dwr 后台推送技术

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实例分为两个页面

sendMsg.jsp页面，发送一段消息给后台，然后后台将消息推送到showMsg.jsp页面

web.xml配置

[html] view plain copy

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xmlns:web="http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" id="WebApp_ID" version="2.5">
<display-name>dwr3</display-name>
<welcome-file-list>
<welcome-file>index.jsp</welcome-file>
</welcome-file-list>
<servlet>
<servlet-name>dwr-invoker</servlet-name>
<servlet-class>uk.ltd.getahead.dwr.DWRServlet</servlet-class>
<init-param>
<param-name>debug</param-name>
<param-value>true</param-value>
</init-param>
<init-param>
<param-name>pollAndCometEnabled</param-name>
<param-value>true</param-value>
</init-param>
<load-on-startup>1</load-on-startup>
</servlet>
<servlet-mapping>
<servlet-name>dwr-invoker</servlet-name>
<url-pattern>/dwr/*</url-pattern>
</servlet-mapping>
</web-app>

sendMsg.java 推送类

[java] view plain copy

package com.zf.dwr;
import java.util.Collection;
import org.directwebremoting.ScriptBuffer;
import org.directwebremoting.ScriptSession;
import org.directwebremoting.WebContext;
import org.directwebremoting.WebContextFactory;
import org.directwebremoting.proxy.dwr.Util;
public class SendMsg {
@SuppressWarnings("deprecation")
public void sendMsg(String msg){
//得到上下文
WebContext contex = WebContextFactory.get();
//得到要推送到的页面 dwr3为项目名称，一定要加上。
Collection<ScriptSession> sessions = contex.getScriptSessionsByPage("/dwr3/showMsg.jsp");
//不知道该怎么解释这个，
Util util = new Util(sessions);
//下面是创建一个javascript脚本，相当于在页面脚本中添加了一句 show(msg);
ScriptBuffer sb = new ScriptBuffer();
sb.appendScript("show(");
sb.appendData(msg);
sb.appendScript(")");
//推送
util.addScript(sb);
}
}

dwr.xml配置

[html] view plain copy

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE dwr PUBLIC
"-//GetAhead Limited//DTD Direct Web Remoting 3.0//EN"
"http://getahead.org/dwr/dwr30.dtd">
<dwr>
<allow>
<create creator="new" javascript="SendMsg">
<param name="class">com.zf.dwr.SendMsg</param>
</create>
</allow>
</dwr>

sendMsg.jsp

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<%@ page language="java" contentType="text/html; charset=utf-8"
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">
<script type="text/javascript" src="dwr/util.js"></script>
<script type="text/javascript" src="dwr/engine.js"></script>
<script type="text/javascript" src="dwr/interface/SendMsg.js"></script>
<script type="text/javascript" src="jquery-1.7.1.js"></script>
<script type="text/javascript">
$(function(){
dwr.engine.setActiveReverseAjax(true);
$("#but").click(function(){
SendMsg.sendMsg($("#msg").val());
});
});
</script>
<title>Insert title here</title>
</head>
<body>
<input type="text" id="msg" />
<input type="button" value="发送" id="but" />
</body>
</html>

showMsg.jsp

[html] view plain copy

<%@ page language="java" contentType="text/html; charset=utf-8"
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">
<script type="text/javascript" src="dwr/util.js"></script>
<script type="text/javascript" src="dwr/engine.js"></script>
<script type="text/javascript" src="jquery-1.7.1.js"></script>
<script type="text/javascript">
$(function(){
//这句话千万不能少，表示允许使用推送技术
dwr.engine.setActiveReverseAjax(true);
});
//这个函数是提供给后台推送的时候调用的
function show(msg){
$("#message").text(msg);
}
</script>
<title>Insert title here</title>
</head>
<body>
<div id="message" style="width: 200px;height: 200px;border: 1px solid red ; text-align: center; padding: 5px;"></div>
</body>
</html>

使用dwr推送技术时需要有WebContext 对象，但是 WebContext 对象需要通过 WebContextFactory.get();获取，而WebContextFactory.get(); 方法只有当页面请求某个java方法时，在该方法中调用WebContextFactory.get(); 才能得到值。否则是取不到值的。

然后通过WebContext 获取指定的页面。再向页面推送消息。

有一个问题是当后来又新开了页面，却收不到消息。

可以使用一个线程，去定时的更新Collection<ScriptSession> 对象

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