java - Hibernate : Why FetchType. LAZY-annotated 集合属性急切加载？

Question

我尝试实现简单的 one-to-many协会。在使用 Debug模式检查项目对象后，我发现 List<Bid> bids已经加载。但是List<Bid> bids 属性用 FetchType.LAZY 注释.一些书籍和网页声称 FetchType.LAZY是 JPA 提供者接受或拒绝的提示。但我想知道 JPA 提供商在什么情况下会忽略 FetchType.LAZY .提前谢谢你。

@Entity
@Table(name = "ITEM")
public class Item implements Serializable {

    @Id
    private Long id = null;

    private String name;
    @ManyToOne(fetch = FetchType.EAGER)
    @JoinColumn(name = "SELLER_ID", nullable = false)
    private User seller;

    @OneToMany(mappedBy = "item", fetch = FetchType.LAZY)
    private List<Bid> bids;

    /**
     * No-arg constructor for JavaBean tools.
     */
    public Item() {}

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public User getSeller() {
        return seller;
    }

    public void setSeller(User seller) {
        this.seller = seller;
    }

    @Override
    public String toString() {
        return "Item{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", seller=" + seller +
                ", bids=" + bids +
                '}';
    }
}

@Entity
@Table(name = "BID")
public class Bid implements Serializable {

    @Id @GeneratedValue
    @Column(name = "BID_ID")
    private Long id = null;

    @ManyToOne
    @JoinColumn(name = "ITEM_ID", nullable = false, updatable = false, insertable = false)
    private Item item;

    @ManyToOne
    @JoinColumn(name = "BIDDER_ID", nullable = false, updatable = false)
    private User bidder;

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public Item getItem() {
        return item;
    }

    public void setItem(Item item) {
        this.item = item;
    }

    public User getBidder() {
        return bidder;
    }

    public void setBidder(User bidder) {
        this.bidder = bidder;
    }

    @Override
    public String toString() {
        return "Bid{" +
                "id=" + id +
                ", bidder=" + bidder +
                '}';
    }
}

private static void itemSeller(EntityManager em) {

    Item item = em.find(Item.class, 1L);
    System.out.println("hello");

}

编辑:我把断点放在语句 System.out.println("hello") 上.我检查了项目对象。看图片:

Answer 1

通过检查调试器中的对象，您要求它调用列表的方法来显示其大小、内容等。当然，这会惰性地初始化列表。

与您的 toString() 方法相同，它隐式地循环遍历列表以打印它。