Ajax请求session失效该如何解决

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;/** * 登录过滤器 *　　拥有Session是否失效和用户是否登录2个条件判断 *　　如果是ajax请求则设置session超时 * @author Merlin.Ma * */public class LoginFilter implements Filter{  private String redirectUrl = "/login.html";  private String sessionKey = "userName";  @Override  public void destroy() {  }  @Override  public void doFilter(ServletRequest request, ServletResponse response,      FilterChain chain) throws IOException, ServletException {    HttpServletRequest req = (HttpServletRequest) request;    HttpServletResponse rep = (HttpServletResponse) response;    HttpSession session = req.getSession();    if( session == null || session.getAttribute(sessionKey) == null){      //如果判断是 AJAX 请求,直接设置为session超时      if( req.getHeader("x-requested-with") != null && req.getHeader("x-requested-with").equals("XMLHttpRequest") ) {        rep.setHeader("sessionstatus", "timeout");       } else {        rep.sendRedirect( req.getContextPath() + redirectUrl);      }    }else {      chain.doFilter(request, response);    }     }  @Override  public void init(FilterConfig filterConfig) throws ServletException {    String url = filterConfig.getInitParameter("redirectUrl");    String key = filterConfig.getInitParameter("sessionKey");    redirectUrl = url == null? redirectUrl:url;    sessionKey = key == null ? sessionKey : key ;  }}

//全局的ajax访问，处理ajax清求时sesion超时 $.ajaxSetup({  contentType : "application/x-www-form-urlencoded;charset=utf-8",  complete : function(XMLHttpRequest, textStatus) {    var sessionstatus = XMLHttpRequest.getResponseHeader("sessionstatus"); // 通过XMLHttpRequest取得响应头，sessionstatus，    if (sessionstatus == "timeout") {      // 如果超时就处理 ，指定要跳转的页面      window.location.replace("login.html");    }  }});