各位请帮忙看下过滤器的问题

实在是不好意思  跑这上面来问这种问题
只是我搞了好久
在网上也找了好久没找到解决的方法
还是得来啊
这是我的过滤器的代码

package com.yourcompany.tools;
import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.Filter;
import javax.servlet.FilterChain;
import javax.servlet.FilterConfig;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class GuoLvQi  implements Filter {
	/**
	 * Destruction of the servlet. <br>
	 */
	public void doFilter(ServletRequest request,
            ServletResponse response,
            FilterChain chain)throws ServletException, IOException
	{
		HttpServletRequest req = (HttpServletRequest)request;
		HttpServletResponse res= (HttpServletResponse)response; 
		String uri=req.getContextPath()+"/form/renshi.jsp";
		try{
		if(req.getSession().getAttribute("用户名")==null)
			res.sendRedirect(uri);
		//req.getRequestDispatcher(uri).forward(req,response);
		else 
			chain.doFilter(request, response); 
		}catch(Exception ex){ex.printStackTrace();}	}
	public void destroy() {
		 // Just puts "destroy" string in log
		// Put your code here
		
	}
	/**
	 * Initialization of the servlet. <br>
	 *
	 * @throws ServletException if an error occurs
	 */
	public void init() throws ServletException { 
		 
		}
	public boolean test(int arg0) {
		// TODO Auto-generated method stub
		return false;
	}
	public void init(FilterConfig arg0) throws ServletException {
		// TODO Auto-generated method stub	
	} 
}

这下面是我的web.xml关于过滤器的配置

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://java.sun.com/xml/ns/j2ee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" version="2.4" xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee   http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd">
   <filter>
  <filter-name>GuoLvQi</filter-name>
  <filter-class>com.yourcompany.tools.GuoLvQi</filter-class>
 </filter>
  <filter-mapping>
  <filter-name>GuoLvQi</filter-name>
  <url-pattern>*.do</url-pattern>
  </filter-mapping>
  <filter-mapping>
  <filter-name>GuoLvQi</filter-name>
  <url-pattern>*.jsp</url-pattern>
  </filter-mapping>

我现在不想过滤器登录页面
如果过滤的话  那浏览器下面老是显示进入/form/renshi.jsp(登录页面)可老是进不去  就在那里打转
下面是我的过滤条件

if(req.getSession().getAttribute("用户名")==null)
			res.sendRedirect(uri);

如果我把过滤条件注释的话

try{
		//if(req.getSession().getAttribute("用户名")==null)
			//res.sendRedirect(uri);
		//req.getRequestDispatcher(uri).forward(req,response);
		//else 
			chain.doFilter(request, response); 
		}catch(Exception ex){ex.printStackTrace();}

那么所有的页面都能进去了
请问怎么配置web.xml让它对登录页面不过滤呢
请大虾们帮我看下啊  谢谢！！！