IBM的一个Comet实现例子的问题

wshq

　　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"  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。