10.2. Making objects persistent

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);
If Cat has a generated identifier, the identifier is generated and assigned to the cat when 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 save().
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.