.NET Core開發日志——HttpContext

之前的文章記述了從ASP.NET Core Module到KestrelServer的請求處理過程。現在該聊聊如何生成ASP.NET中我們所熟悉的HttpContext。

當KestrelServer啟動時，會綁定相應的IP地址，同時在綁定時將加入HttpConnectionMiddleware作為終端連接的中間件。

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;
    });
}

當請求抵達此中間件時，在其OnConnectionAsync方法里會創建HttpConnection對象，並通過該對象處理請求。

public async Task OnConnectionAsync(ConnectionContext connectionContext)
{
    ...

    var connection = new HttpConnection(httpConnectionContext);
    _serviceContext.ConnectionManager.AddConnection(httpConnectionId, connection);

    try
    {
        var processingTask = connection.ProcessRequestsAsync(_application);

        ...
    }
    ...
}

ProcessRequestsAsync方法內部會根據HTTP協議的不同創建Http1Connection或者Http2Connection對象，一般為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父類HttpProtocol里的ProcessRequests方法會創建一個Context對象，但這還不是最終要找到的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類中會看到HttpContext原來是由HttpContextFactory工廠類生成的。

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類才是最后的一站。

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;
}

簡單理了張流程圖總結一下：

生成的HttpContext對象最終傳遞到IHttpApplication的ProcessRequestAsync方法。之后的事情便是WebHost與HostingApplication的工作了。

那么費了這么多工夫，所生成的HttpContext究竟有什么用處呢？

先查看MSDN上對它的定義：

Encapsulates all HTTP-specific information about an individual HTTP request.

可以理解為對於每個單獨的HTTP請求，其間所創建的HttpContext對象封裝了全部所需的HTTP信息。

再看其包含的屬性：

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();
}

請求(Request)，響應(Response)，會話(Session)這些與HTTP接觸時最常見到的名詞，都出現在HttpContext對象中。說明在處理HTTP請求時，若是需要獲取這些相關信息，完全可以通過調用其屬性而得到。

通過傳遞一個上下文環境參數，以協助獲取各環節處理過程中所需的信息，在各種框架中是十分常見的作法。ASP.NET Core里的用法並無特別的創新，但其實用性還是毋庸置疑的。如果想要構建自己的框架時，不妨多參考下ASP.NET Core里的代碼，畢竟它已是一個較成熟的產品，其中有許多值得借鑒的地方。