作爲ASP.NET Core請求處理管道的“龍頭”的服務器負責監聽和接收請求並最終完成對請求的響應。它將原始的請求上下文描述爲相應的特性（Feature），並以此將HttpContext上下文創建出來，中間件針對HttpContext上下文的所有操作將藉助於這些特性轉移到原始的請求上下文上。學習ASP.NET Core框架最有效的方式就是按照它的原理“再造”一個框架，瞭解服務器的本質最好的手段就是試着自定義一個服務器。現在我們自定義一個真正的服務器。在此之前，我們再來回顧一下表示服務器的IServer接口。(本篇提供的實例已經彙總到《ASP.NET Core 6框架揭祕-實例演示版》）
一、IServer
二、請求和響應特性
三、StreamBodyFeature
四、HttpListenerServer
一、IServer
作爲服務器的IServer對象利用如下所示的Features屬性提供了與自身相關的特性。除了利用StartAsync<TContext>和StopAsync方法啓動和關閉服務器之外，它還實現了IDisposable接口，資源的釋放工作可以通過實現的Dispose方法來完成。StartAsync<TContext>方法將IHttpApplication<TContext>類型的參數作爲處理請求的“應用”，該對象是對中間件管道的封裝。從這個意義上講，服務器就是傳輸層和這個IHttpApplication<TContext>對象之間的“中介”。
public interface IServer : IDisposable { IFeatureCollection Features { get; } Task StartAsync<TContext>(IHttpApplication<TContext> application, CancellationToken cancellationToken) where TContext : notnull; Task StopAsync(CancellationToken cancellationToken); }
雖然不同服務器類型的定義方式千差萬別，但是背後的模式基本上與下面這個以僞代碼定義的服務器類型一致。如下這個Server利用IListener對象來監聽和接收請求，該對象是利用構造函數中注入的IListenerFactory工廠根據指定的監聽地址創建出來的。StartAsync<TContext>方法從Features特性集合中提取出IServerAddressesFeature特性，並針對它提供的每個監聽地址創建一個IListener對象。該方法爲每個IListener對象開啓一個“接收和處理請求”的循環，循環中的每次迭代都會調用IListener對象的AcceptAsync方法來接收請求，我們利用RequestContext對象來表示請求上下文。
public class Server : IServer { private readonly IListenerFactory _listenerFactory; private readonly List<IListener> _listeners = new(); public IFeatureCollection Features { get; } = new FeatureCollection(); public Server(IListenerFactory listenerFactory) => _listenerFactory = listenerFactory; public async Task StartAsync<TContext>(IHttpApplication<TContext> application, CancellationToken cancellationToken) where TContext : notnull { var addressFeature = Features.Get<IServerAddressesFeature>()!; foreach (var address in addressFeature.Addresses) { var listener = await _listenerFactory.BindAsync(address); _listeners.Add(listener); _ = StartAcceptLoopAsync(listener); } async Task StartAcceptLoopAsync(IListener listener) { while (true) { var requestContext = await listener.AcceptAsync(); _ = ProcessRequestAsync(requestContext); } } async Task ProcessRequestAsync(RequestContext requestContext) { var feature = new RequestContextFeature(requestContext); var contextFeatures = new FeatureCollection(); contextFeatures.Set<IHttpRequestFeature>(feature); contextFeatures.Set<IHttpResponseFeature>(feature); contextFeatures.Set<IHttpResponseBodyFeature>(feature); var context = application.CreateContext(contextFeatures); Exception? exception = null; try { await application.ProcessRequestAsync(context); } catch (Exception ex) { exception = ex; } finally { application.DisposeContext(context, exception); } } } public Task StopAsync(CancellationToken cancellationToken) => Task.WhenAll(_listeners.Select(listener => listener.StopAsync())); public void Dispose() => _listeners.ForEach(listener => listener.Dispose()); } public interface IListenerFactory { Task<IListener> BindAsync(string listenAddress); } public interface IListener : IDisposable { Task<RequestContext> AcceptAsync(); Task StopAsync(); } public class RequestContext { ... } public class RequestContextFeature : IHttpRequestFeature, IHttpResponseFeature, IHttpResponseBodyFeature { public RequestContextFeature(RequestContext requestContext); ... }
