文章出處

我們在上面對ASP.NET Core默認提供的具有跨平臺能力的KestrelServer進行了詳細介紹（《聊聊ASP.NET Core默認提供的這個跨平臺的服務器——KestrelServer》），為了讓讀者朋友們對管道中的Server具有更加深刻的認識，接下來我們采用實例演示的形式創建一個自定義的Server。這個自定義的Server直接利用HttpListener來完成針對請求的監聽、接收和響應，我們將其命名為HttpListenerServer。在正式介紹HttpListenerServer的設計和實現之前，我們先來顯示一下如何將它應用到一個具體的Web應用中。

一、HttpListenerServer的使用

我們依然采用最簡單的Hello World應用來演示針對HttpListenerServer的應用，所以我們在Startup類的Configure方法中編寫如下的程序直接響應一個“Hello World”字符串。

   1: public class Startup

   2: {

   3:     public void Configure(IApplicationBuilder app)

   4:     {

   5:         app.Run(context => context.Response.WriteAsync("Hello World!"));

   6:     }

   7: }

在作為程序入口的Main方法中，我們直接創建一個WebHostBuilder對象并調用擴展方法UseHttpListener完成針對自定義HttpListenerServer的注冊。我們接下來調用UseStartup方法注冊上面定義的這個啟動類型，然后調用Build方法創建一個WebHost對象，最后調用Run方法運行這個作為宿主的WebHost。

   1: public class Program

   2: {

   3:     public static void Main()

   4:     {

   5:         new WebHostBuilder()

   6:             .UseHttpListener()

   7:             .UseStartup<Startup>()

   8:             .Build()

   9:             .Run();

  10:     }

  11: }

12:

  13: public static class WebHostBuilderExtensions

  14: {

  15:     public static IWebHostBuilder UseHttpListener(this IWebHostBuilder builder)

  16:     {

  17:         builder.ConfigureServices(services => services.AddSingleton<IServer, HttpListenerServer>());

  18:         return builder;

  19:     }

  20: }

我們自定義的擴展方法UseHttpListener的邏輯很簡單，它只是調用WebHostBuilder的ConfigureServices方法將我們自定義的HttpListenerServer類型以單例模式注冊到指定的ServiceCollection上而已。我們直接運行這個程序并利用瀏覽器訪問默認的監聽地址（http://localhost:5000），服務端響應的“Hello World”字符串會按照如下圖所示的形式顯示在瀏覽器上。

二、總體設計

接下來我們來介紹一下HttpListenerServer的大體涉及。除了HttpListenerServer這個實現了IServer的自定義Server類型之外，我們只定義了一個名為HttpListenerServerFeature的特性類型，下圖所示的UML基本上體現了HttpListenerServer的總體設計。

三、HttpListenerServerFeature

如果我們利用HttpListener來監聽請求，它會為接收到的每次請求創建一個屬于自己的上下文，具體來說這是一個類型為HttpListenerContext對象。我們可以利用這個HttpListenerContext對象獲取所有與請求相關的信息，針對請求的任何響應也都是利用它完成的。上面這個HttpListenerServerFeature實際上就是對這個作為原始上下文的HttpListenerContext對象的封裝，或者說它是管道使用的DefaultHttpContext與這個原始上下文之間溝通的中介。

如下所示的代碼片段展示了HttpListenerServerFeature類型的完整定義。簡單起見，我們并沒有實現上面提到過的所有特性接口，而只是選擇性地實現了IHttpRequestFeature和IHttpResponseFeature這兩個最為核心的特性接口。它的構造函數除了具有一個類型為HttpListenerContext的參數之外，還具有一個字符串的參數pathBase用來指定請求URL的基地址（對應IHttpRequestFeature的PathBase屬性），我們利用它來計算請求URL的相對地址（對應IHttpRequestFeature的Path屬性）。IHttpRequestFeature和IHttpResponseFeature中定義的屬性都可以直接利用HttpListenerContext對應的成員來實現，這方面并沒有什么特別之處。

   1: public class HttpListenerServerFeature : IHttpRequestFeature, IHttpResponseFeature

