DWR运行机制简析及简单实例的实现

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  首先，我们讨论什么是真正的DWR。DWR是Direct Web Remoting的缩写，是Java和JavaScript相结合的开源库。通过它，可以简单、容易地构建Ajax程序，无需深入了解XMLRequest的编码 细节。它容许你通过客户端的JavaScript，采用看似调用浏览器本地代码的方法来调用服务器端的代码。这就是DWR的直接含义。相对DWR提高的各种强大功能而言，其体系结构却非常简单。 下面一幅精确的图概要说明了DWR的运行机制。

在图的左侧有一个evenHandler()函数，他调用了AjaxService对象中的getOption()方法。这段代码看上去是执行一段位于本地JavaScript对象的方法，但事实上却是调用服务器上运行的 一个Java对象的方法。看代码时，你无法识别这一点，我认为这是DWR的一大优点。 传送给方法populateList()的参数是另一个JavaScript函数，这是回调函数。它会在远程调用结果返回时 执行，因此带箭头的循环说明双向数据流动。
      下面我们来实现一个简单的DWR实例——基本四则运算。 首先应先配置：到官网下载页面可以发现3个文件，分别是dwr.jar、DWR完整源代码、和dwr.war。dwr.jar是最简洁且必须的，如果时间充裕的话，也许希望将dwr.war下载下来。该程序 是DWR的一个小的范例程序，演示了许多DWR功能（包括聊天程序、基于邮政编码自动调入详细地址信息并进行快速地址查询、采用GI的实时股票报价等）。
      将dwr.jar，放到工程的WEB-INF/lib目录下,让后配置web.xml,加入以下内容：

