEventType() == CometEvent.EventType.ERROR) {
        	log("Error for session: " + request.getSession(true).getId());
            event.close();
        } else if (event.getEventType() == CometEvent.EventType.END) {
        	log("End for session: " + request.getSession(true).getId());
            event.close();
        } else if (event.getEventType() == CometEvent.EventType.READ) {
            throw new UnsupportedOperationException("This servlet does not accept data");
        }

    }

private class Weatherman {

    private final List<URL> zipCodes;
    private final String YAHOO_WEATHER = "http://weather.yahooapis.com/forecastrss?p=";

    public Weatherman(Integer... zips) {
        zipCodes = new ArrayList<URL>(zips.length);
        for (Integer zip : zips) {
            try {
                zipCodes.add(new URL(YAHOO_WEATHER + zip));
            } catch (Exception e) {
                // dont add it if it sucks
            }
        }
    }

    public void start() {
        Runnable r = new Runnable() {

            public void run() {
                int i = 0;
                while (i >= 0) {
                    int j = i % zipCodes.size();
                    SyndFeedInput input = new SyndFeedInput();
                    try {
                        SyndFeed feed = input.build(new InputStreamReader(zipCodes.get(j).openStream()));
                        SyndEntry entry = (SyndEntry) feed.getEntries().get(0);
                        messageSender.send(entryToHtml(entry));
                        Thread.sleep(30000L);
                    } catch (Exception e) {
                        // just eat it, eat it
                    }
                    i++;
                }
            }
        };
        Thread t = new Thread(r);
        t.start();
    }
    private String entryToHtml(SyndEntry entry){
        StringBuilder html = new StringBuilder("<h2>");
        html.append(entry.getTitle());
        html.append("</h2>");
        html.append(entry.getDescription().getValue());
        return html.toString();
    }
}

   private class MessageSender implements Runnable {

        protected boolean running = true;
        protected final ArrayList<String> messages = new ArrayList<String>();
        private ServletResponse connection;

        private synchronized void setConnection(ServletResponse connection){
            this.connection = connection;
            notify();
        }

        public void stop() {
            running = false;
        }

        /**
         * Add message for sending.
         */
        public void send(String message) {
            synchronized (messages) {
                messages.add(message);
                log("Message added #messages=" + messages.size());
                messages.notify();
            }
        }

        public void run() {
        	log("start running!!!");
            while (running) {
                if (messages.size() == 0) {
                    try {
                        synchronized (messages) {
                        	log("start waiting!!!");
                            messages.wait();
                        }
                    } catch (InterruptedException e) {
                        // Ignore
                    }
                }
                String[] pendingMessages = null;
                synchronized (messages) {
                    pendingMessages = messages.toArray(new String[0]);
                    messages.clear();
                }
                try {
                    if (connection == null){
                        try{
                            synchronized(this){
                                wait();
                            }
                        } catch (InterruptedException e){
                            // Ignore
                        }
                    }
                    PrintWriter writer = connection.getWriter();
                    for (int j = 0; j < pendingMessages.length; j++) {
                        final String forecast = pendingMessages[j] + "<br>";
                        writer.println(forecast);
                        log("Writing:" + forecast);
                    }
                    writer.flush();
                    writer.close();
                    //connection = null;
                    log("Closing connection");
                } catch (IOException e) {
                	log("IOExeption sending message", e);
                }
            }
        }
    }
}

 

 

<%@page contentType="text/html" pageEncoding="UTF-8"%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"
   "http://www.w3.org/TR/html4/loose.dtd">

<html>
    <head>
        <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
        <title>Comet Weather</title>
        <SCRIPT TYPE="text/javascript">
            function go(){
                var url = "http://localhost:8080/comet1/Weather";
                var request =  new XMLHttpRequest();
                request.open("GET", url, true);
                request.setRequestHeader("Content-Type","application/x-javascript;");
                request.onreadystatechange = function() {
                    if (request.readyState == 4) {
                        if (request.status == 200){
                            if (request.responseText) {
                                document.getElementById("forecasts").innerHTML = request.responseText;
                            }
                        }else{
                        	document.getElementById("forecasts").innerHTML = "Error!!!!";
                        }
                        //go();
                    }
                };
                request.send(null);
            }
        </SCRIPT>
    </head>
    <body>
        <h1>Rapid Fire Weather</h1>
        <input type="button" onclick="go()" value="Go!"></input>
        <div id="forecasts">
        
        </div>
    </body>
</html>

   

    做如下修改：    

        一、JSP页面中客户端多次调用go()，改为一次，把循环调用go()语句注释掉。

        二、每次request只有一次write，在MessageSender中注释掉 connection = null; 这样保证每次request有多次response。

      

        Tomcat上说明Comet的模型是“BEGIN -> READ -> READ -> READ -> ERROR/TIMEOUT”，这里使用的get请求，不是post请求，没有使用到Read事件。