asp.net core 3.1 源码学习（二）

IWebHost实现类WebHost

internal class WebHost : IWebHost, IAsyncDisposable
    {
        private static readonly string DeprecatedServerUrlsKey = "server.urls";

        private readonly IServiceCollection _applicationServiceCollection;
        private IStartup _startup;
        private ApplicationLifetime _applicationLifetime;
        private HostedServiceExecutor _hostedServiceExecutor;

        private readonly IServiceProvider _hostingServiceProvider;
        private readonly WebHostOptions _options;
        private readonly IConfiguration _config;
        private readonly AggregateException _hostingStartupErrors;

        private IServiceProvider _applicationServices;
        private ExceptionDispatchInfo _applicationServicesException;
        private ILogger _logger =  NullLogger.Instance;

        private bool _stopped;
        private bool _startedServer;

        // Used for testing only
        internal WebHostOptions Options => _options;

        private IServer Server { get; set; }

        public WebHost(
            IServiceCollection appServices,
            IServiceProvider hostingServiceProvider,
            WebHostOptions options,
            IConfiguration config,
            AggregateException hostingStartupErrors)
        {
            if (appServices == null)
            {
                throw new ArgumentNullException(nameof(appServices));
            }

            if (hostingServiceProvider == null)
            {
                throw new ArgumentNullException(nameof(hostingServiceProvider));
            }

            if (config == null)
            {
                throw new ArgumentNullException(nameof(config));
            }

            _config = config;
            _hostingStartupErrors = hostingStartupErrors;
            _options = options;
            _applicationServiceCollection = appServices;
            _hostingServiceProvider = hostingServiceProvider;
            _applicationServiceCollection.AddSingleton<ApplicationLifetime>();
            // There's no way to to register multiple service types per definition. See https://github.com/aspnet/DependencyInjection/issues/360
            _applicationServiceCollection.AddSingleton(services
                => services.GetService<ApplicationLifetime>() as IHostApplicationLifetime);
#pragma warning disable CS0618 // Type or member is obsolete
            _applicationServiceCollection.AddSingleton(services
                => services.GetService<ApplicationLifetime>() as AspNetCore.Hosting.IApplicationLifetime);
            _applicationServiceCollection.AddSingleton(services
                => services.GetService<ApplicationLifetime>() as Extensions.Hosting.IApplicationLifetime);
#pragma warning restore CS0618 // Type or member is obsolete
            _applicationServiceCollection.AddSingleton<HostedServiceExecutor>();
        }

        public IServiceProvider Services
        {
            get
            {
                return _applicationServices;
            }
        }

        public IFeatureCollection ServerFeatures
        {
            get
            {
                EnsureServer();
                return Server?.Features;
            }
        }

        // Called immediately after the constructor so the properties can rely on it.
        public void Initialize()
        {
            try
            {
                EnsureApplicationServices();
            }
            catch (Exception ex)
            {
                // EnsureApplicationServices may have failed due to a missing or throwing Startup class.
                if (_applicationServices == null)
                {
                    _applicationServices = _applicationServiceCollection.BuildServiceProvider();
                }

                if (!_options.CaptureStartupErrors)
                {
                    throw;
                }

                _applicationServicesException = ExceptionDispatchInfo.Capture(ex);
            }
        }

        public void Start()
        {
            StartAsync().GetAwaiter().GetResult();
        }

        public virtual async Task StartAsync(CancellationToken cancellationToken = default)
        {
            HostingEventSource.Log.HostStart();
            _logger = _applicationServices.GetRequiredService<ILoggerFactory>().CreateLogger("Microsoft.AspNetCore.Hosting.Diagnostics");
            _logger.Starting();

            var application = BuildApplication();

            _applicationLifetime = _applicationServices.GetRequiredService<ApplicationLifetime>();
            _hostedServiceExecutor = _applicationServices.GetRequiredService<HostedServiceExecutor>();

            // Fire IHostedService.Start
            await _hostedServiceExecutor.StartAsync(cancellationToken).ConfigureAwait(false);

            var diagnosticSource = _applicationServices.GetRequiredService<DiagnosticListener>();
            var httpContextFactory = _applicationServices.GetRequiredService<IHttpContextFactory>();
            var hostingApp = new HostingApplication(application, _logger, diagnosticSource, httpContextFactory);
            await Server.StartAsync(hostingApp, cancellationToken).ConfigureAwait(false);
            _startedServer = true;

            // Fire IApplicationLifetime.Started
            _applicationLifetime?.NotifyStarted();


            _logger.Started();

