会话超时过滤器

package com.authority.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;

public class SessionTimeOutFilter implements Filter
{

    private static final long serialVersionUID = 1L;

    private FilterConfig      filterConfig;

    private String            forward          = "";


     /**
     * 初始化过滤器配置
     */
    public void init(FilterConfig config) throws ServletException
    {
        this.filterConfig = config;
        forward = filterConfig.getInitParameter("logout");
    }

    /**
     * 运行过滤器
     */
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain filterChain) throws IOException,
            ServletException
    {
        HttpServletRequest servletRequest = (HttpServletRequest) request;
        HttpServletResponse servletResponse = (HttpServletResponse) response;
        HttpSession session = servletRequest.getSession();
        //获取当前页面文件名
        String url = servletRequest.getRequestURI();

        url = url.substring(url.lastIndexOf("/") + 1, url.length());

        try
        {
            //排除后台不作权限控制的页面名
            String exclude = "login.do,base.jsp,error.jsp,menubase.jsp,message.jsp"
                    + "redirect.jsp,taglibs.jsp,login.jsp,forward.jsp";
            //当获取url文件名与不做权限验证的页面不同时
            if (exclude.indexOf(url) == -1)
            {
                //获取网站访问根目录
                String accessPath = servletRequest.getContextPath();

                String userid = (String) session.getAttribute("userid");
                //当用户名,和密码不存在或者为空字符串时
                if (userid == null || userid.trim().length() == 0)
                {
                    servletResponse.sendRedirect(accessPath + forward);
                    //将这个请求url放在session中以便登录成功后能自动跳转到该url
                    session.setAttribute("requrl", url);
                    return;
                }
            }
        }
        catch (Exception e)
        {
            e.printStackTrace();
        }
        filterChain.doFilter(request, response);
    }

     /**
     * 销毁
     */
    public void destroy()
    {
    }

}