<ol style="margin-top: 0px; margin-right: 0px; margin-bottom: 1px; margin-left: 0px; padding-top: 2px; padding-right: 0px; padding-bottom: 2px; padding-left: 0px; background-color: #ffffff; border: 1px solid #d1d7dc;" class="dp-xml"><li style="font-size: 1em; margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 38px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 10px; border-left-width: 1px; border-left-style: solid; border-left-color: #d1d7dc; background-color: #fafafa; line-height: 18px; color: #2b91af; font-family: Monaco, 'DejaVu Sans Mono', 'Bitstream Vera Sans Mono', Consolas, 'Courier New', monospace;"><span style="font-size: small;"><?xml version="1.0" encoding="UTF-8"?>    <li style="font-size: 1em; margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 38px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 10px; border-left-width: 1px; border-left-style: solid; border-left-color: #d1d7dc; background-color: #fafafa; line-height: 18px; color: #2b91af; font-family: Monaco, 'DejaVu Sans Mono', 'Bitstream Vera Sans Mono', Consolas, 'Courier New', monospace;"><!DOCTYPE dwr PUBLIC "-//GetAhead Limited//DTD Direct Web Remoting 2.0//EN" " http://getahead.org/dwr/dwr20.dtd">    <li style="font-size: 1em; margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 38px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 10px; border-left-width: 1px; border-left-style: solid; border-left-color: #d1d7dc; background-color: #fafafa; line-height: 18px; color: #2b91af; font-family: Monaco, 'DejaVu Sans Mono', 'Bitstream Vera Sans Mono', Consolas, 'Courier New', monospace;">    <li style="font-size: 1em; margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 38px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 10px; border-left-width: 1px; border-left-style: solid; border-left-color: #d1d7dc; background-color: #fafafa; line-height: 18px; color: #2b91af; font-family: Monaco, 'DejaVu Sans Mono', 'Bitstream Vera Sans Mono', Consolas, 'Courier New', monospace;"><dwr>    <li style="font-size: 1em; margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 38px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 10px; border-left-width: 1px; border-left-style: solid; border-left-color: #d1d7dc; background-color: #fafafa; line-height: 18px; color: #2b91af; font-family: Monaco, 'DejaVu Sans Mono', 'Bitstream Vera Sans Mono', Consolas, 'Courier New', monospace;">    <li style="font-size: 1em; margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 38px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 10px; border-left-width: 1px; border-left-style: solid; border-left-color: #d1d7dc; background-color: #fafafa; line-height: 18px; color: #2b91af; font-family: Monaco, 'DejaVu Sans Mono', 'Bitstream Vera Sans Mono', Consolas, 'Courier New', monospace;">  <allow>    <li style="font-size: 1em; margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 38px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 10px; border-left-width: 1px; border-left-style: solid; border-left-color: #d1d7dc; background-color: #fafafa; line-height: 18px; color: #2b91af; font-family: Monaco, 'DejaVu Sans Mono', 'Bitstream Vera Sans Mono', Consolas, 'Courier New', monospace;">    <li style="font-size: 1em; margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 38px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 10px; border-left-width: 1px; border-left-style: solid; border-left-color: #d1d7dc; background-color: #fafafa; line-height: 18px; color: #2b91af; font-family: Monaco, 'DejaVu Sans Mono', 'Bitstream Vera Sans Mono', Consolas, 'Courier New', monospace;">    <!-- intro - for the test on index.html -->    <li style="font-size: 1em; margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 38px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 10px; border-left-width: 1px; border-left-style: solid; border-left-color: #d1d7dc; background-color: #fafafa; line-height: 18px; color: #2b91af; font-family: Monaco, 'DejaVu Sans Mono', 'Bitstream Vera Sans Mono', Consolas, 'Courier New', monospace;">    <create creator="new" javascript="MathDelegate">    <li style="font-size: 1em; margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 38px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 10px; border-left-width: 1px; border-left-style: solid; border-left-color: #d1d7dc; background-color: #fafafa; line-height: 18px; color: #2b91af; font-family: Monaco, 'DejaVu Sans Mono', 'Bitstream Vera Sans Mono', Consolas, 'Courier New', monospace;">      <param name="class" value="cn.dwr.test.MathDelegate"/>    <li style="font-size: 1em; margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 38px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 10px; border-left-width: 1px; border-left-style: solid; border-left-color: #d1d7dc; background-color: #fafafa; line-height: 18px; color: #2b91af; font-family: Monaco, 'DejaVu Sans Mono', 'Bitstream Vera Sans Mono', Consolas, 'Courier New', monospace;">    </create>    <li style="font-size: 1em; margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 38px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 10px; border-left-width: 1px; border-left-style: solid; border-left-color: #d1d7dc; background-color: #fafafa; line-height: 18px; color: #2b91af; font-family: Monaco, 'DejaVu Sans Mono', 'Bitstream Vera Sans Mono', Consolas, 'Courier New', monospace;">    <li style="font-size: 1em; margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 38px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 10px; border-left-width: 1px; border-left-style: solid; border-left-color: #d1d7dc; background-color: #fafafa; line-height: 18px; color: #2b91af; font-family: Monaco, 'DejaVu Sans Mono', 'Bitstream Vera Sans Mono', Consolas, 'Courier New', monospace;">  </allow>    <li style="font-size: 1em; margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 38px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 10px; border-left-width: 1px; border-left-style: solid; border-left-color: #d1d7dc; background-color: #fafafa; line-height: 18px; color: #2b91af; font-family: Monaco, 'DejaVu Sans Mono', 'Bitstream Vera Sans Mono', Consolas, 'Courier New', monospace;">    <li style="font-size: 1em; margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 38px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 10px; border-left-width: 1px; border-left-style: solid; border-left-color: #d1d7dc; background-color: #fafafa; line-height: 18px; color: #2b91af; font-family: Monaco, 'DejaVu Sans Mono', 'Bitstream Vera Sans Mono', Consolas, 'Courier New', monospace;"></dwr>    </span>  <li style="font-size: 1em; margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 38px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 10px; border-left-width: 1px; border-left-style: solid; border-left-color: #d1d7dc; background-color: #fafafa; line-height: 18px; color: #2b91af; font-family: Monaco, 'DejaVu Sans Mono', 'Bitstream Vera Sans Mono', Consolas, 'Courier New', monospace;">   然后在工程中建立一个Java类<li style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 38px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 10px; border-left-width: 1px; border-left-style: solid; border-left-color: #d1d7dc; background-color: #fafafa;"><span style="font-family: Arial, sans-serif, Helvetica, Tahoma; font-size: x-small;"><span style="line-height: 19px;"><span style="font-family: Monaco, 'DejaVu Sans Mono', 'Bitstream Vera Sans Mono', Consolas, 'Courier New', monospace; line-height: 18px; font-size: 12px;">

• <span style="font-size: small;">package cn.dwr.test;     
  
• /**   
  
•  * 简单的加减乘除四则运算类，用于测试第一个DWR   
  
•  * @author Sephiroth   
  
•  *   
  
•  */    
  
• public class MathDelegate {     
  
•     
  
• /**   
  
•  * 两个数相加   
  
•  */    
  
•     public int add(int a,int b){     
  
•         return a+b;     
  
•     }     
  
•          
  
•     /**   
  
•      * 两个数相减   
  
•      */    
  
•     public int subtract(int a,int b){     
  
•         return a-b;     
  
•     }     
  
•          
  
•     /**   
  
•      * 两个数相乘   
  
•      */    
  
•     public int multiply(int a,int b){     
  
•         return a*b;     
  
•     }     
  
•          
  
•     /**   
  
•      * 两个数相除   
  
•      */    
  
•     public int divide(int a,int b){     
  
•         if(b!=0){     
  
•             return a/b;     
  
•         }else{     
  
•             return 0;     
  
•         }     
  
•     }     
  
• }  </span>  
  

<span style="font-family: Arial, sans-serif, Helvetica, Tahoma;"> 细心的你们也许发现这端代码不够稳健，之排查了除0的错误，这样做也是为了示例代码的简洁而已。

