ASP.NET Web API ：Web宿主

上一節描述了API的整個運作架構，即分為三層hosting、message handler pipeline 和 controller handling。此節講其中一個宿主，WebHost 寄宿在asp.net 傳統管道上。

Routing（路由）

在asp.net平台，路由是一般由RouteTables.Routes靜态屬性添加的，類型是RouteCollection，例如下面的MVC模闆自帶的添加路由的代碼。

protected void Application_Start()
{
    RegisterRoutes(RouteTable.Routes);
}
  
public static void RegisterRoutes(RouteCollection routes)
{
    routes.IgnoreRoute("{resource}.axd/{*pathInfo}");
  
    routes.MapRoute(
        "Default", // Route name
        "{controller}/{action}/{id}", // URL with parameters
        new { controller = "Home", action = "Index", id = UrlParameter.Optional } 
    );
}
           

大部分路由邏輯都是在UrlRoutingModule裡，屬于PostResolveRequestCache asp.net管道事件。每一次請求，這個module都是重新比對一次這個路由集合，并擷取一個RouteData執行個體，如果比對，則：

1、執行個體化RouteData同時擷取一個route handler（HttpControllerRouteHandler）。

2、從routehandler擷取一個http handle 繼承自IHttphandler接口。IRouteHandler接口方法：IHttpHandler GetHttpHandler(RequestContext requestContext)

3、最後，目前的請求上下文(RequestContext）被映射到上面的這個httphandler。

是以，最後結果是asp.net管道請求被此handler處理。

Web API 內建

當寄宿在asp.net上，Web API特定的配置被定義在一個單例模式的HttpConfiguration對象裡，通過靜态屬性GlobalCnfiguration.Configuration通路。

Web API 也定義了一對新的RouteCollection擴充方法MapHttpRoute，來注冊Web API特殊的路由，下面是配置的例子代碼：

HttpConfiguration config = GlobalConfiguration.Configuration;
config.Routes.MapHttpRoute("default", "{controller}/{id}",  new {id = UrlParameter.Optional});
// other configuration settings
           

注意：

1. 靜态屬性GlobalConfiguration.Configuration是用于擷取配置的引用，内部這個對象指向的是全局的RouteTables.Routes這個集合。
2. 當新增路由時使用的是MapHttpRoute這個擴充方法。

當一個route通過MapHttpRoute增加後比對到一個Request，，HttpControllerRouteHandler會建立一個新的HttpControllerHandler，其繼承自IAsyncHttpHandler，此handler通過RouteData(包含了路由的資訊）初始化。

當被調用，HttpControllerhandler 在他的BeginReocessRequest方法裡有以下行為：

• 為目前上下文建立HttpRequestMessage執行個體
• 使用GlobalConfiguration.Configuration擷取配置并建立一個HttpServer，并且把HttpRequestMessage送入服務管道。

當這個請求被HttpServer接受後，進入宿主的獨立處理階段（Web API的新管道）

下面的類圖是路由解決過程的摘要，并且配置設定給HttpServer（消息處理管道）

原文位址： 

ASP.NET Web API: web hosting

看文字描述比較晦澀難懂，需要根據源碼了解，上圖容易了解。

HttpApplication -> UrlRoutingModule -> RouteCollection -> RouteData -> HttpControllerRouteHandler -> HttpControllerHandler -> HttpRequestMessage -> HttpServer

附加一些源碼：

1、UrlRoutingModule擷取RouteData

public virtual void PostResolveRequestCache(HttpContextBase context)
{
    RouteData routeData = this.RouteCollection.GetRouteData(context);
    if (routeData == null)
    {
        return;
    }
    IRouteHandler routeHandler = routeData.RouteHandler;
    if (routeHandler == null)
    {
        throw new InvalidOperationException(string.Format(CultureInfo.CurrentCulture, SR.GetString("UrlRoutingModule_NoRouteHandler"), new object[0]));
    }
    if (routeHandler is StopRoutingHandler)
    {
        return;
    }
    RequestContext requestContext = new RequestContext(context, routeData);
    context.Request.RequestContext = requestContext;
    IHttpHandler httpHandler = routeHandler.GetHttpHandler(requestContext);
    if (httpHandler == null)
    {
        throw new InvalidOperationException(string.Format(CultureInfo.CurrentUICulture, SR.GetString("UrlRoutingModule_NoHttpHandler"), new object[]
        {
            routeHandler.GetType()
        }));
    }
    if (!(httpHandler is UrlAuthFailureHandler))
    {
        context.RemapHandler(httpHandler);
        return;
    }
    if (FormsAuthenticationModule.FormsAuthRequired)
    {
        UrlAuthorizationModule.ReportUrlAuthorizationFailure(HttpContext.Current, this);
        return;
    }
    throw new HttpException(401, SR.GetString("Assess_Denied_Description3"));
}
           

2、MapHttpRoute（RouteCollectionExtension）

public static Route MapHttpRoute(this RouteCollection routes, string name, string routeTemplate, object defaults, object constraints, HttpMessageHandler handler)
{
    if (routes == null)
    {
        throw Error.ArgumentNull("routes");
    }

    HttpRouteValueDictionary defaultsDictionary = new HttpRouteValueDictionary(defaults);
    HttpRouteValueDictionary constraintsDictionary = new HttpRouteValueDictionary(constraints);
    HostedHttpRoute httpRoute = (HostedHttpRoute)GlobalConfiguration.Configuration.Routes.CreateRoute(routeTemplate, defaultsDictionary, constraintsDictionary, dataTokens: null, handler: handler);
    Route route = httpRoute.OriginalRoute;
    routes.Add(name, route);
    return route;
}
           

3、HttpControllerRouteHandler.GetHandler

protected virtual IHttpHandler GetHttpHandler(RequestContext requestContext)
{
    return new HttpControllerHandler(requestContext.RouteData);
}
           

4、HttpControllerHandler.ProcessRequest

internal async Task ProcessRequestAsyncCore(HttpContextBase contextBase)
{
    HttpRequestMessage request = contextBase.GetHttpRequestMessage() ?? ConvertRequest(contextBase);

    // Add route data
    request.SetRouteData(_routeData);
    CancellationToken cancellationToken = contextBase.Response.GetClientDisconnectedTokenWhenFixed();
    HttpResponseMessage response = null;

    try
    {
        response = await _server.SendAsync(request, cancellationToken);
        await CopyResponseAsync(contextBase, request, response, _exceptionLogger.Value, _exceptionHandler.Value,
            cancellationToken);
    }
    finally
    {
        // The other HttpTaskAsyncHandler is HttpRouteExceptionHandler; it has similar cleanup logic.
        request.DisposeRequestResources();
        request.Dispose();

        if (response != null)
        {
            response.Dispose();
        }
    }
}
           

5、HttpApplication -> HttpControllerHandler

public HttpControllerHandler(RouteData routeData)
    : this(routeData, GlobalConfiguration.DefaultServer)//HttpServer : DelegatingHandler : HttpMessageHandler
{
}

public HttpControllerHandler(RouteData routeData, HttpMessageHandler handler)
{
    if (routeData == null)
    {
        throw Error.ArgumentNull("routeData");
    }
    if (handler == null)
    {
        throw Error.ArgumentNull("handler");
    }

    _routeData = new HostedHttpRouteData(routeData);
    _server = new HttpMessageInvoker(handler);
}