What is source in the Spring's ApplicationEvent?

Question

In Spring Framework classes that represent an event to be published by the ApplicationEventPublisher and listened to by the @EventListener are EventObject <- ApplicationEvent <- PayloadApplicationEvent .

My question is what is the non-nullable source in the initial EventObject constructor and all derived subclasses constructors?

Javadocs give a rather vague explanation that it is
"the object upon which the Event in question initially occurred" .

Is it an associated domain entity or publisher service or something else?

Additionally, I am confused why is it required at all if @EventListener states that "Events can be ApplicationEvent instances as well as arbitrary objects" ?

Answer 1

I understand source to be the where the event is created. For example, a @Service that processes a web request received from a @Controller . Therefore, when you call ApplicationEventPublisher.publishEvent() the source parameter to the ApplicationEvent event is this , the service.

public class AwesomeEvent extends ApplicationEvent {
    private final String howAwesome;

    public AwesomeEvent(Object source, String howAwesome) {
        super(source);
        this.howAwesome = howAwesome;
    }
}

@Service
@RequiredArgsConstructor // because, lazy
public class AwesomeService {
    private final ApplicationEventPublisher eventPublisher;

    public void awesomeMethod() {
        // Do superhero awesome stuff
        eventPublisher.publishEvent(new AwesomeEvent(this, "Extremely"));
    }
}

Answer 2

Just show some code to you. Look at the getApplicationContext() method.

public abstract class ApplicationContextEvent extends ApplicationEvent {

public ApplicationContextEvent(ApplicationContext source) {
    super(source);
}

public final ApplicationContext getApplicationContext() {
    return (ApplicationContext) getSource();
}