Spring 数据 findAll() 没有急切获取

Question

我有两个具有单向 one to many 关系的实体。

@Entity
public class Basket {

    @Id
    @GeneratedValue
    private Long id;

    private int capacity;
}

@Entity
public class Item {

    @Id
    @GeneratedValue
    private Long id;

    private String name;

    @ManyToOne
    private Basket basket;
}

我保存了几个对象：

    Basket basket1 = new Basket(100);
    Basket basket2 = new Basket(200);
    Basket basket3 = new Basket(300);
    basketRepository.save(asList(basket1, basket2, basket3));

    Item item1 = new Item("item1", basket1);
    Item item11 = new Item("item11", basket1);
    Item item2 = new Item("item2", basket2);
    Item item22 = new Item("item22", basket2);
    Item item3 = new Item("item3", basket3);
    Item item33 = new Item("item33", basket3);
    itemRepository.save(asList(item1, item11, item2, item22, item3, item33));

    // Loading one item. Basket fetched eagerly.
    itemRepository.findOne(1L);

    // Loading many items. Baskets are not loaded (n+1 select problem).
    itemRepository.findAll();

@ManyToOne注解默认使用eager fetch。当我使用 findOne() 加载一个 Item 时，Hibernate 使用 left outer join 生成查询并在同一查询中获取 Basket。但是，当我使用 findAll() 时，Hibernate 首先获取所有 Items，然后执行 N selects（每个 Basket 执行一个），从而导致 (n+1) select problem。为什么 Hiberante 不使用 findAll() 方法急切地获取 Basket 对象以及如何解决这个问题？

Answer 1

根据 JPA 2.0 规范，@ManyToOne 默认为 EAGER。

现在，当您使用 findAll() 时，它相当于触发一个像 entityManager.createQuery(...) 这样的 JPQL 查询，默认情况下它首先加载 items 然后再加载 each item加载 basket 实体并导致 N+1 问题。

您可以采用以下两种方法之一：

通过在 findAll 方法上指定 @Query 注释来覆盖使用的默认查询，并使用带有连接的查询，如 select i from Item i left join fetch i.basket.
使用@NamedEntityGraph with name say basket on Item class and specify which part of the Item graph needs to be loaded eagerly. On the findAll method, use @EntityGraph(value = "basket"). Note that as per spring jpa entity graph，我们也可以使用attributePath通过@EntityGraph without the need of having to explicitly add @NamedEntityGraph to your domain types.[=27定义ad-hoc实体图=]

Answer 2

您可以在存储库中使用@Query 注释覆盖findAll 方法。下面是示例代码

public interface ItemRepository extends CrudRepository<Item, Long> {
    @Override
    @Query("select item from Item item left join fetch item.basket")
    Iterable<Item> findAll();
}

然后您可以记录您的 sql 查询以查看只进行了一个查询

Hibernate: select item0_.id as id1_1_0_, basket1_.id as id1_0_1_, item0_.basket_id as basket_i3_1_0_, item0_.name as name2_1_0_, basket1_.capacity as capacity2_0_1_ from item item0_ left outer join basket basket1_ on item0_.basket_id=basket1_.id

之前是

2018-03-09 13:26:52.269  INFO 4268 --- [           main] o.h.h.i.QueryTranslatorFactoryInitiator  : HHH000397: Using ASTQueryTranslatorFactory
Hibernate: select item0_.id as id1_1_, item0_.basket_id as basket_i3_1_, item0_.name as name2_1_ from item item0_
Hibernate: select basket0_.id as id1_0_0_, basket0_.capacity as capacity2_0_0_ from basket basket0_ where basket0_.id=?
Hibernate: select basket0_.id as id1_0_0_, basket0_.capacity as capacity2_0_0_ from basket basket0_ where basket0_.id=?
Hibernate: select basket0_.id as id1_0_0_, basket0_.capacity as capacity2_0_0_ from basket basket0_ where basket0_.id=?

Spring 数据 findAll() 没有急切获取

Spring data findAll() does not fetch eagerly

java

hibernate

jpa

eager-loading

spring-data