UrlRouting的理解

routes.MapRoute(
    "Default",{controller}/{action}/{id}new { controller = Home",action = Index 但是在匹配这个之前，MVC首先要接管请求才能处理，也就是说我们要有对应MVC的HttpHandler（后面知道它的名字叫MvcHandler）被MapRequestHandler周期的处理引擎查找到并且应用上才行，然后后面才能由 Controller/Action执行。另外一方面，由于该URL地址没有扩展名，所以无法进入ASP.NET的RunTime，MVC2的实现方式是：注册通配符（*.*）映射到aspnet_ISPAI.dll，然后通过一个自定义的UrlRoutingModuel来匹配Route规则，再继续处理，但是MVC3的时候，匹配Route规则的处理机制集成到ASP.NET4.0里了，也就是今天我们这篇文章所要讲的主角（UrlRoutingModule）的处理机制。
 
 
 
 先来看UrlRoutingModule的源码，无容置疑地这个类是继承于IHttpModule，首先看一下Init方法的代码：
 
 
 
protected virtual void Init(HttpApplication application) {

    ////////////////////////////////////////////////////////////////// 
    // Check if this module has been already addded
    if (application.Context.Items[_contextKey] != null) { 
        return;  already added to the pipeline 
    }
    application.Context.Items[_contextKey] = _contextKey; 

     Ideally we would use the MapRequestHandler event.  However,MapRequestHandler is not available
     in II6 or IIS7 ISAPI Mode.  Instead,we use PostResolveRequestCache,which is the event immediately
     before MapRequestHandler.  This allows use to use one common codepath for all versions of IIS. 
    application.PostResolveRequestCache += OnApplicationPostResolveRequestCache;
}
 
 
 该代码在PostResolveRequestCache周期事件上添加了我们需要执行的方法，用于URL匹配规则的设置，但是为什么要在这个周期点上添加事件呢？看了注释，再结合我们前面对Pipeline的了解，释然了，要像动态注册自己的HttpHandler，那就需要在MapRequestHandler之前进行注册自己的规则（因为这个周期点就是做这个事情的），但由于IIS6不支持这个事件，所以为了能让IIS6也能运行MVC3，所以我们需要在这个周期之前的PostResolveRequestCache的事件点上去注册我们的规则，也许如果IIS6被微软废弃以后，就会将这个事件添加到真正的开始点MapRequestHandler上哦。
 
 我们继续来看注册该事件的OnApplicationPostResolveRequestCache方法的代码：
 
public void PostResolveRequestCache(HttpContextBase context) { 
     Match the incoming URL against the route table
    RouteData routeData = RouteCollection.GetRouteData(context);  Do nothing if no route found 
    if (routeData == null) {
        return; 
    } 

     If a route was found,get an IHttpHandler from the route's RouteHandler 
    IRouteHandler routeHandler = routeData.RouteHandler; if (routeHandler == throw new InvalidOperationException(
            String.Format( 
                CultureInfo.CurrentUICulture,SR.GetString(SR.UrlRoutingModule_NoRouteHandler))); 
    } 

     This is a special IRouteHandler that tells the routing module to stop processing 
     routes and to let the fallback handler handle the request.
    if (routeHandler is StopRoutingHandler) {
        return;
    } 

    RequestContext requestContext = new RequestContext(context,routeData); 
 
     Dev10 766875    Adding RouteData to HttpContext
    context.Request.RequestContext = requestContext; 

    IHttpHandler httpHandler = routeHandler.GetHttpHandler(requestContext); if (httpHandler == new InvalidOperationException( 
            String.Format(
                CultureInfo.CurrentUICulture,SR.GetString(SR.UrlRoutingModule_NoHttpHandler),routeHandler.GetType()));
    } 

    if (httpHandler is UrlAuthFailureHandler) {
        if (FormsAuthenticationModule.FormsAuthRequired) {
            UrlAuthorizationModule.ReportUrlAuthorizationFailure(HttpContext.Current,255); line-height:1.5!important">this); 
            return;
        } 
        else { 
            new HttpException(401,SR.GetString(SR.Assess_Denied_Description3));
        } 
    }

