asp.net core 3.1 源码学习之 GenericWebHostBuilder

先看下如下的扩展方法

public static class GenericHostWebHostBuilderExtensions
    {
        public static IHostBuilder ConfigureWebHost(this IHostBuilder builder, Action<IWebHostBuilder> configure)
        {
            var webhostBuilder = new GenericWebHostBuilder(builder);
            configure(webhostBuilder);
            builder.ConfigureServices((context, services) => services.AddHostedService<GenericWebHostService>());
            return builder;
        }
    }

当我们调用IHostBuilder的扩展方法ConfigureWebHost时候，系统创建了GenericWebHostBuilder类，并注册GenericWebHostService类

再分析下GenericWebHostBuilder

public GenericWebHostBuilder(IHostBuilder builder)
        {
            _builder = builder;

            _config = new ConfigurationBuilder()
                .AddEnvironmentVariables(prefix: "ASPNETCORE_")
                .Build();

            _builder.ConfigureHostConfiguration(config =>
            {
                config.AddConfiguration(_config);

                // We do this super early but still late enough that we can process the configuration
                // wired up by calls to UseSetting
                ExecuteHostingStartups();
            });

            // IHostingStartup needs to be executed before any direct methods on the builder
            // so register these callbacks first
            _builder.ConfigureAppConfiguration((context, configurationBuilder) =>
            {
                if (_hostingStartupWebHostBuilder != null)
                {
                    var webhostContext = GetWebHostBuilderContext(context);
                    _hostingStartupWebHostBuilder.ConfigureAppConfiguration(webhostContext, configurationBuilder);
                }
            });

            _builder.ConfigureServices((context, services) =>
            {
                var webhostContext = GetWebHostBuilderContext(context);
                var webHostOptions = (WebHostOptions)context.Properties[typeof(WebHostOptions)];

                // Add the IHostingEnvironment and IApplicationLifetime from Microsoft.AspNetCore.Hosting
                services.AddSingleton(webhostContext.HostingEnvironment);
#pragma warning disable CS0618 // Type or member is obsolete
                services.AddSingleton((AspNetCore.Hosting.IHostingEnvironment)webhostContext.HostingEnvironment);
                services.AddSingleton<IApplicationLifetime, GenericWebHostApplicationLifetime>();
#pragma warning restore CS0618 // Type or member is obsolete

                services.Configure<GenericWebHostServiceOptions>(options =>
                {
                    // Set the options
                    options.WebHostOptions = webHostOptions;
                    // Store and forward any startup errors
                    options.HostingStartupExceptions = _hostingStartupErrors;
                });

                // REVIEW: This is bad since we don't own this type. Anybody could add one of these and it would mess things up
                // We need to flow this differently
                var listener = new DiagnosticListener("Microsoft.AspNetCore");
                services.TryAddSingleton<DiagnosticListener>(listener);
                services.TryAddSingleton<DiagnosticSource>(listener);

                services.TryAddSingleton<IHttpContextFactory, DefaultHttpContextFactory>();
                services.TryAddScoped<IMiddlewareFactory, MiddlewareFactory>();
                services.TryAddSingleton<IApplicationBuilderFactory, ApplicationBuilderFactory>();

                // IMPORTANT: This needs to run *before* direct calls on the builder (like UseStartup)
                _hostingStartupWebHostBuilder?.ConfigureServices(webhostContext, services);

                // Support UseStartup(assemblyName)
                if (!string.IsNullOrEmpty(webHostOptions.StartupAssembly))
                {
                    try
                    {
                        var startupType = StartupLoader.FindStartupType(webHostOptions.StartupAssembly, webhostContext.HostingEnvironment.EnvironmentName);
                        UseStartup(startupType, context, services);
                    }
                    catch (Exception ex) when (webHostOptions.CaptureStartupErrors)
                    {
                        var capture = ExceptionDispatchInfo.Capture(ex);

                        services.Configure<GenericWebHostServiceOptions>(options =>
                        {
                            options.ConfigureApplication = app =>
                            {
                                // Throw if there was any errors initializing startup
                                capture.Throw();
                            };
                        });
                    }
                }
            });
        }

GenericWebHostBuilder的构造函数接收IHostBuilder作为参数，其实是把asp.netcore用到的服务和配置加载到IHostBuilder中

