Posted on 2008-11-01 10:07
Neil's NoteBook 阅读(121)
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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.