Asp.Net构架(Http请求处理流程)

Http请求处理流程概述

当用户在浏览器输入一个URL地址后，浏览器会发送一个请求到服务器。这时候在服务器上第一个负责处理请求的是IIS。然后IIS再根据请求的URL扩展名将请求分发给不同的ISAPI处理,然后，IIS 接收返回的数据流，并重新返还给 HTTP.SYS，最后，HTTP.SYS 再将这些数据返回给客户端浏览器。

ISAPI

ISAPI是一个底层的WIN32 API，开发者可以使用这些接口深入到IIS，让IIS支持各种其他处理程序。ISAPI是一个桥接口，通常用于高层次的工具与IIS之间的接驳。例如Windows下的Apache与Tomcat就是构建于ISAPI之上。ISAPI是自定义Web请求处理中第一个IIS入口点。

能够处理各种后缀名的应用程序，通常被称为ISAPI应用程序（NOTE：Internet Server Application Programe Interface，互联网服务器应用程序接口）。虽然这 ISAPI 听上去还挺气派，也算是“应用程序”呢，但仔细看看它的全称就明白了：它实际上只是一个接口，起到一个代理的作用，它的主要工作是映射所请求的页面(文件) 和与此后缀名相对应的实际的处理程序。

Http请求刚刚到达服务器的时候

IIS依赖一个叫做 HTTP.SYS 的内置驱动程序来监听来自外部的 HTTP请求。在操作系统启动的时候，IIS首先在HTTP.SYS中注册自己的虚拟路径。

实际上相当于告诉HTTP.SYS哪些URL是可以访问的，哪些是不可以访问的。为什么你访问不存在的文件会出现 404 错误呢？就是在这一步确定的。

如果请求的是一个可访问的URL，HTTP.SYS会将这个请求交给 IIS 工作者进程。

当服务器接收到一个 Http请求的时候，IIS 如何去处理这个请求,依据什么去处理呢？―― 根据文件的后缀名。对于html页面,txt文件,jpeg和gif图像的请求，IIS就自己处理了，当发现请求是Asp.net的资源时(如.aspx,.asmx,*.ashx)，请求将传递到ASP.NET ISAPI扩展aspnet_isapi.dll。

服务器获取所请求的页面的后缀名以后，接下来会在服务器端寻找可以处理这类后缀名的应用程序，如果IIS找不到可以处理此类文件的应用程序，并且这个文件也没有受到服务器端的保护（NOTE：一个受保护的例子就是 cs文件，一个不受保护的例子就是js脚本），那么IIS将直接把这个文件返还给客户端。

aspnet_isapi.dll可以处理多种资源类型,包括Web服务和HTTP处理程序调用等。

除了以外，ISAPI 还做一些其他的工作：

• 从HTTP.SYS中获取当前的Httq请求信息，并且将这些信息保存到 HttpWorkerRequest 类中。
• 在相互隔离的应用程序域AppDomain中加载HttpRuntime。
• 调用 HttpRuntime的ProcessRequest方法。

IIS与aspnet_isapi的处理逻辑：

• 当请求到达时,IIS检查资源类型并调用aspnet_isapi扩展。如果启用了默认的进程模型。aspnet_isapi会将请求排队。并将请求分配给辅助进程、所有的请求数据都是通过异步I/O发送。
• 收到请求后，ASP.NET辅助进程将通知aspnet_isapi，它将为请求服务(建立管道)。
• 在辅助进程的上下文中执行请求。有时，辅助进程可能需要回调aspnet_isapi以完成请求，也就是需要说枚举服务器变量。这种情况下，辅助进程将使用同步管道，因为这样可以保持请求处理的逻辑顺序。
• 完成后，响应被发送到打开了异步管道的aspnet_isapi。如果aspnet_isapi检测到辅助进程已取消，它将自动终止请求并释放所有相关的IIS资源。

