What is Spring Boot's transaction management with @Transactional?

// Service with transaction management:
@Service
public class OrderService {

    private final OrderRepository orderRepo;
    private final InventoryRepository inventoryRepo;
    private final EventPublisher events;

    // Default: propagation=REQUIRED, isolation=DEFAULT, rollbackFor=RuntimeException
    @Transactional
    public Order placeOrder(CreateOrderRequest request) {
        // All operations in one transaction:
        Order order = orderRepo.save(new Order(request));
        inventoryRepo.reserveStock(request.productId(), request.quantity());
        events.publish(new OrderPlacedEvent(order.getId()));
        return order;
        // Commits on successful return
        // Rolls back on RuntimeException
    }

    // Read-only transaction (performance optimisation):
    @Transactional(readOnly = true)
    public Optional<Order> findById(String id) {
        return orderRepo.findById(id);
        // Hibernate skips dirty checking for read-only transactions
    }

    // Custom rollback rules:
    @Transactional(rollbackFor = {BusinessException.class, IOException.class})
    public void processPayment(String orderId) throws IOException {
        // Rolls back on checked IOException too
    }

    // Propagation types:
    @Transactional(propagation = Propagation.REQUIRES_NEW)
    public void auditLog(String message) {
        // Always runs in its own NEW transaction,
        // suspending any existing outer transaction
        auditRepo.save(new AuditEntry(message));
    }
}