2.11 Spring Events

Use Spring Events to decouple domains and react to "order completed", "user registered", etc., with listeners (e.g. notification, settlement, inventory update).

Reference: Spring Boot 3.2.x

1. Event Class

public class OrderCompletedEvent {

    private final Long orderId;
    private final String userId;

    public OrderCompletedEvent(Long orderId, String userId) {
        this.orderId = orderId;
        this.userId = userId;
    }
    // getters
}

• You can extend ApplicationEvent, or since Spring 4.2+ a plain POJO works fine.

2. Publishing

@Service
@RequiredArgsConstructor
public class OrderService {

    private final ApplicationEventPublisher publisher;

    @Transactional
    public void completeOrder(Long orderId) {
        // business logic
        Order order = orderRepository.findById(orderId).orElseThrow();
        order.complete();
        orderRepository.save(order);

        publisher.publishEvent(new OrderCompletedEvent(orderId, order.getUserId()));
    }
}

• Inject ApplicationEventPublisher and call publishEvent(...) to publish.
• By default, listeners run synchronously — completeOrder returns only after all listeners finish.

3. Listener

@Component
@Slf4j
public class OrderEventListener {

    @EventListener
    public void onOrderCompleted(OrderCompletedEvent event) {
        log.info("Order completed: orderId={}", event.getOrderId());
        notificationService.sendOrderComplete(event.getUserId(), event.getOrderId());
    }
}

• @EventListener: Called when an event matching the method parameter type is published.

4. Async Listener

To run a listener on a separate thread, combine with @Async.

@Async
@EventListener
public void onOrderCompletedAsync(OrderCompletedEvent event) {
    // async processing
}

• Requires @EnableAsync. Transaction boundaries differ from the publisher's, so a new transaction may be needed for DB access inside the listener.

5. Conditional (SpEL)

@EventListener(condition = "#event.orderId != null")
public void onOrderCompletedConditional(OrderCompletedEvent event) {
    // only runs when orderId is not null
}

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tip

Use @TransactionalEventListener for "run only after commit" to keep data consistency — the listener won't run if the transaction is rolled back.