I have an entity that needs to notify observers when some of its properties have changed.

One particular Observer is a NotificationService. The notification service will send out an email when the entity has been changed. The problem I am having is determing when to notify the observer. It could easily be done if I notify the observer right away when the state of the entity has changed. But in the case with the Notification service, what if the changes to the entity never got persisted. An exception could occur, a network problem, or a million other reasons that would prevent the changes from making it to the db. The Notification Service would have sent out an email to notify users about changes, but the changes never actually got saved.

Any ideas about how I could solve this, some other pattern, an Interceptor, etc...

I'd probably use the PostFlush method of an interceptor.

There are some good articles on how to build and use interceptors on the Hibernate wiki - see the section "Interceptors, user types, and other extensions" on this page http://www.hibernate.org/37.html

Cheers,

Symon.

Thanks Symon,
I think that is ultimately what I am going to do. My design plan is to have an Interface called IDelayedNotifier. IDelayedNotifier will have one operation called NotifyDelayedObservers or something like that. then in OnPostFlush I will call NotifyDelayedObservers for IDelayedNotifiers. Then in NotifyDelayedObservers the entity can go about notifying any observers if the correct conditions are met.

Is that something along the lines of what you were thinking?