.net core web 启动过程(3)-Hosting

关于Hosting展示代码来源 .net core的runtime-8.0.7源码：https://github.com/dotnet/runtime

/// <summary>
 /// A program initialization utility.
 /// </summary>
 public partial class HostBuilder : IHostBuilder
 {
     private const string HostBuildingDiagnosticListenerName = "Microsoft.Extensions.Hosting";
     private const string HostBuildingEventName = "HostBuilding";
     private const string HostBuiltEventName = "HostBuilt";
　　　//这部分代码在本系列文章第一章已经描述，此处不过多介绍
     private readonly List<Action<IConfigurationBuilder>> _configureHostConfigActions = new List<Action<IConfigurationBuilder>>();
     private readonly List<Action<HostBuilderContext, IConfigurationBuilder>> _configureAppConfigActions = new List<Action<HostBuilderContext, IConfigurationBuilder>>();
     private readonly List<Action<HostBuilderContext, IServiceCollection>> _configureServicesActions = new List<Action<HostBuilderContext, IServiceCollection>>();
     
     private readonly List<IConfigureContainerAdapter> _configureContainerActions = new List<IConfigureContainerAdapter>();

     private IServiceFactoryAdapter _serviceProviderFactory;
　　　//这些在属性会在Build(）方法中不同阶段中被创建
     private bool _hostBuilt;
     private IConfiguration? _hostConfiguration;
     private IConfiguration? _appConfiguration;
     private HostBuilderContext? _hostBuilderContext;
     private HostingEnvironment? _hostingEnvironment;
     private IServiceProvider? _appServices;
     private PhysicalFileProvider? _defaultProvider;

     /// <summary>
     /// Initializes a new instance of <see cref="HostBuilder"/>.
     /// </summary>
     public HostBuilder()
     {
　　　　　　//构造函数中默认 初始化 ServiceProviderFactory,不过可以通过HostBuilder.UseServiceProviderFactory方法覆盖掉 
　　　　　　_serviceProviderFactory = new ServiceFactoryAdapter<IServiceCollection>(new DefaultServiceProviderFactory()); 
　　  } 
}

 再来回顾一下HostBuilder的Build过程中都经历了哪些阶段。

  public partial class HostBuilder : IHostBuilder
  {
      /// <summary>
      /// Run the given actions to initialize the host. This can only be called once.
      /// </summary>
      /// <returns>An initialized <see cref="IHost"/></returns>
      /// <remarks>Adds basic services to the host such as application lifetime, host environment, and logging.</remarks>
      public IHost Build()
      {
          if (_hostBuilt)
          {
              throw new InvalidOperationException(SR.BuildCalled);
          }
          _hostBuilt = true;

          // REVIEW: If we want to raise more events outside of these calls then we will need to
          // stash this in a field.
          using DiagnosticListener diagnosticListener = LogHostBuilding(this);

          // 1.初始化主机（Host）配置
          InitializeHostConfiguration();
          // 2.初始化 HostingEnvironment
          InitializeHostingEnvironment();
          // 3.初始化 HostBuilderContext
          InitializeHostBuilderContext();
          // 4.初始化应用（App）配置
          InitializeAppConfiguration();
          // 5.初始化服务并创建 Service Provider
          InitializeServiceProvider();

          return ResolveHost(_appServices, diagnosticListener);
      }
}

 首先看一下第一阶段初始化HostConfiguretion

 [MemberNotNull(nameof(_hostConfiguration))]
 private void InitializeHostConfiguration()
 {
     IConfigurationBuilder configBuilder = new ConfigurationBuilder()
         .AddInMemoryCollection(); // Make sure there's some default storage since there are no default providers

     foreach (Action<IConfigurationBuilder> buildAction in _configureHostConfigActions)
     {
         buildAction(configBuilder);
     }　
　　　　
　　
　　　　//此阶段结束，私有属性 _hostConfiguretion已经被创建，在第二个阶段中就可以使用了。
     _hostConfiguration = configBuilder.Build();
 }