JavaPersistenceWithHibernate第二版笔记-第六章-Mapping inheritance-003Table per concrete class with unions(@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)、<union-subclass>)

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.