宿主环境（Hosting）与管道（Pipeline）

从本质上讲，Asp.Net 主要是由一系列的类组成，这些类的主要目的就是将Http请求转变为对客户端的响应。

当 Web.config文件的内容发生改变 或者 .aspx文件发生变动的时候，为了能够卸载运行在同一个进程中的应用程序（NOTE：卸载也是为了重新加载），Http请求被分放在相互隔离的应用程序域中(应用程序域就是 AppDomain)。

• Asp.net处理管道的第一步是创建HttpWorkerRequest对象，它包含于当前请求有关的所有信息。
• HttpWorkerRequest把请求传递给HttpRuntime类的静态ProcessRequest方法。HttpRuntime首先要做的事是创建HttpContext对象，并用HttpWorkerRequest进行初始化。

        /// <devdoc>
        ///    <para><SPAN>The method that drives
        ///       all ASP.NET web processing execution.</SPAN></para>
        /// </devdoc>
        [AspNetHostingPermission(SecurityAction.Demand, Level = AspNetHostingPermissionLevel.Medium)]
        public static void ProcessRequest(HttpWorkerRequest wr) {
            if (wr == null)
                throw new ArgumentNullException("wr");

            if (HttpRuntime.UseIntegratedPipeline) {//判断是否为集成管道
                throw new PlatformNotSupportedException(SR.GetString(SR.Method_Not_Supported_By_Iis_Integrated_Mode, "HttpRuntime.ProcessRequest"));
            }

            ProcessRequestNoDemand(wr);
        }
        internal static void ProcessRequestNoDemand(HttpWorkerRequest wr) {
            RequestQueue rq = _theRuntime._requestQueue;

            wr.UpdateInitialCounters();

            if (rq != null)  // could be null before first request
                wr = rq.GetRequestToExecute(wr);

            if (wr != null) {
                CalculateWaitTimeAndUpdatePerfCounter(wr);
                wr.ResetStartTime();
                ProcessRequestNow(wr);
            }
        }
        private void ProcessRequestInternal(HttpWorkerRequest wr) {
            // Count active requests
            Interlocked.Increment(ref _activeRequestCount);

            if (_disposingHttpRuntime) {//当请求数量超出程序处理速度，服务器返回503，服务器繁忙的响应
                // Dev11 333176: An appdomain is unloaded before all requests are served, resulting in System.AppDomainUnloadedException during isapi completion callback
                //
                // HttpRuntim.Dispose could have already finished on a different thread when we had no active requests
                // In this case we are about to start or already started unloading the appdomain so we will reject the request the safest way possible
                try {
                    wr.SendStatus(503, "Server Too Busy");
                    wr.SendKnownResponseHeader(HttpWorkerRequest.HeaderContentType, "text/html; charset=utf-8");
                    byte[] body = Encoding.ASCII.GetBytes("<html><body>Server Too Busy</body></html>");
                    wr.SendResponseFromMemory(body, body.Length);
                    // this will flush synchronously because of HttpRuntime.ShutdownInProgress
                    wr.FlushResponse(true);
                    wr.EndOfRequest();
                } finally {
                    Interlocked.Decrement(ref _activeRequestCount);
                }
                return;
            }

            // Construct the Context on HttpWorkerRequest, hook everything together
            HttpContext context;

            try {
                context = new HttpContext(wr, false /* initResponseWriter */);
            } 
            catch {//HttpContext 组装错误，返回400错误
                try {
                    // If we fail to create the context for any reason, send back a 400 to make sure
                    // the request is correctly closed (relates to VSUQFE3962)
                    wr.SendStatus(400, "Bad Request");
                    wr.SendKnownResponseHeader(HttpWorkerRequest.HeaderContentType, "text/html; charset=utf-8");
                    byte[] body = Encoding.ASCII.GetBytes("<html><body>Bad Request</body></html>");
                    wr.SendResponseFromMemory(body, body.Length);
                    wr.FlushResponse(true);
                    wr.EndOfRequest();
                    return;
                } finally {
                    Interlocked.Decrement(ref _activeRequestCount);
                }
            }

            wr.SetEndOfSendNotification(_asyncEndOfSendCallback, context);

