Event-driven architecture pattern in Spring Boot - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when an event is published in Spring Boot?

In Spring Boot's event-driven architecture, when an event is published using ApplicationEventPublisher, what is the immediate behavior?

Spring Boot

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

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

// Somewhere in a service
applicationEventPublisher.publishEvent(new MyEvent(this));

A The event is queued and listeners are called only when the application context is refreshed.

B Listeners are called asynchronously in separate threads after publishEvent returns.

C All listeners for MyEvent are called synchronously in the same thread before publishEvent returns.

D Listeners are called only if explicitly polled by the publisher after publishing.

💡 Hint

Think about how Spring's default event publishing works in terms of thread execution.

By default, Spring publishes events synchronously in the same thread that calls publishEvent. All listeners are invoked before the method returns.

📝 Syntax

intermediate

Identify the correct way to declare an event listener method in Spring Boot

Which of the following method declarations correctly listens to a custom event OrderCreatedEvent in Spring Boot?

Spring Boot

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

A

@EventListener
public void onOrderCreated(OrderCreatedEvent event) { /* handle event */ }

B

@EventListener(OrderCreatedEvent.class)
public void onOrderCreated() { /* handle event */ }

C public void onOrderCreated(@EventListener OrderCreatedEvent event) { /* handle event */ }

D

@EventListener
public void onOrderCreated(Object event) { /* handle event */ }

💡 Hint

Look for the correct annotation placement and method signature.

The correct way is to annotate the method with @EventListener and accept the event type as a parameter. Option A matches this pattern.

🔧 Debug

advanced

Why does this Spring Boot event listener not get called?

Given this listener class, why is the event handler method never invoked when the event is published?

Spring Boot

@Component
public class PaymentListener {
    public void handlePaymentEvent(PaymentEvent event) {
        System.out.println("Payment processed");
    }
}

// Event published elsewhere
applicationEventPublisher.publishEvent(new PaymentEvent(this));

A The method is missing the @EventListener annotation, so Spring does not recognize it as an event handler.

B The event publisher must call handlePaymentEvent directly for the listener to work.

C The event class PaymentEvent must extend RuntimeException to be handled.

D The class is not annotated with @Service, so it cannot listen to events.

💡 Hint

Check if the listener method is properly marked to receive events.

Spring only calls methods annotated with @EventListener (or implementing ApplicationListener). Without the annotation, the method is ignored.

❓ state_output

advanced

What is the output order when multiple listeners handle the same event?

Consider two listeners for the same event UserRegisteredEvent with different @Order annotations. What is the order of their execution?

Spring Boot

@Component
public class ListenerA {
    @EventListener
    @Order(2)
    public void onUserRegistered(UserRegisteredEvent event) {
        System.out.println("Listener A");
    }
}

@Component
public class ListenerB {
    @EventListener
    @Order(1)
    public void onUserRegistered(UserRegisteredEvent event) {
        System.out.println("Listener B");
    }
}

// Event published
applicationEventPublisher.publishEvent(new UserRegisteredEvent(this));

A Only Listener A prints because it has higher order.

B Listener B prints first, then Listener A.

C Listener A prints first, then Listener B.

D Listeners print in random order.

💡 Hint

Lower @Order value means higher priority.

Spring calls listeners in ascending order of their @Order value. So ListenerB (order 1) runs before ListenerA (order 2).

🧠 Conceptual

expert

Which statement best describes the advantage of event-driven architecture in Spring Boot?

Select the option that best explains why using event-driven architecture benefits a Spring Boot application.

A It eliminates the need for dependency injection by using events instead.

B It forces all components to run synchronously, ensuring strict execution order.

C It requires all events to be stored in a database before processing, improving reliability.

D It decouples components by allowing them to communicate through events, improving modularity and scalability.

💡 Hint

Think about how events help components work independently.

Event-driven architecture allows components to react to events without tight coupling, making the system more modular and easier to scale.