What is source in the Spring's ApplicationEvent?

user_7781169 0
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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"?

events spring java
2 Answers

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"));
    }
}
user_16511160

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();
}
user_113972140
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