to everybody: the problem is solved I put the jar file containing MyValidator.class into lib2 folder, and it worked. Apparently, myfaces uses some tricks to load jar files from "lib2" (not "lib") folder. You were right if I put my class in lib2 folder the same Class Loader that loads myfaces libs loads my validator too and makes it castable to Validator interface. View the original post : http://www.jboss.com/index.html?module=bb&op=viewtopic&p=4133525#... Reply to the post : http://www.jboss.com/index.html?module=bb&op=posting&mode=reply&a...