Hibernate:一对一延迟加载,可选 = false [英] Hibernate: one-to-one lazy loading, optional = false
问题描述
我遇到了一对一延迟加载在休眠中不起作用的问题.我已经解决了,但仍然不正确理解会发生什么.
I faced the problem that one-to-one lazy loading doesn't work in hibernate. I've already solved it, but still don't properly understand what happens.
我的代码(延迟加载在这里不起作用,当我拉人 - 地址也被提取):
My code (lazy loading doesn't work here, when I pull Person - Address is also fetched):
@Entity
public class Person{
@Id
@SequenceGenerator(name = "person_sequence", sequenceName = "sq_person")
@GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "person_sequence")
@Column(name = "id")
private long personID;
@OneToOne(mappedBy="person", cascade=CascadeType.ALL, fetch = FetchType.LAZY)
private Adress address;
//.. getters, setters
}
@Entity
public class Address {
@Id
@Column(name="id", unique=true, nullable=false)
@GeneratedValue(generator="gen")
@GenericGenerator(name="gen", strategy="foreign", parameters=@Parameter(name="property", value="person"))
private long personID;
@PrimaryKeyJoinColumn
@OneToOne
private FileInfo person;
}
但是:如果我在 OneToOne 关系中添加 optional=false
,延迟加载工作正常!>
But: if I add optional=false
in OneToOne relationship, lazy loading works fine!
@OneToOne(mappedBy="person", cascade=CascadeType.ALL, optional = false, fetch = FetchType.LAZY)
private Adress address;
问题/恳求:请向我解释 optional=false
注释如何帮助实现延迟加载.
Question/Entreaty: please, explain to me how optional=false
annotation helps to achieve lazy loading.
PS 我读过帖子 post1 和post2,明白为什么简单的OneToOne不能偷懒,但我还是可以不要掌握 optional=false
魔法.
P.S. I've read posts post1 and post2, and understand why simple OneToOne can't be lazy, but I still can't grasp optional=false
magic.
推荐答案
如果关联是可选的,则 Hibernate 无法在不发出查询的情况下知道给定人员的地址是否存在.所以它不能用代理填充地址字段,因为可能没有引用这个人的地址,也不能用 null
填充它,因为可能有一个引用这个人的地址.
If the association is optional, Hibernate has no way of knowing if an address exists for a given person without issuing a query. So it can't populate the address field with a proxy, because there could be no address referencing the person, and it can't populate it with null
, because there might be an address referencing the person.
当您强制关联(即 optional=false
)时,它会信任您并假定地址存在,因为关联是强制性的.所以它直接用代理填充地址字段,知道有一个地址引用了这个人.
When you make the association mandatory (i.e. optional=false
), it trusts you and assumes that an address exists, since the association is mandatory. So it directly populates the address field with a proxy, knowing that there is an address referencing the person.
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