     Remap IIS7 to our handler
 context.RemapHandler(httpHandler); 
}

context.Handler = _application.MapHttpHandler(
    context,request.RequestType,request.FilePathObject,request.PhysicalPathInternal,255); line-height:1.5!important">false /*useAppConfig*/); 

internal IHttpHandler MapHttpHandler(HttpContext context,String requestType,VirtualPath path,String pathTranslated,bool useAppConfig) { Don't use remap handler when HttpServerUtility.Execute called IHttpHandler handler = (context.ServerExecuteDepth == 0) ? context.RemapHandlerInstance : null; using (new ApplicationImpersonationContext()) { Use remap handler if possible if (handler != null){ return handler; } Map new handler HttpHandlerAction mapping = GetHandlerMapping(context,requestType,path,useAppConfig); If a page developer has removed the default mappings with <httpHandlers><clear> without replacing them then we need to give a more descriptive error than a null parameter exception. if (mapping == null) { PerfCounters.IncrementCounter(AppPerfCounter.REQUESTS_NOT_FOUND); PerfCounters.IncrementCounter(AppPerfCounter.REQUESTS_FAILED); new HttpException(SR.GetString(SR.Http_handler_not_found_for_request_type,requestType)); } Get factory from the mapping IHttpHandlerFactory factory = GetFactory(mapping); Get factory from the mapping try { Check if it supports the more efficient GetHandler call that can avoid a VirtualPath object creation. IHttpHandlerFactory2 factory2 = factory as IHttpHandlerFactory2; if (factory2 != null) { handler = factory2.GetHandler(context,pathTranslated); } else { handler = factory.GetHandler(context,path.VirtualPathString,pathTranslated); } } catch (FileNotFoundException e) { if (HttpRuntime.HasPathDiscoveryPermission(pathTranslated)) 404,255); line-height:1.5!important">null,e); else null); } catch (DirectoryNotFoundException e) { if (HttpRuntime.HasPathDiscoveryPermission(pathTranslated)) else catch (PathTooLongException e) { 414,255); line-height:1.5!important">null); } Remember for recycling if (_handlerRecycleList == null) _handlerRecycleList = new ArrayList(); _handlerRecycleList.Add(new HandlerWithFactory(handler,factory)); } return handler; }

[TypeForwardedFrom(System.Web.Routing,Version=3.5.0.0,Culture=Neutral,PublicKeyToken=31bf3856ad364e35")]
class Route : RouteBase
{    
    public Route(string url,IRouteHandler routeHandler)
    {
        Url = url;
        RouteHandler = routeHandler;
    }
     
     url;
            Defaults = defaults; 
            Constraints = constraints; 
            RouteHandler = routeHandler;
        }

    省略部分代码
    override RouteData GetRouteData(HttpContextBase httpContext)
    {
         Parse incoming URL (we trim off the first two chars since they're always "~/")
        string requestPath = httpContext.Request.AppRelativeCurrentExecutionFilePath.Substring(2) + httpContext.Request.PathInfo;

        RouteValueDictionary values = _parsedRoute.Match(requestPath,Defaults);

        if (values == null)
        {
             If we got back a null value set,that means the URL did not match
            return null;
        }

        RouteData routeData = new RouteData(this,RouteHandler);

                 Validate the values
        if (!ProcessConstraints(httpContext,values,RouteDirection.IncomingRequest)) { 
            null; 
        }
 
         Copy the matched values
        foreach (var value in values) {
            routeData.Values.Add(value.Key,value.Value);
        } 

         Copy the DataTokens from the Route to the RouteData 
        if (DataTokens != null) { 
            var prop in DataTokens) {
                routeData.DataTokens[prop.Key] = prop.Value; 
            }
        }
        return routeData;
    }       
    }

public RouteCollection RouteCollection { get { if (_routeCollection == null) { _routeCollection = RouteTable.Routes; } return _routeCollection; } set { _routeCollection = value; } }

interface IRouteHandler { IHttpHandler GetHttpHandler(RequestContext requestContext); }

void Application_Start() { RegisterRoutes(RouteTable.Routes); } static void RegisterRoutes(RouteCollection routes) { routes.MapRoute( UrlParameter.Optional } ); }