ASP.NET Core[源碼分析篇] - WebHost - ⎝⎛CodingNote.cc ⎞⎠

ASP.NET Core[源碼分析篇] – WebHost

2019 年 10 月 3 日
筆記

　_configureServicesDelegates的承接

　　在【ASP.NET Core[源碼分析篇] – Startup】這篇文章中，我們得知了目前為止（UseStartup），所有的動作都是在_configureServicesDelegates裡面添加了註冊的委託，那麼系統是什麼時候執行這些委託完成註冊的呢？

　　真正的註冊　

　　通過之前的一系列眼花繚亂的操作，我們得到了所有需要註冊的委託_configureServicesDelegates，我們看一下WebHostBuilder.Build如何實現真正的註冊。

　　WebHostBuilder.Build()

　　public IWebHost Build()      {        if (this._webHostBuilt)          throw new InvalidOperationException(Resources.WebHostBuilder_SingleInstance);        this._webHostBuilt = true;        AggregateException hostingStartupErrors;        IServiceCollection serviceCollection1 = this.BuildCommonServices(out hostingStartupErrors);        IServiceCollection serviceCollection2 = serviceCollection1.Clone();        IServiceProvider providerFromFactory = GetProviderFromFactory(serviceCollection1);        .....    　　　　WebHost webHost = new WebHost(serviceCollection2, providerFromFactory, this._options, this._config, hostingStartupErrors);        try        {          webHost.Initialize();          return (IWebHost) webHost;        }        catch        {          webHost.Dispose();          throw;        }        IServiceProvider GetProviderFromFactory(IServiceCollection collection)        {          ServiceProvider serviceProvider = collection.BuildServiceProvider();          IServiceProviderFactory<IServiceCollection> service = ((IServiceProvider) serviceProvider).GetService<IServiceProviderFactory<IServiceCollection>>();          if (service == null)            return (IServiceProvider) serviceProvider;          using (serviceProvider)            return service.CreateServiceProvider(service.CreateBuilder(collection));        }      }

　　這裡面有個最重要的方法BuildCommonServices，這個方法實現了委託的真正的執行。

private IServiceCollection BuildCommonServices(        out AggregateException hostingStartupErrors)      {          .....  　　　　　ServiceCollection services = new ServiceCollection();
          services.AddTransient<IApplicationBuilderFactory, ApplicationBuilderFactory>();          services.AddTransient<IHttpContextFactory, HttpContextFactory>();          services.AddScoped<IMiddlewareFactory, MiddlewareFactory>();          services.AddOptions();          services.AddLogging();          services.AddTransient<IStartupFilter, AutoRequestServicesStartupFilter>();          services.AddTransient<IServiceProviderFactory<IServiceCollection>, DefaultServiceProviderFactory>();    　　　　.....          foreach (Action<WebHostBuilderContext, IServiceCollection> servicesDelegate in this._configureServicesDelegates)          servicesDelegate(this._context, (IServiceCollection) services);        return (IServiceCollection) services;      }

　　從上面的程式碼我們可以看到，首先創建了一個真正的ServiceCollection實例，然後基於這個實例添加了一些額外的重要的註冊（ApplicationBuilderFactory，HttpContextFactory，DefaultServiceProviderFactory等），然後把這個ServiceCollection實例作為參數傳遞到_configureServicesDelegates列表的各個委託中並執行，這樣的話所有在Startup需要註冊的實例都已經註冊在services這個ServiceCollection實例中。

　　需要注意的是，到此為止程式並沒有執行Startup裡面的方法。

　　WebHost

　　當我們的BuildCommonServices完成後，返回一個ServiceCollection實例，並且基於這個ServiceCollection實例生成了一個ServiceProvider對象，然後做為生成WebHost對象的參數傳遞到WebHost中。

WebHost webHost = new WebHost(serviceCollection2, providerFromFactory, this._options, this._config, hostingStartupErrors);    webHost.Initialize();

　　WebHost.Initialize()

　　我們先看一下WebHost的Initialize方法

　　public void Initialize()      {        try        {          this.EnsureApplicationServices();        }        catch (Exception ex)        {          if (this._applicationServices == null)            this._applicationServices = (IServiceProvider) this._applicationServiceCollection.BuildServiceProvider();          if (!this._options.CaptureStartupErrors)            throw;          else            this._applicationServicesException = ExceptionDispatchInfo.Capture(ex);        }      }    　　private void EnsureApplicationServices()      {        if (this._applicationServices != null)          return;        this.EnsureStartup();        this._applicationServices = this._startup.ConfigureServices(this._applicationServiceCollection);      }        private void EnsureStartup()      {        if (this._startup != null)          return;        this._startup = this._hostingServiceProvider.GetService<IStartup>();        if (this._startup == null)          throw new InvalidOperationException(string.Format("No startup configured. Please specify startup via WebHostBuilder.UseStartup, WebHostBuilder.Configure, injecting {0} or specifying the startup assembly via {1} in the web host configuration.", (object) "IStartup", (object) "StartupAssemblyKey"));      }

