Question

为什么Spring JPA没有初始化LAZY属性MyChildEntity.myParentEntity（所有字段都为空）？

我尝试使用Hibernate.initialize和@Transactional，但这没有帮助。

我的服务：

@Service
@Transactional
public class MyService {

    @Resource
    private MyChildEntityRepository myChildEntityRepository;

    @Resource
    private MyParentEntityRepository myParentEntityRepository;

    @PostConstruct
    public void init() {
        MyParentEntity p = myParentEntityRepository.save(new MyParentEntity("my name"));

        myChildEntityRepository.save(new MyChildEntity(p, "first value"));
        myChildEntityRepository.save(new MyChildEntity(new MyParentEntity(1L, "another name"), "another value"));

        // At this point both MyChildEntity's are in database and have correct foreign key value
        List<MyChildEntity> result = myChildEntityRepository.findAll();
        //even this doesn't help, myParentEntity property still has all fields equals to null
        Hibernate.initialize(result.get(0).getMyParentEntity());

        MyParentEntity p2 = result.get(0).getMyParentEntity();

        //trigger proxy's method to initialize lazy field
        System.out.print(p2.getName()); // null
        System.out.println(p2.getId()); // null

        // PROBLEM: p2 has all fields equals null
        // the same for result.get(1)

        // BUT, this works correct - returns (1L, "my name") entity
        myParentEntityRepository.findAll(); 
    }
}

子实体：

@Entity
public class MyChildEntity {

    @Id
    @SequenceGenerator(sequenceName = "CHILD_SEQ", name = "ChildSeq", allocationSize = 1)
    @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "ChildSeq")
    private Long id;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "my_parent_entity_id", referencedColumnName = "id")
    private MyParentEntity myParentEntity;

    @Column
    private String value;

    // constructors, getters, setters...

父实体：

@Entity
public class MyParentEntity {

    @Id
    @SequenceGenerator(sequenceName = "WORKFLOW_SEQ", name = "WorkflowSeq", allocationSize = 1)
    @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "WorkflowSeq")
    private Long id;

    @Column
    private String name;

    //constructors, getters, setters...

Answer 1

fetch属性指示何时应从中检索相关实体使用javax.persistence.FetchType枚举的数据库。 FetchType.EAGER表示JPA 提供程序必须在检索实体时检索值。另一方面，FetchType.LAZY 作为JPA提供者的提示，它只能在属性时等待并获取值首先访问（可能永远不会，因此节省了数据库之旅）。 然而，JPA提供商不需要支持延迟加载，因此无论如何都可能会急切地加载这些值。

来自 Nicholas S Williams 的 Web应用程序专业Java

编辑：

我真的道歉我花了很长时间。这是我认为错的。我没有在父实体中看到子实体的实例。它应该是这样的：

public class MyParentEntity {
... //other fields
@OneToMany(fetch = FetchType.LAZY, mappedBy = "myParentEntity")
private Set<MyChildEntity> myChildEntities = new HashSet<MyChildEntity>;
... //other fields or constructors or getters or setters 
...     
}

我希望这有效。如果没有，那么在MyChildEntity课程中，@JoinColumn内有一个名为referencedColumnName的奇怪注释。我不知道那是什么。请删除它。

由于

Spring JPA甚至在事务中也没有初始化延迟属性

1 个答案: