I would like impelement WFS SERVER using geotools and eg. jetty. Jetty will provide web container and geotools should parse whf requests (getcapabilities etc) and generate resposses (gml, json?) but I am unable to find any solutions. Geotools can works as wsf client but can as server?
You are looking for GeoServer which is implemented using GeoTools. Please see my answer to this question as to why starting to write your own WFS is a bad idea :-)
If you really must spend the time and effort to reinvent the wheel then GeoTools is fine - you will want to make use of a DataStore to read your data in and look at the XML output modules to write out your responses. Don't forget to check the WFS specification to make sure you're compliant.