servlet filter mapping empty string not working

the special url pattern "" (empty string) works in servlet mapping, but not in servlet filter mapping.

12.2 Specification of Mappings
...
The empty string ("") is a special URL pattern that exactly maps to the
application's context root, i.e., requests of the form http://host:port/<context- root>/.

The @WebFilter(urlPatterns=

) is not triggered at request time.