11.6. Modifying detached objects

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();

// in a higher layer of the application

// 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 certain that the session does not contain an already persistent instance with the same identifier. Use 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 the reattachment of your detached instances is the first operation that is executed.
The application should individually update() detached instances that are reachable from the given detached instance only if it wants their state to be updated. This can be automated using transitive persistence. See Section 11.11, “Transitive persistence” for more information.
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 LockModes. 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 12.3, “Optimistic concurrency control”.