asp.net mvc源码分析路由篇 如何找到 IHttpHandler

学习是使用asp.net已经有很长一段时间了，现在就来分析一下mvc的整过过程吧。个人计划写一个mvc系列的博文，仅从源代码的角度来分析mvc。在接触mvc时我们一定会经历路由，那么路由这东东是怎么搞出来的啊。在我们的web.config中有这么一句：    <add assembly="System.Web.Routing, Version=4.0.0.0, Culture=neutral, PublicKeyToken=31BF3856AD364E35" /> 看来路由是它咋负责了。在这个dll中有一个很特殊的类UrlRoutingModule

我们来看看它里面主要的核心代码吧：

protected virtual void Init(HttpApplication application)
    {
        if (application.Context.Items[_contextKey] == null)
        {
            application.Context.Items[_contextKey] = _contextKey;
            application.PostResolveRequestCache += new EventHandler(this.OnApplicationPostResolveRequestCache);
        }
    }

    private void OnApplicationPostResolveRequestCache(object sender, EventArgs e)
    {
        HttpContextBase context = new HttpContextWrapper(((HttpApplication) sender).Context);
        this.PostResolveRequestCache(context);
    }

    public virtual void PostResolveRequestCache(HttpContextBase context)
    {
        RouteData routeData = this.RouteCollection.GetRouteData(context);
        if (routeData != null)
        {
            IRouteHandler routeHandler = routeData.RouteHandler;
            if (routeHandler == null)
            {
                throw new InvalidOperationException(string.Format(CultureInfo.CurrentUICulture, SR.GetString("UrlRoutingModule_NoRouteHandler"), new object[0]));
            }
            if (!(routeHandler is StopRoutingHandler))
            {
                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)
                {
                    if (!FormsAuthenticationModule.FormsAuthRequired)
                    {
                        throw new HttpException(0x191, SR.GetString("Assess_Denied_Description3"));
                    }
                    UrlAuthorizationModule.ReportUrlAuthorizationFailure(HttpContext.Current, this);
                }
                else
                {
                    context.RemapHandler(httpHandler);
                }
            }
        }
    }

在IHttpModule.Init中注册了一个PostResolveRequestCache事件，而该事件主要是调用PostResolveRequestCache这个方法，在这个方法里面有几句很重要的代码是

  RouteData routeData = this.RouteCollection.GetRouteData(context);

IRouteHandler routeHandler = routeData.RouteHandler;

 RequestContext requestContext = new RequestContext(context, routeData);
  context.Request.RequestContext = requestContext;
 IHttpHandler httpHandler = routeHandler.GetHttpHandler(requestContext);

context.RemapHandler(httpHandler);

让我们来分析第一句RouteData routeData = this.RouteCollection.GetRouteData(context) ，这句我们猜测是获取路由信息。要想理解这句代码又得回到我们程序中来，我们在Global.asax.cs文件中的RegisterRoutes方法中，默认有这么一句

 routes.MapRoute(
                "Default", // 路由名称
                "{controller}/{action}/{id}", // 带有参数的 URL
                new { controller = "Home", action = "Index", id = UrlParameter.Optional } // 参数默认值
            );

这句代码主要是注册一个路由，这里的url要注意不能随便写，需要有controller和action。具体是怎么实现的了？

 public static Route MapRoute(this RouteCollection routes, string name, string url, object defaults, object constraints, string[] namespaces) {
            Route route = new Route(url, new MvcRouteHandler()) {
                Defaults = new RouteValueDictionary(defaults),
                Constraints = new RouteValueDictionary(constraints),
                DataTokens = new RouteValueDictionary()
            };

            if ((namespaces != null) && (namespaces.Length > 0)) {
                route.DataTokens["Namespaces"] = namespaces;
            }
            routes.Add(name, route);
            return route;
        }

