GH-181 - Delay lookup of EventPublicationRegistry as much as possible.

We now avoid to initialize the EventPublicationRegistry in the ApplicationEventMulticaster unless there is a transactional event listener interested in the event to be published. This should help us avoid premature initialization of the registry for application events published early in the application context bootstrap phase.

Author: odrotbohm

spring-modulith-events/spring-modulith-events-core/src/main/java/org/springframework/modulith/events/support/PersistentApplicationEventMulticaster.java

@@ -94,12 +94,8 @@ public void multicastEvent(ApplicationEvent event, ResolvableType eventType) {
 			return;
 		}
 
-		var txListeners = new TransactionalEventListeners(listeners);
-		var eventToPersist = getEventToPersist(event);
-
-		registry.get().store(eventToPersist, txListeners.stream() //
-				.map(TransactionalApplicationListener::getListenerId) //
-				.map(PublicationTargetIdentifier::of));
+		new TransactionalEventListeners(listeners)
+				.ifPresent(it -> storePublications(it, getEventToPersist(event)));
 
 		for (ApplicationListener listener : listeners) {
 			listener.onApplicationEvent(event);
@@ -146,6 +142,15 @@ private ApplicationListener<ApplicationEvent> executeListenerWithCompletion(Even
 		return listener;
 	}
 
+	private void storePublications(Stream<TransactionalApplicationListener<ApplicationEvent>> listeners,
+			Object eventToPersist) {
+
+		var identifiers = listeners.map(TransactionalApplicationListener::getListenerId) //
+				.map(PublicationTargetIdentifier::of);
+
+		registry.get().store(eventToPersist, identifiers);
+	}
+
 	private static Object getEventToPersist(ApplicationEvent event) {
 
 		return PayloadApplicationEvent.class.isInstance(event) //
@@ -257,5 +262,9 @@ public void doWithListener(String identifier,
 					.findFirst()
 					.ifPresent(callback);
 		}
+
+		public boolean hasListeners() {
+			return !listeners.isEmpty();
+		}
 	}
 }