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());
    }
}

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@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())
    );
}

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Custom Events

// Generic event
public class EntityEvent<T> {
    private final T entity;
    private final EventType type;
    private final Instant timestamp = Instant.now();

    public enum EventType { CREATED, UPDATED, DELETED }
}

// Generic publisher
@Component
public class EntityEventPublisher {
    private final ApplicationEventPublisher publisher;

    public <T> void publishCreated(T entity) {
        publisher.publishEvent(new EntityEvent<>(entity, EventType.CREATED));
    }
}

// Typed listener
@EventListener
public void handleUserEvent(EntityEvent<User> event) {
    switch (event.getType()) {
        case CREATED -> handleUserCreated(event.getEntity());
        case UPDATED -> handleUserUpdated(event.getEntity());
    }
}

Full Reference: See transactional.md for @TransactionalEventListener, Async Events.

---

@TransactionalEventListener

// Execute AFTER transaction commit (default)
@TransactionalEventListener
public void handleAfterCommit(OrderCreatedEvent event) {
    emailService.sendOrderConfirmation(event.orderId());
}

// Execute AFTER rollback
@TransactionalEventListener(phase = TransactionPhase.AFTER_ROLLBACK)
public void handleAfterRollback(OrderCreatedEvent event) {
    alertService.notifyRollback(event);
}

// Execute BEFORE commit
@TransactionalEventListener(phase = TransactionPhase.BEFORE_COMMIT)
public void handleBeforeCommit(OrderCreatedEvent event) {
    validateOrderBeforeCommit(event);
}

// Fallback if no transaction
@TransactionalEventListener(fallbackExecution = true)
public void handleWithFallback(OrderCreatedEvent event) { }

Full Reference: See patterns.md for Domain Events, Aggregate Root, Event Store.

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Best Practices

• ✅ Use @TransactionalEventListener for side effects
• ✅ Use @Async for non-critical operations
• ✅ Implement retry for fallible listeners
• ✅ Use immutable events (records)
• ✅ Define order with @Order if needed
• ❌ Don't modify state in sync listeners
• ❌ Don't assume execution order without @Order

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Production Checklist

• Event classes immutabili
• TransactionalEventListener for external calls
• Async for non-critical operations
• Error handling implemented
• Retry for transient operations
• Monitoring events

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When NOT to Use This Skill

• Distributed events - Use spring-kafka or spring-amqp
• Guaranteed delivery - Use messaging systems
• Event sourcing - Consider Axon Framework

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Common Pitfalls

Error	Cause	Solution
Listener not executed	Missing @Component	Add annotation
Event lost on rollback	Using @EventListener	Use @TransactionalEventListener
Deadlock	Sync listener calls same service	Use @Async
Exception hidden	Async void	Implement error handler

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Anti-Patterns

Anti-Pattern	Problem	Solution
Sync events in transaction	Long transactions	Use @Async or @TransactionalEventListener
Circular event publishing	Infinite loop	Guard with flags
Heavy processing in sync	Blocks publisher	Use async listeners
Modifying event after publish	Shared state issues	Make events immutable

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Quick Troubleshooting

Problem	Diagnostic	Fix
Listener not invoked	Check @EventListener	Verify component scanned
Transaction not committed	Check event phase	Use AFTER_COMMIT
Async not working	Check @EnableAsync	Add to config
Order matters	Listeners random	Use @Order

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Reference Files

File	Content
transactional.md	@TransactionalEventListener, Async Events, Lifecycle
patterns.md	Domain Events, Aggregate Root, Event Store

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External Documentation

• Application Events
• TransactionalEventListener