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; }