JPA中的EntityGraph

前言

在使用JPA进行数据库访问时，可能会遇到n+1问题，也就是在查询关联实体时，jpa会额外执行查询操作，使用EntityGraph时，这是JPA推出优化解决效率问题的注解，EntityGraph则直接在查询语句的时候，直接用到用到Left Join，优化了数据库的性能

JPA关联表

实体类

定义一个实体类

@Data
@Entity
@FieldNameConstants
@Table(name = "machine_room")
@NamedEntityGraph(name = "MachineRoom.machineRoomInformationList", attributeNodes = {
        @NamedAttributeNode(value = "machineRoomInformationList"),
})
public class MachineRoom {


    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;


    private String projectName;


    @OneToMany(mappedBy = "machineRoom", cascade = CascadeType.ALL)
    private List<MachineRoomInformation> machineRoomInformationList;

}

关联实体类

@Data
@Entity
@FieldNameConstants
@Table(name = "machine_room_information")
public class MachineRoomInformation {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;

    private String attributeKey;


    private String attributeValue;


    @ManyToOne
    @JoinColumn(name = "machine_room_id")
    private MachineRoom machineRoom;
}

定义dao层

public interface IMachineRoomRepository extends JpaRepository<MachineRoom, Long> {


    @Transactional(rollbackFor = Exception.class)
    void deleteByIdIn(List<Long> ids);
    
    List<MachineRoom> findByIdIn(List<Long> ids);
}

数据库插入一条数据

接口

@GetMapping("/hello3")
public String hello3() {
    List<MachineRoom> machineRoomList = iMachineRoomRepository.findByIdIn(List.of(1L, 2L, 3L, 4L, 5L, 6L, 7L, 8L, 102L));
    MachineRoomDto machineRoomDto = new MachineRoomDto();
    for (MachineRoom machineRoom : machineRoomList) {
        BeanUtils.copyProperties(machineRoom, machineRoomDto);
    }
    return "success";
}

会发两条sql语句

加上@EntityGraph注解

public interface IMachineRoomRepository extends JpaRepository<MachineRoom, Long> {


    @Transactional(rollbackFor = Exception.class)
    void deleteByIdIn(List<Long> ids);

    @EntityGraph(value = "MachineRoom.machineRoomInformationList")
    List<MachineRoom> findByIdIn(List<Long> ids);
}

会产生一条left join语句

总结

使用EntityGraph可以加快级联查询