session的增加读取
session.setAttribute("username", "张三");
String u = (String) session.getAttribute("username");
application(服务器级别的，如果服务关了/或断电了，那么他就会消失)
List onlineUserList = (List) application.getAttribute("onlineUserList");
if(onlineUserList == null)
{
onlineUserList = new ArrayList();
application.setAttribute("onlineUserList", onlineUserList);
}
onlineUserList.add("张三");