之前的文章记述了从ASP.NET Core Module到KestrelServer的请求处理过程。现在该聊聊如何生成ASP.NET中我们所熟悉的HttpContext。
当KestrelServer启动时，会绑定相应的IP地址，同时在绑定时将加入HttpConnectionMiddleware作为终端连接的中间件。
public async Task StartAsync<TContext>(IHttpApplication<TContext> application, CancellationToken cancellationToken){ ???try ???{ ???????... ???????async Task OnBind(ListenOptions endpoint) ???????{ ???????????// Add the HTTP middleware as the terminal connection middleware ???????????endpoint.UseHttpServer(endpoint.ConnectionAdapters, ServiceContext, application, endpoint.Protocols); ???????????var connectionDelegate = endpoint.Build(); ???????????// Add the connection limit middleware ???????????if (Options.Limits.MaxConcurrentConnections.HasValue) ???????????{ ???????????????connectionDelegate = new ConnectionLimitMiddleware(connectionDelegate, Options.Limits.MaxConcurrentConnections.Value, Trace).OnConnectionAsync; ???????????} ???????????var connectionDispatcher = new ConnectionDispatcher(ServiceContext, connectionDelegate); ???????????var transport = _transportFactory.Create(endpoint, connectionDispatcher); ???????????_transports.Add(transport); ???????????await transport.BindAsync().ConfigureAwait(false); ???????} ???????await AddressBinder.BindAsync(_serverAddresses, Options, Trace, OnBind).ConfigureAwait(false); ???} ???...}public static IConnectionBuilder UseHttpServer<TContext>(this IConnectionBuilder builder, IList<IConnectionAdapter> adapters, ServiceContext serviceContext, IHttpApplication<TContext> application, HttpProtocols protocols){ ???var middleware = new HttpConnectionMiddleware<TContext>(adapters, serviceContext, application, protocols); ???return builder.Use(next => ???{ ???????return middleware.OnConnectionAsync; ???});}当请求抵达此中间件时，在其OnConnectionAsync方法里会创建HttpConnection对象，并通过该对象处理请求。
public async Task OnConnectionAsync(ConnectionContext connectionContext){ ???... ???var connection = new HttpConnection(httpConnectionContext); ???_serviceContext.ConnectionManager.AddConnection(httpConnectionId, connection); ???try ???{ ???????var processingTask = connection.ProcessRequestsAsync(_application); ???????... ???} ???...}ProcessRequestsAsync方法内部会根据HTTP协议的不同创建Http1Connection或者Http2Connection对象，一般为Http1Connection。
public async Task ProcessRequestsAsync<TContext>(IHttpApplication<TContext> httpApplication){ ???try ???{ ???????... ???????lock (_protocolSelectionLock) ???????{ ???????????// Ensure that the connection hasn‘t already been stopped. ???????????if (_protocolSelectionState == ProtocolSelectionState.Initializing) ???????????{ ???????????????switch (SelectProtocol()) ???????????????{ ???????????????????case HttpProtocols.Http1: ???????????????????????// _http1Connection must be initialized before adding the connection to the connection manager ???????????????????????requestProcessor = _http1Connection = CreateHttp1Connection(_adaptedTransport, application); ???????????????????????_protocolSelectionState = ProtocolSelectionState.Selected; ???????????????????????break; ???????????????????case HttpProtocols.Http2: ???????????????????????// _http2Connection must be initialized before yielding control to the transport thread, ???????????????????????// to prevent a race condition where _http2Connection.Abort() is called just as ???????????????????????// _http2Connection is about to be initialized. ???????????????????????requestProcessor = CreateHttp2Connection(_adaptedTransport, application); ???????????????????????_protocolSelectionState = ProtocolSelectionState.Selected; ???????????????????????break; ???????????????????case HttpProtocols.None: ???????????????????????