LazyInitializationException

A lazy proxy can load its data only while the persistence context (Session) is open, which in Spring means inside the @Transactional method. Touch it after the transaction ends and you get:

public Order getOrder(Long id) {                 // no @Transactional
    return orderRepository.findById(id).orElseThrow();
}
// in the controller:
order.getItems().size();

org.hibernate.LazyInitializationException: Cannot lazily initialize collection of role 'com.shopease.order.Order.items' with key '501' (no session)

org.hibernate.LazyInitializationException: Could not initialize proxy [com.shopease.catalog.Category#3] - no session

Controller ──► OrderService.getOrder() ──► repository.findById()
                    │  transaction + Session open
                    │  Order loaded, items = uninitialized collection
                    ▼  transaction ends, Session closed, Order is now DETACHED
 Controller: order.getItems().size()  ──►  no Session to run SELECT  ──►  LazyInitializationException

1. Load what you need inside the transaction with a fetch join or @EntityGraph (next topics).
2. Map to a DTO inside the service (the transaction is still open), and return the DTO to the controller.
3. Use a DTO projection query that selects exactly the needed columns.

@Transactional(readOnly = true)
public OrderDetailsDto getOrder(Long id) {
    Order order = orderRepository.findWithItemsById(id)       // JOIN FETCH query
            .orElseThrow(() -> new OrderNotFoundException(id));
    return OrderDetailsDto.from(order);                       // mapped while the Session is open
}