Observer Pattern Real-World Examples in Java

Direct Answer

Observer-style APIs in Java include Swing and JavaFX listeners, JavaBeans PropertyChangeListener, servlet lifecycle listeners, and Spring application events. In Spring, ApplicationEventPublisher hands an event to the configured multicaster and matching @EventListener methods receive it. Delivery is synchronous by default but can be configured differently. @TransactionalEventListener binds delivery to a transaction phase; its default is AFTER_COMMIT when a transaction exists.

Code Example

@Service
class OrderService {
    private final ApplicationEventPublisher publisher; // the subject
    OrderService(ApplicationEventPublisher publisher) { this.publisher = publisher; }

    @Transactional
    public void placeOrder(Order order) {
        order.setStatus(OrderStatus.CONFIRMED);
        // notify every observer without naming a single one of them
        publisher.publishEvent(new OrderPlacedEvent(order.getId()));
    }
}

record OrderPlacedEvent(Long orderId) {} // carries the data to observers

@Component
class EmailListener {                 // observer 1
    @EventListener                     // synchronous: runs in publisher's thread
    void onOrder(OrderPlacedEvent e) { System.out.println("Email for " + e.orderId()); }
}

@Component
class LoyaltyListener {               // observer 2
    @TransactionalEventListener(phase = TransactionPhase.AFTER_COMMIT)
    void onOrder(OrderPlacedEvent e) { System.out.println("Award points for " + e.orderId()); }
}

Add a third listener tomorrow — OrderService never changes. That is Observer's decoupling.

Common Mistakes & Pitfalls

• Using java.util.Observable. It's deprecated since Java 9. Use Spring events or PropertyChangeSupport.
• Assuming @EventListener is async. It runs synchronously in the publisher's thread; add @Async (with @EnableAsync) for background execution.
• Ignoring transaction boundaries. A plain @EventListener can run before the DB commit. If the transaction rolls back, you've already emailed the customer. Use @TransactionalEventListener(AFTER_COMMIT).
• Only quoting stock-ticker textbook examples. Name real APIs — that's what separates a fresher who has used Spring from one who memorized a diagram.