Real World Example

A wallet transfer must debit one wallet and credit another in one transaction. Dirty checking means the service only changes fields; Hibernate writes both UPDATEs at commit, or nothing if an exception rolls back:

@Entity
@Table(name = "wallets")
public class Wallet {
    @Id @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private Long userId;
    @Column(nullable = false, precision = 12, scale = 2)
    private BigDecimal balance;

    public void debit(BigDecimal amount) {
        if (balance.compareTo(amount) < 0) throw new InsufficientBalanceException(id);
        balance = balance.subtract(amount);
    }
    public void credit(BigDecimal amount) { balance = balance.add(amount); }
}

@Service
public class WalletService {
    private final EntityManager em;
    public WalletService(EntityManager em) { this.em = em; }

    @Transactional
    public void transfer(Long fromId, Long toId, BigDecimal amount) {
        Wallet from = em.find(Wallet.class, fromId);   // MANAGED
        Wallet to = em.find(Wallet.class, toId);       // MANAGED
        from.debit(amount);                            // may throw -> rollback, no SQL written
        to.credit(amount);
        em.persist(new WalletTransaction(fromId, toId, amount));   // TRANSIENT -> MANAGED
    }   // commit: INSERT transaction, UPDATE from, UPDATE to
}