.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：

 1 public IWebHost Build()
 2         {
 3             if (!_webHostBuilt)
 4             {
 5                 _webHostBuilt = true;
 6                 AggregateException hostingStartupErrors;
 7                 IServiceCollection serviceCollection = BuildCommonServices(out hostingStartupErrors);
 8                 IServiceCollection serviceCollection2 = serviceCollection.Clone();
 9                 IServiceProvider hostingServiceProvider = _003CBuild_003Eg__GetProviderFromFactory_007C13_0(serviceCollection);
10                 if (!_options.SuppressStatusMessages)
11                 {
12                     if (Environment.GetEnvironmentVariable("Hosting:Environment") != null)
13                     {
14                         Console.WriteLine("The environment variable 'Hosting:Environment' is obsolete and has been replaced with 'ASPNETCORE_ENVIRONMENT'");
15                     }
16                     if (Environment.GetEnvironmentVariable("ASPNET_ENV") != null)
17                     {
18                         Console.WriteLine("The environment variable 'ASPNET_ENV' is obsolete and has been replaced with 'ASPNETCORE_ENVIRONMENT'");
19                     }
20                     if (Environment.GetEnvironmentVariable("ASPNETCORE_SERVER.URLS") != null)
21                     {
22                         Console.WriteLine("The environment variable 'ASPNETCORE_SERVER.URLS' is obsolete and has been replaced with 'ASPNETCORE_URLS'");
23                     }
24                 }
25                 AddApplicationServices(serviceCollection2, hostingServiceProvider);
26                 WebHost webHost = new WebHost(serviceCollection2, hostingServiceProvider, _options, _config, hostingStartupErrors);
27                 try
28                 {
29                     webHost.Initialize();
30                     ILogger<WebHost> requiredService = webHost.Services.GetRequiredService<ILogger<WebHost>>();
31                     foreach (IGrouping<string, string> item in from g in _options.GetFinalHostingStartupAssemblies().GroupBy((string a) => a, StringComparer.OrdinalIgnoreCase)
32                                                                where g.Count() > 1
33                                                                select g)
34                     {
35                         requiredService.LogWarning($"The assembly {item} was specified multiple times. Hosting startup assemblies should only be specified once.");
36                     }
37                     return webHost;
38                 }
39                 catch
40                 {
41                     webHost.Dispose();
42                     throw;
43                 }
44             }
45             throw new InvalidOperationException(Resources.WebHostBuilder_SingleInstance);
46         }

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