            // Log the fact that we did load hosting startup assemblies.
            if (_logger.IsEnabled(LogLevel.Debug))
            {
                foreach (var assembly in _options.GetFinalHostingStartupAssemblies())
                {
                    _logger.LogDebug("Loaded hosting startup assembly {assemblyName}", assembly);
                }
            }

            if (_hostingStartupErrors != null)
            {
                foreach (var exception in _hostingStartupErrors.InnerExceptions)
                {
                    _logger.HostingStartupAssemblyError(exception);
                }
            }
        }

        private void EnsureApplicationServices()
        {
            if (_applicationServices == null)
            {
                EnsureStartup();
                _applicationServices = _startup.ConfigureServices(_applicationServiceCollection);
            }
        }

        private void EnsureStartup()
        {
            if (_startup != null)
            {
                return;
            }

            _startup = _hostingServiceProvider.GetService<IStartup>();

            if (_startup == null)
            {
                throw new InvalidOperationException($"No application configured. Please specify startup via IWebHostBuilder.UseStartup, IWebHostBuilder.Configure, injecting {nameof(IStartup)} or specifying the startup assembly via {nameof(WebHostDefaults.StartupAssemblyKey)} in the web host configuration.");
            }
        }

        private RequestDelegate BuildApplication()
        {
            try
            {
                _applicationServicesException?.Throw();
                EnsureServer();

                var builderFactory = _applicationServices.GetRequiredService<IApplicationBuilderFactory>();
                var builder = builderFactory.CreateBuilder(Server.Features);
                builder.ApplicationServices = _applicationServices;

                var startupFilters = _applicationServices.GetService<IEnumerable<IStartupFilter>>();
                Action<IApplicationBuilder> configure = _startup.Configure;
                foreach (var filter in startupFilters.Reverse())
                {
                    configure = filter.Configure(configure);
                }

                configure(builder);

                return builder.Build();
            }
            catch (Exception ex)
            {
                if (!_options.SuppressStatusMessages)
                {
                    // Write errors to standard out so they can be retrieved when not in development mode.
                    Console.WriteLine("Application startup exception: " + ex.ToString());
                }
                var logger = _applicationServices.GetRequiredService<ILogger<WebHost>>();
                logger.ApplicationError(ex);

                if (!_options.CaptureStartupErrors)
                {
                    throw;
                }

                EnsureServer();

                // Generate an HTML error page.
                var hostingEnv = _applicationServices.GetRequiredService<IHostEnvironment>();
                var showDetailedErrors = hostingEnv.IsDevelopment() || _options.DetailedErrors;

                var model = new ErrorPageModel
                {
                    RuntimeDisplayName = RuntimeInformation.FrameworkDescription
                };
                var systemRuntimeAssembly = typeof(System.ComponentModel.DefaultValueAttribute).GetTypeInfo().Assembly;
                var assemblyVersion = new AssemblyName(systemRuntimeAssembly.FullName).Version.ToString();
                var clrVersion = assemblyVersion;
                model.RuntimeArchitecture = RuntimeInformation.ProcessArchitecture.ToString();
                var currentAssembly = typeof(ErrorPage).GetTypeInfo().Assembly;
                model.CurrentAssemblyVesion = currentAssembly
                    .GetCustomAttribute<AssemblyInformationalVersionAttribute>()
                    .InformationalVersion;
                model.ClrVersion = clrVersion;
                model.OperatingSystemDescription = RuntimeInformation.OSDescription;
                model.ShowRuntimeDetails = showDetailedErrors;

                if (showDetailedErrors)
                {
                    var exceptionDetailProvider = new ExceptionDetailsProvider(
                        hostingEnv.ContentRootFileProvider,
                        sourceCodeLineCount: 6);

                    model.ErrorDetails = exceptionDetailProvider.GetDetails(ex);
                }
                else
                {
                    model.ErrorDetails = new ExceptionDetails[0];
                }

                var errorPage = new ErrorPage(model);
                return context =>
                {
                    context.Response.StatusCode = 500;
                    context.Response.Headers[HeaderNames.CacheControl] = "no-cache";
                    return errorPage.ExecuteAsync(context);
                };
            }
        }

        private void EnsureServer()
        {
            if (Server == null)
            {
                Server = _applicationServices.GetRequiredService<IServer>();

                var serverAddressesFeature = Server.Features?.Get<IServerAddressesFeature>();
                var addresses = serverAddressesFeature?.Addresses;
                if (addresses != null && !addresses.IsReadOnly && addresses.Count == 0)
                {
                    var urls = _config[WebHostDefaults.ServerUrlsKey] ?? _config[DeprecatedServerUrlsKey];
                    if (!string.IsNullOrEmpty(urls))
                    {
                        serverAddressesFeature.PreferHostingUrls = WebHostUtilities.ParseBool(_config, WebHostDefaults.PreferHostingUrlsKey);

