servlet3.0的学习（二）

Servlet 3.0 异步传输的使用 

AjaxCometServlet.java 

 import java.io.IOException;
 import java.io.PrintWriter;
 import java.util.Queue;
 import java.util.concurrent.ConcurrentLinkedQueue;
 import java.util.concurrent.BlockingQueue;
 import java.util.concurrent.LinkedBlockingQueue;

 import javax.servlet.AsyncContext;
 import javax.servlet.AsyncEvent;
 import javax.servlet.AsyncListener;
 import javax.servlet.ServletConfig;
 import javax.servlet.ServletException;
 import javax.servlet.annotation.WebServlet;
 import javax.servlet.http.HttpServlet;
 import javax.servlet.http.HttpServletRequest;
 import javax.servlet.http.HttpServletResponse;

 @WebServlet(urlPatterns = {"/chat"}, asyncSupported = true)
 public class AjaxCometServlet extends HttpServlet {

     private static final Queue<AsyncContext> queue = new ConcurrentLinkedQueue<AsyncContext>();

     private static final BlockingQueue<String> messageQueue = new LinkedBlockingQueue<String>();

     private static final String BEGIN_SCRIPT_TAG = "<script type='text/javascript'>\n";

     private static final String END_SCRIPT_TAG = "</script>\n";

     private static final long serialVersionUID = -2919167206889576860L;

     private Thread notifierThread = null;

     @Override
     public void init(ServletConfig config) throws ServletException {
         Runnable notifierRunnable = new Runnable() {
             public void run() {
                 boolean done = false;
                 while (!done) {
                     String cMessage = null;
                     try {
                         cMessage = messageQueue.take();
                         for (AsyncContext ac : queue) {
                             try {
                                 PrintWriter acWriter = ac.getResponse().getWriter();
                                 acWriter.println(cMessage);
                                 acWriter.flush();
                             } catch(IOException ex) {
                                 System.out.println(ex);
                                 queue.remove(ac);
                             }
                         }
                     } catch(InterruptedException iex) {
                         done = true;
                         System.out.println(iex);
                     }
                 }
             }
         };
         notifierThread = new Thread(notifierRunnable);
         notifierThread.start();
     }

     @Override
     protected void doGet(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException {
         res.setContentType("text/html");
         res.setHeader("Cache-Control", "private");
         res.setHeader("Pragma", "no-cache");
         PrintWriter writer = res.getWriter();
         // for IE
         writer.println("<!-- Comet is a programming technique that enables web servers to send data to the client without having any need for the client to request it. -->\n");
         writer.flush();

         final AsyncContext ac = req.startAsync();
         ac.setTimeout(10  * 60 * 1000);
         ac.addListener(new AsyncListener() {
             public void onComplete(AsyncEvent event) throws IOException {
                 queue.remove(ac);
             }

             public void onTimeout(AsyncEvent event) throws IOException {
                 queue.remove(ac);
             }

             public void onError(AsyncEvent event) throws IOException {
                 queue.remove(ac);
             }

             public void onStartAsync(AsyncEvent event) throws IOException {
             }
         });
         queue.add(ac);
     }

     @Override
     @SuppressWarnings("unchecked")
     protected void doPost(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException {
         res.setContentType("text/plain");
         res.setHeader("Cache-Control", "private");
         res.setHeader("Pragma", "no-cache");

         req.setCharacterEncoding("UTF-8");
         String action = req.getParameter("action");
         String name = req.getParameter("name");

         if ("login".equals(action)) {
             String cMessage = BEGIN_SCRIPT_TAG + toJsonp("System Message", name + " has joined.") + END_SCRIPT_TAG;
             notify(cMessage);

             res.getWriter().println("success");
         } else if ("post".equals(action)) {
             String message = req.getParameter("message");
             String cMessage = BEGIN_SCRIPT_TAG + toJsonp(name, message) + END_SCRIPT_TAG;
             notify(cMessage);

             res.getWriter().println("success");
         } else {
             res.sendError(422, "Unprocessable Entity");
         }
     }

     @Override
     public void destroy() {
         queue.clear();
         notifierThread.interrupt();
     }

     private void notify(String cMessage) throws IOException {
         try {
             messageQueue.put(cMessage);
         } catch(Exception ex) {
             IOException t = new IOException();
             t.initCause(ex);
             throw t;
         }
     }

     private String escape(String orig) {
         StringBuffer buffer = new StringBuffer(orig.length());

         for (int i = 0; i < orig.length(); i++) {
             char c = orig.charAt(i);
             switch (c) {
             case '\b':
                 buffer.append("\\b");
                 break;
             case '\f':
                 buffer.append("\\f");
                 break;
             case '\n':
                 buffer.append("<br />");
                 break;
             case '\r':
                 // ignore
                 break;
             case '\t':
                 buffer.append("\\t");
                 break;
             case '\'':
                 buffer.append("\\'");
                 break;
             case '\"':
                 buffer.append("\\\"");
                 break;
             case '\\':
                 buffer.append("\\\\");
                 break;
             case '<':
                 buffer.append("&lt;");
                 break;
             case '>':
                 buffer.append("&gt;");
                 break;
             case '&':
                 buffer.append("&amp;");
                 break;
             default:
                 buffer.append(c);
             }
         }

         return buffer.toString();
     }

     private String toJsonp(String name, String message) {
         return "window.parent.update({ name: \"" + escape(name) + "\", message: \"" + escape(message) + "\" });\n";
     }
 }

ajax.js 

 var count=0;
 $(document).ready(function () {

     var url='/ajax/chat';
     $('#login-name').focus();

