请求如何进入ASP.NET MVC框架

　　在创建Route时，除了传递url模式外，还默认帮我们传递了一个MvcRouteHandler，它实现了IRouteHandler接口。
步骤三、获取IHttpHandler接口对象

　　有了MvcRouteHandler，就可以调用它的GetHttpHandler方法获取IHttpHandler了，该方法实现如下：

protected virtual IHttpHandler GetHttpHandler(RequestContext requestContext)
{
  //设置session状态
  requestContext.HttpContext.SetSessionStateBehavior(GetSessionStateBehavior(requestContext));
 
  //返回一个实现了IHttpHandler的MvcHandler
  return new MvcHandler(requestContext);
}

　　可以看到，它返回了一个MvcHandler，MvcHandler就实现了IHttpHandler接口。所以开头说的，请求本质都是交给HttpHandler的，其实MVC也是这样的，请求交给了MvcHandler处理。我们可以看MvcHandler定义和主要方法：

public class MvcHandler : IHttpAsyncHandler, IHttpHandler, IRequiresSessionState
{
   protected internal virtual IAsyncResult BeginProcessRequest(HttpContextBase httpContext, AsyncCallback callback, object state)
  {
    IController controller;
    IControllerFactory factory;
 
    //这个方法里会激活Controller对象
    ProcessRequestInit(httpContext, out controller, out factory);
 
    IAsyncController asyncController = controller as IAsyncController;
    if (asyncController != null)
    {
      // asynchronous controller
      BeginInvokeDelegate beginDelegate = delegate(AsyncCallback asyncCallback, object asyncState)
      {
        try
        {
          //调用Controller的BeginExecute方法
          return asyncController.BeginExecute(RequestContext, asyncCallback, asyncState);
        }
        catch
        {
          factory.ReleaseController(asyncController);
          throw;
        }
      };
 
      EndInvokeDelegate endDelegate = delegate(IAsyncResult asyncResult)
      {
        try
        {
          asyncController.EndExecute(asyncResult);
        }
        finally
        {
          factory.ReleaseController(asyncController);
        }
      };
 
      SynchronizationContext syncContext = SynchronizationContextUtil.GetSynchronizationContext();
      AsyncCallback newCallback = AsyncUtil.WrapCallbackForSynchronizedExecution(callback, syncContext);
      return AsyncResultWrapper.Begin(newCallback, state, beginDelegate, endDelegate, _processRequestTag);
    }
    else
    {
      // synchronous controller
      Action action = delegate
      {
        try
        {
          controller.Execute(RequestContext);
        }
        finally
        {
          factory.ReleaseController(controller);
        }
      };
 
      return AsyncResultWrapper.BeginSynchronous(callback, state, action, _processRequestTag);
    }
  }
}

　　可以看到，MvcHandler的任务就是激活Controller，并执行它的Execute方法。这个过程和Webform里的页面处理是很相似的，.aspx请求到来，会根据虚拟路径找到实现IHttpHandler的Page（类似于路由机制根据url模式找到MvcHandler）,然后进入Page的页面周期（类似于Mvc的激活Controller，然后执行Action过程）。