添加一“ASPNETCORE_”开头的环境变量

寻找程序集有HostingStartupAttribute的特性，调用IHostingStartup接口对IWebHostBuilder进行配置

services.TryAddSingleton<IHttpContextFactory, DefaultHttpContextFactory>();
                services.TryAddScoped<IMiddlewareFactory, MiddlewareFactory>();
                services.TryAddSingleton<IApplicationBuilderFactory, ApplicationBuilderFactory>();

接下来调用实现IStartUp接口的类

private void UseStartup(Type startupType, HostBuilderContext context, IServiceCollection services)
        {
            var webHostBuilderContext = GetWebHostBuilderContext(context);
            var webHostOptions = (WebHostOptions)context.Properties[typeof(WebHostOptions)];

            ExceptionDispatchInfo startupError = null;
            object instance = null;
            ConfigureBuilder configureBuilder = null;

            try
            {
                // We cannot support methods that return IServiceProvider as that is terminal and we need ConfigureServices to compose
                if (typeof(IStartup).IsAssignableFrom(startupType))
                {
                    throw new NotSupportedException($"{typeof(IStartup)} isn't supported");
                }
                if (StartupLoader.HasConfigureServicesIServiceProviderDelegate(startupType, context.HostingEnvironment.EnvironmentName))
                {
                    throw new NotSupportedException($"ConfigureServices returning an {typeof(IServiceProvider)} isn't supported.");
                }

                instance = ActivatorUtilities.CreateInstance(new HostServiceProvider(webHostBuilderContext), startupType);
                context.Properties[_startupKey] = instance;

                // Startup.ConfigureServices
                var configureServicesBuilder = StartupLoader.FindConfigureServicesDelegate(startupType, context.HostingEnvironment.EnvironmentName);
                var configureServices = configureServicesBuilder.Build(instance);

                configureServices(services);

                // REVIEW: We're doing this in the callback so that we have access to the hosting environment
                // Startup.ConfigureContainer
                var configureContainerBuilder = StartupLoader.FindConfigureContainerDelegate(startupType, context.HostingEnvironment.EnvironmentName);
                if (configureContainerBuilder.MethodInfo != null)
                {
                    var containerType = configureContainerBuilder.GetContainerType();
                    // Store the builder in the property bag
                    _builder.Properties[typeof(ConfigureContainerBuilder)] = configureContainerBuilder;

                    var actionType = typeof(Action<,>).MakeGenericType(typeof(HostBuilderContext), containerType);

                    // Get the private ConfigureContainer method on this type then close over the container type
                    var configureCallback = GetType().GetMethod(nameof(ConfigureContainer), BindingFlags.NonPublic | BindingFlags.Instance)
                                                     .MakeGenericMethod(containerType)
                                                     .CreateDelegate(actionType, this);

                    // _builder.ConfigureContainer<T>(ConfigureContainer);
                    typeof(IHostBuilder).GetMethods().First(m => m.Name == nameof(IHostBuilder.ConfigureContainer))
                        .MakeGenericMethod(containerType)
                        .InvokeWithoutWrappingExceptions(_builder, new object[] { configureCallback });
                }

                // Resolve Configure after calling ConfigureServices and ConfigureContainer
                configureBuilder = StartupLoader.FindConfigureDelegate(startupType, context.HostingEnvironment.EnvironmentName);
            }
            catch (Exception ex) when (webHostOptions.CaptureStartupErrors)
            {
                startupError = ExceptionDispatchInfo.Capture(ex);
            }

            // Startup.Configure
            services.Configure<GenericWebHostServiceOptions>(options =>
            {
                options.ConfigureApplication = app =>
                {
                    // Throw if there was any errors initializing startup
                    startupError?.Throw();

                    // Execute Startup.Configure
                    if (instance != null && configureBuilder != null)
                    {
                        configureBuilder.Build(instance)(app);
                    }
                };
            });
        }