   2: {

   3:     private readonly HttpListenerContext     httpListenerContext;

   4:     private string            queryString;

   5:     private IHeaderDictionary         requestHeaders;

   6:     private IHeaderDictionary         responseHeaders;

   7:     private string            protocol;

   8:     private readonly string       pathBase;

9:

  10:     public HttpListenerServerFeature(HttpListenerContext httpListenerContext, string pathBase)

  11:     {

  12:         this.httpListenerContext     = httpListenerContext;

  13:         this.pathBase         = pathBase;

  14:     }

15:

  16:     #region IHttpRequestFeature

17:

  18:     Stream IHttpRequestFeature.Body

  19:     {

  20:         get { return httpListenerContext.Request.InputStream; }

  21:         set { throw new NotImplementedException(); }

  22:     }

23:

  24:     IHeaderDictionary IHttpRequestFeature.Headers

  25:     {

  26:         get { return requestHeaders

  27:          ?? (requestHeaders = GetHttpHeaders(httpListenerContext.Request.Headers)); }

  28:         set { throw new NotImplementedException(); }

  29:     }

30:

  31:     string IHttpRequestFeature.Method

  32:     {

  33:         get { return httpListenerContext.Request.HttpMethod; }

  34:         set { throw new NotImplementedException(); }

  35:     }

36:

  37:     string IHttpRequestFeature.Path

  38:     {

  39:         get { return httpListenerContext.Request.RawUrl.Substring(pathBase.Length);}

  40:         set { throw new NotImplementedException(); }

  41:     }

42:

  43:     string IHttpRequestFeature.PathBase

  44:     {

  45:         get { return pathBase; }

  46:         set { throw new NotImplementedException(); }

  47:     }

48:

  49:     string IHttpRequestFeature.Protocol

  50:     {

  51:         get{ return protocol ?? (protocol = this.GetProtocol());}

  52:         set { throw new NotImplementedException(); }

  53:     }

54:

  55:     string IHttpRequestFeature.QueryString

  56:     {

  57:         Get { return queryString ?? (queryString = this.ResolveQueryString());}

  58:         set { throw new NotImplementedException(); }

  59:     }

60:

  61:     string IHttpRequestFeature.Scheme

  62:     {

  63:         get { return httpListenerContext.Request.IsWebSocketRequest ? "https" : "http"; }

  64:         set { throw new NotImplementedException(); }

  65:     }

  66:     #endregion

67:

  68:     #region IHttpResponseFeature

  69:     Stream IHttpResponseFeature.Body

  70:     {

  71:         get { return httpListenerContext.Response.OutputStream; }

  72:         set { throw new NotImplementedException(); }

  73:     }

74:

  75:     string IHttpResponseFeature.ReasonPhrase

  76:     {

  77:         get { return httpListenerContext.Response.StatusDescription; }

  78:         set { httpListenerContext.Response.StatusDescription = value; }

  79:     }

80:

  81:     bool IHttpResponseFeature.HasStarted

  82:     {

  83:         get { return httpListenerContext.Response.SendChunked; }

  84:     }

85:

  86:     IHeaderDictionary IHttpResponseFeature.Headers

  87:     {

  88:         get { return responseHeaders

  89:             ?? (responseHeaders = GetHttpHeaders(httpListenerContext.Response.Headers)); }

  90:         set { throw new NotImplementedException(); }

  91:     }

  92:     int IHttpResponseFeature.StatusCode

  93:     {

  94:         get { return httpListenerContext.Response.StatusCode; }

  95:         set { httpListenerContext.Response.StatusCode = value; }

  96:     }

97:

  98:     void IHttpResponseFeature.OnCompleted(Func<object, Task> callback, object state)

  99:     {

 100:         throw new NotImplementedException();

 101:     }

 102:

 103:     void IHttpResponseFeature.OnStarting(Func<object, Task> callback, object state)

 104:     {

 105:         throw new NotImplementedException();

 106:     }

 107:     #endregion

 108:

 109:     private string ResolveQueryString()

 110:     {

 111:         string queryString = "";

