基于AJAX的长轮询(long-polling)方式实现COMET例子

什么是 Comet?

解释： Alex Russell （ Dojo Toolkit 的项目 Lead ）称这种基于 HTTP 长连接、无须在浏览器端安装插件的 “ 服务器推 ” 技术为 “Comet” 。

有两种实现 Comet 应用的实现模型，目前主要讨论的是基于 AJAX 的长轮询 (long-polling) 方式

例子如下：

Servlet实现类：TestComet

 

public class TestComet extends HttpServlet implements CometProcessor {
	
	private static final long serialVersionUID = 1L;

	// 发送器
	private MessageSender messageSender = null;

	private static final Integer TIMEOUT = 60 * 1000;

	@Override
	public void destroy() {
		messageSender.stop();
		messageSender = null;

	}

	@Override
	public void init() throws ServletException {
		System.out.println("--init-----------------");
		
		// 初始化发送器
		messageSender = new MessageSender();
		Thread messageSenderThread = new Thread(messageSender, "MessageSender["
				+ getServletContext().getContextPath() + "]");
		messageSenderThread.setDaemon(true);
		
		// 启动发送线程
		messageSenderThread.start();
	}

	public void event(final CometEvent event) throws IOException,
			ServletException {
		System.out.println("--event-----------------");
		
		// 获取事件对应的REQUEST 和 RESPONSE
		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());
			// 注入RESPONSE
			messageSender.setConnection(response);
			
			Weatherman weatherman = new Weatherman(messageSender, 95118, 32408);
			new Thread(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");
		}
	}
}

 信息发送器:

public class MessageSender implements Runnable {

	// 标志位
	protected boolean running = true;

	// 信息列表
	protected final ArrayList<String> messages = new ArrayList<String>();

	// HTTP RESPONSE
	private ServletResponse connection;

	// 注入HTTP RESPONSE
	public synchronized void setConnection(ServletResponse connection) {
		this.connection = connection;
		notify();
	}

	// 发送信息
	public void send(String message) {
		// 同步队列，加入发送信息
		synchronized (messages) {
			messages.add(message);
			log("Message added #messages=" + messages.size());
			// 唤醒
			messages.notify();
		}
	}

	public void run() {

		// 线程启动
		log("start");
		while (running) {
			if (messages.size() == 0) {
				try {
					synchronized (messages) {
						log("MessageSender wait[空闲状态，线程等待]");
						// 释放锁
						messages.wait();
					}
				} catch (InterruptedException e) {
					e.printStackTrace();
					// Ignore
				}
			}
			String[] pendingMessages = null;
			synchronized (messages) {
				
				// 导出发送的信息至数组
				pendingMessages = messages.toArray(new String[0]);
				// 清空信息队列
				messages.clear();
			}
			try {
				if (connection == null) {
					try {
						synchronized (this) {
							// 等待注入HTTP RESPONSE
							wait();
						}
					} catch (InterruptedException e) {
						// Ignore
						e.printStackTrace();
					}
				}
				
				// 输出流操作
				OutputStream out = connection.getOutputStream();
				for (int j = 0; j < pendingMessages.length; j++) {
					final String forecast = pendingMessages[j] + "<br>";
					out.write(forecast.getBytes());
					out.flush();
					connection.flushBuffer();
					log("Writing[写入]:" + forecast);
				}
			} catch (IOException e) {
				log("IOExeption sending message", e);
			}
		}
	}

	// 停止
	public void stop() {
		running = false;
	}

	// 日志
	private void log(Object obj) {
		System.out.println(obj);
	}

	// 日志
	private void log(Object obj, Throwable e) {
		System.out.println(obj);
		e.printStackTrace();
	}

}

 YAHOO天气预报：

public class Weatherman implements Runnable {

	// 链接列表
	private final List<URL> zipCodes;
	
	// YAHOO WEATHER
	private final String YAHOO_WEATHER = "http://weather.yahooapis.com/forecastrss?p=";
	
	// 发送器
	private MessageSender messageSender;

	public Weatherman(MessageSender messageSender, Integer... zips) {
		this.messageSender = messageSender;
		zipCodes = new ArrayList<URL>(zips.length);
		for (Integer zip : zips) {
			try {
				// 添加具体链接
				zipCodes.add(new URL(YAHOO_WEATHER + zip));
			} catch (Exception e) {
				e.printStackTrace();
			}
		}
	}

	public void run() {
		System.out.println("Weatherman run[天气预报员启动]");
		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);
				
				// 发送数据
				messageSender.send(entryToHtml(entry));
				
				// 线程休眠
				Thread.sleep(10000L);
			} catch (Exception e) {
				e.printStackTrace();
			}
			i++;
		}
	}

	// 格式转换
	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();
	}
}

 WEB.XML配置：

	<servlet>
	    <description>TestComet</description>
	    <display-name>TestComet</display-name>
	    <servlet-name>TestComet</servlet-name>
	    <servlet-class>cn.test.TestComet</servlet-class>
	</servlet>

	<servlet-mapping>
	    <servlet-name>TestComet</servlet-name>
	    <url-pattern>/TestComet</url-pattern>
	</servlet-mapping>

 TOMCAT配置，NIO

<Connector connectionTimeout="20000" port="8888" protocol="org.apache.coyote.http11.Http11NioProtocol" redirectPort="8443"/>

 

 

参考文档：

Comet ：基于 HTTP 长连接的 “ 服务器推 ” 技术

http://www.ibm.com/developerworks/cn/web/wa-lo-comet/

 

使用 Java 实现 Comet 风格的 Web 应用（一）

http://blog.csdn.net/ligaoyang/archive/2009/08/03/4402889.aspx

 

使用 Java 实现 Comet 风格的 Web 应用（二）

http://blog.csdn.net/ligaoyang/archive/2009/08/03/4402898.aspx

 

注：

以上例子只支持FIREFOX，不支持IE

可能会出现一些问题，例如有些包可能有冲突，需要在context.xml中添加

<Loader delegate="true" />

 

评论

2 楼 zhdalong 2011-12-30  

public void event(final CometEvent event) throws IOException, 
            ServletException { }

什么时候调用

1 楼 neal 2011-04-12  

按照这个搭出来运行报错，能否打包整个发给我