In my work I use Hibernate annotations and have been developing with them for a number of years now.

I have recently started re-engineering the plan viewer facility management application to basically take advantage of everything I have learned over the years.

As part of the implementation of associations using JPA (rather than the cumbersome hibernate xml configurations), I have needed to map zero to many associations. Hibernate does not have such a annotation, but it does have the following:

@ManyToOne
@org.hibernate.annotations.NotFound(action=org.hibernate.annotations.NotFoundAction.IGNORE)
This basically tells hibernate to ignore any missing associations thereby rendering a many to one relationship into a many to zero.
Hibernate annotations