The persistence context is a cache of persistent objects. Every object in persistent state is known to the persistence context, and a duplicate, a snapshot of each persistent instance, is held in the ache. This snapshot is used internally for dirty checking, to detect any modifications you made to your persistent objects.
Many Hibernate users who ignore this simple fact run into an OutOfmemorryException. This is typically the case when you load thousands of objects in a Session but never intend to modify them. Hibernate still has to create a snapshot of each object in the persistence context cache and keep a reference to the managed object, which can lead to memory exhaustion. (Obviously, you should execute a bulk data operation if you modify thousands of objects.
The persistence context cache never shrinks automatically. To reduce or regain the memory consumed by the persistence context in a particular unit of work, you have to do the following:
. Keep the size of your persistence context to the necessary minimum. Often, many persistent instances in your session are there by accident(e.g. you needed only a few but queried for many), you can also execute a query in Hibernate that returns objects in read-only state, without creating a persistence context snapshot.
. You can call session.evict(object) to detach a persistent instance manually from the persistence context cache. You can call session.clear() to detach all persistent instance from the persistence context. Detached objects aren't checked for dirty state; they aren't managed.
. with session.setReadOnly(object, true), you can disable dirty checking for a particular instance. The persistence context will no longer maintain the snapshot if it's read-only. With session.setReadOnly(object, false), you can re-enable dirty checking for an instance and force the recreation of a snapshot. Note that these operations don't change the object's state.