                        foreach (var value in urls.Split(new[] { ';' }, StringSplitOptions.RemoveEmptyEntries))
                        {
                            addresses.Add(value);
                        }
                    }
                }
            }
        }

        public async Task StopAsync(CancellationToken cancellationToken = default)
        {
            if (_stopped)
            {
                return;
            }
            _stopped = true;

            _logger.Shutdown();

            var timeoutToken = new CancellationTokenSource(Options.ShutdownTimeout).Token;
            if (!cancellationToken.CanBeCanceled)
            {
                cancellationToken = timeoutToken;
            }
            else
            {
                cancellationToken = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken, timeoutToken).Token;
            }

            // Fire IApplicationLifetime.Stopping
            _applicationLifetime?.StopApplication();

            if (Server != null && _startedServer)
            {
                await Server.StopAsync(cancellationToken).ConfigureAwait(false);
            }

            // Fire the IHostedService.Stop
            if (_hostedServiceExecutor != null)
            {
                await _hostedServiceExecutor.StopAsync(cancellationToken).ConfigureAwait(false);
            }

            // Fire IApplicationLifetime.Stopped
            _applicationLifetime?.NotifyStopped();

            HostingEventSource.Log.HostStop();
        }

        public void Dispose()
        {
            DisposeAsync().ConfigureAwait(false).GetAwaiter().GetResult();
        }

        public async ValueTask DisposeAsync()
        {
            if (!_stopped)
            {
                try
                {
                    await StopAsync().ConfigureAwait(false);
                }
                catch (Exception ex)
                {
                    _logger.ServerShutdownException(ex);
                }
            }

            await DisposeServiceProviderAsync(_applicationServices).ConfigureAwait(false);
            await DisposeServiceProviderAsync(_hostingServiceProvider).ConfigureAwait(false);
        }

        private async ValueTask DisposeServiceProviderAsync(IServiceProvider serviceProvider)
        {
            switch (serviceProvider)
            {
                case IAsyncDisposable asyncDisposable:
                    await asyncDisposable.DisposeAsync();
                    break;
                case IDisposable disposable:
                    disposable.Dispose();
                    break;
            }
        }
    }

WebHost初始化时，先验证IStartup接口是否已经注册了

接下来看下StartAsync方法

首先调用BuildApplication创建RequestDelegate

RequestDelegate是一个委托delegate Task RequestDelegate(HttpContext context)，用于处理请求

该方法主要构建处理请求的委托链，看下如何具体实现的

这里涉及到两个关键接口IApplicationBuilder、IApplicationBuilderFactory

 IApplicationBuilder的实现类

public class ApplicationBuilder : IApplicationBuilder
    {
        private const string ServerFeaturesKey = "server.Features";
        private const string ApplicationServicesKey = "application.Services";

        private readonly IList<Func<RequestDelegate, RequestDelegate>> _components = new List<Func<RequestDelegate, RequestDelegate>>();

        public ApplicationBuilder(IServiceProvider serviceProvider)
        {
            Properties = new Dictionary<string, object>(StringComparer.Ordinal);
            ApplicationServices = serviceProvider;
        }

        public ApplicationBuilder(IServiceProvider serviceProvider, object server)
            : this(serviceProvider)
        {
            SetProperty(ServerFeaturesKey, server);
        }

        private ApplicationBuilder(ApplicationBuilder builder)
        {
            Properties = new CopyOnWriteDictionary<string, object>(builder.Properties, StringComparer.Ordinal);
        }

        public IServiceProvider ApplicationServices
        {
            get
            {
                return GetProperty<IServiceProvider>(ApplicationServicesKey);
            }
            set
            {
                SetProperty<IServiceProvider>(ApplicationServicesKey, value);
            }
        }

        public IFeatureCollection ServerFeatures
        {
            get
            {
                return GetProperty<IFeatureCollection>(ServerFeaturesKey);
            }
        }

        public IDictionary<string, object> Properties { get; }

        private T GetProperty<T>(string key)
        {
            object value;
            return Properties.TryGetValue(key, out value) ? (T)value : default(T);
        }

        private void SetProperty<T>(string key, T value)
        {
            Properties[key] = value;
        }

        public IApplicationBuilder Use(Func<RequestDelegate, RequestDelegate> middleware)
        {
            _components.Add(middleware);
            return this;
        }

        public IApplicationBuilder New()
        {
            return new ApplicationBuilder(this);
        }

