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Event Aggregator

StructuralReactiveAbout 2 min


Event Aggregator


A system with lots of objects can lead to complexities when a
client wants to subscribe to events. The client has to find and register for
each object individually, if each object has multiple events then each event
requires a separate subscription. An Event Aggregator acts as a single source
of events for many objects. It registers for all the events of the many objects
allowing clients to register with just the aggregator.


Real-world example

King Joffrey sits on the iron throne and rules the seven kingdoms of Westeros. He receives most
of his critical information from King's Hand, the second in command. King's hand has many
close advisors himself, feeding him with relevant information about events occurring in the

In Plain Words

Event Aggregator is an event mediator that collects events from multiple sources and delivers
them to registered observers.

Programmatic Example

In our programmatic example, we demonstrate the implementation of an event aggregator pattern. Some of
the objects are event listeners, some are event emitters, and the event aggregator does both.

public interface EventObserver {
  void onEvent(Event e);

public abstract class EventEmitter {

  private final Map<Event, List<EventObserver>> observerLists;

  public EventEmitter() {
    observerLists = new HashMap<>();

  public final void registerObserver(EventObserver obs, Event e) {

  protected void notifyObservers(Event e) {

KingJoffrey is listening to events from KingsHand.

public class KingJoffrey implements EventObserver {
  public void onEvent(Event e) {"Received event from the King's Hand: {}", e.toString());

KingsHand is listening to events from his subordinates LordBaelish, LordVarys, and Scout.
Whatever he hears from them, he delivers to KingJoffrey.

public class KingsHand extends EventEmitter implements EventObserver {

  public KingsHand() {

  public KingsHand(EventObserver obs, Event e) {
    super(obs, e);

  public void onEvent(Event e) {

For example, LordVarys finds a traitor every Sunday and notifies the KingsHand.

public class LordVarys extends EventEmitter implements EventObserver {
  public void timePasses(Weekday day) {
    if (day == Weekday.SATURDAY) {

The following snippet demonstrates how the objects are constructed and wired together.

    var kingJoffrey = new KingJoffrey();

    var kingsHand = new KingsHand();
    kingsHand.registerObserver(kingJoffrey, Event.TRAITOR_DETECTED);
    kingsHand.registerObserver(kingJoffrey, Event.STARK_SIGHTED);
    kingsHand.registerObserver(kingJoffrey, Event.WARSHIPS_APPROACHING);
    kingsHand.registerObserver(kingJoffrey, Event.WHITE_WALKERS_SIGHTED);

    var varys = new LordVarys();
    varys.registerObserver(kingsHand, Event.TRAITOR_DETECTED);
    varys.registerObserver(kingsHand, Event.WHITE_WALKERS_SIGHTED);

    var scout = new Scout();
    scout.registerObserver(kingsHand, Event.WARSHIPS_APPROACHING);
    scout.registerObserver(varys, Event.WHITE_WALKERS_SIGHTED);

    var baelish = new LordBaelish(kingsHand, Event.STARK_SIGHTED);

    var emitters = List.of(
        .<Consumer<? super EventEmitter>>map(day -> emitter -> emitter.timePasses(day))

The console output after running the example.

18:21:52.955 [main] INFO com.iluwatar.event.aggregator.KingJoffrey - Received event from the King's Hand: Warships approaching
18:21:52.960 [main] INFO com.iluwatar.event.aggregator.KingJoffrey - Received event from the King's Hand: White walkers sighted
18:21:52.960 [main] INFO com.iluwatar.event.aggregator.KingJoffrey - Received event from the King's Hand: Stark sighted
18:21:52.960 [main] INFO com.iluwatar.event.aggregator.KingJoffrey - Received event from the King's Hand: Traitor detected

Class diagram

alt text
Event Aggregator


Use the Event Aggregator pattern when

  • Event Aggregator is a good choice when you have lots of objects that are
    potential event sources. Rather than have the observer deal with registering
    with them all, you can centralize the registration logic to the Event
    Aggregator. As well as simplifying registration, an Event Aggregator also
    simplifies the memory management issues in using observers.