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>

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
        
    }

}