关闭浏览器后session不失效 in servlet.

重新学习session，为了使一个session在浏览器关闭仍能使用，看了看response的set-cookie格式。写了servlet测试如下：

/**
	 * @author Zhou Hao, Fri, 20 Jul 2012 20:11:14 EST
	 * 
	 */
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		// This method is used to test the functionality through which session information
		// can be retrieved even if a browser has been closed.
		// This is achieved by setting related HTTP response header, that it, "Set-Cookie".
		// The implementation of getSession() will not specify the age of the cookie (named as JSESSIONID),
		// which means that a programmer has to reset the expiration information manually. 
		
		response.setContentType("text/html;charset=utf-8");
		PrintWriter out = response.getWriter();
		
		HttpSession session = request.getSession(false);
		if(session !=null){
			// if session has been set, just show it.
			out.print(session.getId());
		}else{
			out.print("no session<br/>");
			HttpSession session2 = request.getSession();
			Collection<String> names = response.getHeaderNames();
			for (String name : names) {
				if("Set-Cookie".equals(name)){
					// Get GMT timezone
					TimeZone zone = TimeZone.getTimeZone("GMT");
					// For testing purpose, I just set the expiration time as 2 minute after from now on.
					Date d = new Date(System.currentTimeMillis()+60*1000);
					// Set the date format in Set-Cookie header in accordance with HTTP specification.
					// Example: "Set-Cookie ...;Expires=Wed, 09 Jun 2021 10:18:14 GMT".
					SimpleDateFormat sdf = new SimpleDateFormat("EEE, d MMM yyyy HH:mm:ss z");
					// Set timezone as GMT. 
					//The SimpleDateFormat will convert the default timezone (in this machine is EST)
					// to GMT timezone.
					sdf.setTimeZone(zone);
					// Format the date.
					String date = sdf.format(d);
					// Append the expiration information to response header Set-Cookie
					response.setHeader(name, response.getHeader(name)+"; Expires="+date);
				}
			}
		}	
	}