// An error was already logged in SelectProtocol(), but we should close the connection. ???????????????????????Abort(ex: null); ???????????????????????break; ???????????????????default: ???????????????????????// SelectProtocol() only returns Http1, Http2 or None. ???????????????????????throw new NotSupportedException($"{nameof(SelectProtocol)} returned something other than Http1, Http2 or None."); ???????????????} ???????????????_requestProcessor = requestProcessor; ???????????} ???????} ???????if (requestProcessor != null) ???????{ ???????????await requestProcessor.ProcessRequestsAsync(httpApplication); ???????} ???????await adaptedPipelineTask; ???????await _socketClosedTcs.Task; ???} ???...}Http1Connection父类HttpProtocol里的ProcessRequests方法会创建一个Context对象，但这还不是最终要找到的HttpContext。
private async Task ProcessRequests<TContext>(IHttpApplication<TContext> application){ ???// Keep-alive is default for HTTP/1.1 and HTTP/2; parsing and errors will change its value ???_keepAlive = true; ???while (_keepAlive) ???{ ???????... ???????var httpContext = application.CreateContext(this); ???????try ???????{ ???????????KestrelEventSource.Log.RequestStart(this); ???????????// Run the application code for this request ???????????await application.ProcessRequestAsync(httpContext); ???????????if (_ioCompleted == 0) ???????????{ ???????????????VerifyResponseContentLength(); ???????????} ???????} ???????... ???}}在HostingApplication类中会看到HttpContext原来是由HttpContextFactory工厂类生成的。
public Context CreateContext(IFeatureCollection contextFeatures){ ???var context = new Context(); ???var httpContext = _httpContextFactory.Create(contextFeatures); ???_diagnostics.BeginRequest(httpContext, ref context); ???context.HttpContext = httpContext; ???return context;}HttpContextFactory类才是最后的一站。
public HttpContext Create(IFeatureCollection featureCollection){ ???if (featureCollection == null) ???{ ???????throw new ArgumentNullException(nameof(featureCollection)); ???} ???var httpContext = new DefaultHttpContext(featureCollection); ???if (_httpContextAccessor != null) ???{ ???????_httpContextAccessor.HttpContext = httpContext; ???} ???var formFeature = new FormFeature(httpContext.Request, _formOptions); ???featureCollection.Set<IFormFeature>(formFeature); ???return httpContext;}简单理了张流程图总结一下：
生成的HttpContext对象最终传递到IHttpApplication的ProcessRequestAsync方法。之后的事情便是HttpHost与HttpApplication的工作了。
那么费了这么多工夫，所生成的HttpContext究竟有什么用处呢？
先查看MSDN上对它的定义：
Encapsulates all HTTP-specific information about an individual HTTP request.
可以理解为对于每个单独的HTTP请求，其间所创建的HttpContext对象封装了全部所需的HTTP信息。
再看其包含的属性：
public abstract class HttpContext{ ???public abstract IFeatureCollection Features { get; } ???public abstract HttpRequest Request { get; } ???public abstract HttpResponse Response { get; } ???public abstract ConnectionInfo Connection { get; } ???public abstract WebSocketManager WebSockets { get; } ???public abstract AuthenticationManager Authentication { get; } ???public abstract ClaimsPrincipal User { get; set; } ???public abstract IDictionary<object, object> Items { get; set; } ???public abstract IServiceProvider RequestServices { get; set; } ???public abstract CancellationToken RequestAborted { get; set; } ???public abstract string TraceIdentifier { get; set; } ???public abstract ISession Session { get; set; } ???public abstract void Abort();}请求(Request)，响应(Response)，会话(Session)这些与HTTP接触时最常见到的名词，都出现在HttpContext对象中。说明在处理HTTP请求时，若是需要获取这些相关信息，完全可以通过调用其属性而得到。
通过传递一个上下文环境参数，以协助获取各环节处理过程中所需的信息，在各种框架中是十分常见的作法。ASP.NET Core里的用法并无特别的创新，但其实用性还是毋庸置疑的。如果想要构建自己的框架时，不妨多参考下ASP.NET Core里的代码，毕竟它已是一个较成熟的产品，其中有许多值得借鉴的地方。
.NET Core开发日志——HttpContext
原文地址：https://www.cnblogs.com/kenwoo/p/9369637.html