StartAsync<TContext>方法接下來利用此RequestContext上下文將RequestContextFeature特性創建出來。RequestContextFeature特性類型同時實現了IHttpRequestFeature, IHttpResponseFeature和 IHttpResponseBodyFeature這三個核心接口，我們特性針對這三個接口將特性對象添加到創建的FeatureCollection集合中。特性集合隨後作爲參數調用IHttpApplication<TContext>的CreateContext方法將TContext上下文創建出來，後者將進一步作爲參數調用另一個ProcessRequestAsync方法將請求分發給中間件管道進行處理。待處理結束，IHttpApplication<TContext>對象的DisposeContext方法被調用，創建的TContext上下文承載的資源得以釋放。
二、請求和響應特性
接下來我們將採用類似的模式來定義一個基於HttpListener的服務器。提供的HttpListenerServer的思路就是利用自定義特性來封裝表示原始請求上下文的HttpListenerContext對象，我們使用HttpRequestFeature和HttpResponseFeature這個兩個現成特性。
public class HttpRequestFeature : IHttpRequestFeature { public string Protocol { get; set; } public string Scheme { get; set; } public string Method { get; set; } public string PathBase { get; set; } public string Path { get; set; } public string QueryString { get; set; } public string RawTarget { get; set; } public IHeaderDictionary Headers { get; set; } public Stream Body { get; set; } }
public class HttpResponseFeature : IHttpResponseFeature { public int StatusCode { get; set; } public string? ReasonPhrase { get; set; } public IHeaderDictionary Headers { get; set; } public Stream Body { get; set; } public virtual bool HasStarted => false; public HttpResponseFeature() { StatusCode = 200; Headers = new HeaderDictionary(); Body = Stream.Null; } public virtual void OnStarting(Func<object, Task> callback, object state){} public virtual void OnCompleted(Func<object, Task> callback, object state){} }
如果我們使用HttpRequestFeature來描述請求，意味着HttpListener在接受到請求之後需要將請求信息從HttpListenerContext上下文轉移到該特性上。如果使用HttpResponseFeature來描述響應，待中間件管道在完成針對請求的處理後，我們還需要將該特性承載的響應數據應用到HttpListenerContext上下文上。
三、StreamBodyFeature
現在我們有了描述請求和響應的兩個特性，還需要一個描述響應主體的特性，爲此我們定義瞭如下這個StreamBodyFeature特性類型。StreamBodyFeature直接使用構造函數提供的Stream對象作爲響應主體的輸出流，並根據該對象創建出Writer屬性返回的PipeWriter對象。本着“一切從簡”的原則，我們並沒有實現用來發送文件的SendFileAsync方法，其他成員也採用最簡單的方式進行了實現。
public class StreamBodyFeature : IHttpResponseBodyFeature { public Stream Stream { get; } public PipeWriter Writer { get; } public StreamBodyFeature(Stream stream) { Stream = stream; Writer = PipeWriter.Create(Stream); } public Task CompleteAsync() => Task.CompletedTask; public void DisableBuffering() { } public Task SendFileAsync(string path, long offset, long? count, CancellationToken cancellationToken = default)=> throw new NotImplementedException(); public Task StartAsync(CancellationToken cancellationToken = default) => Task.CompletedTask; }
四、HttpListenerServer
在如下這個自定義的HttpListenerServer服務器類型中，與傳輸層交互的HttpListener體現在_listener字段上。服務器在初始化過程中，它的Features屬性返回的IFeatureCollection對象中添加了一個ServerAddressesFeature特性，因爲我們需要用它來存放註冊的監聽地址。實現StartAsync<TContext>方法將監聽地址從這個特性中取出來應用到HttpListener對象上。
public class HttpListenerServer : IServer { private readonly HttpListener _listener = new(); public IFeatureCollection Features { get; } = new FeatureCollection(); public HttpListenerServer() => Features.Set<IServerAddressesFeature>(new ServerAddressesFeature()); public Task StartAsync<TContext>(IHttpApplication<TContext> application,CancellationToken cancellationToken) where TContext : notnull { var pathbases = new HashSet<string>(StringComparer.OrdinalIgnoreCase); var addressesFeature = Features.Get<IServerAddressesFeature>()!; foreach (string address in addressesFeature.Addresses) { _listener.Prefixes.Add(address.TrimEnd('/') + "/"); pathbases.Add(new Uri(address).AbsolutePath.TrimEnd('/')); } _listener.Start(); while (true) { var listenerContext = _listener.GetContext(); _ = ProcessRequestAsync(listenerContext); } async Task ProcessRequestAsync( HttpListenerContext