为什么typedquery.getresultlist()通过单独的select解析每个manytoone关联?

考虑到以下简单的实体关联: (实体A)*-1(实体B) 在数据库中用EntityA(EntityB_ID)中的外键生成。

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,每行EntityA都会执行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热切的(看起来是这样),EntityB也应该通过implizit join来获取,不是吗? 怎么了?

但是,如果只加载一个EntityA对象,这会变得更加奇怪:

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

然后Hibernate似乎理解了工作:

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=?

以上测试是用Hibernate 3.5和JPA 2.0进行的。