        public RequestDelegate Build()
        {
            RequestDelegate app = context =>
            {
                // If we reach the end of the pipeline, but we have an endpoint, then something unexpected has happened.
                // This could happen if user code sets an endpoint, but they forgot to add the UseEndpoint middleware.
                var endpoint = context.GetEndpoint();
                var endpointRequestDelegate = endpoint?.RequestDelegate;
                if (endpointRequestDelegate != null)
                {
                    var message =
                        $"The request reached the end of the pipeline without executing the endpoint: '{endpoint.DisplayName}'. " +
                        $"Please register the EndpointMiddleware using '{nameof(IApplicationBuilder)}.UseEndpoints(...)' if using " +
                        $"routing.";
                    throw new InvalidOperationException(message);
                }

                context.Response.StatusCode = 404;
                return Task.CompletedTask;
            };

            foreach (var component in _components.Reverse())
            {
                app = component(app);
            }

            return app;
        }
    }

字段_components保存一个委托列表，当调用Use方法时，添加委托到字段中

最后调用Build方法创建RequestDelegate

public class ApplicationBuilderFactory : IApplicationBuilderFactory
    {
        private readonly IServiceProvider _serviceProvider;

        public ApplicationBuilderFactory(IServiceProvider serviceProvider)
        {
            _serviceProvider = serviceProvider;
        }

        public IApplicationBuilder CreateBuilder(IFeatureCollection serverFeatures)
        {
            return new ApplicationBuilder(_serviceProvider, serverFeatures);
        }
    }

ApplicationBuilderFactory用于创建ApplicationBuilder

我们再回到BuildApplication中

应用找到所有实现IStartupFilter的类型，再结合IStartup类型的Configure方法构建委托链，再构建RequestDelegate

构建好RequestDelegate后，再调用HostedServiceExecutor来执行IHostedService

internal class HostedServiceExecutor
    {
        private readonly IEnumerable<IHostedService> _services;
        private readonly ILogger<HostedServiceExecutor> _logger;

        public HostedServiceExecutor(ILogger<HostedServiceExecutor> logger, IEnumerable<IHostedService> services)
        {
            _logger = logger;
            _services = services;
        }

        public Task StartAsync(CancellationToken token)
        {
            return ExecuteAsync(service => service.StartAsync(token));
        }

        public Task StopAsync(CancellationToken token)
        {
            return ExecuteAsync(service => service.StopAsync(token), throwOnFirstFailure: false);
        }

        private async Task ExecuteAsync(Func<IHostedService, Task> callback, bool throwOnFirstFailure = true)
        {
            List<Exception> exceptions = null;

            foreach (var service in _services)
            {
                try
                {
                    await callback(service);
                }
                catch (Exception ex)
                {
                    if (throwOnFirstFailure)
                    {
                        throw;
                    }

                    if (exceptions == null)
                    {
                        exceptions = new List<Exception>();
                    }

                    exceptions.Add(ex);
                }
            }

            // Throw an aggregate exception if there were any exceptions
            if (exceptions != null)
            {
                throw new AggregateException(exceptions);
            }
        }
    }

接下来看下IHttpContextFactory怎么创建HttpContext的

 IHttpContextFactory接口的默认实现DefaultHttpContextFactory

public class DefaultHttpContextFactory : IHttpContextFactory
    {
        private readonly IHttpContextAccessor _httpContextAccessor;
        private readonly FormOptions _formOptions;
        private readonly IServiceScopeFactory _serviceScopeFactory;

        // This takes the IServiceProvider because it needs to support an ever expanding
        // set of services that flow down into HttpContext features
        public DefaultHttpContextFactory(IServiceProvider serviceProvider)
        {
            // May be null
            _httpContextAccessor = serviceProvider.GetService<IHttpContextAccessor>();
            _formOptions = serviceProvider.GetRequiredService<IOptions<FormOptions>>().Value;
            _serviceScopeFactory = serviceProvider.GetRequiredService<IServiceScopeFactory>();
        }

        public HttpContext Create(IFeatureCollection featureCollection)
        {
            if (featureCollection is null)
            {
                throw new ArgumentNullException(nameof(featureCollection));
            }

            var httpContext = new DefaultHttpContext(featureCollection);
            Initialize(httpContext);
            return httpContext;
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        internal void Initialize(DefaultHttpContext httpContext, IFeatureCollection featureCollection)
        {
            Debug.Assert(featureCollection != null);
            Debug.Assert(httpContext != null);

            httpContext.Initialize(featureCollection);

