Struts2访问或添加元素
1、完全脱离ServletAPI
ActionContext acx = ActionContext.getContext();
acx.put("request1", "request");
acx.getApplication().put("application1","application");
acx.getSession().put("session1", "session");
依靠拦截器的核心功能实现Map与request,session,application之间的转化

2、得到HttpRequest,ServletContex,HttpResponse对象
a.借助ServletActionContext
HttpServletRequest request = ServletActionContext.getRequest();
HttpSession session = request.getSession();
ServletContext application = ServletActionContext.getServletContext();
HttpServletResponse response = ServletActionContext.getResponse();
b.DI(IOC)
Action实现ServletRequestAware,ServletResponseAware,ServletContextAware接口
提供HttpServletRequest,HttpServletResponse,ServletContext属性