.net core mvc启动顺序以及主要部件2

前一篇提到WebHost.CreateDefaultBuilder(args)方法创建了WebHostBuilder实例，WebHostBuilder实例有三个主要功能 1、构建了IConfiguration实例和基础环境配置，2、构建了IServiceCollection服务，也就是依赖注入的容器，3、创建了webhost实例，这个webhost就是我们的接收请求的第一个管道，其中暴露出来的主要方法Build，请看源代码：

public IWebHost Build()
        {
            if (!_webHostBuilt)
            {
                _webHostBuilt = true;
                AggregateException hostingStartupErrors;
                IServiceCollection serviceCollection = BuildCommonServices(out hostingStartupErrors);
                IServiceCollection serviceCollection2 = serviceCollection.Clone();
                IServiceProvider hostingServiceProvider = _003CBuild_003Eg__GetProviderFromFactory_007C13_0(serviceCollection);
                if (!_options.SuppressStatusMessages)
                {
                    if (Environment.GetEnvironmentVariable("Hosting:Environment") != null)
                    {
                        Console.WriteLine("The environment variable 'Hosting:Environment' is obsolete and has been replaced with 'ASPNETCORE_ENVIRONMENT'");
                    }
                    if (Environment.GetEnvironmentVariable("ASPNET_ENV") != null)
                    {
                        Console.WriteLine("The environment variable 'ASPNET_ENV' is obsolete and has been replaced with 'ASPNETCORE_ENVIRONMENT'");
                    }
                    if (Environment.GetEnvironmentVariable("ASPNETCORE_SERVER.URLS") != null)
                    {
                        Console.WriteLine("The environment variable 'ASPNETCORE_SERVER.URLS' is obsolete and has been replaced with 'ASPNETCORE_URLS'");
                    }
                }
                AddApplicationServices(serviceCollection2, hostingServiceProvider);
                WebHost webHost = new WebHost(serviceCollection2, hostingServiceProvider, _options, _config, hostingStartupErrors);
                try
                {
                    webHost.Initialize();
                    ILogger<WebHost> requiredService = webHost.Services.GetRequiredService<ILogger<WebHost>>();
                    foreach (IGrouping<string, string> item in from g in _options.GetFinalHostingStartupAssemblies().GroupBy((string a) => a, StringComparer.OrdinalIgnoreCase)
                                                               where g.Count() > 1
                                                               select g)
                    {
                        requiredService.LogWarning($"The assembly {item} was specified multiple times. Hosting startup assemblies should only be specified once.");
                    }
                    return webHost;
                }
                catch
                {
                    webHost.Dispose();
                    throw;
                }
            }
            throw new InvalidOperationException(Resources.WebHostBuilder_SingleInstance);
        }