            Initialize(httpContext);
        }

        [MethodImpl(MethodImplOptions.AggressiveInlining)]
        private DefaultHttpContext Initialize(DefaultHttpContext httpContext)
        {
            if (_httpContextAccessor != null)
            {
                _httpContextAccessor.HttpContext = httpContext;
            }

            httpContext.FormOptions = _formOptions;
            httpContext.ServiceScopeFactory = _serviceScopeFactory;

            return httpContext;
        }

        public void Dispose(HttpContext httpContext)
        {
            if (_httpContextAccessor != null)
            {
                _httpContextAccessor.HttpContext = null;
            }
        }

        internal void Dispose(DefaultHttpContext httpContext)
        {
            if (_httpContextAccessor != null)
            {
                _httpContextAccessor.HttpContext = null;
            }

            httpContext.Uninitialize();
        }
    }

再来看下IHttpApplication接口

 实现类HostingApplication：

internal class HostingApplication : IHttpApplication<HostingApplication.Context>
    {
        private readonly RequestDelegate _application;
        private readonly IHttpContextFactory _httpContextFactory;
        private readonly DefaultHttpContextFactory _defaultHttpContextFactory;
        private HostingApplicationDiagnostics _diagnostics;

        public HostingApplication(
            RequestDelegate application,
            ILogger logger,
            DiagnosticListener diagnosticSource,
            IHttpContextFactory httpContextFactory)
        {
            _application = application;
            _diagnostics = new HostingApplicationDiagnostics(logger, diagnosticSource);
            if (httpContextFactory is DefaultHttpContextFactory factory)
            {
                _defaultHttpContextFactory = factory;
            }
            else
            {
                _httpContextFactory = httpContextFactory;
            }
        }

        // Set up the request
        public Context CreateContext(IFeatureCollection contextFeatures)
        {
            Context hostContext;
            if (contextFeatures is IHostContextContainer<Context> container)
            {
                hostContext = container.HostContext;
                if (hostContext is null)
                {
                    hostContext = new Context();
                    container.HostContext = hostContext;
                }
            }
            else
            {
                // Server doesn't support pooling, so create a new Context
                hostContext = new Context();
            }

            HttpContext httpContext;
            if (_defaultHttpContextFactory != null)
            {
                var defaultHttpContext = (DefaultHttpContext)hostContext.HttpContext;
                if (defaultHttpContext is null)
                {
                    httpContext = _defaultHttpContextFactory.Create(contextFeatures);
                    hostContext.HttpContext = httpContext;
                }
                else
                {
                    _defaultHttpContextFactory.Initialize(defaultHttpContext, contextFeatures);
                    httpContext = defaultHttpContext;
                }
            }
            else
            {
                httpContext = _httpContextFactory.Create(contextFeatures);
                hostContext.HttpContext = httpContext;
            }

            _diagnostics.BeginRequest(httpContext, hostContext);
            return hostContext;
        }

        // Execute the request
        public Task ProcessRequestAsync(Context context)
        {
            return _application(context.HttpContext);
        }

        // Clean up the request
        public void DisposeContext(Context context, Exception exception)
        {
            var httpContext = context.HttpContext;
            _diagnostics.RequestEnd(httpContext, exception, context);

            if (_defaultHttpContextFactory != null)
            {
                _defaultHttpContextFactory.Dispose((DefaultHttpContext)httpContext);
            }
            else
            {
                _httpContextFactory.Dispose(httpContext);
            }

            _diagnostics.ContextDisposed(context);

            // Reset the context as it may be pooled
            context.Reset();
        }


        internal class Context
        {
            public HttpContext HttpContext { get; set; }
            public IDisposable Scope { get; set; }
            public Activity Activity { get; set; }

            public long StartTimestamp { get; set; }
            internal bool HasDiagnosticListener { get; set; }
            public bool EventLogEnabled { get; set; }

            public void Reset()
            {
                // Not resetting HttpContext here as we pool it on the Context

                Scope = null;
                Activity = null;

                StartTimestamp = 0;
                HasDiagnosticListener = false;
                EventLogEnabled = false;
            }
        }
    }

其中内部类Context对请求进行了封装

介绍上面的两个类型，主要是IServer的StartAsync方法调用到

var diagnosticSource = _applicationServices.GetRequiredService<DiagnosticListener>();
            var httpContextFactory = _applicationServices.GetRequiredService<IHttpContextFactory>();
            var hostingApp = new HostingApplication(application, _logger, diagnosticSource, httpContextFactory);
            await Server.StartAsync(hostingApp, cancellationToken).ConfigureAwait(false);
            _startedServer = true;

            // Fire IApplicationLifetime.Started
            _applicationLifetime?.NotifyStarted();

这时候进入到了IServer处理请求的环节了

以上主要介绍了IWebHost的运行原理。