stence.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。
子类实体每保存一条数据，会在EMP表中插入一条记录，如FT_EMP表插入一条数据，会先在EMP表中插入name，并生成empId，再在FT_EMP表中插入empId和salary。PT_EMP同理。
不管超类是抽象类还是具体类，都会生成对应的表。
三、每个具体的类一个表的策略
这种映射策略每个类都会映射成一个单独的表，类的所有属性，包括继承的属性都会映射成表的列。
这种映射策略的缺点是：对多态关系的支持有限，当查询涉及到类继承结构时通常需要发起SQL UNION查询。实例如下：
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 s