Newly instantiated instances of a a persistent class are considered transient by Hibernate. We can make a transient instance persistent by associating it with a session:
DomesticCat fritz = new DomesticCat();
Long generatedId = (Long) sess.save(fritz);
Cat has a generated identifier, the identifier is generated and assigned to the
save() is called. If
Cat has an
assigned identifier, or a composite key, the identifier should be assigned to the
cat instance before calling
save(). You may also use
persist() instead of
save(), with the semantics defined in the EJB3 early draft.
Alternatively, you may assign the identifier using an overloaded version of
DomesticCat pk = new DomesticCat();
pk.setKittens( new HashSet() );
sess.save( pk, new Long(1234) );
If the object you make persistent has associated objects (e.g. the
kittens collection in the previous example), these objects may be made persistent in any order you like unless you have a
NOT NULL constraint upon a foreign key column. There is never a risk of violating foreign key constraints. However, you might violate a
NOT NULL constraint if you
save() the objects in the wrong order.
Usually you don't bother with this detail, as you'll very likely use Hibernate's transitive persistence feature to save the associated objects automatically. Then, even
NOT NULL constraint violations don't occur - Hibernate will take care of everything. Transitive persistence is discussed later in this chapter.