java - JPA继承两个或多个父类(super class)

Question

我对JPA中的继承进行了研究，并resources我发现每个实体只使用一个父类(super class)。但没有使用 2 个或更多父类(super class)的示例。

这个怎么样:

@Entity
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(name = “Abstract_One”)
public abstract class AbstractOne {

    @Id
    protected Long id;
    …
}

@Entity(name = “A”)
@DiscriminatorValue(“A”)
public class A extends AbstractOne {

    @Column
    private int a;

    …
}

@Entity(name = “B”)
@DiscriminatorValue(“B”)
public class B extends A {

    @Column
    private int b;

        …
}

可以这样做吗？

如果可能，哪种继承策略允许这样做并提供最佳的数据一致性？

Answer 1

我只能想象下面的例子

@MappedSuperclass
public class A
{
   ...
   @Id
   @Column(name = "RECID")
   public Long getId()
...
}

@MappedSuperclass
public class B extends A
{
...
   @Column(name = "COL1")
   public String getColumn1()
...
}

@Entity(name="INH_TAB1")
public class C extends B
{
   ...
   @Column(name = "COL2")
   public String getColumn2()
   ...
}

另外，在 Bauer、King、Gregory 所著的优秀著作《Java Persistence with Hibernate》中，我发现以下内容对于解决此问题可能有用:

6.5 Mixing inheritance strategies

You can map an entire inheritance hierarchy with the TABLE_PER_CLASS, SINGLE_TABLE, or JOINED strategy. You can’t mix them — for example, to switch from a table-per-class hierarchy with a discriminator to a normalized table-per-subclass strategy. Once you’ve made a decision for an inheritance strategy, you have to stick with it. This isn’t completely true, however. By using some tricks, you can switch the mapping strategy for a particular subclass. For example, you can map a class hierarchy to a single table, but, for a particular subclass, switch to a separate table with a foreign key–mapping strategy, just as with table-per-subclass.

但是，我无法想象任何实际情况，这种复杂的继承层次结构是必需的/有用的，而且它还会影响性能。