标签：jpa继承映射策略 jpa继承映射 jpa映射策略 jpa jpaj映射
注：这里所说的实体指的是@Entity注解的类
继承映射使用@Inheritance来注解，它的strategy属性的取值由枚举InheritanceType来定义（包括SINGLE_TABLE、TABLE_PER_CLASS、JOINED，分别对应三种继承策略）。@Inheritance注解只能作用于继承结构的超类上。如果不指定继承策略，默认使用SINGLE_TABLE。
实例如下：
package com.mikan;
import java.io.Serializable;
import javax.persistence.Column;
import javax.persistence.DiscriminatorColumn;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.Inheritance;
import javax.persistence.InheritanceType;
import javax.persistence.Table;
@Entity
@Table(name = "EMP")
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(name = "emp_type")
public class Employee implements Serializable {
private static final long serialVersionUID = -7674269980281525370L;
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
protected Integer empId;
@Column
protected String name;
// getter/setter方法
}
package com.mikan;
import javax.persistence.Column;
import javax.persistence.DiscriminatorValue;
import javax.persistence.Entity;
@Entity
@DiscriminatorValue("FT")
public class FullTimeEmployee extends Employee {
private static final long serialVersionUID = 9115429216382631425L;
@Column
private Double salary;
// getter/setter方法
}
package com.mikan;
import javax.persistence.Column;
import javax.persistence.DiscriminatorValue;
import javax.persistence.Entity;
@Entity
@DiscriminatorValue("PT")
public class PartTimeEmployee extends Employee {
private static final long serialVersionUID = -6122347374515830424L;
@Column(name = "hourly_wage")
private Float hourlyWage;
// getter/setter方法
}其中，超类的@DiscriminatorColumn注解可以省略，默认的“discriminator列”名为DTYPE，默认类型为STRING。package com.mikan;
import java.io.Serializable;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.Inheritance;
import javax.persistence.InheritanceType;
import javax.persistence.Table;
@Entity
@Table(name = "EMP")
@Inheritance(strategy = InheritanceType.JOINED)
public class Employee implements Serializable {
private static final long serialVersionUID = -7674269980281525370L;
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
protected Integer empId;
@Column
protected String name;
// getter/setter方法
}
package com.mikan;
import javax.persistence.Column;
import javax.persistence.DiscriminatorValue;
import javax.persistence.Entity;
import javax.persistence.Table;
@Entity
@Table(name = "FT_EMP")
public class FullTimeEmployee extends Employee {
private static final long serialVersionUID = 9115429216382631425L;
@Column
private Double salary;
// getter/setter方法
}
package com.mikan;
import javax.persistence.Column;
import javax.persistence.DiscriminatorValue;
import javax.persistence.Entity;
import javax.persistence.Table;
@Entity
@Table(name = "PT_EMP")
public class PartTimeEmployee extends Employee {
private static final long serialVersionUID = -6122347374515830424L;
@Column(name = "hourly_wage")
private Float hourlyWage;
// getter/setter方法
}这会映射成三个具体的表，分别是，Employee对应EMP表，字段包括empId、name；FullTimeEmployee对应FT_EMP表，字段包括empId、salary；PartTimeEmployee对应PT_EMP表，字段包括empId、hourly_wage。其中，表FT_EMP和PT_EMP中的empId作为表EMP的外键，同是它也是主键。默认情况下，使用超类的主键作为子类的主键和外键。当然，可以通过@PrimaryKeyJoinColumn注解来自己指定外键的名称，如FullTimeEmployee使用@PrimaryKeyJoinColumn(name
= "FT_EMPID")注解，那么该子类实体的字段为FT_EMPID、name，FT_EMPID作为表FT_TIME的主键，同时它也是EMP表的外键。package com.mikan;
import java.io.Serializable;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.Inheritance;
import javax.persistence.InheritanceType;
import javax.persistence.Table;
@Entity
@Table(name = "EMP")
@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)
public class Employee implements Serializable {
private static final long serialVersionUID = -7674269980281525370L;
@Id
@GeneratedValue(strategy = GenerationType.TABLE)
protected Integer empId;
@Column
protected String name;
// getter/setter方法
}
package com.mikan;
import javax.persistence.Column;
import javax.persistence.DiscriminatorValue;
import javax.persistence.Entity;
import javax.persistence.Table;
@Entity
@Table(name = "FT_EMP")
public class FullTimeEmployee extends Employee {
private static final long serialVersionUID = 9115429216382631425L;
@Column
private Double salary;
// getter/setter方法
}
package com.mikan;
import javax.persistence.Column;
import javax.persistence.DiscriminatorValue;
import javax.persistence.Entity;
import javax.persistence.Table;
@Entity
@Table(name = "PT_EMP")
public class PartTimeEmployee extends Employee {
private static final long serialVersionUID = -6122347374515830424L;
@Column(name = "hourly_wage")
private Float hourlyWage;
// getter/setter方法
}这会映射成三个具体的表，分别是，Employee对应EMP表，字段包括empId、name；FullTimeEmployee对应FT_EMP表，字段包括empId、salary；PartTimeEmployee对应PT_EMP表，字段包括empId、hourly_wage。其中，表FT_EMP和PT_EMP中的empId和EMP表的empId没有任何关系。子类实体每保存一条数据，EMP表中不会插入记录。因为TABLE_PER_CLASS策略每个表都是单独的，没有并且各表的主键没有任何关系，所以不能使用GenerationType.AUTO或GenerationType.IDENTITY主键生成策略，可以使用GenerationType.TABLE。
如果超类是抽象类，那么不会生成对应的表。如果超类是具体的类，那么会生成对应的表。以上实例使用JPA的hibernate实现测试通过。
标签：jpa继承映射策略 jpa继承映射 jpa映射策略 jpa jpaj映射
原文地址：http://blog.csdn.net/mhmyqn/article/details/37996673
