.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 variables
    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'");
    }

    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server
    EnsureServer();

    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Host
	 VerifyResponseContentLength();
}

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<HostingApplication.Context>

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

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
<h3 id='id3'>3 mvc接受请求，开始处理流程</h3>
mvc大致调用顺序：Startup.Configure方法中
```C#
//1
app.UseMvc(routes =>
            {
                routes.MapRoute(
                    name: "default",
                    template: "{controller=Home}/{action=Index}/{id?}");
            });
//2
public 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);
}

至此，mvc框架才真正开始处理我们的web请求。host的配置，启动，监听，接受请求，转交给上层服务的大概脉络逻辑就说完了。