　　從上面的程式碼流程可以看出

解析Startup類
執行Startup類的ConfigureServices方法註冊自定義的服務並返回一個IServiceProvider對象

　　至此，我們的Startup類中的ConfigureServices已經執行過，並且WebHost已經具有了IServiceProvider對象　　

　　WebHost.Run()

　　當我們調用WebHost的擴展方法Run啟動應用的時候，本質上是調用了WebHost的StartAsync方法，這個過程創建了我們應用程式最為重要的用於監聽、接收、處理和響應HTTP請求的管道。　

　　public virtual async Task StartAsync(CancellationToken cancellationToken = default (CancellationToken))      {        HostingEventSource.Log.HostStart();        this._logger = this._applicationServices.GetRequiredService<ILogger<WebHost>>();        this._logger.Starting();        RequestDelegate application = this.BuildApplication();        this._applicationLifetime = this._applicationServices.GetRequiredService<Microsoft.AspNetCore.Hosting.IApplicationLifetime>() as ApplicationLifetime;        this._hostedServiceExecutor = this._applicationServices.GetRequiredService<HostedServiceExecutor>();        DiagnosticListener requiredService1 = this._applicationServices.GetRequiredService<DiagnosticListener>();        IHttpContextFactory requiredService2 = this._applicationServices.GetRequiredService<IHttpContextFactory>();        ILogger<WebHost> logger = this._logger;        DiagnosticListener diagnosticSource = requiredService1;        IHttpContextFactory httpContextFactory = requiredService2;        await this.Server.StartAsync<HostingApplication.Context>((IHttpApplication<HostingApplication.Context>) new HostingApplication(application, (ILogger) logger, diagnosticSource, httpContextFactory), cancellationToken).ConfigureAwait(false);        this._applicationLifetime?.NotifyStarted();        await this._hostedServiceExecutor.StartAsync(cancellationToken).ConfigureAwait(false);       .....      }    　　private RequestDelegate BuildApplication()      {          this._applicationServicesException?.Throw();          this.EnsureServer();          IApplicationBuilder builder = this._applicationServices.GetRequiredService<IApplicationBuilderFactory>().CreateBuilder(this.Server.Features);          builder.ApplicationServices = this._applicationServices;          IEnumerable<IStartupFilter> service = this._applicationServices.GetService<IEnumerable<IStartupFilter>>();          Action<IApplicationBuilder> next = new Action<IApplicationBuilder>(this._startup.Configure);          foreach (IStartupFilter startupFilter in service.Reverse<IStartupFilter>())            next = startupFilter.Configure(next);          next(builder);          return builder.Build();        }    　　private void EnsureServer()      {        if (this.Server != null)          return;        this.Server = this._applicationServices.GetRequiredService<IServer>();        IServerAddressesFeature addressesFeature = this.Server.Features?.Get<IServerAddressesFeature>();        ICollection<string> addresses = addressesFeature?.Addresses;        if (addresses == null || addresses.IsReadOnly || addresses.Count != 0)          return;        string str1 = this._config[WebHostDefaults.ServerUrlsKey] ?? this._config[WebHost.DeprecatedServerUrlsKey];        if (string.IsNullOrEmpty(str1))          return;        addressesFeature.PreferHostingUrls = WebHostUtilities.ParseBool(this._config, WebHostDefaults.PreferHostingUrlsKey);        string str2 = str1;        char[] separator = new char[1]{ ';' };        foreach (string str3 in str2.Split(separator, StringSplitOptions.RemoveEmptyEntries))          addresses.Add(str3);      }

　　這塊主要是Server的創建，管道的創建和監聽Http請求的Server啟動，我們將分步進行剖析。

　　1. EnsureServer

　　我們先看一下這個Server是什麼

public interface IServer : IDisposable  {      IFeatureCollection Features { get; }        Task StartAsync<TContext>(IHttpApplication<TContext> application, CancellationToken cancellationToken);        Task StopAsync(CancellationToken cancellationToken);  }

　　IServer的實例其實是在開始Program裡面的CreateDefaultBuilder中，已經指定了KestrelServer作為默認的Server實例。　　

public static IWebHostBuilder UseKestrel(this IWebHostBuilder hostBuilder)  {      hostBuilder.UseLibuv();        return hostBuilder.ConfigureServices(services =>      {          services.AddTransient<IConfigureOptions<KestrelServerOptions>, KestrelServerOptionsSetup>();          services.AddSingleton<IServer, KestrelServer>();      });  }

　　那麼這個Server是做什麼用的呢？Server 是一個HTTP伺服器，負責HTTP的監聽，接收一組 FeatureCollection 類型的原始請求，並將其包裝成 HttpContext 以供我們的應用程式完成響應的處理。那它負責監聽哪裡？從程式碼可以看到Addresses 是通過在UseUrls裡面指定的參數（WebHostDefaults.ServerUrlsKey）或者是DeprecatedServerUrlsKey（配置文件裡面的server.urls）中來查找的。

　　2. BuildApplication

　　在上面我們獲取了一個Server用來監聽請求，那麼下一步我們是要構建處理Http請求的管道，IApplicationBuilder 就是用於構建應用程式的請求管道。