            HostingEnvironment.IncrementBusyCount();

            try {
                // First request initialization
                try {
                    EnsureFirstRequestInit(context);
                }
                catch {
                    // If we are handling a DEBUG request, ignore the FirstRequestInit exception.
                    // This allows the HttpDebugHandler to execute, and lets the debugger attach to
                    // the process (VSWhidbey 358135)
                    if (!context.Request.IsDebuggingRequest) {
                        throw;
                    }
                }

                // Init response writer (after we have config in first request init)
                // no need for impersonation as it is handled in config system
                context.Response.InitResponseWriter();

                // Get application instance
                IHttpHandler app = HttpApplicationFactory.GetApplicationInstance(context);

                if (app == null)
                    throw new HttpException(SR.GetString(SR.Unable_create_app_object));

                if (EtwTrace.IsTraceEnabled(EtwTraceLevel.Verbose, EtwTraceFlags.Infrastructure)) EtwTrace.Trace(EtwTraceType.ETW_TYPE_START_HANDLER, context.WorkerRequest, app.GetType().FullName, "Start");

                if (app is IHttpAsyncHandler) {
                    // asynchronous handler
                    IHttpAsyncHandler asyncHandler = (IHttpAsyncHandler)app;
                    context.AsyncAppHandler = asyncHandler;
                    asyncHandler.BeginProcessRequest(context, _handlerCompletionCallback, context);
                }
                else {
                    // synchronous handler
                    app.ProcessRequest(context);
                    FinishRequest(context.WorkerRequest, context, null);
                }
            }
            catch (Exception e) {
                context.Response.InitResponseWriter();
                FinishRequest(wr, context, e);
            }
        }

• 当Http请求进入 Asp.Net Runtime以后，它的管道由托管模块（NOTE：Managed Modules）和处理程序（NOTE：Handlers）组成，并且由管道来处理这个 Http请求。
• 创建了HttpContext实例之后，HttpRuntime类就通过调用HttpApplicationFactory的静态GetApplicationInstance()方法，为该应用程序请求HttpApplication派生类的一个示例。GetApplicationInstance()方法要么创建一个HttpApplication类的一个新实例，要么从应用程序对象池中取出一个实例。

        internal static IHttpHandler GetApplicationInstance(HttpContext context) {
            if (_customApplication != null)
                return _customApplication;

            // Check to see if it's a debug auto-attach request
            if (context.Request.IsDebuggingRequest)
                return new HttpDebugHandler();

            _theApplicationFactory.EnsureInited();

            _theApplicationFactory.EnsureAppStartCalled(context);

            return _theApplicationFactory.GetNormalApplicationInstance(context);
        }
        private void EnsureInited() {
            if (!_inited) {
                lock (this) {
                    if (!_inited) {
                        Init();
                        _inited = true;
                    }
                }
            }
        }
        //
        // Initialization on first request
        //

        private void Init() {
            if (_customApplication != null)
                return;

            try {
                try {
                    _appFilename = GetApplicationFile();//获取global.asax 地址

                    CompileApplication();//编译应用程序
                }
                finally {
                    // Always set up global.asax file change notification, even if compilation
                    // failed.  This way, if the problem is fixed, the appdomain will be restarted.
                    SetupChangesMonitor();//启动变化监控
                }
            }
            catch { // Protect against exception filters
                throw;
            }
        }

