@Service
public class TransferService {
    @Transactional
    public void transfer(Long fromId, Long toId, BigDecimal amount) {
        accountRepo.withdraw(fromId, amount);
        accountRepo.deposit(toId, amount); // if this throws, the withdraw rolls back too
    }
}

• By default, rolls back on unchecked (RuntimeException) exceptions
• propagation controls how nested @Transactional calls behave (REQUIRED, REQUIRES_NEW...)
• readOnly = true is a useful hint/optimization for query-only methods