Bean Scopes, Lifecycle and Events

Beans are singletons by default — one instance shared across threads — so they must be stateless or thread-safe. Other scopes include prototype (new instance per injection) and web scopes (request, session). Use @PostConstruct/@PreDestroy or ApplicationRunner for startup logic, and application events (ApplicationEventPublisher, @EventListener, @TransactionalEventListener) to decouple side effects from core logic.

public record OrderPlaced(long orderId, String email) {}

@Service
class OrderService {
    private final ApplicationEventPublisher events;
    OrderService(ApplicationEventPublisher events) { this.events = events; }

    @Transactional
    public Order place(Cart cart) {
        Order order = /* save */ null;
        events.publishEvent(new OrderPlaced(order.getId(), cart.email()));
        return order;
    }
}

@Component
class OrderEmails {
    @TransactionalEventListener(phase = TransactionPhase.AFTER_COMMIT)   // only if the transaction commits
    void onOrderPlaced(OrderPlaced event) {
        // send confirmation email
    }
}

Common pitfalls

• Mutable fields in singleton beans (shared across requests → race conditions).
• Heavy work in constructors/@PostConstruct slowing startup.
• Sending emails in plain @EventListener before the transaction commits.

Key points

• Singletons must be thread-safe
• Events decouple side effects
• AFTER_COMMIT listeners for post-transaction work

Further reading: Spring: Bean scopes

Go deeper: Spring Boot: Filters, DispatcherServlet and Interceptors in the Request Lifecycle course traces this step of the request, with a production bug and a runnable simulation.