作为ASP.NET Core请求处理管道的“龙头”的服务器负责监听和接收请求并最终完成对请求的响应。它将原始的请求上下文描述为相应的特性（Feature），并以此将HttpContext上下文创建出来，中间件针对HttpContext上下文的所有操作将借助于这些特性转移到原始的请求上下文上。学习ASP.NET Core框架最有效的方式就是按照它的原理“再造”一个框架，了解服务器的本质最好的手段就是试着自定义一个服务器。现在我们自定义一个真正的服务器。在此之前，我们再来回顾一下表示服务器的IServer接口。[本文节选《ASP.NET Core 6框架揭秘》第18章]
一、IServer
二、请求和响应特性
三、StreamBodyFeature
四、HttpListenerServer
一、IServer
作为服务器的IServer对象利用如下所示的Features属性提供了与自身相关的特性。除了利用StartAsync<TContext>和StopAsync方法启动和关闭服务器之外，它还实现了IDisposable接口，资源的释放工作可以通过实现的Dispose方法来完成。StartAsync<TContext>方法将IHttpApplication<TContext>类型的参数作为处理请求的“应用”，该对象是对中间件管道的封装。从这个意义上讲，服务器就是传输层和这个IHttpApplication<TContext>对象之间的“中介”。
public interface IServer : IDisposable
{IFeatureCollection Features { get; }Task StartAsync<TContext>(IHttpApplication<TContext> application, CancellationToken cancellationToken) where TContext : notnull;Task StopAsync(CancellationToken cancellationToken);
}虽然不同服务器类型的定义方式千差万别，但是背后的模式基本上与下面这个以伪代码定义的服务器类型一致。如下这个Server利用IListener对象来监听和接收请求，该对象是利用构造函数中注入的IListenerFactory工厂根据指定的监听地址创建出来的。StartAsync<TContext>方法从Features特性集合中提取出IServerAddressesFeature特性，并针对它提供的每个监听地址创建一个IListener对象。该方法为每个IListener对象开启一个“接收和处理请求”的循环，循环中的每次迭代都会调用IListener对象的AcceptAsync方法来接收请求，我们利用RequestContext对象来表示请求上下文。
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>方法接下来利用此RequestContext上下文将RequestContextFeature特性创建出来。RequestContextFeature特性类型同时实现了IHttpRequestFeature, IHttpResponseFeature和 IHttpResponseBodyFeature这三个核心接口，我们特性针对这三个接口将特性对象添加到创建的FeatureCollection集合中。特性集合随后作为参数调用IHttpApplication<TContext>的CreateContext方法将TContext上下文创建出来，后者将进一步作为参数调用另一个ProcessRequestAsync方法将请求分发给中间件管道进行处理。待处理结束，IHttpApplication<TContext>对象的DisposeContext方法被调用，创建的TContext上下文承载的资源得以释放。
二、请求和响应特性
接下来我们将采用类似的模式来定义一个基于HttpListener的服务器。提供的HttpListenerServer的思路就是利用自定义特性来封装表示原始请求上下文的HttpListenerContext对象，我们使用HttpRequestFeature和HttpResponseFeature这个两个现成特性。
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) { }
}如果我们使用HttpRequestFeature来描述请求，意味着HttpListener在接受到请求之后需要将请求信息从HttpListenerContext上下文转移到该特性上。如果使用HttpResponseFeature来描述响应，待中间件管道在完成针对请求的处理后，我们还需要将该特性承载的响应数据应用到HttpListenerContext上下文上。
三、StreamBodyFeature
现在我们有了描述请求和响应的两个特性，还需要一个描述响应主体的特性，为此我们定义了如下这个StreamBodyFeature特性类型。StreamBodyFeature直接使用构造函数提供的Stream对象作为响应主体的输出流，并根据该对象创建出Writer属性返回的PipeWriter对象。本着“一切从简”的原则，我们并没有实现用来发送文件的SendFileAsync方法，其他成员也采用最简单的方式进行了实现。
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
在如下这个自定义的HttpListenerServer服务器类型中，与传输层交互的HttpListener体现在_listener字段上。服务器在初始化过程中，它的Features属性返回的IFeatureCollection对象中添加了一个ServerAddressesFeature特性，因为我们需要用它来存放注册的监听地址。实现StartAsync<TContext>方法将监听地址从这个特性中取出来应用到HttpListener对象上。
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;}
}在调用Start方法将HttpListener启动后，StartAsync<TContext>方法开始“请求接收处理”循环。接收到的请求上下文被封装成HttpListenerContext上下文，其承载的请求信息利用CreateRequestFeature方法转移到创建的HttpRequestFeature特性上。StartAsync<TContext>方法创建的“空”HttpResponseFeature对象来描述响应，另一个描述响应主体的StreamBodyFeature特性则根据创建的MemoryStream对象构建而成，意味着中间件管道写入的响应主体的内容将暂存到这个内存流中。我们将这三个特性注册到创建的FeatureCollection集合上，并将后者作为参数调用了IHttpApplication<TContext>对象的CreateContext方法将TContext上下文创建出来。此上下文进一步作为参数调用了IHttpApplication<TContext>对象的ProcessRequestAsync方法，中间件管道得以接管请求。
待中间件管道的处理工作完成后，响应的内容还暂存在两个特性中，我们还需要将它们应用到代表原始HttpListenerContext上下文上。StartAsync<TContext>方法从HttpResponseFeature特性提取出响应状态码和响应报头转移到HttpListenerContext上下文上，然后上述这个MemoryStream对象“拷贝”到HttpListenerContext上下文承载的响应主体输出流中。
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/");我们采用上面的演示程序来检测HttpListenerServer能否正常工作。我们为HttpListenerServer类型创建了一个ServiceDescriptor对象将现有的服务器的服务注册替换掉。在调用WebApplication对象的Run方法时显式指定了具有PathBase（“/foobar”）的监听地址“http://localhost:5000/foobar/”，如图1所示的浏览器以此地址访问应用，会得到我们希望的结果。
图1 HttpListenerServer返回的结果
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