FetchMode如何在Spring Data JPA中工作

Question

我确实在项目中的三个模型对象之间有关系（文章末尾的模型和存储库片段）。当我调用PlaceRepository.findById它时，会触发三个选择查询：（“ sql”）SELECT * FROM place p where id = argSELECT * FROM user u where u.id = place.user.idSELECT * FROM city c LEFT OUTER JOIN state s on c.woj_id = s.id where c.id = place.city.id（对我而言）这是非常不正常的行为。据阅读Hibernate文档后所知，它应该始终使用JOIN查询。在类中FetchType.LAZY更改为 查询时（带有附加SELECT 的查询）没有任何区别，而在类更改为 （使用JOIN查询时）则没有变化。FetchType.EAGERPlaceCityFetchType.LAZYFetchType.EAGER当我使用CityRepository.findById抑制射击时，有两个选择：SELECT * FROM city c where id = argSELECT * FROM state s where id = city.state.id我的目标是在所有情况下都具有sam行为（始终为JOIN或SELECT，但首选JOIN）。型号定义：地点：@Entity@Table(name = "place")public class Place extends Identified {    @Fetch(FetchMode.JOIN)    @ManyToOne(fetch = FetchType.LAZY)    @JoinColumn(name = "id_user_author")    private User author;    @Fetch(FetchMode.JOIN)    @ManyToOne(fetch = FetchType.LAZY)    @JoinColumn(name = "area_city_id")    private City city;    //getters and setters}市：@Entity@Table(name = "area_city")public class City extends Identified {    @Fetch(FetchMode.JOIN)    @ManyToOne(fetch = FetchType.LAZY)    @JoinColumn(name = "area_woj_id")    private State state;    //getters and setters}仓库：PlaceRepositorypublic interface PlaceRepository extends JpaRepository<Place, Long>, PlaceRepositoryCustom {    Place findById(int id);}UserRepository：public interface UserRepository extends JpaRepository<User, Long> {        List<User> findAll();    User findById(int id);}城市资料库：public interface CityRepository extends JpaRepository<City, Long>, CityRepositoryCustom {        City findById(int id);}

Answer 1

我认为Spring Data会忽略FetchMode。在使用Spring Data时，我总是使用@NamedEntityGraph和@EntityGraph批注

@Entity

@NamedEntityGraph(name = "GroupInfo.detail",

  attributeNodes = @NamedAttributeNode("members"))

public class GroupInfo {

  // default fetch mode is lazy.

  @ManyToMany

  List<GroupMember> members = new ArrayList<GroupMember>();

  …

}

@Repository

public interface GroupRepository extends CrudRepository<GroupInfo, String> {

  @EntityGraph(value = "GroupInfo.detail", type = EntityGraphType.LOAD)

  GroupInfo getByGroupName(String name);

}

Answer 2

首先，@Fetch(FetchMode.JOIN)并且@ManyToOne(fetch = FetchType.LAZY)是对立的，一个指示EAGER的获取，而另一个则指示LAZY的获取。

急切的获取很少是一个好的选择，对于可预期的行为，最好使用query-time JOIN FETCH指令：

public interface PlaceRepository extends JpaRepository<Place, Long>, PlaceRepositoryCustom {

    @Query(value = "SELECT p FROM Place p LEFT JOIN FETCH p.author LEFT JOIN FETCH p.city c LEFT JOIN FETCH c.state where p.id = :id")

    Place findById(@Param("id") int id);

}

public interface CityRepository extends JpaRepository<City, Long>, CityRepositoryCustom { 

    @Query(value = "SELECT c FROM City c LEFT JOIN FETCH c.state where c.id = :id")   

    City findById(@Param("id") int id);

}