Implementing Object-Relational Mapping

@Entity
@Table(name = "orders")
public class Order {
    @Id @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    @Column(nullable = false) private String status;
    @Column(name = "placed_at", nullable = false) private Instant placedAt;
}

@Embeddable
public class Address {
    private String street; private String city; private String zip;
}

@Entity
public class Customer {
    @Embedded
    @AttributeOverride(name="city", column=@Column(name="billing_city"))
    private Address billing;
}

@NamedQuery(name = "Order.findOpen",
    query = "select o from Order o where o.status = 'OPEN'")
@Entity
public class Order { ... }

em.createNamedQuery("Order.findOpen", Order.class).getResultList();

@Converter(autoApply = true)
public class MoneyConverter implements AttributeConverter<Money, Long> {
    public Long convertToDatabaseColumn(Money m) { return m == null ? null : m.cents(); }
    public Money convertToEntityAttribute(Long c) { return c == null ? null : Money.usd(c); }
}

@Embeddable
public record OrderLineId(Long orderId, int lineNo) implements Serializable {}

@Entity
public class OrderLine {
    @EmbeddedId private OrderLineId id;
}

@Entity
public class Account {
    @Id private Long id;
    @Version private Long version;  // auto-incremented; throws OptimisticLockException on stale write
    private long balanceCents;
}