Question

Hibernate newb尝试理解LAZY和EAGER获取之间的差异。

地址模型：

@Entity
public class Department {

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

    @Column
    private String name;

    @OneToOne(mappedBy = "department", fetch = FetchType.LAZY)
    private Address address;

    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 Address getAddress() {
        return address;
    }

    public void setAddress(Address address) {
        this.address = address;
    }

    @Override
    public String toString() {
        return "Department [id=" + id + ", name=" + name + "]";
    }
}

部门模特：

@Test
    public void testDepartment() {
        Long departmentID;
        Department department;

        Long addressId;
        Address address;

        try (Session session = SessionUtil.getSession()) {
            Transaction tx = session.beginTransaction();

            department = new Department();
            department.setId(69L);
            department.setName("company");

            address = new Address();
            address.setCity("city");
            address.setStreet("street");
            address.setDepartment(department);

            session.save(department);
            session.save(address);

            departmentID = department.getId();
            addressId = address.getId();
            tx.commit();
        }

        try (Session session = SessionUtil.getSession()) {          
            department = session.load(Department.class, departmentID);
            System.out.println(department.getId()); 
        }
    }

测试方法：

Hibernate: select department0_.id as id1_2_0_, department0_.name as name2_2_0_ from Department department0_ where department0_.id=?
Hibernate: select address0_.id as id1_0_1_, address0_.city as city2_0_1_, address0_.department_id as departme4_0_1_, address0_.street as street3_0_1_, department1_.id as id1_2_0_, department1_.name as name2_2_0_ from Address address0_ left outer join Department department1_ on address0_.department_id=department1_.id where address0_.department_id=?
1

当我在执行查询后从db加载部门对象时。

{{1}}

我很惊讶看到连接选择查询，因为我从未明确要求地址对象，应该懒得加载。

Answer 1

如果你看一下FetchType.LAZY的javadoc，你会看到没有保证（而不是EAGER fetchinig）：

LAZY策略是对持久性提供程序运行时的暗示首次访问时，应该懒散地获取数据。该允许实现急切地获取LAZY的数据策略提示已被指定。

这对于OneToOne可选依赖项来说非常正确。

如果你真的需要为这种关系实现延迟加载，可以使用一些解决方法：post 1，post 2

Hibernate - FetchType.LAZY急切地加载数据

1 个答案: