学习ASP.NET Core, 怎能不了解请求处理管道[3]: 自定义一个服务器感受一下管道是如何监听、接收和响应请求的
学习ASP.NET Core, 怎能不了解请求处理管道[3]: 自定义一个服务器感受一下管道是如何监听、接收和响应请求的
我们在《服务器在管道中的“龙头”地位》中对ASP.NET Core默认提供的具有跨平台能力的KestrelServer进行了介绍，为了让读者朋友们对管道中的服务器具有更加深刻的认识，接下来我们采用实例演示的形式创建一个自定义的服务器。这个自定义的服务器直接利用HttpListener来完成针对请求的监听、接收和响应，我们将其命名为HttpListenerServer。在正式介绍HttpListenerServer的设计和实现之前，我们先来显示一下如何将它应用到 一个具体的Web应用中。我们依然采用最简单的Hello World应用来演示针对HttpListenerServer的应用，所以我们在Startup类的Configure方法中编写如下的程序直接响应一个“Hello World”字符串。[本文已经同步到《ASP.NET Core框架揭秘》之中]
1: public class Startup 2: { 3: public void Configure(IApplicationBuilder app) 4: { 5: app.Run(async context => await context.Response.WriteAsync("Hello World!")); 6: } 7: }在作为程序入口的Main方法中，我们直接创建一个WebHostBuilder对象并调用扩展方法UseHttpListener完成针对自定义HttpListenerServer的注册。我们接下来调用UseStartup方法注册上面定义的这个启动类型，然后调用Build方法创建一个WebHost对象，最后调用Run方法运行这个作为宿主的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: }我们自定义的扩展方法UseHttpListener的逻辑很简单，它只是调用WebHostBuilder的ConfigureServices方法将我们自定义的HttpListenerServer类型以单例模式注册到指定的ServiceCollection上而已。我们直接运行这个程序并利用浏览器访问默认的监听地址（http://localhost:5000），服务端响应的“Hello World”字符串会按照如下图所示的形式显示在浏览器上。
接下来我们来介绍一下HttpListenerServer的大体涉及。除了HttpListenerServer这个实现了IServer的自定义Server类型之外，我们只定义了一个名为HttpListenerServerFeature的特性类型，图7所示的UML基本上体现了HttpListenerServer的总体设计。
如果我们利用HttpListener来监听请求，它会为接收到的每次请求创建一个属于自己的上下文，具体来说这是一个类型为HttpListenerContext对象。我们可以利用这个HttpListenerContext对象获取所有与请求相关的信息，针对请求的任何响应也都是利用它完成的。上面这个HttpListenerServerFeature实际上就是对这个作为原始上下文的HttpListenerContext对象的封装，或者说它是管道使用的DefaultHttpContext与这个原始上下文之间沟通的中介。
如下所示的代码片段展示了HttpListenerServerFeature类型的完整定义。简单起见，我们并没有实现上面提到过的所有特性接口，而只是选择性地实现了IHttpRequestFeature和IHttpResponseFeature这两个最为核心的特性接口。它的构造函数除了具有一个类型为HttpListenerContext的参数之外，还具有一个字符串的参数pathBase用来指定请求URL的基地址（对应IHttpRequestFeature的PathBase属性），我们利用它来计算请求URL的相对地址（对应IHttpRequestFeature的Path属性）。IHttpRequestFeature和IHttpResponseFeature中定义的属性都可以直接利用HttpListenerContext对应的成员来实现，这方面并没有什么特别之处。
/*
* 提示：该行代码过长，系统自动注释不进行高亮。一键复制会移除系统注释
* 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 ?? (requestHeaders = GetHttpHeaders(httpListenerContext.Request.Headers)); } 27: set { throw new NotImplementedException(); } 28: } 29: 30: string IHttpRequestFeature.Method 31: { 32: get { return httpListenerContext.Request.HttpMethod; } 33: set { throw new NotImplementedException(); } 34: } 35: 36: string IHttpRequestFeature.Path 37: { 38: get { return httpListenerContext.Request.RawUrl.Substring(pathBase.Length);} 39: set { throw new NotImplementedException(); } 40: } 41: 42: string IHttpRequestFeature.PathBase 43: { 44: get { return pathBase; } 45: set { throw new NotImplementedException(); } 46: } 47: 48: string IHttpRequestFeature.Protocol 49: { 50: get{ return protocol ?? (protocol = this.GetProtocol());} 51: set { throw new NotImplementedException(); } 52: } 53: 54: string IHttpRequestFeature.QueryString 55: { 56: Get { return queryString ?? (queryString = this.ResolveQueryString());} 57: set { throw new NotImplementedException(); } 58: } 59: 60: string IHttpRequestFeature.Scheme 61: { 62: get { return httpListenerContext.Request.IsWebSocketRequest ? "https" : "http"; } 63: set { throw new NotImplementedException(); } 64: } 65: #endregion 66: 67: #region IHttpResponseFeature 68: Stream IHttpResponseFeature.Body 69: { 70: get { return httpListenerContext.Response.OutputStream; } 71: set { throw new NotImplementedException(); } 