JDO and JPA allow the use of a level 2 (L2) cache, with the cache shared between
PersistenceManagers/EntityManagers. The objects in the level 2 cache don't pertain to any
one manager. The L2 cache in DataNucleus is as follows :-
The L2 cache stores an object of type
org.datanucleus.cache.CachedPC
and is keyed
by the identity of the object. The
CachedPC
contains a persistable object (
not
connected to a StateManager), together with the indicators for which fields are loaded, as
well as values for any relation fields. The persistable object will have values as null for
any relation fields, and it is these relation field values stored in the
CachedPC
that provide linkage to other objects.
The relation field values also do not store actual objects; they store the identities of the
related objects. For example if an object X has a 1-1 relation with another persistable
object Y then in the relation field values for X for that field we store the identity of
Y. Similarly if the field is a Collection, then the relation field values will be a
Collection of identities of the related objects. This provides isolation of each object
in the L2 cache (so objects aren't storing references to other objects and so allowing garbage
collection etc).
Objects are stored in the L2 cache in the following situations
-
An object is retrieved (from the datastore) within a transaction, and it is
stored in the L2 cache if no object with that identity already exists there.
-
At commit() of the transaction any object that has been modified during that
transaction will be stored/updated in the L2 cache if its persistable object is
still in memory in the PM/EM (could have been garbage collected since flushing)
Each class can be configured to be
cacheable
or not. The default for a persistable
class is to be cacheable. Configuration is performed via annotations or XML metadata.
If a class is not cacheable then objects of that type aren't stored in the L2 cache.
