A
PersistenceCapable
class that provides callback methods for life
cycle events implements this interface.
For JDO 2.0,
InstanceCallbacks
has been refactored to extend
four other interfaces, without changing any of the methods or semantics.
This allows fine-grained control over callbacks, for
example to allow a class to implement the load callback without
implementing any of the other callbacks. For backward compatibility
with JDO 1.0, the
InstanceCallbacks
interface is preserved.
Classes which include non-persistent fields whose values depend
on the values of persistent fields require callbacks on specific
JDO instance life cycle events in order to correctly populate the
values in these fields.
The callbacks might also be used if the persistent instances
need to be put into the runtime infrastructure of the application.
For example, a persistent instance might notify other instances
on changes to state. The persistent instance might be in a list of
managed instances. When the persistent instance is made hollow,
it can no longer generate change events, and the persistent
instance should be removed from the list of managed instances.
To implement this, the application programmer would implement
jdoPostLoad
to put itself into the list of managed
instances, and implement
jdoPreClear
to remove itself from
the list. With JDO 1.0, the domain class would be declared to implement
InstanceCallbacks
. With JDO 2.0, the domain class
would be declared to implement
javax.jdo.listener.LoadCallback
and
javax.jdo.listener.ClearCallback
.
Note that JDO does not manage the state of non-persistent
fields, and when a JDO instance transitions to hollow, JDO clears
the persistent fields. It is the programmer's responsibility to
clear non-persistent fields so that garbage collection of
referred instances can occur.