        //
        //  Application instance management
        //

        private HttpApplication GetNormalApplicationInstance(HttpContext context) {
            HttpApplication app = null;

            if (!_freeList.TryTake(out app)) {
                // If ran out of instances, create a new one
                app = (HttpApplication)HttpRuntime.CreateNonPublicInstance(_theApplicationType);

                using (new ApplicationImpersonationContext()) {
                    app.InitInternal(context, _state, _eventHandlerMethods);
                }
            }

            if (AppSettings.UseTaskFriendlySynchronizationContext) {
                // When this HttpApplication instance is no longer in use, recycle it.
                app.ApplicationInstanceConsumersCounter = new CountdownTask(1); // representing required call to HttpApplication.ReleaseAppInstance
                app.ApplicationInstanceConsumersCounter.Task.ContinueWith((_, o) => RecycleApplicationInstance((HttpApplication)o), app, TaskContinuationOptions.ExecuteSynchronously);
            }
            return app;
        }

• 在创建完成HttpApplication实例之后，就对它进行初始化，并在初始化期间分配应用程序定义的所
  有模块。模块式实现IHttpModule接口的类，作用就是为了实现那经典的19个标准处理事件。

        private void CompileApplication() {
            // Get the Application Type and AppState from the global file

            _theApplicationType = BuildManager.GetGlobalAsaxType();

            BuildResultCompiledGlobalAsaxType result = BuildManager.GetGlobalAsaxBuildResult();

            if (result != null) {

                // Even if global.asax was already compiled, we need to get the collections
                // of application and session objects, since they are not persisted when
                // global.asax is compiled.  Ideally, they would be, but since <object> tags
                // are only there for ASP compat, it's not worth the trouble.
                // Note that we only do this is the rare case where we know global.asax contains
                // <object> tags, to avoid always paying the price (VSWhidbey 453101)
                if (result.HasAppOrSessionObjects) {
                    GetAppStateByParsingGlobalAsax();
                }

                // Remember file dependencies
                _fileDependencies = result.VirtualPathDependencies;
            }

            if (_state == null) {
                _state = new HttpApplicationState();
            }


            // Prepare to hookup event handlers via reflection

            ReflectOnApplicationType();
        }
        private void ReflectOnApplicationType() {//这个方法为应用程序注册事件
            ArrayList handlers = new ArrayList();
            MethodInfo[] methods;

            Debug.Trace("PipelineRuntime", "ReflectOnApplicationType");

            // get this class methods
            methods = _theApplicationType.GetMethods(BindingFlags.NonPublic | BindingFlags.Public | BindingFlags.Instance | BindingFlags.Static);
            foreach (MethodInfo m in methods) {
                if (ReflectOnMethodInfoIfItLooksLikeEventHandler(m))
                    handlers.Add(m);
            }
            
            // get base class private methods (GetMethods would not return those)
            Type baseType = _theApplicationType.BaseType;
            if (baseType != null && baseType != typeof(HttpApplication)) {
                methods = baseType.GetMethods(BindingFlags.NonPublic | BindingFlags.Instance | BindingFlags.Static);
                foreach (MethodInfo m in methods) {
                    if (m.IsPrivate && ReflectOnMethodInfoIfItLooksLikeEventHandler(m))
                        handlers.Add(m);
                }
            }

            // remember as an array
            _eventHandlerMethods = new MethodInfo[handlers.Count];
            for (int i = 0; i < _eventHandlerMethods.Length; i++)
                _eventHandlerMethods[i] = (MethodInfo)handlers[i];
        }
        private bool ReflectOnMethodInfoIfItLooksLikeEventHandler(MethodInfo m) {
            if (m.ReturnType != typeof(void))
                return false;

            // has to have either no args or two args (object, eventargs)
            ParameterInfo[] parameters = m.GetParameters();

            switch (parameters.Length) {
                case 0:
                    // ok
                    break;
                case 2:
                    // param 0 must be object
                    if (parameters[0].ParameterType != typeof(System.Object))
                        return false;
                    // param 1 must be eventargs
                    if (parameters[1].ParameterType != typeof(System.EventArgs) &&
                        !parameters[1].ParameterType.IsSubclassOf(typeof(System.EventArgs)))
                        return false;
                    // ok
                    break;

                default:
                    return false;
            }

            // check the name (has to have _ not as first or last char)
            String name = m.Name;
            int j = name.IndexOf('_');
            if (j <= 0 || j > name.Length-1)
                return false;

