JavaWeb项目设置Session失效时长，失效后自动跳转页面

<!-- 配置session30分钟后过期 -->
 <session-config>
 　　<session-timeout>30</session-timeout>
 </session-config>  

<!-- 配置过滤器，filter-class路径为所编写的过滤器类的路径 -->  
  <filter>  
     <filter-name>loginFilter</filter-name>  
     <filter-class>com.sale.filter.LoginFilter</filter-class>  
  </filter>  
  <filter-mapping>  
     <filter-name>loginFilter</filter-name>  
     <url-pattern>*.action</url-pattern>  
  </filter-mapping>  
 

package com.sale.filter;
import java.io.IOException;  

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.HttpServletRequest;  
import javax.servlet.http.HttpServletResponse;  
import javax.servlet.http.HttpSession;  
/**
 * 过滤器类
 * @author Saffi
 * @date 2018年1月31日
 */

public class LoginFilter implements Filter {  
     @Override
    public void destroy() {  

  
    }  
    @Override
    public void doFilter(ServletRequest req, ServletResponse res,  
            FilterChain chain) throws IOException, ServletException {  
  
        HttpServletRequest httpReq=(HttpServletRequest)req;  
        HttpServletResponse httpRes=(HttpServletResponse)res;  
        HttpSession httpSession=httpReq.getSession();  
        String path = httpReq.getRequestURI();    //当前请求相对url
        String loginUrl = httpReq.getContextPath()+ "/loginout.action";    //登录界面url
        String initUrl = httpReq.getContextPath()+   "loginInit.action";    //初始化界面url
        String userName = (String)httpSession.getAttribute("currentUsername");    //在session中获取当前用户名
        // 登陆页面、初始化页面均无需过滤
        if(loginUrl.equals(path) || initUrl.equals(path)) {
            chain.doFilter(req, res);
            return;
        }
        if(userName==null){  
            httpRes.sendRedirect(loginUrl);
            return;
        }else{  
            chain.doFilter(req, res); 
            return;
        }  
    }  
    @Override
    public void init(FilterConfig arg0) throws ServletException {  

  
    }  
  
}