 receive requests, we iterate over the list in order and
    instantiate an instance of the first request class whose regexp
    matches the request path. The request class can be specified as
    either a class object or a (fully-qualified) name.

    A dictionary may be passed as the third element (``target_kwargs``)
    of the tuple, which will be used as keyword arguments to the handler's
    constructor and `~RequestHandler.initialize` method. This pattern
    is used for the `StaticFileHandler` in this example (note that a
    `StaticFileHandler` can be installed automatically with the
    static_path setting described below)::

        application = web.Application([
            (r"/static/(.*)", web.StaticFileHandler, {"path": "/var/www"}),
        ])

    We support virtual hosts with the `add_handlers` method, which takes in
    a host regular expression as the first argument::

        application.add_handlers(r"www\.myhost\.com", [
            (r"/article/([0-9]+)", ArticleHandler),
        ])

    If there's no match for the current request's host, then ``default_host``
    parameter value is matched against host regular expressions.


    .. warning::

       Applications that do not use TLS may be vulnerable to :ref:`DNS
       rebinding <dnsrebinding>` attacks. This attack is especially
       relevant to applications that only listen on ``127.0.0.1`` or
       other private networks. Appropriate host patterns must be used
       (instead of the default of ``r'.*'``) to prevent this risk. The
       ``default_host`` argument must not be used in applications that
       may be vulnerable to DNS rebinding.

    You can serve static files by sending the ``static_path`` setting
    as a keyword argument. We will serve those files from the
    ``/static/`` URI (this is configurable with the
    ``static_url_prefix`` setting), and we will serve ``/favicon.ico``
    and ``/robots.txt`` from the same directory.  A custom subclass of
    `StaticFileHandler` can be specified with the
    ``static_handler_class`` setting.

    .. versionchanged:: 4.5
       Integration with the new `tornado.routing` module.

    NÚhandlersÚ