Spring 即使设置了 FetchType.EAGER 也没有加载数据

Question

我有两个模型正在尝试从 REST API（宠物和媒体）取回数据。我试图通过 FetchType.EAGER 注释获取宠物和媒体之间的 oneToMany 关系，但是当我编写 MediaRepository 时数据没有出现。如果我不实施该文件，媒体关系和数据会在响应中返回。

实施 MediaRepository.java，GET /pets returns:

{
  "id": 72,
  "name": "Spot",
  "description": "Annoying as hell",
  "media": [], <-- why is this here only if I don't implement MediaRepository?
  ...
}

没有 MediaRepository.java 实施，GET /pets returns:

{
  "id": 72,
  "name": "Spot",
  "description": "Annoying as hell",
  ... (No media array in response)
}

Pet.java

@Entity
public class Pet implements Serializable {

    private static final long serialVersionUID = 1L;

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

    @Column(name="name")
    private String name;

    @Column(name="description")
    private String description;

    @OneToMany(cascade=CascadeType.ALL, mappedBy="pet", FetchType.EAGER, orphanRemoval=true)
    private List<Media> media;

    @ManyToOne
    private Category category;

    @Enumerated(EnumType.STRING)
    private Status status;
}

Media.java

@Entity
public class Media implements Serializable {
    private static final long serialVersionUID = 1L;

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

    @Column(name="url")
    private String url;

    @Column(name="title")
    private String title;

    @ManyToOne
    private Pet pet;
}

PetRepository.java

import org.springframework.data.jpa.repository.JpaRepository;

public interface PetRepository extends JpaRepository<Pet, Long> {
}

MediaRepository.java

import org.springframework.data.jpa.repository.JpaRepository;

public interface MediaRepository extends JpaRepository<Media, Long> {
}

Answer 1

这与 Hibernate 获取策略无关。

您看到的行为是 Spring Data Rest 设计的工作方式。为媒体定义存储库后，您会看到响应中提供了 link 供客户端检索关联的媒体项。如果没有存储库，则关联必须内嵌在响应中，因为当然没有办法独立检索集合。

如果您希望在响应中有选择地内联集合，那么您可以通过定义投影来实现。

@Projection(name = "inlineData", types=Pet.class)
public interface PetProjection{

    Long getId();
    String getName();
    String getDescription();
    List<Media> getMedia();
}

您可以将此投影自动应用于集合资源：

@RepositoryRestResource(excerptProjection = PetProjection.class)
public interface PetRepository extends JpaRepository<Pet, Long> {}

对于项目资源，客户通常会指定他们希望内联此数据：

例如

http://example.com/api/pets/1?projection=inlineData

http://docs.spring.io/spring-data/rest/docs/current/reference/html/#projections-excerpts

Spring 即使设置了 FetchType.EAGER 也没有加载数据

Spring not loading data even with FetchType.EAGER set

java

spring

hibernate

spring-data-jpa

spring-data-rest