spring-events

Spring Application Events

Quick Start

// Custom Event (POJO - preferred)
public record OrderCreatedEvent(
    Long orderId,
    Long customerId,
    BigDecimal totalAmount,
    Instant createdAt
) {}

// Publisher
@Service
@RequiredArgsConstructor
public class OrderService {

    private final ApplicationEventPublisher eventPublisher;

    @Transactional
    public Order createOrder(CreateOrderRequest request) {
        Order order = orderRepository.save(new Order(request));
        eventPublisher.publishEvent(new OrderCreatedEvent(
            order.getId(), order.getCustomerId(),
            order.getTotalAmount(), order.getCreatedAt()
        ));
        return order;
    }
}

// Listener
@Component
@Slf4j
public class OrderEventListener {

    @EventListener
    public void handleOrderCreated(OrderCreatedEvent event) {
        log.info("Order created: {}", event.orderId());
    }
}

@EventListener

@Component
public class EventListeners {

    // Basic listener
    @EventListener
    public void handleOrderCreated(OrderCreatedEvent event) {
        log.info("Processing order: {}", event.orderId());
    }

    // Conditional listener
    @EventListener(condition = "#event.totalAmount > 1000")
    public void handleLargeOrder(OrderCreatedEvent event) {
        notifyManager(event);
    }

    // Ordered execution
    @EventListener
    @Order(1)  // Executed first
    public void validateOrder(OrderCreatedEvent event) { }

    @EventListener
    @Order(2)  // Executed second
    public void processOrder(OrderCreatedEvent event) { }

    // Multiple event types
    @EventListener({OrderCreatedEvent.class, OrderUpdatedEvent.class})
    public void handleOrderChange(Object event) { }
}

Event Chain (Publish New Event from Listener)

@EventListener
public NotificationEvent handleOrderCreated(OrderCreatedEvent event) {
    return new NotificationEvent(event.customerId(), "Order created!");
}

@EventListener
public Collection<Object> handleOrderShipped(OrderShippedEvent event) {
    return List.of(
        new NotificationEvent(event.customerId(), "Order shipped!"),
        new AnalyticsEvent("order_shipped", event.orderId())
    );
}