Spring数据findAll（）不会急于获取

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;
}

我保存了两个对象：

    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注释eager fetch默认使用。当我Item使用加载一个时findOne()，Hibernate 使用生成查询，left outer join并Basket在同一查询中获取。但是，当我使用findAll()Hibernate 时，它首先获取所有内容Items，然后执行N selects（每个执行一个Basket），从而导致(n+1) select problem。为什么Hiberante不急于Basket使用findAll()method来获取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()它时，就相当于触发了JPQL查询entityManager.createQuery(...)，并且默认情况下，它items首先加载第一个查询，随后each item又加载该basket实体并导致N + 1问题。

您可以采用以下两种方法之一：

1. 覆盖通过在方法上指定@Query注释findAll而使用的默认查询，并将查询与join类似使用select i from Item i left join fetch i.basket。

2. 在类上使用@NamedEntityGraph命名basket，Item并指定Item需要急于加载图形的哪一部分。在findAll方法上，使用@EntityGraph(value = "basket")。请注意，根据spring jpa实体图，我们还可以使用attributePath通过定义临时实体图@EntityGraph without the need of having to explicitly add @NamedEntityGraph to your domain types。

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();
}

然后你可以记录你的sql查询，看看只做了一个查询

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

而在此之前

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=?