Hibernate 延迟加载在 @OneToOne 映射中不起作用

Question

虽然应用了延迟加载，但它仍在运行所有 4 个查询， 但我只取：

   System.out.println("Author's Name: " + b.getEmail());


    public class Author  {

        @Id
        @Column(name = "AUTHOR_ID")
        @GeneratedValue(strategy = GenerationType.IDENTITY)
        private long id;
        private String name;
        private String email;

        @OneToOne(mappedBy = "author", fetch = FetchType.LAZY)
        private Book book;
        @OneToOne(mappedBy = "author", fetch = FetchType.LAZY)
        private Address address;
        @OneToOne(mappedBy = "author", fetch = FetchType.LAZY)
        private Location location;
    }
public class BooksManager {
    public static void main(String[] args) {
        Author b = (Author) session.get(Author.class, 6L);
        System.out.println("Author's Name: " + b.getEmail());
    }
}

它正在运行上述代码的所有 4 个查询：

Hibernate: select author0_.AUTHOR_ID as AUTHOR_I1_1_0_, author0_.email as email2_1_0_, author0_.name as name3_1_0_ from AUTHOR author0_ where author0_.AUTHOR_ID=?

Hibernate: select address0_.address_id as address_1_0_0_, address0_.description as descript2_0_0_, address0_.PUBLISHED as PUBLISHE3_0_0_, address0_.title as title4_0_0_ from ADDRESS address0_ where address0_.address_id=?

Hibernate: select book0_.book_id as book_id1_2_0_, book0_.description as descript2_2_0_, book0_.PUBLISHED as PUBLISHE3_2_0_, book0_.title as title4_2_0_ from BOOK book0_ where book0_.book_id=?

Hibernate: select location0_.location_id as location1_3_0_, location0_.description as descript2_3_0_, location0_.PUBLISHED as PUBLISHE3_3_0_, location0_.title as title4_3_0_ from LOCATION location0_ where location0_.location_id=?

Answer 1

在您的定义中，所有 3 个 OneToOne 关系都是可选的（可为空的）。 当休眠加载实体作者时，它必须为关系创建 3 个代理。但是如果一个属性是 null ，hibernate 不应该为那个属性创建代理。 现在休眠以某种方式必须知道该属性是否为空。 所以它必须查找所有其他 3 个表才能检测到这一点。 最后，hibernate 足够聪明，可以忽略为延迟加载所做的提示并急切地加载它们。