04 | Startup：掌握ASP.NET Core的启动过程

新建一个 ASP.NET Core Web 应用程序

选择 API

public class Program
{public static void Main(string[] args){CreateHostBuilder(args).Build().Run();}public static IHostBuilder CreateHostBuilder(string[] args) =>Host.CreateDefaultBuilder(args).ConfigureWebHostDefaults(webBuilder =>{webBuilder.UseStartup<Startup>();});
}

在 Program.cs 的 Main 函数中

CreateHostBuilder 方法返回了一个 IHostBuilder

它是应用程序启动的核心接口

IHostBuilder 接口有六个方法：

主要关注以下三个：

• ConfigureAppConfiguration

• ConfigureHostConfiguration

• ConfigureServices

接下来，我们添加一些代码演示整个应用程序的启动过程：

public static IHostBuilder CreateHostBuilder(string[] args) =>Host.CreateDefaultBuilder(args).ConfigureAppConfiguration(builder =>{Console.WriteLine("ConfigureAppConfiguration");}).ConfigureServices(service =>{Console.WriteLine("ConfigureServices");}).ConfigureHostConfiguration(builder =>{Console.WriteLine("ConfigureHostConfiguration");}).ConfigureWebHostDefaults(webBuilder =>{Console.WriteLine("ConfigureWebHostDefaults");webBuilder.UseStartup<Startup>();});

接着，在 Startup 的三个方法中添加一些代码

public Startup(IConfiguration configuration)
{Console.WriteLine("Startup");...public void ConfigureServices(IServiceCollection services)
{Console.WriteLine("ConfigureServices");...public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{Console.WriteLine("Configure");...

启动程序查看输出：

ConfigureWebHostDefaults
ConfigureHostConfiguration
ConfigureAppConfiguration
ConfigureServices
Startup
Startup.ConfigureServices
Startup.Configure

调整一下委托的注册顺序

public static IHostBuilder CreateHostBuilder(string[] args) =>Host.CreateDefaultBuilder(args).ConfigureWebHostDefaults(webBuilder =>{Console.WriteLine("ConfigureWebHostDefaults");webBuilder.UseStartup<Startup>();}).ConfigureServices(service =>{Console.WriteLine("ConfigureServices");}).ConfigureAppConfiguration(builder =>{Console.WriteLine("ConfigureAppConfiguration");}).ConfigureHostConfiguration(builder =>{Console.WriteLine("ConfigureHostConfiguration");});

会得到不同的结果

ConfigureWebHostDefaults
ConfigureHostConfiguration
ConfigureAppConfiguration
Startup
Startup.ConfigureServices
ConfigureServices
Startup.Configure

本质上，如果查看源码会发现，委托注册进去之后，实际上是按照一定的顺序来执行的：

1、ConfigureWebHostDefaults

这个阶段注册了应用程序必要的几个组件，比如配置的组件、容器组件

2、ConfigureHostConfiguration

用于配置应用程序启动时必要的配置，比如应用程序启动时所需要监听的端口，URL 地址

在这个过程可以嵌入一些自己配置的内容，注入到配置的框架中

3、ConfigureAppConfiguration

用于嵌入自己的配置文件，供应用程序读取，这些配置将会在后续的应用程序执行过程中间每个组件读取

4、ConfigureServices, ConfigureLogging, Startup, Startup.ConfigureServices

用于往容器里注入应用的组件

5、Startup.Configure

用于注入中间件，处理 HttpContext 整个请求过程

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{Console.WriteLine("Startup.Configure");if (env.IsDevelopment()){app.UseDeveloperExceptionPage();}app.UseHttpsRedirection();app.UseRouting();app.UseAuthorization();app.UseFileServer();app.UseWebSockets();app.UseEndpoints(endpoints =>{endpoints.MapControllers();});
}

在整个启动的过程中，Startup 这个类不是必要的，只是让代码结构更加合理

public static IHostBuilder CreateHostBuilder(string[] args) =>Host.CreateDefaultBuilder(args).ConfigureWebHostDefaults(webBuilder =>{Console.WriteLine("ConfigureWebHostDefaults");//webBuilder.UseStartup<Startup>();webBuilder.ConfigureServices(services =>{Console.WriteLine("webBuilder.ConfigureServices");services.AddControllers();});webBuilder.Configure(app =>{Console.WriteLine("webBuilder.Configure");app.UseHttpsRedirection();app.UseRouting();app.UseAuthorization();app.UseFileServer();app.UseWebSockets();app.UseEndpoints(endpoints =>{endpoints.MapControllers();});});}).ConfigureServices(service =>{Console.WriteLine("ConfigureServices");}).ConfigureAppConfiguration(builder =>{Console.WriteLine("ConfigureAppConfiguration");}).ConfigureHostConfiguration(builder =>{Console.WriteLine("ConfigureHostConfiguration");});
}

启动程序查看输出：

ConfigureWebHostDefaults
ConfigureHostConfiguration
ConfigureAppConfiguration
webBuilder.ConfigureServices
ConfigureServices
webBuilder.Configure

服务注册一般放在 Startup 的 ConfigureServices，一般是services.AddXXX

public void ConfigureServices(IServiceCollection services)
{services.AddMvc();services.AddAuthentication();services.AddAuthorization();Console.WriteLine("Startup.ConfigureServices");services.AddControllers();
}

中间件的注册一般放在 Startup 的 Configure

public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
{Console.WriteLine("Startup.Configure");if (env.IsDevelopment()){app.UseDeveloperExceptionPage();}app.UseHttpsRedirection();app.UseRouting();app.UseAuthorization();app.UseFileServer();app.UseWebSockets();app.UseEndpoints(endpoints =>{endpoints.MapControllers();});
}