根据对Http Runtime和Http Pipeline的分析,我们知道一个ASP.NET应用程序可以有多个HttpModuel,但是只能有一个HttpHandler,并且通过这个HttpHandler的BeginProcessRequest(或ProcessRequest)来处理并返回请求,前面的章节将到了再MapHttpHandler这个周期将会根据请求的URL来查询对应的HttpHandler,那么它是如何查找的呢?
一起我们在做自定义HttpHandler的时候,需要执行URL以及扩展名匹配规则,然后查找HttpHandler的时候就是根据相应的规则来查找哪个HttpHandler可以使用。另一方面我们本系列教材讲的MVC就是通过注册路由(Route)来匹配到对应的Controller和Action上的,例如Global.asax里的代码:
routes.MapRoute( "Default", "{controller}/{action}/{id}", new { controller = "Home", action = "Index", id = UrlParameter.Optional }
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 virtual 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 == null) { 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 == null) { throw new InvalidOperationException( String.Format( CultureInfo.CurrentUICulture, SR.GetString(SR.UrlRoutingModule_NoHttpHandler), routeHandler.GetType())); } if (httpHandler is UrlAuthFailureHandler) { if (FormsAuthenticationModule.FormsAuthRequired) { UrlAuthorizationModule.ReportUrlAuthorizationFailure(HttpContext.Current, this); return; } else { throw new HttpException(401, SR.GetString(SR.Assess_Denied_Description3)); } } // Remap IIS7 to our handler context.RemapHandler(httpHandler); }
我已经加粗了4行重要的代码,第一行是通过传递HttpContext参数,从RouteCollection找到对应的静态属性RouteData( GetRouteData方法里会先判断真实文件是否存在,如果不存在才去找RouteData),第二行然后从RouteData的属性RouteHandler获取一个IRouteHandler的实例,第三行是从该实例里获取对应的IHttpHandler实例,第4行是调用HttpContext的RemapHandler方法重新map新的handler(这行代码的注释虽然说是remap IIS7,其实IIS6也是用了,只不过判断该方法里对IIS7集成模式多了一点特殊处理而已),然后可以通过HttpContext. RemapHandlerInstance属性来得到这个实例。
关于Route/RouteData/RouteCollection/IRouteHandler的作用主要就是定义URL匹配到指定的IHttpHandler,然后注册进去,具体实现我们稍后再讲,现在先看一下Http Pipeline里是如何找到这个IHttpHandler实例的,由于IIS6和IIS7集成模式是差不多的,前面的文章我们提到了都是最终调用到IHttpHandlerFactory的实例,然后从中获取IHttpHandler,所以我们这里只分析IIS6和IIS7经典模式的实现。
先来看BuildSteps里查找HttpHandler的方法MapHandlerExecutionStep的代码,只有几行代码,最重要的是:
context.Handler = _application.MapHttpHandler( context, request.RequestType, request.FilePathObject, request.PhysicalPathInternal, false /*useAppConfig*/);
MapHttpHandler就是我们要查找Handler的方法了,来仔细看看代码:
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); throw 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, requestType, path, pathTranslated); } else { handler = factory.GetHandler(context, requestType, path.VirtualPathString, pathTranslated); } } catch (FileNotFoundException e) { if (HttpRuntime.HasPathDiscoveryPermission(pathTranslated)) throw new HttpException(404, null, e); else throw new HttpException(404, null); } catch (DirectoryNotFoundException e) { if (HttpRuntime.HasPathDiscoveryPermission(pathTranslated)) throw new HttpException(404, null, e); else throw new HttpException(404, null); } catch (PathTooLongException e) { if (HttpRuntime.HasPathDiscoveryPermission(pathTranslated)) throw new HttpException(414, null, e); else throw new HttpException(414, null); } // Remember for recycling if (_handlerRecycleList == null) _handlerRecycleList = new ArrayList(); _handlerRecycleList.Add(new HandlerWithFactory(handler, factory)); } return handler; }
