struts2的拦截器拦截ajax请求

拦截器代码：
@Override
public String intercept(ActionInvocation invocation) throws Exception {

ActionContext ctx = invocation.getInvocationContext();
Map session = ctx.getSession();
User user = (User) session.get(Constants.SESSION_USER_KEY);
if (user == null) {
HttpServletRequest request = ServletActionContext.getRequest();
HttpServletResponse response = ServletActionContext.getResponse();
PrintWriter pw = response.getWriter();

String flag = "";

//对ajax请求的拦截

if (request.getHeader("X-Requested-With") != null
&& request.getHeader("X-Requested-With").equalsIgnoreCase(     
"XMLHttpRequest")) {  
log.info("用户没登录或登录过期，不能访问");
response.setCharacterEncoding("text/html;charset=utf-8");
response.setContentType("text/html;charset=utf-8");
flag = "9999";
pw.write(flag);

return null;

//不是异步请求的拦截

} else {
response.setCharacterEncoding("text/html;charset=utf-8");
response.sendRedirect("/businessTest/login.jsp");
log.info("用户没登录或登录过期，不能访问");
return "login";
}
}
return invocation.invoke();

}

jsp页面代码

// 登录验证 
Ext.Ajax.on('requestcomplete',checkUserSessionStatus, this);   //所有的异步请求都会走的事件
function checkUserSessionStatus(conn,response,options){
var str = response.responseText;
if(str == '9999'){
alert('连接已超时,请重新登录!');
window.location.href = 'login.jsp';
}
}