HttpRequest在整个HttpModule中的生命周期

Http Request开始
                        |
                       HttpModule
                            |
                     HttpModule.BeginRequest()
                        |
                HttpModule.AuthenticateRequest()
                        |
                      HttpModule.AuthorizeRequest()
                        |
                  HttpModule.ResolveRequestCache()
                        |
                    建立HttpHandler控制点
                        |
                接着处理（HttpHandler已经建立,此后Session可用)
                        |
                  HttpModule.AcquireRequestState()
                        |
                HttpModule.PreRequestHandlerExecute()
                        |
                   进入HttpHandler处理HttpRequest
                        |
                    HttpHandler.ProcessRequest()
                        |
            返回到HttpModule接着处理（HttpHandler生命周期结束，Session失效）
                        |
                HttpModule.PostRequestHandlerExecute()
                        |
                HttpModule.ReleaseRequestState()
                        |
                HttpModule.UpdateRequestCache()
                        |
                    HttpModule.EndRequest()
                        |
                HttpModule.PreSendRequestHeaders()
                        |
                HttpModule.PreSendRequestContent()
                        |
                    将处理后的数据返回客户端
                        |
                         整个Http Request处理结束