FetchMode 和 FetchType 的区别 [英] difference between FetchMode and FetchType

问题描述

指定 lazy = "true" 和使用 fetch = "select" 或 "join" 有什么区别?哪一个比另一个更受欢迎?

what is the difference in specifying lazy = "true" and using fetch = "select" or "join" ? which one is preferred over the other ?

问候贾延德拉

推荐答案

假设我们有这样的实体:

Let's say we have entities like this:

@Entity
@Table
public class Parent {
    @Id
    private Long id;

    @OneToMany(mappedBy="parent", fetch = FetchType.EAGER)
    @Fetch(FetchMode.JOIN)
    private List<Child> child;    
    //getter setters
}


@Entity
@Table
public class Child {    
    @Id
    private Long id;

    @ManyToOne(fetch = FetchType.LAZY)
    private Parent parent;

    //getter setter
}

在上面的例子中，当得到 Parent 实体时，hibernate 会使用 join 自动加载所有 child 实体.另一方面，当您获取 Child 时，不会选择 Parent 实体，除非您在代码 child.getParent().

In above example, when getting Parent entity, hibernate will automaticly load all child entities eagerly using join. On the other hand, when you fetch Child, Parent entity won't be selected unless you call it explicity in your code child.getParent().

FetchType (Lazy/Eager) 告诉我们在代码中有调用时是希望实体加载还是延迟加载.

FetchType (Lazy/Eager) tells whether we want entity to be loaded eagerly or lazy, when there's call in code.

FetchMode (Select/Join) 告诉我们是希望我们的实体加载额外的选择，还是在一个带有连接或子选择的查询中加载.

FetchMode (Select/Join) tells whether we want our entitity to be loaded with additional select or in one query with join or subselect.

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