Hibernate中的Entity类之间的继承关系之三JOINED

对于Hibernate提供的4种兼容JPA的映射策略，这里介绍第三种Joined table，也被称为table-per-subclass策略。 

在这种策略中，存在如下特征：

• 父子Entity类都对应各自的数据库表
• 父表中无需设置discriminator列
• 子表中的主键ID，也是子表的外键，用以指向父表的主键ID。默认父子表中的主键名字相同，也可以在子Entity类中通过如下标注给出列名。

  @javax.persistence.PrimaryKeyJoinColumn(name="CUST_ID")
  

• 访问子Entity类时需要在数据库中join访问父Entity类

父Entity类定义如下：

@Entity(name = "Account")
@javax.persistence.Inheritance(strategy = InheritanceType.JOINED)
public static class Account {@Idprivate Long id;private String owner;private BigDecimal balance;private BigDecimal interestRate;public Long getId() {return id;}public void setId(Long id) {this.id = id;}public String getOwner() {return owner;}public void setOwner(String owner) {this.owner = owner;}public BigDecimal getBalance() {return balance;}public void setBalance(BigDecimal balance) {this.balance = balance;}public BigDecimal getInterestRate() {return interestRate;}public void setInterestRate(BigDecimal interestRate) {this.interestRate = interestRate;}
}

@Entity(name = "DebitAccount")
@javax.persistence.PrimaryKeyJoinColumn(name = "account_id")
public static class DebitAccount extends Account {private BigDecimal overdraftFee;public BigDecimal getOverdraftFee() {return overdraftFee;}public void setOverdraftFee(BigDecimal overdraftFee) {this.overdraftFee = overdraftFee;}
}

另一个子Entity类定义如下：

@Entity(name = "CreditAccount")
@javax.persistence.PrimaryKeyJoinColumn(name = "account_id")
public static class CreditAccount extends Account {private BigDecimal creditLimit;public BigDecimal getCreditLimit() {return creditLimit;}public void setCreditLimit(BigDecimal creditLimit) {this.creditLimit = creditLimit;}
}

数据库表结构如下：

CREATE TABLE Account (id BIGINT NOT NULL ,balance NUMERIC(19, 2) ,interestRate NUMERIC(19, 2) ,owner VARCHAR(255) ,PRIMARY KEY ( id )
)CREATE TABLE CreditAccount (creditLimit NUMERIC(19, 2) ,account_id BIGINT NOT NULL REFERENCES Account(id),PRIMARY KEY ( id )
)CREATE TABLE DebitAccount (overdraftFee NUMERIC(19, 2) ,account_id BIGINT NOT NULL REFERENCES Account(id),PRIMARY KEY ( id )
)

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