从代码可以看出,首先如果当前页面使用了HttpServerUtility.Execute进行页面内跳转,就不使用我们通过路由设置的HttpHandler(也就是HttpContent.RemapHandlerInstance属性),如果没有跳转,就使用,并且优先级是第一的,只有当不设置任何基于Route的HttpHandler,才走剩余的匹配规则(也就是之前ASP.NET默认的按照扩展名类匹配的,这部分和我们关系不大就不做详细分析了)。
好了,知道了UrlRouteModuel的大概机制,我们再回头看看如何通过Route/RouteData/RouteCollection/IRouteHandler这几个类来实现动态注册Route规则的,先来看Route的代码:
[TypeForwardedFrom("System.Web.Routing, Version=3.5.0.0, Culture=Neutral, PublicKeyToken=31bf3856ad364e35")] public class Route : RouteBase { public Route(string url, IRouteHandler routeHandler) { Url = url; RouteHandler = routeHandler; } public Route(string url, RouteValueDictionary defaults, RouteValueDictionary constraints, IRouteHandler routeHandler) { Url = url; Defaults = defaults; Constraints = constraints; RouteHandler = routeHandler; } //省略部分代码 public 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)) { return 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) { foreach (var prop in DataTokens) { routeData.DataTokens[prop.Key] = prop.Value; } } return routeData; } }
Route代码提供了一系列的构造函数重载(我们这里只列出了两个),构造函数主要是传入URL和对应的IRouteHandler实例以及约束规则(比如正则等),然后提供了一个最重要的GetRouteData方法,用于将Route自身和IRouteHandler组装成RouteData,然后返回(中途也会验证相应的约束条件,比如是否符合某个正则表达式),RouteData类本身没有什么逻辑,只是暴露了Route和RouteHandler属性。
我们再来看RouteCollection,该类保存了所有的Route规则(即URL和对应的IRouteHandler),通过静态属性RouteTable.Routes来获取RouteCollection实例,通过UrlRoutingModule里暴露的RouteCollection属性我们可以验证这一点:
public RouteCollection RouteCollection { get { if (_routeCollection == null) { _routeCollection = RouteTable.Routes; } return _routeCollection; } set { _routeCollection = value; } }
还有一个需要注意的,RouteHandler继承的IRouteHandler的代码:
public interface IRouteHandler { IHttpHandler GetHttpHandler(RequestContext requestContext); }
该代码只提供了一个GetHttpHandler方法,所有实现这个接口的类需要实现这个方法,MVCHandler就是这么实现的(下一章节我们再细看)。
至此,我们应该有一个清晰的认识了,我们通过全局静态属性集合(RouteTable.Routes)去添加各种各样的Route(但应该在HttpModule初始化周期之前),然后通过UrlRoutingModule负责注册Route以及对应的IRouteHandler实例(IRouteHandler实例可以通过GetHttpHandler获取IHttpHandler),最终实现根据不同URL来接管不同的HttpHandler。
MVC正是利用HttpApplication创建的周期(Application_Start方法)来添加了我们所需要的Route规则,当然在添加规则的时候带上了MVCHandler这个重要的HttpHandler,
代码如下:
protected void Application_Start() { RegisterRoutes(RouteTable.Routes); } public static void RegisterRoutes(RouteCollection routes) { routes.MapRoute( "Default", "{controller}/{action}/{id}", new { controller = "Home", action = "Index", id = UrlParameter.Optional } ); }
MapRoute方法是一个扩展方法,通过该扩展方法注册Route是个不错的方法,下一章节,我们讲讲解MVC是如何注册自己的MVCRouteHandler实例以及如何实现MVCHandler的调用的。
http://www.cnblogs.com/me-sa/archive/2009/06/01/MVCLifecycle.html
http://www.cnblogs.com/zhaoyang/archive/2011/11/16/2251200.html
本文已同步至目录索引:MVC之前的那点事儿系列
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MVC之前的那点事儿系列(8):UrlRouting的理解,布布扣,bubuko.com
原文:http://www.cnblogs.com/TomXu/p/3756858.html