Hibernate在关系中创建错误的实体子类型

Question

我有一个奇怪的问题,hibernate不会在多对一关系中创建预期的实体类型.我们有以下具有子类层次结构的实体(简化):

@Entity
@Table(name = "A")
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(name = "DISCRIMINATOR", discriminatorType = DiscriminatorType.STRING, length = 1)
public abstract class A {

    @Id
    ...
    public Long getId() { ... }
    ...
}

@Entity
@DiscriminatorValue("1")
public class A1 extends A {
    ...
}

@Entity
@DiscriminatorValue("2")
public class A2 extends A {
    ...
}


@Entity
@Table(name = "B")
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(name = "DISCRIMINATOR", discriminatorType = DiscriminatorType.STRING, length = 1)
public abstract class B<AClass extends A> {

    protected AClass a;

    @Id
    ...
    public Long getId() { ... }
    ...

    public abstract AClass getA();
    public void setA(AClass a) { ... }
}

@Entity
@DiscriminatorValue("1")
public class B1 extends B<A1> {
    ...

    @Override
    @ManyToOne(fetch = EAGER)
    @JoinColumn(name = "A_ID")
    public A1 getA() { ... }
}

@Entity
@DiscriminatorValue("2")
public class B2 extends B<A2> {
    ...

    @Override
    @ManyToOne(fetch = EAGER)
    @JoinColumn(name = "A_ID")
    public A2 getA() { ... }
}

在persistence.xml两个实体中都按顺序声明

A2
A1
B2
B1

现在我在DB中创建A1和B1的实例:

A1 a1 = new A1();
entityManager.persist(a1);
B1 b1 = new B1();
b1.setA(a1);
entityManager.persist(b1);

我可以看到实例正确保存到数据库中每个都有ID 1,DISCRIMINATOR也是1,B中的A_ID也是1.

当我现在尝试获取B(在另一个休眠会话中):

B b = entityManager.find(B.class, 1L);

我得到了例外:

org.hibernate.PropertyAccessException: Exception occurred inside getter of B
Caused by: java.lang.ClassCastException: A2 cannot be cast to A1
at B1.getA(B1.java:61)
... 108 more 

通过调试,我发现hibernate正在创建B1类型的正确实体,并为A的关系创建一个类型为A2的错误实体.如果persistence.xml更改了该订单,则会创建正确的A1类型.在这种情况下,似乎hibernate不会将A表的DISCRIMINATOR列考虑在内,但始终会创建在配置中声明的第一个子类型.怎么解决这个问题？注释有问题吗？

(我也getA()首先在超类型B中使用其注释的方法的具体实现,但这会导致类似的问题.)

Answer 1

使用 Hibernate 5.0.2.Final 我能够使用@ManyToOne(..., targetEntity = A.class). 我也换成public abstract AClass getA();了普通的吸气剂。

@Entity
@Table(name = "B")
@Inheritance(strategy = InheritanceType.SINGLE_TABLE)
@DiscriminatorColumn(name = "DISCRIMINATOR", discriminatorType = DiscriminatorType.STRING, length = 1)
public abstract class B<AClass extends A> {
    private Long id;
    private AClass a;

    @Id
    @GeneratedValue
    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    @ManyToOne(fetch = FetchType.EAGER, targetEntity = A.class)
    @JoinColumn(name = "A_ID")
    public AClass getA() {
        return a;
    }

    public void setA(AClass a) {
        this.a = a;
    }
}

@Entity
@DiscriminatorValue("1")
public class B1 extends B<A1> {
    // no need to override getA()
}

@Entity
@DiscriminatorValue("2")
public class B2 extends B<A2> {
    // no need to override getA()
}

我在文档中没有找到有关此行为的任何信息。所以我只有我的观察：

• 如果没有targetEntity = A.classHibernate，那么在急切地从with 中获取行时甚至不会查询DISCRIMINATOR表的列，就像它已经决定了. AABA
• 当我添加targetEntity = A.class,A.DISCRIMINATOR出现在查询中时，对象是用 class 的正确子类创建的A。