Event publishing with ApplicationEventPublisher in Spring Boot - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when an event is published using ApplicationEventPublisher?

Consider a Spring Boot application where a custom event is published using ApplicationEventPublisher. What is the expected behavior after publishing the event?

Spring Boot

public class CustomEvent extends ApplicationEvent {
    public CustomEvent(Object source) {
        super(source);
    }
}

@Component
public class EventPublisher {
    private final ApplicationEventPublisher publisher;

    public EventPublisher(ApplicationEventPublisher publisher) {
        this.publisher = publisher;
    }

    public void publish() {
        publisher.publishEvent(new CustomEvent(this));
    }
}

@Component
public class EventListener {
    @EventListener
    public void handleCustomEvent(CustomEvent event) {
        System.out.println("Event received: " + event.getSource());
    }
}

A The event triggers a restart of the Spring Boot application context.

B The event is queued and processed asynchronously by default in a separate thread pool.

C The event is published synchronously and all matching @EventListener methods are called immediately in the same thread.

D The event is ignored unless explicitly registered in a configuration file.

💡 Hint

Think about how Spring handles events by default without extra configuration.

By default, ApplicationEventPublisher publishes events synchronously in the same thread. All listeners annotated with @EventListener for that event type are called immediately.

📝 Syntax

intermediate

Which code snippet correctly publishes a custom event in Spring Boot?

Select the code snippet that correctly publishes a custom event using ApplicationEventPublisher in a Spring Boot component.

A publisher.publishEvent(new CustomEvent(this));

B publisher.publish(new CustomEvent(this));

C publisher.fireEvent(new CustomEvent(this));

D publisher.sendEvent(new CustomEvent(this));

💡 Hint

Check the exact method name in ApplicationEventPublisher interface.

The correct method to publish an event is publishEvent. Other method names do not exist in ApplicationEventPublisher.

🔧 Debug

advanced

Why does the event listener not receive the published event?

Given the following code, why does the event listener not receive the event? @Component public class MyListener { public void onCustomEvent(CustomEvent event) { System.out.println("Event received"); } } @Component public class MyPublisher { private final ApplicationEventPublisher publisher; public MyPublisher(ApplicationEventPublisher publisher) { this.publisher = publisher; } public void publish() { publisher.publishEvent(new CustomEvent(this)); } }

A The listener method is missing the @EventListener annotation.

B The event class CustomEvent is not annotated with @Component.

C The ApplicationEventPublisher is not injected properly.

D The event must be published asynchronously to be received.

💡 Hint

Check how Spring knows which methods listen to events.

Spring detects event listener methods by the @EventListener annotation. Without it, the method is not called.

🧠 Conceptual

advanced

What is the effect of publishing an event with a payload object instead of extending ApplicationEvent?

In Spring Framework 4.2 and later, you can publish any object as an event without extending ApplicationEvent. What is the effect of publishing a plain payload object instead of a subclass of ApplicationEvent?

A The event triggers a runtime exception due to missing ApplicationEvent superclass.

B The event is published normally and listeners can receive the payload object directly.

C The event is ignored because it does not extend ApplicationEvent.

D The event is published but listeners must cast the event to ApplicationEvent manually.

💡 Hint

Spring Framework supports publishing any object as event since version 4.2.

Since Spring 4.2, any object can be published as an event. Listeners receive the object directly without needing to extend ApplicationEvent.

❓ state_output

expert

What is the console output after publishing two events synchronously?

Given the following Spring Boot components, what will be printed to the console after calling publisher.publish()? public class CustomEvent extends ApplicationEvent { public CustomEvent(Object source) { super(source); } } @Component public class EventPublisher { private final ApplicationEventPublisher publisher; public EventPublisher(ApplicationEventPublisher publisher) { this.publisher = publisher; } public void publish() { publisher.publishEvent(new CustomEvent("First")); publisher.publishEvent(new CustomEvent("Second")); } } @Component public class EventListener { @EventListener public void onEvent(CustomEvent event) { System.out.println("Received: " + event.getSource()); } }

A No output because events are asynchronous

B

Received: Second
Received: First

C Received: First

D

Received: First
Received: Second

💡 Hint

Events are published synchronously in order by default.

Events are published synchronously in the order they are called, so the listener prints both in sequence.