Java 为什么延迟加载在一对一关联中不起作用？
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Why Lazy loading not working in one to one association?
提问by M Sach
@Entity
public class Person {
@Id
@GeneratedValue
private int personId;
@OneToOne(cascade=CascadeType.ALL, mappedBy="person", fetch=FetchType.LAZY)
private PersonDetail personDetail;
//getters and setters
}
@Entity
public class PersonDetail {
@Id
@GeneratedValue
private int personDetailId;
@OneToOne(cascade=CascadeType.ALL,fetch=FetchType.LAZY)
private Person person;
//getters and setters
}
when i do
当我做
Person person1=(Person)session.get(Person.class, 1);
i see two queries being fired. one for fetching person data and another for person detail data.
我看到两个查询被触发。一个用于获取个人数据，另一个用于获取个人详细信息。
As per my understanding only 1 query should have been fired that is for fetching person data not for person detail data as i have mentioned lazy loading. Why personDetail data is getting fetched along with person data ?
根据我的理解，只有 1 个查询应该被触发，用于获取个人数据而不是个人详细信息数据，正如我提到的延迟加载。为什么 personDetail 数据与 person 数据一起被获取？
采纳答案by gipinani
Hibernate cannot proxy your own object as it does for Sets / Lists in a @ToManyrelation, so Lazy loading does not work.
Hibernate 不能代理您自己的对象，因为它为@ToMany关系中的 Sets/Lists所做的那样，因此延迟加载不起作用。
I think this link could be useful to understand your problem: http://justonjava.blogspot.co.uk/2010/09/lazy-one-to-one-and-one-to-many.html
我认为此链接可能有助于了解您的问题：http: //justonjava.blogspot.co.uk/2010/09/lazy-one-to-one-and-one-to-many.html
回答by Sean Mickey
Based on your comment and since the PersonDetailentity contains a foreign key column that references the Personentity, it looks like you only have 1 problem:
根据您的评论，并且由于PersonDetail实体包含引用实体的外键列Person，因此您似乎只有 1 个问题：
Entity relationships include the concept of a relationship owner(in this case PersonDetail), which means that you want to add a @JoinColumnannotation in the PersonDetailentity.
实体关系包括关系所有者的概念（在本例中为PersonDetail），这意味着您要@JoinColumn在PersonDetail实体中添加注释。
You have already correctly defined the inverse side of the relationship (the entity that is not the ownerof the relationship) with the mappedByattribute that was added to the association annotation (@OneToOnein your case) to make the relationship bi-directional, which means that the associated PersonDetailmay be reached from a Personinstance.
您已经使用添加到关联注释（在您的情况下）的属性正确定义了关系的反面（不是关系所有者的实体）以使关系双向，这意味着可以从实例访问相关联。mappedBy@OneToOnePersonDetailPerson
Given the relationship that is clarified in your comment, you should only have to make 1 change to your code as shown here:
鉴于您在评论中阐明的关系，您只需对代码进行 1 处更改，如下所示：
@Entity
public class Person {
@Id
@GeneratedValue
private int personId;
//Retain the mappedBy attribute here:
@OneToOne(cascade=CascadeType.ALL, mappedBy="person",
fetch=FetchType.LAZY)
private PersonDetail personDetail;
//getters and setters...
}
@Entity
public class PersonDetail {
@Id
@GeneratedValue
private int personDetailId;
@OneToOne(cascade=CascadeType.ALL, fetch=FetchType.LAZY)
//Change: add the @JoinColumn annotation here:
@JoinColumn(name="PERSON_FK_COLUMN")
private Person person;
//getters and setters...
}