        接着在WEB-INF目录下，新建dwr.xml（具体的配置文件详解请参照网上相关介绍）：

<span style="font-size: small;"><span style="font-family: Monaco, 'DejaVu Sans Mono', 'Bitstream Vera Sans Mono', Consolas, 'Courier New', monospace; font-size: 12px;"><ol style="font-size: 1em; line-height: 1.4em; margin-top: 0px; margin-right: 0px; margin-bottom: 1px; margin-left: 0px; padding-top: 2px; padding-right: 0px; padding-bottom: 2px; padding-left: 0px; background-color: #ffffff; color: #2b91af; border: 1px solid #d1d7dc;" class="dp-xml"><li style="font-size: 1em; margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 38px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 10px; border-left-width: 1px; border-left-style: solid; border-left-color: #d1d7dc; background-color: #fafafa; line-height: 18px;"><span style="font-size: small;"><?xml version="1.0" encoding="UTF-8"?>    <li style="font-size: 1em; margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 38px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 10px; border-left-width: 1px; border-left-style: solid; border-left-color: #d1d7dc; background-color: #fafafa; line-height: 18px;"><!DOCTYPE dwr PUBLIC "-//GetAhead Limited//DTD Direct Web Remoting 2.0//EN" "http://getahead.org/dwr/dwr20.dtd">    <li style="font-size: 1em; margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 38px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 10px; border-left-width: 1px; border-left-style: solid; border-left-color: #d1d7dc; background-color: #fafafa; line-height: 18px;">    <li style="font-size: 1em; margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 38px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 10px; border-left-width: 1px; border-left-style: solid; border-left-color: #d1d7dc; background-color: #fafafa; line-height: 18px;"><dwr>    <li style="font-size: 1em; margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 38px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 10px; border-left-width: 1px; border-left-style: solid; border-left-color: #d1d7dc; background-color: #fafafa; line-height: 18px;">    <li style="font-size: 1em; margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 38px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 10px; border-left-width: 1px; border-left-style: solid; border-left-color: #d1d7dc; background-color: #fafafa; line-height: 18px;">  <allow>    <li style="font-size: 1em; margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 38px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 10px; border-left-width: 1px; border-left-style: solid; border-left-color: #d1d7dc; background-color: #fafafa; line-height: 18px;">    <li style="font-size: 1em; margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 38px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 10px; border-left-width: 1px; border-left-style: solid; border-left-color: #d1d7dc; background-color: #fafafa; line-height: 18px;">    <!-- intro - for the test on index.html -->    <li style="font-size: 1em; margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 38px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 10px; border-left-width: 1px; border-left-style: solid; border-left-color: #d1d7dc; background-color: #fafafa; line-height: 18px;">    <create creator="new" javascript="MathDelegate">    <li style="font-size: 1em; margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 38px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 10px; border-left-width: 1px; border-left-style: solid; border-left-color: #d1d7dc; background-color: #fafafa; line-height: 18px;">      <param name="class" value="cn.dwr.test.MathDelegate"/>    <li style="font-size: 1em; margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 38px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 10px; border-left-width: 1px; border-left-style: solid; border-left-color: #d1d7dc; background-color: #fafafa; line-height: 18px;">    </create>    <li style="font-size: 1em; margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 38px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 10px; border-left-width: 1px; border-left-style: solid; border-left-color: #d1d7dc; background-color: #fafafa; line-height: 18px;">    <li style="font-size: 1em; margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 38px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 10px; border-left-width: 1px; border-left-style: solid; border-left-color: #d1d7dc; background-color: #fafafa; line-height: 18px;">  </allow>    <li style="font-size: 1em; margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 38px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 10px; border-left-width: 1px; border-left-style: solid; border-left-color: #d1d7dc; background-color: #fafafa; line-height: 18px;">    <li style="font-size: 1em; margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 38px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 10px; border-left-width: 1px; border-left-style: solid; border-left-color: #d1d7dc; background-color: #fafafa; line-height: 18px;"></dwr>    <li style="font-size: 1em; margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 38px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 10px; border-left-width: 1px; border-left-style: solid; border-left-color: #d1d7dc; background-color: #fafafa; line-height: 18px;"></span>  <li style="font-size: 1em; margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 38px; padding-top: 0px; padding-right: 0px; padding-bottom: 0px; padding-left: 10px; border-left-width: 1px; border-left-style: solid; border-left-color: #d1d7dc; background-color: #fafafa; line-height: 18px;"><span style="color: black;"><span style="color: black;"><span style="font-family: Arial, sans-serif, Helvetica, Tahoma;">该文件目前在这实例中只完成一个任务，就是告诉DWR哪些类可以被远程访问，本例仅有MathDelegate类。可以为用于表示服务器端对象的JavaScript对象重新指定名称，但是通常情况二者取同名，本例就是如此。
   
      最后，附上本示例的唯一而简洁的页面index.html代码：
<div style="font-family: Monaco, 'DejaVu Sans Mono', 'Bitstream Vera Sans Mono', Consolas, 'Courier New', monospace; font-size: 12px; background-color: transparent; width: 694px; margin-left: 9px; padding: 1px;" class="dp-highlighter"><div class="bar"><div style="text-align: left; color: black; font-weight: bold; padding: 3px; margin: 0px;" class="tools">Html代码