Hibernate: Safely reattach an object to the session

Question

I'm maintaining a cache of objects across hibernate sessions by storing (possibly-detached) objects in a map. When the cache gets a hit, I test if the object is already part of the session with Session.contains(object). If not, I re-attach it with Session.lock(object, LockMode.NONE).

The problem is, if the same object was loaded previously in the session, it throws a org.hibernate.NonUniqueObjectException. Given a detached instance, I see no way to find out in advance if this exception will get thrown, without possibly hitting the database.

There are a couple workarounds:

1. Reattach all cached objects at the start of every transaction.
2. Catch the NonUniqueObjectException and then call session.load(object.class, object.getId());

Neither of these are as clean as checking the session in advance for an object class + id.

Is there any way to do that?

Answer 1

Session.merge() should do it:

obj = session.merge(obj);