各参数如下

routeName="Default", // 路由名称
routeUrl= "{controller}/{action}/{id}", // 带有参数的 URL
defaults=new { controller = "Home", action = "Index", id = UrlParameter.Optional } // 参数默认值
constraints=null
namespaces=null

在这里创建了一个Route实例并且把它加入到RouteCollection中了。

如果我们的项目中有特殊的需要，需要创建自己的HttpHandler这么办了？

其实很简单只要我们注册自己的IRouteHandler了，routes.Add(new Route("{controller}/{action}/{id}",new MvcRouteHandler())); 然后在里面GetHttpHandler实现自己的逻辑处理

现在又让我们回到 RouteData routeData = this.RouteCollection.GetRouteData(context);这句代码中来，GetRouteData的主要代码如下：

 public RouteData GetRouteData(HttpContextBase httpContext)
{
     using (this.GetReadLock())
        {
            foreach (RouteBase base2 in this)
            {
                RouteData routeData = base2.GetRouteData(httpContext);
                if (routeData != null)
                {
                    return routeData;
                }
            }
        }
    return null;
}
在这里的base2就是我们先前调用MapRoute是添加的Route的。而Route的GetRouteData的方法如下：

public override RouteData GetRouteData(HttpContextBase httpContext)
{
    string virtualPath = httpContext.Request.AppRelativeCurrentExecutionFilePath.Substring(2) + httpContext.Request.PathInfo;
    RouteValueDictionary values = this._parsedRoute.Match(virtualPath, this.Defaults);
    if (values == null)
    {
        return null;
    }
    RouteData data = new RouteData(this, this.RouteHandler);
    if (!this.ProcessConstraints(httpContext, values, RouteDirection.IncomingRequest))
    {
        return null;
    }
    foreach (KeyValuePair<string, object> pair in values)
    {
        data.Values.Add(pair.Key, pair.Value);
    }
    if (this.DataTokens != null)
    {
        foreach (KeyValuePair<string, object> pair2 in this.DataTokens)
        {
            data.DataTokens[pair2.Key] = pair2.Value;
        }
    }
    return data;
}

这个方法很复杂，有许多验证和检查，我们主要关心一句 RouteData data = new RouteData(this, this.RouteHandler);

当然剩下 RequestContext requestContext = new RequestContext(context, routeData);
  context.Request.RequestContext = requestContext;这2句没什么特别了。

现在让我们来看看IHttpHandler httpHandler = routeHandler.GetHttpHandler(requestContext);这句究竟干了些什么，意思很明白获取Httphandler。

那么MvcRouteHandler是如何获取一个Httphandler的了，
        protected virtual IHttpHandler GetHttpHandler(RequestContext requestContext) {
            requestContext.HttpContext.SetSessionStateBehavior(GetSessionStateBehavior(requestContext));
            return new MvcHandler(requestContext);
        }

直接返回了一个MvcHandler实例。

最有一句context.RemapHandler(httpHandler); 很简单很好明白吧，在HttpContext的RemapHandler方法中有这么一句 this._remapHandler = handler;

在HttpContext中有这个属性

 internal IHttpHandler RemapHandlerInstance
    {
        get
        {
            return this._remapHandler;
        }
    }

那么这个东西又是什么时候调用的了，在HttpApplication的内部类MaterializeHandlerExecutionStep中的  void HttpApplication.IExecutionStep.Execute()方法调用

 if (httpContext.RemapHandlerInstance != null)
        {
            httpContext.Handler = httpContext.RemapHandlerInstance;
        }

看到MaterializeHandlerExecutionStep这个了类名，我想大家都能猜到吧。在内部类PipelineStepManager中BuildSteps方法有

 HttpApplication.IExecutionStep step = new HttpApplication.MaterializeHandlerExecutionStep(app);
            app.AddEventMapping("ManagedPipelineHandler", RequestNotification.MapRequestHandler, false, step);

我想大家看到这里对mvc整个路由应该有个大致的理解了吧。