这个方法中有几个笔记重要的部分，下面我给标记一下

调用BuildCommonServices私有方法主要是注入一些程序所需的基础组件，请看源代码：

 private IServiceCollection BuildCommonServices(out AggregateException hostingStartupErrors)
        {
            hostingStartupErrors = null;
            _options = new WebHostOptions(_config, Assembly.GetEntryAssembly()?.GetName().Name);
            if (!_options.PreventHostingStartup)
            {
                List<Exception> list = new List<Exception>();
                foreach (string item in _options.GetFinalHostingStartupAssemblies().Distinct(StringComparer.OrdinalIgnoreCase))
                {
                    try
                    {
                        foreach (HostingStartupAttribute customAttribute in Assembly.Load(new AssemblyName(item)).GetCustomAttributes<HostingStartupAttribute>())
                        {
                            ((IHostingStartup)Activator.CreateInstance(customAttribute.HostingStartupType)).Configure(this);
                        }
                    }
                    catch (Exception innerException)
                    {
                        list.Add(new InvalidOperationException("Startup assembly " + item + " failed to execute. See the inner exception for more details.", innerException));
                    }
                }
                if (list.Count > 0)
                {
                    hostingStartupErrors = new AggregateException(list);
                }
            }
            string contentRootPath = ResolveContentRootPath(_options.ContentRootPath, AppContext.BaseDirectory);
            _hostingEnvironment.Initialize(contentRootPath, _options);
            _context.HostingEnvironment = _hostingEnvironment;
            ServiceCollection serviceCollection = new ServiceCollection();
            serviceCollection.AddSingleton(_options);
            ((IServiceCollection)serviceCollection).AddSingleton((IHostingEnvironment)_hostingEnvironment);
            ((IServiceCollection)serviceCollection).AddSingleton((Microsoft.Extensions.Hosting.IHostingEnvironment)_hostingEnvironment);
            serviceCollection.AddSingleton(_context);
            IConfigurationBuilder configurationBuilder = new ConfigurationBuilder().SetBasePath(_hostingEnvironment.ContentRootPath).AddConfiguration(_config);
            foreach (Action<WebHostBuilderContext, IConfigurationBuilder> configureAppConfigurationBuilderDelegate in _configureAppConfigurationBuilderDelegates)
            {
                configureAppConfigurationBuilderDelegate(_context, configurationBuilder);
            }
            IConfigurationRoot configurationRoot = configurationBuilder.Build();
            ((IServiceCollection)serviceCollection).AddSingleton((IConfiguration)configurationRoot);
            _context.Configuration = configurationRoot;
            DiagnosticListener implementationInstance = new DiagnosticListener("Microsoft.AspNetCore");
            serviceCollection.AddSingleton(implementationInstance);
            ((IServiceCollection)serviceCollection).AddSingleton((DiagnosticSource)implementationInstance);
            serviceCollection.AddTransient<IApplicationBuilderFactory, ApplicationBuilderFactory>();
            serviceCollection.AddTransient<IHttpContextFactory, HttpContextFactory>();
            serviceCollection.AddScoped<IMiddlewareFactory, MiddlewareFactory>();
            serviceCollection.AddOptions();
            serviceCollection.AddLogging();
            serviceCollection.AddTransient<IStartupFilter, AutoRequestServicesStartupFilter>();
            serviceCollection.AddTransient<IServiceProviderFactory<IServiceCollection>, DefaultServiceProviderFactory>();
            serviceCollection.AddSingleton<ObjectPoolProvider, DefaultObjectPoolProvider>();
            if (!string.IsNullOrEmpty(_options.StartupAssembly))
            {
                try
                {
                    Type startupType = StartupLoader.FindStartupType(_options.StartupAssembly, _hostingEnvironment.EnvironmentName);
                    if (IntrospectionExtensions.GetTypeInfo(typeof(IStartup)).IsAssignableFrom(startupType.GetTypeInfo()))
                    {
                        serviceCollection.AddSingleton(typeof(IStartup), startupType);
                    }
                    else
                    {
                        serviceCollection.AddSingleton(typeof(IStartup), delegate (IServiceProvider sp)
                        {
                            IHostingEnvironment requiredService = sp.GetRequiredService<IHostingEnvironment>();
                            return new ConventionBasedStartup(StartupLoader.LoadMethods(sp, startupType, requiredService.EnvironmentName));
                        });
                    }
                }
                catch (Exception source)
                {
                    ExceptionDispatchInfo capture = ExceptionDispatchInfo.Capture(source);
                    ((IServiceCollection)serviceCollection).AddSingleton((Func<IServiceProvider, IStartup>)delegate
                    {
                        capture.Throw();
                        return null;
                    });
                }
            }
            foreach (Action<WebHostBuilderContext, IServiceCollection> configureServicesDelegate in _configureServicesDelegates)
            {
                configureServicesDelegate(_context, serviceCollection);
            }
            return serviceCollection;
        }

这个方法中其中就包含了注入IConfiguration的代码和构建IServiceCollection容器的代码，当然还有一些其他的主要中间件注入啊IApplicationBuilderFactory，包括HttpContextFactory等等都在这个方法里面被IServiceCollection容器管理，具体请看下图

这里讲清楚之后，我们直接进入UseStartup方法中，这个方法是注入了Startup的实例，请看主要源代码

public static IWebHostBuilder UseStartup(this IWebHostBuilder hostBuilder, Type startupType)
        {
            string name = startupType.GetTypeInfo().Assembly.GetName().Name;
            return hostBuilder.UseSetting(WebHostDefaults.ApplicationKey, name).ConfigureServices(delegate (IServiceCollection services)
            {
                if (IntrospectionExtensions.GetTypeInfo(typeof(IStartup)).IsAssignableFrom(startupType.GetTypeInfo()))
                {
                    services.AddSingleton(typeof(IStartup), startupType);
                }
                else
                {
                    services.AddSingleton(typeof(IStartup), delegate (IServiceProvider sp)
                    {
                        IHostingEnvironment requiredService = sp.GetRequiredService<IHostingEnvironment>();
                        return new ConventionBasedStartup(StartupLoader.LoadMethods(sp, startupType, requiredService.EnvironmentName));
                    });
                }
            });
        }

上面只是被IServiceCollection所管理  并没有真正起作用，真正调用的是Main方法中的Build().Run()，这里的Build就是上面那个IWebHostBuilder中的主要的构建方法和调用Startup中的ConfigureServices方法，而Run则是正式启动应用程序

请看代码：

分享Program类的过程大致就到这里了