Jsp通过Filter实现UrlRewriter原理

web.xml文件：

 <?xml version="1.0" encoding="UTF-8"?>
 <web-app version="2.5" xmlns="http://java.sun.com/xml/ns/javaee"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee   http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd">
  <welcome-file-list>
   <welcome-file>index.jsp</welcome-file>
  </welcome-file-list>
     <filter>
         <filter-name>UrlRewriterHelper</filter-name>
         <filter-class>helper.UrlRewriterHelper</filter-class>
     </filter>
     <filter-mapping>
         <filter-name>UrlRewriterHelper</filter-name>
         <url-pattern>/*</url-pattern>
     </filter-mapping>
 </web-app>

web.xml

UrlRewriterHelper文件：

 package helper;

 import java.io.IOException;
 import java.net.HttpRetryException;

 import javax.servlet.Filter;
 import javax.servlet.FilterChain;
 import javax.servlet.FilterConfig;
 import javax.servlet.RequestDispatcher;
 import javax.servlet.ServletException;
 import javax.servlet.ServletRequest;
 import javax.servlet.ServletResponse;
 import javax.servlet.http.HttpServletRequest;

 public class UrlRewriterHelper implements Filter {

     @Override
     public void destroy() {
         // TODO Auto-generated method stub

     }

     @Override
     public void doFilter(ServletRequest arg0, ServletResponse response,
             FilterChain arg2) throws IOException, ServletException {
         // TODO Auto-generated method stub
         if (arg0 instanceof HttpServletRequest) {
             HttpServletRequest request = (HttpServletRequest) arg0;
             String url = request.getRequestURI();
             int beginIndex = url.lastIndexOf(".htm");
             if (beginIndex == url.length() - 4) {
                 String newUrl = url.substring(0, beginIndex) + ".jsp";
                 String contextRoot = request.getContextPath();
                 beginIndex = newUrl.indexOf(contextRoot);
                 if (beginIndex == 0) {
                     newUrl = newUrl.substring(contextRoot.length());
                     RequestDispatcher dispatcher = request.getRequestDispatcher(newUrl);
                     dispatcher.forward(request, response);
                     return;
                 }
             }
         }
         arg2.doFilter(arg0, response);
     }

     @Override
     public void init(FilterConfig arg0) throws ServletException {
         // TODO Auto-generated method stub

     }

 }

UrlRewriterHelper