I want to develop a WFS Server. When a client request read a layer, What the response of format should be?
Note: The language of this project is java servlet.
Why are creating your own WFS implementation? Can't you just use geoserver? WFS is a complex type of service, this won't be easy.
Anyway the response for WFS-GetFeature should include GML and optionally any other format you want. Geotools can provide you with the right JAVA API's to do this.
Just look at the documentation of geoserver for WFS if you want examples.