Events

The Persistence Infrastructure provides some helper classes which allow you to raise and listen for events. You can define your own events or write listeners for ones that are already defined "out-of-the-box".

Events can be a useful tool in removing an explicit dependency from one class (the event raiser) to another (the event listener). If you require to add another listener to an event, you can do so without having to "open up" the code that raises the event - the event raiser and listener only depend on the event interface, not on each other's implementation.

The Persistence Infrastructure supports:

an instance of a class raising events declared on an interface;
zero, one or more "listener" instances wired to listen for these events; and
special classes of "persistence events" which are automatically raised for all persistence operations, signalling when various standard operations are performed on entities.

To implement events and listeners, follow these steps (described in detail below):

identify where an event must be raised;
define the Event interface;
create an EventDispatcherFactory;
raise events;
create an event listener; and
configure Guice.

After these steps there is a description of how to add a listener for generic persistence events.