Many applications need to retrieve an object in one transaction, send it to the UI layer for manipulation, then save the changes in a new transaction. Applications that use this kind of approach in a high-concurrency environment usually use versioned data to ensure isolation for the "long" unit of work.
Hibernate supports this model by providing for reattachment of detached instances
using the Session.update()
or Session.merge()
methods:
// in the first session Cat cat = (Cat) firstSession.load(Cat.class, catId); Cat potentialMate = new Cat(); firstSession.save(potentialMate); // in a higher layer of the application cat.setMate(potentialMate); // later, in a new session secondSession.update(cat); // update cat secondSession.update(mate); // update mate
If the Cat
with identifier catId
had already
been loaded by secondSession
when the application tried to
reattach it, an exception would have been thrown.
Use update()
if you are sure that the session does
not contain an already persistent instance with the same identifier, and
merge()
if you want to merge your modifications at any time
without consideration of the state of the session. In other words, update()
is usually the first method you would call in a fresh session, ensuring that
reattachment of your detached instances is the first operation that is executed.
The application should individually update()
detached instances
reachable from the given detached instance if and only if it wants
their state also updated. This can be automated of course, using transitive
persistence, see Section 10.11, “Transitive persistence”.
The lock()
method also allows an application to reassociate
an object with a new session. However, the detached instance has to be unmodified!
//just reassociate: sess.lock(fritz, LockMode.NONE); //do a version check, then reassociate: sess.lock(izi, LockMode.READ); //do a version check, using SELECT ... FOR UPDATE, then reassociate: sess.lock(pk, LockMode.UPGRADE);
Note that lock()
can be used with various
LockMode
s, see the API documentation and the
chapter on transaction handling for more information. Reattachment is not
the only usecase for lock()
.
Other models for long units of work are discussed in Section 11.3, “Optimistic concurrency control”.