     $('#comet-frame')[0].src=url + '?' + count;
     count ++;

     $("#login-button").click(function () {
             var name = $('#login-name').val();
             if(! name.length > 0) {
                 $('#system-message').css("color","red");
                 $('#login-name').focus();
                 return;
             }
             $('#system-message').css("color","#2d2b3d") ;
             $('#system-message').html(name + ':');

             $('#login-button').disabled = true;
             $('#login-form').css("display","none");
             $('#message-form').css("display","");

             var query =
             'action=login' +
             '&name=' + encodeURI($('#login-name').val());
             $.ajax({
                 type:"post",
                 url:url,
                 data:query,
                 success:function (data,status) {
                     $('#message').focus();
                 },
                 error:function () {
                     alert("error occured!!!");
                 }
             });
     });


     $("#post-button").click(function () {
         var message = $('#message').val();
         if(!message > 0) {
             return;
         }
         $('#message').disabled = true;
         $('#post-button').disabled = true;

         var query =
         'action=post' +
         '&name=' + encodeURI($('#login-name').val()) +
         '&message=' + encodeURI(message);
         $.ajax({
             type:"post",
             url:url,
             data:query,
             success:function (data,status) {
                 $('#message').disabled = false;
                 $('#post-button').disabled = false;
                 $('#message').focus();
                 $('#message').val("");
             },
             error:function () {
                 alert("error occured!!!");
             }
         });
     }
     )
 });

 function update(data){
     var p = document.createElement('p');
     p.innerHTML = data.name + ':<br/>' + data.message;

     $('#display')[0].appendChild(p);
 }

index.html 

 <?xml version="1.0" encoding="UTF-8" ?>
 <!DOCTYPE html
 PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
 "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
 <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en" lang="en">
     <head>
         <meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
         <title>Glassfish Chat</title>
         <link rel="stylesheet" href="css/default.css" type="text/css" />
         <script type="text/javascript" src="js/jquery-1.4.min.js"></script>
         <script type="text/javascript" src="js/ajax.js"></script>
     </head>
     <body>
         <div id="container">
             <div id="container-inner">
                 <div id="header">
                     <h1>Glassfish Chat</h1>
                 </div>
                 <div id="main">
                     <div id="display">
                     </div>
                     <div id="form">
                         <div id="system-message">Please input your name:</div>
                         <div id="login-form">
                             <input id="login-name" type="text" />
                             <br />
                             <input id="login-button" type="button" value="Login" />
                         </div>
                         <div id="message-form" style="display: none;">
                             <div>
                                 <textarea id="message" name="message" rows="2" cols="40"></textarea>
                                 <br />
                                 <input id="post-button" type="button" value="Post Message" />
                             </div>
                         </div>
                     </div>
                 </div>
             </div>
         </div>
         <iframe id="comet-frame" style="display: none;"></iframe>
     </body>
 </html>

servlet形成异步处理的过程 

在init()方法里，开启一条线程从消息队列中读取消息，并且在AsyncContext（异步上下文）中处理这些消息  

 PrintWriter acWriter = ac.getResponse().getWriter();
 acWriter.println(cMessage);
 acWriter.flush();

  前面的代码写入一个信息，每个异步请求对应AsyncContext。

在doGet方法里，设置超时时间为10分钟，开始异步并且增加AsyncContext到队列中 

 final AsyncContext ac = req.startAsync();
 ac.setTimeout(10 * 60 * 1000);
 queue.add(ac);

  对AsyncContext注册AsyncListener 监听器，在这种情况下，只有从队列中清理AsyncContext 
 

 ac.addListener(new AsyncListener() {
     public void onComplete(AsyncEvent event) throws IOException {
         queue.remove(ac);
     }

     public void onTimeout(AsyncEvent event) throws IOException {
         queue.remove(ac);
     }

     public void onError(AsyncEvent event) throws IOException {
         queue.remove(ac);
     }

     public void onStartAsync(AsyncEvent event) throws IOException {
     }
 });

在doPost方法里，将消息加入到消息队列里，消息在init方法里开启线程执行


问题：不明白在ajax.js中为什么要加上$('#comet-frame')[0].src=url + '?' + count;这么一句，不加上的话，页面显示不了！ 

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