72: } 73: 74: string IHttpResponseFeature.ReasonPhrase 75: { 76: get { return httpListenerContext.Response.StatusDescription; } 77: set { httpListenerContext.Response.StatusDescription = value; } 78: } 79: 80: bool IHttpResponseFeature.HasStarted 81: { 82: get { return httpListenerContext.Response.SendChunked; } 83: } 84: 85: IHeaderDictionary IHttpResponseFeature.Headers 86: { 87: get { return responseHeaders ?? (responseHeaders = GetHttpHeaders(httpListenerContext.Response.Headers)); } 88: set { throw new NotImplementedException(); } 89: } 90: int IHttpResponseFeature.StatusCode 91: { 92: get { return httpListenerContext.Response.StatusCode; } 93: set { httpListenerContext.Response.StatusCode = value; } 94: } 95: 96: void IHttpResponseFeature.OnCompleted(Func<object, Task> callback, object state) 97: { 98: throw new NotImplementedException(); 99: } 100: 101: void IHttpResponseFeature.OnStarting(Func<object, Task> callback, object state) 102: { 103: throw new NotImplementedException(); 104: } 105: #endregion 106: 107: private string ResolveQueryString() 108: { 109: string queryString = ""; 110: var collection = httpListenerContext.Request.QueryString; 111: for (int i = 0; i < collection.Count; i++) 112: { 113: queryString += $"{collection.GetKey(i)}={collection.Get(i)}&"; 114: } 115: return queryString.TrimEnd('&'); 116: } 117: 118: private IHeaderDictionary GetHttpHeaders(NameValueCollection headers) 119: { 120: HeaderDictionary dictionary = new HeaderDictionary(); 121: foreach (string name in headers.Keys) 122: { 123: dictionary[name] = new StringValues(headers.GetValues(name)); 124: } 125: return dictionary; 126: } 127: 128: private string GetProtocol() 129: { 130: HttpListenerRequest request = httpListenerContext.Request; 131: Version version = request.ProtocolVersion; 132: return string.Format("{0}/{1}.{2}", request.IsWebSocketRequest ? "HTTPS" : "HTTP", version.Major, version.Minor); 133: } 134: }
*/接下来我们来看看HttpListenerServer的定义。如下面的代码片段所示，用来监听请求的HttpListener在构造函数中被创建，与此同时，我们会创建一个用于获取监听地址的ServerAddressesFeature对象并将其添加到属于自己的特性列表中。当HttpListenerServer随着Start方法的调用而被启动后，它将这个ServerAddressesFeature对象提取出来，然后利用它得到所有的地址并添加到HttpListener的Prefixes属性表示的监听地址列表中。接下来，HttpListener的Start方法被调用，并在一个无限循环中开启请求的监听与接收。
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 31: .First(address => httpListenerContext.Request.Url.IsBaseOf(new Uri(address))); 32: string pathBase = new Uri(listenUrl).LocalPath.TrimEnd('/') ; 33: HttpListenerServerFeature feature = new HttpListenerServerFeature(httpListenerContext, pathBase); 34: 35: FeatureCollection features = new FeatureCollection(); 36: features.Set<IHttpRequestFeature>(feature); 37: features.Set<IHttpResponseFeature>(feature); 38: TContext context = application.CreateContext(features); 39: 40: application.ProcessRequestAsync(context).ContinueWith(task => 41: { 42: httpListenerContext.Response.Close(); 43: application.DisposeContext(context, task.Exception); 44: }); 45: } 46: } 47: }HttpListener的GetContext方法以同步的方式监听请求，并利用接收到的请求创建返回的HttpListenerContext对象。我们利用它解析出当前请求的基地址，并进一步创建出描述当前原始上下文的HttpListenerServerFeature。接下来我们将这个对象分别采用特性接口IHttpRequestFeature和IHttpResponseFeature添加到创建的FeatureCollection对象中。然后我们将这个FeatureCollection作为参数调用HttpApplication的CreateContext创建出上下文对象，并将其作为参数调用HttpApplication的ProcessContext方法让注册的中间件来逐个地对请求进行处理。
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