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.