　　我們一般的管道創建是在 Startup 類的 Configure 方法中對 IApplicationBuilder 進行配置，嗯其實在這裡還有一個 IStartupFilter 也可以用來配置 IApplicationBuilder，並且在 Startup 類的Configure 方法之前執行，所有我們看到在BuildApplication方法中，一個大概的步驟是這樣的：

基於IApplicationBuilderFactory創建IApplicationBuilder對象
基於IStartupFilter的管道構建
調用IApplicationBuilder對象的Build方法完成完整的管道

public RequestDelegate Build()      {        RequestDelegate requestDelegate = (RequestDelegate) (context =>        {          context.Response.StatusCode = 404;          return Task.CompletedTask;        });        foreach (Func<RequestDelegate, RequestDelegate> func in this._components.Reverse<Func<RequestDelegate, RequestDelegate>>())          requestDelegate = func(requestDelegate);        return requestDelegate;      }

　　3. Server.StartAsync

　　在這裡，Server的啟動是需要一個IHttpApplication類型的參數的，來負責 HttpContext 的創建，我們看一下這個參數

public interface IHttpApplication<TContext>  {      TContext CreateContext(IFeatureCollection contextFeatures);        Task ProcessRequestAsync(TContext context);        void DisposeContext(TContext context, Exception exception);  }

　　它的默認實現類是它的默認實現是 HostingApplication 類

public class HostingApplication : IHttpApplication<HostingApplication.Context>    {      private readonly RequestDelegate _application;      private readonly IHttpContextFactory _httpContextFactory;public Task ProcessRequestAsync(HostingApplication.Context context)      {        return this._application(context.HttpContext);      }
　　......  }

　　我們來看一下Server的Http監聽綁定

public async Task StartAsync<TContext>(        IHttpApplication<TContext> application,        CancellationToken cancellationToken)      {        try        {          if (!BitConverter.IsLittleEndian)            throw new PlatformNotSupportedException(CoreStrings.BigEndianNotSupported);          this.ValidateOptions();          if (this._hasStarted)            throw new InvalidOperationException(CoreStrings.ServerAlreadyStarted);          this._hasStarted = true;          this._heartbeat.Start();          await AddressBinder.BindAsync(this._serverAddresses, this.Options, (ILogger) this.Trace, new Func<ListenOptions, Task>(OnBind)).ConfigureAwait(false);        }        catch (Exception ex)        {          this.Trace.LogCritical((EventId) 0, ex, "Unable to start Kestrel.");          this.Dispose();          throw;        }          async Task OnBind(ListenOptions endpoint)        {          endpoint.UseHttpServer<TContext>((IList<IConnectionAdapter>) endpoint.ConnectionAdapters, this.ServiceContext, application, endpoint.Protocols);          ConnectionDelegate connectionDelegate = endpoint.Build();          if (this.Options.Limits.MaxConcurrentConnections.HasValue)            connectionDelegate = new ConnectionDelegate(new ConnectionLimitMiddleware(connectionDelegate, this.Options.Limits.MaxConcurrentConnections.Value, this.Trace).OnConnectionAsync);          ConnectionDispatcher connectionDispatcher = new ConnectionDispatcher(this.ServiceContext, connectionDelegate);          ITransport transport = this._transportFactory.Create((IEndPointInformation) endpoint, (IConnectionDispatcher) connectionDispatcher);          this._transports.Add(transport);          await transport.BindAsync().ConfigureAwait(false);        }      }

　　至此為止，Server已經綁定一個監聽埠，註冊了HTTP連接事件，剩下的就是開啟監聽了。　　

　　4. HostedService

　　HostedService 為我們開啟了一個後台運行服務，它會在隨著程式啟動而啟動。　　

public class HostedServiceExecutor    {      private readonly IEnumerable<IHostedService> _services;public async Task StartAsync(CancellationToken token)      {
　　　　await this.ExecuteAsync((Func<IHostedService, Task>) (service => service.StartAsync(token)));            }        public async Task StopAsync(CancellationToken token)      {
　　　　await this.ExecuteAsync((Func<IHostedService, Task>) (service => service.StopAsync(token)));            }        private async Task ExecuteAsync(Func<IHostedService, Task> callback)      {        List<Exception> exceptions = (List<Exception>) null;        foreach (IHostedService service in this._services)        {          try          {            await callback(service);          }          catch (Exception ex)          {            if (exceptions == null)              exceptions = new List<Exception>();            exceptions.Add(ex);          }        }        if (exceptions != null)          throw new AggregateException((IEnumerable<Exception>) exceptions);      }    }

　　總結

　　這兩篇文章從Startup開始到最後的Http管道創建和HttpServer的啟動監聽，涉及到了很多關鍵點，從程式碼流程來看，只要抓住幾個關鍵點即可理解整體的一個流程。大家可以帶著以下這些問題去跟著文章走：

Startup有多少種實例化方式？
IStartup在哪裡被實例化的？
IServiceCollection何時實例化的？
IServiceProvider何時實例化的？
Startup的ConfigureService方法何時被執行？
IApplicationBuilder何時實例化的？
Startup的Configure方法何時被執行？
Http監聽管道是何時和如何構建的？