hibernate - 为什么 TypedQuery.getResultList() 用单独的 SELECT 解析每个 ManyToOne 关联？

Question

考虑以下简单的实体关联:
(实体A)*-1(实体B)
在数据库中使用 EntityA (entityB_id) 中的外键创建。

JPA 实体正在单向映射此关系:

@Entity
EntityA {
    @Id
    @GeneratedValue
    private long id;

    @Column(nullable=false,length=250)
    private String name;

    @ManyToOne(optional=false)
    private EntityB entityB;

    ... getter/setter ...
}

@Entity
EntityB {
    @Id
    @GeneratedValue
    private long id;

    @Column(nullable=false,length=250)
    private String name;

    ... getter/setter ...
}

如果进行简单查询:

EntityManager em = ...;
TypedQuery<EntityA> tq = em.createQuery("from EntityA a", EntityA.class);
tq.getResultList();

我在 Hibernate 的 SQL 调试输出中看到，对 EntityA 的每一行都进行了 EntityB 查询:

Hibernate: 
    select
        entitya0_.id as id8_,
        entitya0_.entityB_id as entityB3_8_,
        entitya0_.name as name8_ 
    from
        EntityA entitya0_
Hibernate: 
    select
        entityb0_.id as id4_0_,
        entityb0_.name as name4_0_ 
    from
        EntityB entityb0_ 
    where
        entityb0_.id=?

即使默认的获取策略是 EAGER(似乎是这种情况)，EntityB 也应该通过 implizit join 获取，不是吗？
怎么了？

但它变得更加奇怪——如果只加载了一个 EntityA 对象:

EntityA a = em.find(EntityA.class, new Long(1));

然后 Hibernate 似乎了解这项工作:

Hibernate: 
    select
        entitya0_.id as id1_1_,
        entitya0_.entityB_id as entityB3_1_1_,
        entitya0_.name as name1_1_,
        entityb1_.id as id12_0_,
        entityb1_.name as name12_0_ 
    from
        EntityA entitya0_ 
    inner join
        EntityB entityb1_ 
        on entitya0_.entityB_id=entityb1_.id 
    where
        entitya0_.id=?

以上测试是使用 Hibernate 3.5 和 JPA 2.0 进行的。

Answer 1

Even if the default fetch strategy is EAGER (which seems to be the case), EntityB should be fetched via implicit join, shouldn't it? What is wrong?

确实，默认 FetchType的 ManyToOne是 EAGER .但这只是说 One侧应该加载 当 Many侧加载，而不是 怎么样 .如何由持久性提供者自行决定(并且 JPA 不允许调整策略)。

Hibernate 有一个特定的 Fetch注释允许调整获取模式。从文档:

2.4.5.1. Lazy options and fetching modes

JPA comes with the fetch option to define lazy loading and fetching modes, however Hibernate has a much more option set in this area. To fine tune the lazy loading and fetching strategies, some additional annotations have been introduced:

• [...]

• @Fetch: defines the fetching strategy used to load the association. FetchMode can be SELECT (a select is triggered when the association needs to be loaded), SUBSELECT (only available for collections, use a subselect strategy - please refers to the Hibernate Reference Documentation for more information) or JOIN (use a SQL JOIN to load the association while loading the owner entity). JOIN overrides any lazy attribute (an association loaded through a JOIN strategy cannot be lazy).

您可能想尝试以下操作(如果您不介意使用提供者特定的注释):

@ManyToOne(optional=false)
@Fetch(FetchMode.JOIN)
private EntityB entityB;