.net core 源码解析-web app是如何启动并接收处理请求

最近.net core 1.1也发布了，蹒跚学步的小孩又长高了一些，园子里大家也都非常积极的在学习，闲来无事，扒拔源码，涨涨见识。

先来见识一下web站点是如何启动的，如何接受请求,.net core web app最简单的例子,大约长这样

 public static void Main(string[] args)    
{           
  //dotnet NetCoreWebApp.dll --server.urls="http://localhost:5000/;http://localhost:5001/"var config = new ConfigurationBuilder().AddCommandLine(args).Build();       new WebHostBuilder().UseConfiguration(config).UseKestrel().UseContentRoot(Directory.GetCurrentDirectory())                //.UseIISIntegration().UseStartup<Startup>()                //.Configure(confApp =>//{//    confApp.Run(context =>//    {//        return context.Response.WriteAsync("hello");//    });//}).Build().Run();}

WebHostBuilder看名字也知道是为了构建WebHost而存在的。在构建WebHost的路上他都做了这些：如加载配置，注册服务，配置功能等。

1.1 加载配置

builder内部维护了一个IConfiguration _config，可以简单的理解为key-value集合对象。可以通过UseSetting增加，也可以通过UseConfiguration增加

WebHostBuilder对UseStartup ()的解析实现

我们从官方代码例子中能看到Startup类只是一个普通的类，builder是如何调用到这个类的方法的呢？
Build方法关于这一块的代码大概如下：

private IServiceCollection BuildHostingServices(){   

 var startupType = StartupLoader.FindStartupType(_options.StartupAssembly, _hostingEnvironment.EnvironmentName); 
    if (typeof(IStartup).GetTypeInfo().IsAssignableFrom(startupType.GetTypeInfo())){services.AddSingleton(typeof(IStartup), startupType);}   
   else{services.AddSingleton(typeof(IStartup), sp =>{         
     var hostingEnvironment = sp.GetRequiredService<IHostingEnvironment>();          
      var methods = StartupLoader.LoadMethods(sp, startupType, hostingEnvironment.EnvironmentName);           
       return new ConventionBasedStartup(methods);});}
}

能看出来其实Startup可以是一个实现了IStartup接口的类。为什么官方还需要搞一个普通类的方式呢？其实这里还有一个小技巧：
针对Configure和ConfigureServices方法我们还可以做的更多，那就是根据不同的environmentName调用不同的方法。
Configure方法可以是Configure+EnvironmentName，ConfigureServices则是Configure+EnvironmentName+Services。这样的话还能做到区分环境进去不同的配置。
下面代码展示了builder是如何选择这2个方法的

 private static ConfigureBuilder FindConfigureDelegate(Type startupType, string environmentName)       
 {           
 
  var configureMethod = FindMethod(startupType, "Configure{0}", environmentName, typeof(void), required: true);   
  return new ConfigureBuilder(configureMethod);}       
  
private static ConfigureServicesBuilder FindConfigureServicesDelegate(Type startupType, string environmentName)    
{            
  var servicesMethod = FindMethod(startupType, "Configure{0}Services", environmentName, typeof(IServiceProvider), required: false)?? FindMethod(startupType, "Configure{0}Services", environmentName, typeof(void), required: false);      
        return servicesMethod == null ? null : new ConfigureServicesBuilder(servicesMethod);
}

1.2 Build()

根据之前use的各类配置，服务，参数等构建WebHost

public IWebHost Build(){    // Warn about deprecated environment variablesif (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'");} 
   var hostingServices = BuildHostingServices();  
   var hostingContainer = hostingServices.BuildServiceProvider();   
   var host = new WebHost(hostingServices, hostingContainer, _options, _config);host.Initialize();  
   return host;
}

2.1 构建WebHost

调用Initialize完成，host的初始化工作。Initialize 调用一次BuildApplication();

public void Initialize(){   
 if (_application == null){_application = BuildApplication();}
}
 
 private RequestDelegate BuildApplication(){   
  //获取ServiceCollection中的IStartup，完成我们Startup.ConfigureService方法的调用，将我们代码注册的service加入到系统EnsureApplicationServices();   
   //解析可以为urls或server.urls的value为绑定的address。以；分割的多个地址//初始化UseKestrel()，UseIISIntegration()等指定的 实现了IServer接口的serverEnsureServer();   
    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();
}

2.2 ApplicationBuilderFactory.Build();

根据Server.Features build ApplicationBuilderFactory对象。完成ApplicationBuilderFactory的build过程。
大致就是注册各类中间件_components(middleware)，也就是说的这个 https://docs.asp.net/en/latest/fundamentals/middleware.html
借用官方的图说明一下什么是middleware。

public RequestDelegate Build()      
  {RequestDelegate app = context =>{context.Response.StatusCode = 404;                return TaskCache.CompletedTask;};         
   foreach (var component in _components.Reverse()){app = component(app);}          
     return app;}

