Using Spring Framework’s Application Events and Listeners

Without any special configuration, Spring Framework automatically acts as a message broker in a pub/sub environment. To publish messages, you call the publishEvent method on an instance of org.springframework.context.ApplicationEventPublisher and pass it an instance of org.springframework.context.ApplicationEvent. You can obtain an ApplicationEventPublisher in a Spring-managed bean either through dependency injection or by implementing org.springframework.context.ApplicationEventPublisherAware. With an ApplicationEventPublisher instance, you can publish as many events as you want as often as you need to.
Subscribing to events is equally easy. Given an event FooEvent extends ApplicationEvent, a Spring bean needs only to implement org.springframework.context.ApplicationListener<FooEvent> to subscribe to FooEvent messages. To subscribe to BarEvent, implement ApplicationListener<BarEvent>. You can create and subscribe to a hierarchy of events, too. For example, consider the following event definitions:

public class TopEvent extends ApplicationEvent { }
public class MiddleEvent extends TopEvent { }
public class CenterEvent extends TopEvent { }
public class BottomEvent extends MiddleEvent { }

Given this, a bean that implements:

- ApplicationListener<ApplicationEvent> subscribes to all events
- ApplicationListener<TopEvent> subscribes to TopEvent, MiddleEvent, CenterEvent, and BottomEvent
- ApplicationListener<MiddleEvent> subscribes to MiddleEvent and BottomEvent
- ApplicationListener<CenterEvent> subscribes only to CenterEvent
- ApplicationListener<BottomEvent> subscribes only to BottomEvent