I have a form. And I want to have one url for form itself and for submitting it.
Controller should look at method and if it is GET, show form, if POST - make validations and so on.
There's nothing to do that built-in; what I've usually done (because it's really easy :) is use the @Action annotation to map either the input method or a "process" method that'll do validation. I suppose you could do something similar with an interceptor, although I've never tried it with Convention, only a "plain" S2 app.
Rather than specifically checking the request type ("GET" or "POST") I've just had different action mappings to display the form and process the form submission.
In Struts 1 I had a base action class that checked for GET/POST and would call validation manually on a POST. Struts 2's architecture makes this impractical on the action side.
In Struts 2 there are (at least) two simple approaches: interceptor side, and action mapping side.
On the interceptor side it's trivial to tweak the validation interceptor to skip validation on GET requests--essentially creating an exact duplicate of the shipping validation interceptor plus a request type check.
On the action mapping side you could display the form with one action mapping and process it with another. The initial form display could be handled by the input() method (which validation skips) and processed with a process() (or re-mapped execute() method?) method, which validation wouldn't skip.
I don't think I've explained myself well--feel free to follow up.