在设计bs模式的时候，必要的一个listener

response.setHeader("Cache-Control","no-cache"); //Forces caches to obtain a new copy of the page from the origin server
response.setHeader("Cache-Control","no-store"); //Directs caches not to store the page under any circumstance
response.setDateHeader("Expires", 0); //Causes the proxy cache to see the page as "stale"
response.setHeader("Pragma","no-cache"); //HTTP 1.0 backward compatibility

HttpSession session = response.getSession();
String userName = (String) session.getAttribute("User");
if (null == userName) {
   request.setAttribute("Error", "Session has ended.  Please login.");
   RequestDispatcher rd = request.getRequestDispatcher("login.jsp");
   rd.forward(request, response);
}