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();
fritz.setColor(Color.GINGER);
fritz.setSex('M');
fritz.setName("Fritz");
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.
persist() makes a transient instance persistent.
However, it doesn't guarantee that the identifier value will be assigned to
the persistent instance immediately, the assignment might happen at flush time.
persist() also guarantees that it will not execute an
INSERT statement if it is called outside of transaction
boundaries. This is useful in long-running conversations with an extended
Session/persistence context.
save() does guarantee to return an identifier. If an INSERT
has to be executed to get the identifier ( e.g. "identity" generator, not
"sequence"), this INSERT happens immediately, no matter if you are inside or
outside of a transaction. This is problematic in a long-running conversation
with an extended Session/persistence context.
Alternatively, you may assign the identifier using an overloaded version
of save().
DomesticCat pk = new DomesticCat();
pk.setColor(Color.TABBY);
pk.setSex('F');
pk.setName("PK");
pk.setKittens( new HashSet() );
pk.addKitten(fritz);
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.