            // special pseudo-events
            if (StringUtil.EqualsIgnoreCase(name, "Application_OnStart") ||
                StringUtil.EqualsIgnoreCase(name, "Application_Start")) {
                _onStartMethod = m;
                _onStartParamCount = parameters.Length;
            }
            else if (StringUtil.EqualsIgnoreCase(name, "Application_OnEnd") ||
                     StringUtil.EqualsIgnoreCase(name, "Application_End")) {
                _onEndMethod = m;
                _onEndParamCount = parameters.Length;
            }
            else if (StringUtil.EqualsIgnoreCase(name, "Session_OnEnd") ||
                     StringUtil.EqualsIgnoreCase(name, "Session_End")) {
                _sessionOnEndMethod = m;
                _sessionOnEndParamCount = parameters.Length;
            }

            return true;
        }
        private void EnsureAppStartCalled(HttpContext context) {
            if (!_appOnStartCalled) {
                lock (this) {
                    if (!_appOnStartCalled) {
                        using (new DisposableHttpContextWrapper(context)) {
                            // impersonation could be required (UNC share or app credentials)

                            WebBaseEvent.RaiseSystemEvent(this, WebEventCodes.ApplicationStart);

                            // fire outside of impersonation as HttpApplication logic takes
                            // care of impersonation by itself
                            FireApplicationOnStart(context);
                        }

                        _appOnStartCalled = true;
                    }
                }
            }
        }
        //
        //  Application on_start 
        //

        private void FireApplicationOnStart(HttpContext context) {
            if (_onStartMethod != null) {
                HttpApplication app = GetSpecialApplicationInstance();

                app.ProcessSpecialRequest(
                                         context,
                                         _onStartMethod,
                                         _onStartParamCount,
                                         this, 
                                         EventArgs.Empty, 
                                         null);//执行前面注册的启动事件_onStartMethod

                RecycleSpecialApplicationInstance(app);
            }
        }
        private HttpApplication GetSpecialApplicationInstance() {
            return GetSpecialApplicationInstance(IntPtr.Zero, null);
        }
        private HttpApplication GetSpecialApplicationInstance(IntPtr appContext, HttpContext context) {
            HttpApplication app = null;

            if (!_specialFreeList.TryTake(out app)) {
                //
                //  Put the context on the thread, to make it available to anyone calling
                //  HttpContext.Current from the HttpApplication constructor or module Init
                //
                using (new DisposableHttpContextWrapper(context)) {
                    // If ran out of instances, create a new one
                    app = (HttpApplication)HttpRuntime.CreateNonPublicInstance(_theApplicationType);

                    using (new ApplicationImpersonationContext()) {
                        app.InitSpecial(_state, _eventHandlerMethods, appContext, context);
                    }
                }
            }

            return app;
        }

• 在创建了模块之后，HttpRuntime类通过调用它的BeginProcessRequest方法，要求最新检索到的HttpApplication类对当前请求提供服务。然后，为当前请求找到合适的处理程序工厂。
  创建处理程序，传递当前HttpContext，一旦ProcessRequest方法返回，请求完成。

本文参考文档

• .NET Framework源码：https://referencesource.microsoft.com/download.html
• https://www.cnblogs.com/kissdodog/p/3527379.html
• https://www.cnblogs.com/kissdodog/p/3527871.html
• https://www.cnblogs.com/JimmyZhang/archive/2007/09/04/880967.html
• https://www.cnblogs.com/JimmyZhang/archive/2007/09/15/894124.html
• https://www.cnblogs.com/JimmyZhang/archive/2007/11/25/971878.html