DomainEvent emitted again in EventListener

[shardt68]

A DomainEvent is emitted again when calling save or delete on the emitting Entity in the EventListener.
there is an Entity

• implementing AbstractAggregateRoot
• with a method registering a DomainEvent

a Service

• is calling the method registering a DomainEvent
• saving the Entity with the repository
  in a @eventlistener method
• the entity is taken from the event or looked up via repository
• the entity is saved or deleted via the repository
• the DomainEvent is emitted again
  an infinite loop is starting

If save or delete is called more then one time the event is emitted only once.
There is a demo attached showing the behavior.
demo-reemit-event-in-listener.zip

[odrotbohm]

There are a couple of conceptual issues with this arrangement. First, it's unusual to attach the aggregate to the event as it breaks the value object nature of an event in the first place. Value objects never refer to aggregates, only the other way around. Second, the event listener trying to save a remote aggregate is unusual, too. If at all, it needs to load the state of the aggregate separately and manipulate that. (I.e. repo.findById(event.getId()).map(…).ifPresent(repo::save)).

That said, I wonder if there's a better way to detect that problem and provide a better error message, as I can see that a StackOverflow error is not the way you want to see this issue surface.

[shardt68]

Sorry I oversimplified the problem.
It happens especially if the entity is looked up from the repository as you proposed.
As it is the same object from the entity manager cache, the event gets emitted again when saving.
If save is called twice in the service the event isn't emitted aging.
But saving the looked up entity in the synchronous event listener the event is emitted again and the infinite loop starts.
Try and see the corrected example attached.
demo-reemit-event-in-listener.zip