JPA中的InheritanceType有哪些类型？

JPA中的InheritanceType用于定义实体的继承映射策略,有以下类型:

1. SINGLE_TABLE:单表继承映射。父类和子类共享同一张表,使用discriminator列区分类型。

@Entity
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(name = "TYPE")  
public class Person { ... }  

@Entity
public class Employee extends Person { ... }  

2. TABLE_PER_CLASS:各类一表继承映射。每一个实体类对应一张表,不共享表。

@Entity
@Inheritance(strategy = InheritanceType.TABLE_PER_CLASS)
public class Person { ... }   

@Entity 
public class Employee extends Person { ... }

3. JOINED:联接继承映射。父类和子类各自的表,并且有外键相连。

@Entity
@Inheritance(strategy = InheritanceType.JOINED)
public class Person { ... }   

@Entity
public class Employee extends Person { ... } 

4. MappedSuperclass:将父类映射为抽象类,其不映射为表,仅用于提供字段映射信息。

@MappedSuperclass
public abstract class Person { ... }

@Entity
public class Employee extends Person { ... } 

代码示例:

@Entity
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(name = "TYPE")  
public class Person {
    @Id
    private int id; 
}

@Entity  
public class Employee extends Person {
    private double salary;
}