ASP.NET Core[源码分析篇] - WebHost

　_configureServicesDelegates的承接

　　在【ASP.NET Core[源码分析篇] - Startup】这篇文章中，我们得知了目前为止（UseStartup），所有的动作都是在_configureServicesDelegates里面添加了注册的委托，那么系统是什么时候执行这些委托完成注册的呢？

　　真正的注册　

　　通过之前的一系列眼花缭乱的操作，我们得到了所有需要注册的委托_configureServicesDelegates，我们看一下WebHostBuilder.Build如何实现真正的注册。

　　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));
      }
    }

　　这里面有个最重要的方法BuildCommonServices，这个方法实现了委托的真正的执行。

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;
    }

　　从上面的代码我们可以看到，首先创建了一个真正的ServiceCollection实例，然后基于这个实例添加了一些额外的重要的注册（ApplicationBuilderFactory，HttpContextFactory，DefaultServiceProviderFactory等），然后把这个ServiceCollection实例作为参数传递到_configureServicesDelegates列表的各个委托中并执行，这样的话所有在Startup需要注册的实例都已经注册在services这个ServiceCollection实例中。

　　需要注意的是，到此为止程序并没有执行Startup里面的方法。

　　WebHost

 　　当我们的BuildCommonServices完成后，返回一个ServiceCollection实例，并且基于这个ServiceCollection实例生成了一个ServiceProvider对象，然后做为生成WebHost对象的参数传递到WebHost中。

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

webHost.Initialize();　　

　　WebHost.Initialize()

　　我们先看一下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"));
    }

　　从上面的代码流程可以看出

1. 解析Startup类
2. 执行Startup类的ConfigureServices方法注册自定义的服务并返回一个IServiceProvider对象

　　至此，我们的Startup类中的ConfigureServices已经执行过，并且WebHost已经具有了IServiceProvider对象　　

　　WebHost.Run()

　　当我们调用WebHost的扩展方法Run启动应用的时候，本质上是调用了WebHost的StartAsync方法，这个过程创建了我们应用程序最为重要的用于监听、接收、处理和响应HTTP请求的管道。　

　　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);
    }

　　这块主要是Server的创建，管道的创建和监听Http请求的Server启动，我们将分步进行剖析。

　　1. EnsureServer

 　　我们先看一下这个Server是什么

public interface IServer : IDisposable
{
    IFeatureCollection Features { get; }

    Task StartAsync<TContext>(IHttpApplication<TContext> application, CancellationToken cancellationToken);

    Task StopAsync(CancellationToken cancellationToken);
}

　　IServer的实例其实是在开始Program里面的CreateDefaultBuilder中，已经指定了KestrelServer作为默认的Server实例。　　

public static IWebHostBuilder UseKestrel(this IWebHostBuilder hostBuilder)
{
    hostBuilder.UseLibuv();

    return hostBuilder.ConfigureServices(services =>
    {
        services.AddTransient<IConfigureOptions<KestrelServerOptions>, KestrelServerOptionsSetup>();
        services.AddSingleton<IServer, KestrelServer>();
    });
}

 　　那么这个Server是做什么用的呢？Server 是一个HTTP服务器，负责HTTP的监听，接收一组 FeatureCollection 类型的原始请求，并将其包装成 HttpContext 以供我们的应用程序完成响应的处理。那它负责监听哪里？从代码可以看到Addresses 是通过在UseUrls里面指定的参数（WebHostDefaults.ServerUrlsKey） 或者是DeprecatedServerUrlsKey（配置文件里面的server.urls）中来查找的。

　　2. BuildApplication

　　在上面我们获取了一个Server用来监听请求，那么下一步我们是要构建处理Http请求的管道，IApplicationBuilder 就是用于构建应用程序的请求管道。

　　我们一般的管道创建是在 Startup 类的 Configure 方法中对 IApplicationBuilder 进行配置，嗯其实在这里还有一个 IStartupFilter 也可以用来配置 IApplicationBuilder，并且在 Startup 类的Configure 方法之前执行，所有我们看到在BuildApplication方法中，一个大概的步骤是这样的：

1. 基于IApplicationBuilderFactory创建IApplicationBuilder对象
2. 基于IStartupFilter的管道构建
3. 调用IApplicationBuilder对象的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

　　在这里，Server的启动是需要一个IHttpApplication类型的参数的，来负责 HttpContext 的创建，我们看一下这个参数

public interface IHttpApplication<TContext>
{
    TContext CreateContext(IFeatureCollection contextFeatures);

    Task ProcessRequestAsync(TContext context);

    void DisposeContext(TContext context, Exception exception);
}

　　它的默认实现类是它的默认实现是 HostingApplication 类

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);
    }
　　......
}

　　我们来看一下Server的Http监听绑定

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);
      }
    }

　　至此为止，Server已经绑定一个监听端口，注册了HTTP连接事件，剩下的就是开启监听了。　　

　　4. HostedService

　　HostedService 为我们开启了一个后台运行服务，它会在随着程序启动而启动。　　

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);
    }
  }

　　总结

　　这两篇文章从Startup开始到最后的Http管道创建和HttpServer的启动监听，涉及到了很多关键点，从代码流程来看，只要抓住几个关键点即可理解整体的一个流程。大家可以带着以下这些问题去跟着文章走：

1. Startup有多少种实例化方式？
2. IStartup在哪里被实例化的？
3. IServiceCollection何时实例化的？
4. IServiceProvider何时实例化的？
5. Startup的ConfigureService方法何时被执行？
6. IApplicationBuilder何时实例化的？
7. Startup的Configure方法何时被执行？
8. Http监听管道是何时和如何构建的？