hibernate - 使用延迟初始化属性分离 JPA 对象

Question

有两个 JPA 实体:具有一对多关系的 User 和 Order。

/**
 * User DTO
 */
@Entity
@Table(name="user")
public class User implements Serializable {
    private static final long serialVersionUID = 8372128484215085291L;

    private Long id;
    private Set<Order> orders;

    public User() {}

    @Id @GeneratedValue(strategy=GenerationType.SEQUENCE, generator="sequenceUser")
    public Long getId() {
        return this.id;
    }
    private void setId(Long id) {
        this.id = id;
    }

    @OneToMany(mappedBy="user", cascade=CascadeType.PERSIST, fetch=FetchType.LAZY)
    @LazyCollection(LazyCollectionOption.EXTRA)
    public Set<Order> getOrders() {
        return orders;
    }
    public void setOrders(Set<Order> orders) {
        this.orders = orders;
    }
 }


/**
 * Order DTO
 */
@Entity
@Table(name="order")
public class Order implements Serializable {
    private static final long serialVersionUID = 84504362507297200L;

    private Long id;
    private User user;

    public Order() {
    }

    @Id @GeneratedValue(strategy=GenerationType.SEQUENCE, generator="sequenceOrder")
    public Long getId() {
        return this.id;
    }
    private void setId(Long id) {
        this.id = id;
    }

    @ManyToOne
    @JoinColumn(name="user_id")
    public User getUser(){
        return user;
    }
    public void setUser(User user){
        this.user = user;
    }
}

我在我的服务层类中使用这些实体，其中每个方法都在事务中运行。除了服务层类的方法必须返回这些实体的情况外，一切都很好。

@Transactional(readOnly=true)
public Set<Order> getOrders() {
    Set<Order> orders = user.getOrders();

    return orders;
}

该方法返回数据良好。但是当我尝试访问接收到的集合元素时，我捕获了异常:“org.hibernate.LazyInitializationException:未能延迟初始化角色集合:package.User.orders，没有 session 或 session 被关闭”。

所以，它被排除在外。我认为分离结果会解决我的问题，但像这样的技巧

@Transactional(readOnly=true)
public Set<Order> getOrders() {
    Set<Order> orders = user.getOrders();

    for(Order order: orders)
        entityManager.detach(order);
    return orders;
}

没有改变任何东西:(

对我来说，有关用户是否按订单参加的信息无关紧要。我只想使用这个集合而不打算修改它。

有谁能够帮助我？ :)

Answer 1

This method returns data well. But when I try access to received collection elements I catch exception: "org.hibernate.LazyInitializationException: failed to lazily initialize a collection of role: package.User.orders, no session or session was closed".

该错误是自我解释的:您正在尝试加载(延迟)加载的集合，但不再有事件 session ，因为 User实例已分离。

So, it was excepted. I thought that detaching result will solve my problem, but trick like this

这不会改变任何事情。 EntityManager#detach(Object) 方法不加载任何东西，它从持久化上下文中删除传递的实体，使其分离。

It doesn't matter for me will info about users attend in set of orders or not. I just want to work with this set and not going to modify it.

您需要建立关联 EAGER(以便在检索用户时加载订单)或在您的服务中检索用户时使用 FETCH JOIN:

SELECT u
FROM User u LEFT JOIN FETCH u.orders
WHERE u.id = :id

Do I understand correctly that hibernate has no standard mechanism for force load all lazy associations of current object

Hibernate 有一个 Hibernate.initialize方法，但这显然不是标准的 JPA(对于可移植代码更喜欢 fetch join)。

I have no way to make hibernate ignore lazy associations of current object (set them as null)

什么？无视是什么意思？你为什么要这样做？