 112:         var collection = httpListenerContext.Request.QueryString;

 113:         for (int i = 0; i < collection.Count; i++)

 114:         {

 115:             queryString += $"{collection.GetKey(i)}={collection.Get(i)}&";

 116:         }

 117:         return queryString.TrimEnd('&');

 118:     }

 119:

 120:     private IHeaderDictionary GetHttpHeaders(NameValueCollection headers)

 121:     {

 122:         HeaderDictionary dictionary = new HeaderDictionary();

 123:         foreach (string name in headers.Keys)

 124:         {

 125:             dictionary[name] = new StringValues(headers.GetValues(name));

 126:         }

 127:         return dictionary;

 128:     }

 129:

 130:     private string GetProtocol()

 131:     {

 132:         HttpListenerRequest request = httpListenerContext.Request;

 133:         Version version = request.ProtocolVersion;

 134:         return string.Format("{0}/{1}.{2}", request.IsWebSocketRequest ? "HTTPS" : "HTTP", version.Major, version.Minor);

 135:     }

 136: }

四、HttpListenerServer

接下來我們來看看HttpListenerServer的定義。如下面的代碼片段所示，用來監聽請求的HttpListener在構造函數中被創建，與此同時，我們會創建一個用于獲取監聽地址的ServerAddressesFeature對象并將其添加到屬于自己的特性列表中。當HttpListenerServer隨著Start方法的調用而被啟動后，它將這個ServerAddressesFeature對象提取出來，然后利用它得到所有的地址并添加到HttpListener的Prefixes屬性表示的監聽地址列表中。接下來，HttpListener的Start方法被調用，并在一個無限循環中開啟請求的監聽與接收。

   1: public class HttpListenerServer : IServer

   2: {

   3:     private readonly HttpListener listener;

4:

   5:     public IFeatureCollection Features { get; } = new FeatureCollection();

6:

   7:     public HttpListenerServer()

   8:     {

   9:         listener = new HttpListener();

  10:         this.Features.Set<IServerAddressesFeature>(new ServerAddressesFeature());

  11:     }

12:

  13:     public void Dispose()

  14:     {

  15:         listener.Stop();

  16:     }

17:

  18:     public void Start<TContext>(IHttpApplication<TContext> application)

  19:     {

  20:         foreach (string address in this.Features.Get<IServerAddressesFeature>().Addresses)

  21:         {

  22:             listener.Prefixes.Add(address.TrimEnd('/') + "/");

  23:         }

24:

  25:         listener.Start();

  26:         while (true)

  27:         {

  28:             HttpListenerContext httpListenerContext = listener.GetContext();

29:

  30:             string listenUrl = this.Features.Get<IServerAddressesFeature>().Addresses.First(address => httpListenerContext.Request.Url.IsBaseOf(new Uri(address)));

  31:             string pathBase = new Uri(listenUrl).LocalPath.TrimEnd('/') ;

  32:             HttpListenerServerFeature feature = new HttpListenerServerFeature(httpListenerContext, pathBase);

33:

  34:             FeatureCollection features = new FeatureCollection();

  35:             features.Set<IHttpRequestFeature>(feature);

  36:             features.Set<IHttpResponseFeature>(feature);

  37:             TContext context = application.CreateContext(features);

38:

  39:             application.ProcessRequestAsync(context).ContinueWith(task =>

  40:             {

  41:                 httpListenerContext.Response.Close();

  42:                 application.DisposeContext(context, task.Exception);

  43:             });

  44:         }

  45:     }

  46: }

HttpListener的GetContext方法以同步的方式監聽請求，并利用接收到的請求創建返回的HttpListenerContext對象。我們利用它解析出當前請求的基地址，并進一步創建出描述當前原始上下文的HttpListenerServerFeature。接下來我們將這個對象分別采用特性接口IHttpRequestFeature和IHttpResponseFeature添加到創建的FeatureCollection對象中。然后我們將這個FeatureCollection作為參數調用HttpApplication的CreateContext創建出上下文對象，并將其作為參數調用HttpApplication的ProcessContext方法讓注冊的中間件來逐個地對請求進行處理。

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