先找到ConfigureServices方法注册服务

再找到ConfigureContainer方法配置容器

最后找到Configure方法配置请求链

我们再看下GenericWebHostService类

internal class GenericWebHostService : IHostedService
    {
        public GenericWebHostService(IOptions<GenericWebHostServiceOptions> options,
                                     IServer server,
                                     ILoggerFactory loggerFactory,
                                     DiagnosticListener diagnosticListener,
                                     IHttpContextFactory httpContextFactory,
                                     IApplicationBuilderFactory applicationBuilderFactory,
                                     IEnumerable<IStartupFilter> startupFilters,
                                     IConfiguration configuration,
                                     IWebHostEnvironment hostingEnvironment)
        {
            Options = options.Value;
            Server = server;
            Logger = loggerFactory.CreateLogger("Microsoft.AspNetCore.Hosting.Diagnostics");
            LifetimeLogger = loggerFactory.CreateLogger("Microsoft.Hosting.Lifetime");
            DiagnosticListener = diagnosticListener;
            HttpContextFactory = httpContextFactory;
            ApplicationBuilderFactory = applicationBuilderFactory;
            StartupFilters = startupFilters;
            Configuration = configuration;
            HostingEnvironment = hostingEnvironment;
        }

        public GenericWebHostServiceOptions Options { get; }
        public IServer Server { get; }
        public ILogger Logger { get; }
        // Only for high level lifetime events
        public ILogger LifetimeLogger { get; }
        public DiagnosticListener DiagnosticListener { get; }
        public IHttpContextFactory HttpContextFactory { get; }
        public IApplicationBuilderFactory ApplicationBuilderFactory { get; }
        public IEnumerable<IStartupFilter> StartupFilters { get; }
        public IConfiguration Configuration { get; }
        public IWebHostEnvironment HostingEnvironment { get; }

        public async Task StartAsync(CancellationToken cancellationToken)
        {
            HostingEventSource.Log.HostStart();

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

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

            RequestDelegate application = null;

            try
            {
                Action<IApplicationBuilder> configure = Options.ConfigureApplication;

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

                var builder = ApplicationBuilderFactory.CreateBuilder(Server.Features);

                foreach (var filter in StartupFilters.Reverse())
                {
                    configure = filter.Configure(configure);
                }

                configure(builder);

                // Build the request pipeline
                application = builder.Build();
            }
            catch (Exception ex)
            {
                Logger.ApplicationError(ex);

                if (!Options.WebHostOptions.CaptureStartupErrors)
                {
                    throw;
                }

                application = BuildErrorPageApplication(ex);
            }

            var httpApplication = new HostingApplication(application, Logger, DiagnosticListener, HttpContextFactory);

            await Server.StartAsync(httpApplication, cancellationToken);

            if (addresses != null)
            {
                foreach (var address in addresses)
                {
                    LifetimeLogger.LogInformation("Now listening on: {address}", address);
                }
            }

            if (Logger.IsEnabled(LogLevel.Debug))
            {
                foreach (var assembly in Options.WebHostOptions.GetFinalHostingStartupAssemblies())
                {
                    Logger.LogDebug("Loaded hosting startup assembly {assemblyName}", assembly);
                }
            }

            if (Options.HostingStartupExceptions != null)
            {
                foreach (var exception in Options.HostingStartupExceptions.InnerExceptions)
                {
                    Logger.HostingStartupAssemblyError(exception);
                }
            }
        }

        private RequestDelegate BuildErrorPageApplication(Exception exception)
        {
            if (exception is TargetInvocationException tae)
            {
                exception = tae.InnerException;
            }

            var showDetailedErrors = HostingEnvironment.IsDevelopment() || Options.WebHostOptions.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(
                    HostingEnvironment.ContentRootFileProvider,
                    sourceCodeLineCount: 6);

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

            var errorPage = new ErrorPage(model);
            return context =>
            {
                context.Response.StatusCode = 500;
                context.Response.Headers[HeaderNames.CacheControl] = "no-cache";
                context.Response.ContentType = "text/html; charset=utf-8";
                return errorPage.ExecuteAsync(context);
            };
        }

        public async Task StopAsync(CancellationToken cancellationToken)
        {
            try
            {
                await Server.StopAsync(cancellationToken);
            }
            finally
            {
                HostingEventSource.Log.HostStop();
            }
        }
    }

看下核心方法StartAsync

从GenericWebHostServiceOptions的ConfigureApplication属性中获取RequestDelegate委托链

调用IApplicationBuilderFactory接口的CreateBuilder创建IApplicationBuilder

再调用IStartupFilter的Configure方法配置整个委托链

接着调用IApplicationBuilder方法构建最终的RequestDelegate

接着创建HostingApplication对象

最后调用IServer接口的StartAsync开始处理请求