/// <summary>
 /// 默认返回 Http404Handler 来处理没有任何匹配的IHttpHandler
 /// </summary>
 /// <param name="context"></param>
 /// <returns></returns>
 protected virtual IHttpHandler CreateHandler404(HttpContext context)
 {
     return(new Http404Handler());
 }
 /// <summary>
 /// 尝试根据路由规则来获取一个匹配的IHttpHandler实例,
 /// 如果没有找到任何匹配,则返回 null
 /// </summary>
 /// <param name="context"></param>
 /// <returns></returns>
 protected virtual IHttpHandler CreateHandlerByRouting(HttpContext context)
 {
     return(RoutingManager.Instance.CreateHandler(context));
 }