我们知道任何asp.net web程序的处理都是由IHttpHandler来实现的，那么这里我看看web api是如何让获取IHttpHandler的。这里假设你已经能熟练的使用web api，我还是沿用以前的风格以一个简单的demo来说明吧。默认在我们的Global.asax.cs有这么一句
WebApiConfig.Register(GlobalConfiguration.Configuration);而WebApiConfig.Register的默认实现也很简单：
public static void Register(HttpConfiguration config)
{
config.Routes.MapHttpRoute(
name: "DefaultApi",
routeTemplate: "api/{controller}/{id}",
defaults: new { id = RouteParameter.Optional }
);
}
一看这个方法我们就知道这是在注册一个路由信息，同时也提示我们以后自己开发asp.net mvc的时候，尽量把web api的路由分来处理，这里单独放到WebApiConfig.Register来处理的。
首先我们还是来看看这里的HttpConfiguration是个什么东东，在GlobalConfiguration中有这么一句
HttpConfiguration config = new HttpConfiguration(new HostedHttpRouteCollection(RouteTable.Routes));
其中这里用到的HostedHttpRouteCollection、HttpConfiguration构造函数如下：
private readonly RouteCollection _routeCollection;
public HostedHttpRouteCollection(RouteCollection routeCollection)
{
if (routeCollection == null)
{
throw Error.ArgumentNull("routeCollection");
}
_routeCollection = routeCollection;
}
public HttpConfiguration(HttpRouteCollection routes)
{
if (routes == null)
{
throw Error.ArgumentNull("routes");
}
_routes = routes;
Services = new DefaultServices(this);
ParameterBindingRules = DefaultActionValueBinder.GetDefaultParameterBinders();
}
到这里我们可以知道HttpConfiguration是可以操作路由的，里面有一个 public HttpRouteCollection Routes属性。
现在我们来看看路由究竟是怎么添加的，MapHttpRoute方法具体实现如下：
public static IHttpRoute MapHttpRoute(this HttpRouteCollection
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);
IHttpRoute route =
routes.CreateRoute(routeTemplate, defaultsDictionary,
constraintsDictionary, dataTokens: null, handler: handler);
routes.Add(name, route);
return route;
}
}
这里的routes是HostedHttpRouteCollection实例，其CreateRoute方法实现如下：
public override IHttpRoute CreateRoute(string uriTemplate,
IDictionary<string, object> defaults, IDictionary<string,
object> constraints, IDictionary<string, object> dataTokens,
HttpMessageHandler handler)
{
return new HostedHttpRoute(uriTemplate, defaults, constraints, dataTokens, handler);
}
其中HostedHttpRoute的构造函数如下：
public HostedHttpRoute(string uriTemplate, IDictionary<string,
object> defaults, IDictionary<string, object> constraints,
IDictionary<string, object> dataTokens, HttpMessageHandler
handler)
{
RouteValueDictionary routeDefaults = defaults != null ? new RouteValueDictionary(defaults) : null;
RouteValueDictionary routeConstraints = constraints != null ? new RouteValueDictionary(constraints) : null;
RouteValueDictionary routeDataTokens = dataTokens != null ? new RouteValueDictionary(dataTokens) : null;
OriginalRoute = new
HttpWebRoute(uriTemplate, routeDefaults, routeConstraints,
routeDataTokens, HttpControllerRouteHandler.Instance, this);
Handler = handler;
}
HostedHttpRoute的OriginalRoute属性有点特殊，是一个HttpWebRoute实例，HttpWebRoute的构造函数如下：
internal class HttpWebRoute : Route
public HttpWebRoute(string url,
RouteValueDictionary defaults, RouteValueDictionary constraints,
RouteValueDictionary dataTokens, IRouteHandler routeHandler, IHttpRoute httpRoute)
: base(url, defaults, constraints, dataTokens, routeHandler)
{
if (httpRoute == null)
{
throw Error.ArgumentNull("httpRoute");
}
HttpRoute = httpRoute;
}
我们知道路由信息表里面的实例都是Route，看来这个的HttpWebRoute实例是我们真正需要的路由信息。这个路由处理的handler是 HttpControllerRouteHandler，其中HttpControllerRouteHandler实现了一个单例模式：
public class HttpControllerRouteHandler : IRouteHandler
{
private static readonly Lazy<HttpControllerRouteHandler> _instance =
new Lazy<HttpControllerRouteHandler>(() => new HttpControllerRouteHandler(), isThreadSafe: true);
public static HttpControllerRouteHandler Instance
{
get { return _instance.Value; }
}
}
现在我们回到MapHttpRoute方法中来，这里已经获取到了一个IHttpRoute的实例（HostedHttpRoute实例），现在剩下就只有一句了 routes.Add(name, route);，它的实现如下：
public override void Add(string name, IHttpRoute route)
{
_routeCollection.Add(name, route.ToRoute());
}
在HostedHttpRouteCollection的构造函数中曾把 _routeCollection设置为RouteTable.Routes，这里实际是在往RouteTable.Routes添加路由信息。这里的 route的IHttpRoute方法实现如下：
public static Route ToRoute(this IHttpRoute httpRoute)
{
if (httpRoute == null)
{
throw Error.ArgumentNull("httpRoute");
}
HostedHttpRoute hostedHttpRoute = httpRoute as HostedHttpRoute;
if (hostedHttpRoute != null)
{
return hostedHttpRoute.OriginalRoute;
}
return new HttpWebRoute(
httpRoute.RouteTemplate,
MakeRouteValueDictionary(httpRoute.Defaults),
MakeRouteValueDictionary(httpRoute.Constraints),
MakeRouteValueDictionary(httpRoute.DataTokens),
HttpControllerRouteHandler.Instance,
httpRoute);
}
到这里我们应该知道web api 默认的Route其实就是HttpWebRoute，其对应的IRouteHandler默认是HttpControllerRouteHandler，至于程序是如何通过路由信息表来找到路由这里我们就忽略了，大家可以参考
asp.net mvc源码分析-路由篇 如何找到 IHttpHandler
asp.net mvc源码分析-Route的GetRouteData
找到了IRouteHandler那么后面就调用它的GetHttpHandler来获取IHttpHandler实例，这里HttpControllerRouteHandler的GetHttpHandler实现非常简单：
return new HttpControllerHandler(requestContext.RouteData);
那么我们来看看HttpControllerHandler的构造函数：
public class HttpControllerHandler : IHttpAsyncHandler
public HttpControllerHandler(RouteData routeData)
{
if (routeData == null)
{
throw Error.ArgumentNull("routeData");
}
_routeData = new HostedHttpRouteData(routeData);
}
而HostedHttpRouteData的构造函数如下：
public HostedHttpRouteData(RouteData routeData)
{
if (routeData == null)
{
throw Error.ArgumentNull("routeData");
}
OriginalRouteData = routeData;
HttpWebRoute route = routeData.Route as HttpWebRoute;
Route = route == null ? null : route.HttpRoute;
}
至于这里的HostedHttpRoute 为什么需要一个OriginalRoute ，HostedHttpRouteData 需要一个OriginalRouteData 属性，在这里我就不多说了吧，相信大家应该都知道他们的作用吧，在后面需要的地方我再说说这2个属性干什么东东了。
这里我们还是总结一下吧：web api默认的路由都是在WebApiConfig.Register方法中添加，默认的route是HttpWebRoute实例，默认的 IRouteHandler是HttpControllerRouteHandler实例，它返回的handler是一个实现 IHttpAsyncHandler接口HttpControllerHandler实例。