2.3 builder完成之后，接着执行Run方法启动web服务

启动host。host.Run();最终调用到WebHost.Start(),并调用当前app指定的Server对象启动web服务

public virtual void Start()      
  {Initialize();_logger = _applicationServices.GetRequiredService<ILogger<WebHost>>();            var diagnosticSource = _applicationServices.GetRequiredService<DiagnosticSource>();            var httpContextFactory = _applicationServices.GetRequiredService<IHttpContextFactory>();_logger.Starting();Server.Start(new HostingApplication(_application, _logger, diagnosticSource, httpContextFactory));_applicationLifetime.NotifyStarted();_logger.Started();}

2.4 KestrelHttpServer的Start方法，启动对监听的监听接收请求

简化代码大约这样子

public void Start<TContext>(IHttpApplication<TContext> application)
{    
    var engine = new KestrelEngine(new ServiceContext{                            //接收到请求之后，回调FrameFactory方法，开始处理请求FrameFactory = context =>{                            return new Frame<TContext>(application, context);},                            //启动完成，停止等通知事件AppLifetime = _applicationLifetime,Log = trace,ThreadPool = new LoggingThreadPool(trace),DateHeaderValueManager = dateHeaderValueManager,ServerOptions = Options});    //启动工作线程engine.Start(threadCount);   
     foreach (var address in _serverAddresses.Addresses.ToArray()){        //判断ipv4，ipv6，localhosts得到监听的地址，并启动对该端口的监听，等待请求进来engine.CreateServer(address)}
}//engine.Start(threadCount);


public void Start(int count)     
   {         
        for (var index = 0; index < count; index++){Threads.Add(new KestrelThread(this));}            foreach (var thread in Threads){thread.StartAsync().Wait();}}

engine.CreateServer(address)
先不说了，是tcpListener的一堆代码。看了代码感觉这里又是深不可测，先放着，有空了在撸这一部分。需要理解tcpListener为何如此设计，需要精读这部分代码

2.5 接收请求后的处理

listerner接到请求之后实例化Microsoft.AspNetCore.Server.Kestrel.Internal.Http.Connection,并调用该对象的Start()
接着由Microsoft.AspNetCore.Server.Kestrel.Internal.Http.Frame .Start() 异步启动task开始处理请求。

KestrelHttpServer处理请求：Frame .RequestProcessingAsync();

public override async Task RequestProcessingAsync(){    
var messageBody = MessageBody.For(_httpVersion, FrameRequestHeaders, this);_keepAlive = messageBody.RequestKeepAlive;_upgrade = messageBody.RequestUpgrade;InitializeStreams(messageBody);   
 var context = _application.CreateContext(this);  
    await _application.ProcessRequestAsync(context).ConfigureAwait(false);    //经过一系列的检查，各种判断，请求终于由KestrelHttpServer交给了统一的HostVerifyResponseContentLength();
}

这里的application 就是Server.Start(new HostingApplication(_application, _logger, diagnosticSource, httpContextFactory));这里实例化的HostingApplication
也就是Microsoft.AspNetCore.Hosting.Internal下面的public class HostingApplication : IHttpApplication

2.6 httpcontext的创建 _application.CreateContext(this);

public Context CreateContext(IFeatureCollection contextFeatures)        {    
 var httpContext = _httpContextFactory.Create(contextFeatures);     
  
  var diagnoticsEnabled = 
_diagnosticSource.IsEnabled("Microsoft.AspNetCore.Hosting.BeginRequest");   
 var startTimestamp = (diagnoticsEnabled || _logger.IsEnabled(LogLevel.Information)) ? Stopwatch.GetTimestamp() : 0;            var scope = _logger.RequestScope(httpContext);_logger.RequestStarting(httpContext);      
       if (diagnoticsEnabled){_diagnosticSource.Write("Microsoft.AspNetCore.Hosting.BeginRequest", new { httpContext = httpContext, timestamp = startTimestamp });}           
        return new Context{HttpContext = httpContext,Scope = scope,StartTimestamp = startTimestamp,};}

2.7 Host处理请求

```C#
public Task ProcessRequestAsync(Context context)
{
return _application(context.HttpContext);
}

~~~
这里的_application就是Server.Start(new HostingApplication(_application, _logger, diagnosticSource, httpContextFactory));中的_application，也就是BuildApplication()构建出来的RequestDelegate。开启mvc处理流程

3 mvc接受请求，开始处理流程

mvc大致调用顺序：Startup.Configure方法中

//1app.UseMvc(routes =>{routes.MapRoute(name: "default",template: "{controller=Home}/{action=Index}/{id?}");});//2public static IApplicationBuilder UseMvc(this IApplicationBuilder app, Action<IRouteBuilder> onfigureRoutes)      
  {            return app.UseRouter(routes.Build());}//3

public static IApplicationBuilder UseRouter(this IApplicationBuilder builder, IRouter router){   
 if (builder.ApplicationServices.GetService(typeof(RoutingMarkerService)) == null){       
  throw new InvalidOperationException(Resources.FormatUnableToFindServices(nameof(IServiceCollection),nameof(RoutingServiceCollectionExtensions.AddRouting),            "ConfigureServices(...)"));}    //注册一个Middleware接收请求，开始处理.如2.2所展示的代码，RouterMiddleware将加入到_components，由2.7完成调用return builder.UseMiddleware<RouterMiddleware>(router);
}