listenerContext) { FeatureCollection features = new(); var requestFeature = CreateRequestFeature(pathbases, listenerContext); var responseFeature = new HttpResponseFeature(); var body = new MemoryStream(); var bodyFeature = new StreamBodyFeature(body); features.Set<IHttpRequestFeature>(requestFeature); features.Set<IHttpResponseFeature>(responseFeature); features.Set<IHttpResponseBodyFeature>(bodyFeature); var context = application.CreateContext(features); Exception? exception = null; try { await application.ProcessRequestAsync(context); var response = listenerContext.Response; response.StatusCode = responseFeature.StatusCode; if (responseFeature.ReasonPhrase is not null) { response.StatusDescription = responseFeature.ReasonPhrase; } foreach (var kv in responseFeature.Headers) { response.AddHeader(kv.Key, kv.Value); } body.Position = 0; await body.CopyToAsync(listenerContext.Response.OutputStream); } catch (Exception ex) { exception = ex; } finally { body.Dispose(); application.DisposeContext(context, exception); listenerContext.Response.Close(); } } } public void Dispose() => _listener.Stop(); private static HttpRequestFeature CreateRequestFeature(HashSet<string> pathbases,HttpListenerContext listenerContext) { var request = listenerContext.Request; var url = request.Url!; var absolutePath = url.AbsolutePath; var protocolVersion = request.ProtocolVersion; var requestHeaders = new HeaderDictionary(); foreach (string key in request.Headers) { requestHeaders.Add(key, request.Headers.GetValues(key)); } var requestFeature = new HttpRequestFeature { Body = request.InputStream, Headers = requestHeaders, Method = request.HttpMethod, QueryString = url.Query, Scheme = url.Scheme, Protocol = $"{url.Scheme.ToUpper()}/{protocolVersion.Major}.{protocolVersion.Minor}" }; var pathBase = pathbases.First(it => absolutePath.StartsWith(it, StringComparison.OrdinalIgnoreCase)); requestFeature.Path = absolutePath[pathBase.Length..]; requestFeature.PathBase = pathBase; return requestFeature; } public Task StopAsync(CancellationToken cancellationToken) { _listener.Stop(); return Task.CompletedTask; } }
在調用Start方法將HttpListener啓動後，StartAsync<TContext>方法開始“請求接收處理”循環。接收到的請求上下文被封裝成HttpListenerContext上下文，其承載的請求信息利用CreateRequestFeature方法轉移到創建的HttpRequestFeature特性上。StartAsync<TContext>方法創建的“空”HttpResponseFeature對象來描述響應，另一個描述響應主體的StreamBodyFeature特性則根據創建的MemoryStream對象構建而成，意味着中間件管道寫入的響應主體的內容將暫存到這個內存流中。我們將這三個特性註冊到創建的FeatureCollection集合上，並將後者作爲參數調用了IHttpApplication<TContext>對象的CreateContext方法將TContext上下文創建出來。此上下文進一步作爲參數調用了IHttpApplication<TContext>對象的ProcessRequestAsync方法，中間件管道得以接管請求。
待中間件管道的處理工作完成後，響應的內容還暫存在兩個特性中，我們還需要將它們應用到代表原始HttpListenerContext上下文上。StartAsync<TContext>方法從HttpResponseFeature特性提取出響應狀態碼和響應報頭轉移到HttpListenerContext上下文上，然後上述這個MemoryStream對象“拷貝”到HttpListenerContext上下文承載的響應主體輸出流中。
using App; using Microsoft.AspNetCore.Hosting.Server; using Microsoft.Extensions.DependencyInjection.Extensions; var builder = WebApplication.CreateBuilder(args); builder.Services.Replace(ServiceDescriptor.Singleton<IServer, HttpListenerServer>()); var app = builder.Build(); app.Run(context => context.Response.WriteAsync("Hello World!")); app.Run("http://localhost:5000/foobar/");
我們採用上面的演示程序來檢測HttpListenerServer能否正常工作。我們爲HttpListenerServer類型創建了一個ServiceDescriptor對象將現有的服務器的服務註冊替換掉。在調用WebApplication對象的Run方法時顯式指定了具有PathBase（“/foobar”）的監聽地址“http://localhost:5000/foobar/”，如圖1所示的瀏覽器以此地址訪問應用，會得到我們希望的結果。
圖1 HttpListenerServer返回的結果