OK, I understand better, this happens at validation time.
Thanks Joe
> Joe Fialli wrote:
>
> The schema components for identity constraints, key, keyref and
> unique, are supported
> as part of JAXP 1.3 validation that can optionally be enabled as
> part of JAXB 2.0 unmarshal and marshal.
>
> >From JAXB 2.0 specification:
>
> 6.11 Identity Constraint
> An identity constraint does not represent any data, it represents a
> constraint that is enforced by validation.
> These constraints can be checked by optional validation that can be
> enabled at either unmarshal and/or marshal time.
>