一、代码
1.
package org.jpwh.model.inheritance.tableperclass; import org.jpwh.model.Constants; import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.Inheritance;
import javax.persistence.InheritanceType;
import javax.validation.constraints.NotNull; @Entity
//in Hibernate, it’s equivalent to a <union-subclass> mapping in the old native Hibernate XML metadata
@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)
public abstract class BillingDetails { @Id
@GeneratedValue(generator = Constants.ID_GENERATOR)
protected Long id; @NotNull
protected String owner; // ... protected BillingDetails() {
} protected BillingDetails(String owner) {
this.owner = owner;
} public Long getId() {
return id;
} public String getOwner() {
return owner;
} public void setOwner(String owner) {
this.owner = owner;
}
}
2.
package org.jpwh.model.inheritance.tableperclass; import javax.persistence.Entity;
import javax.validation.constraints.NotNull; @Entity
public class CreditCard extends BillingDetails { @NotNull
protected String cardNumber; @NotNull
protected String expMonth; @NotNull
protected String expYear; // ... public CreditCard() {
super();
} public CreditCard(String owner, String cardNumber, String expMonth, String expYear) {
super(owner);
this.cardNumber = cardNumber;
this.expMonth = expMonth;
this.expYear = expYear;
} public String getCardNumber() {
return cardNumber;
} public void setCardNumber(String cardNumber) {
this.cardNumber = cardNumber;
} public String getExpMonth() {
return expMonth;
} public void setExpMonth(String expMonth) {
this.expMonth = expMonth;
} public String getExpYear() {
return expYear;
} public void setExpYear(String expYear) {
this.expYear = expYear;
}
}
3.
package org.jpwh.model.inheritance.tableperclass; import javax.persistence.Entity;
import javax.validation.constraints.NotNull; @Entity
public class BankAccount extends BillingDetails { @NotNull
protected String account; @NotNull
protected String bankname; @NotNull
protected String swift; public BankAccount() {
super();
} public BankAccount(String owner, String account, String bankname, String swift) {
super(owner);
this.account = account;
this.bankname = bankname;
this.swift = swift;
} public String getAccount() {
return account;
} public void setAccount(String account) {
this.account = account;
} public String getBankname() {
return bankname;
} public void setBankname(String bankname) {
this.bankname = bankname;
} public String getSwift() {
return swift;
} public void setSwift(String swift) {
this.swift = swift;
}
}
二、相比上一个方法的优点
1.查询父类不用再通过多次select,而是通过union all,the query select bd from BillingDetails bd generates the following SQL statement:
elect
ID, OWNER, EXPMONTH, EXPYEAR, CARDNUMBER,
ACCOUNT, BANKNAME, SWIFT, CLAZZ_
from
(select ID, OWNER, EXPMONTH, EXPYEAR, CARDNUMBER,
null as ACCOUNT,
null as BANKNAME,
null as SWIFT,
1 as CLAZZ_
from CREDITCARD
union all
select id, OWNER,
null as EXPMONTH,
null as EXPYEAR,
null as CARDNUMBER,
ACCOUNT, BANKNAME, SWIFT,
2 as CLAZZ_ from BANKACCOUNT
) as BILLINGDETAILS
2.Another much more important advantage is the ability to handle polymorphic associations; for example, an association mapping from User to BillingDetails would now be possible. Hibernate can use a UNION query to simulate a single table as the target of the association mapping.