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 {

@Id
private 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;
}
}

子Entity类定义如下：

@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 )
)