java Servlet Filter 拦截Ajax请求

/**
 * 版权：Copyright 2016-2016 AudaqueTech. Co. Ltd. All Rights Reserved.
 * 描述：
 * 创建人：赵巍
 * 创建时间：2016年11月28日
 * 修改人：
 * 修改时间：
 * 修改内容：    
 */
package com.thinkgem.jeesite.common.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;

import org.apache.shiro.SecurityUtils;
import org.apache.shiro.subject.Subject;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

public class CustomFilter implements Filter
{
    private final Logger logger = LoggerFactory.getLogger(getClass());
    private String failureUrl;

    @Override
    public void init(FilterConfig filterConfig) throws ServletException {
        //empty implement
    }

    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain)
            throws IOException, ServletException {
        HttpServletRequest httpRequest = (HttpServletRequest) request;
        HttpServletResponse httpResponse = (HttpServletResponse) response;
        HttpSession session = httpRequest.getSession();

       // String userid = (String)session.getAttribute("userid");
        Subject subject = SecurityUtils.getSubject();

       /* if(uri.contains("/user/login") || uri.contains("/user/regPage") ){
            chain.doFilter(request, response);
        }else{ */ 
            if(subject.getPrincipals() == null){
                String contextPath = httpRequest.getContextPath();
                String redirect =  contextPath ;
                //ajax session 过期处理
                //1:判断是否是ajax请求
                if (httpRequest.getHeader("x-requested-with") != null 
                        && "XMLHttpRequest".equalsIgnoreCase(httpRequest.getHeader("x-requested-with"))) {   
                    //向http头添加 状态 sessionstatus
                    httpResponse.setHeader("sessionstatus","timeout");
                    httpResponse.sendError(518, "session timeout."); 
                    /*httpResponse.setStatus(403);*/
                    //向http头添加登录的url
                    httpResponse.addHeader("loginPath", failureUrl);
                    chain.doFilter(request, response);
                    logger.debug("ajax request");
                    return ;
                }
                httpResponse.sendRedirect(redirect);
                return;
            }else{
                chain.doFilter(request, response);
            }
        /*}*/
    }

    @Override
    public void destroy() {
        //empty implement
    }

    public String getFailureUrl() {
        return failureUrl;
    }

    public void setFailureUrl(String failureUrl) {
        this.failureUrl = failureUrl;
    }

}

$.ajax({
                       type: "get",
                       url: $(this).attr("data-href"),
                       success: function(data){
                           //
                       },
                       complete: function(xhr,status){
                           var sessionStatus = xhr.getResponseHeader('sessionstatus');
                           if(sessionStatus == 'timeout') {
                               top.location = "${ctx}";
                               return false;
                           }
                       }
                    });
}