FetchMode和FetchType之间的区别

问题描述

指定 lazy =true并使用 fetch =select或join？哪一个比另一个优先？

问候
jayendra

解决方案

  @Entity 
 @Table 
 public class父母{
 @Id 
私人长ID; 
 
 @OneToMany（mappedBy =parent，fetch = FetchType.EAGER）
 @Fetch（FetchMode.JOIN）
 private List< Child>儿童; 
 // getter setters 
} 
 
 
 @Entity 
 @Table 
 public class Child {
 @Id 
私人长ID; 
 
 @ManyToOne（fetch = FetchType.LAZY）
私有父母; 
 
 // getter setter 
} 
  

在上面的例子中，当获取 Parent 实体时，hibernate会使用join自动加载所有子实体。另一方面，当你读取 Child 时， Parent 实体将不会被选中，除非您在您的代码 child.getParent（）。

FetchType（Lazy / Eager）告诉我们是否希望实体被加载或者懒惰，当有代码调用的时候。

FetchMode（Select / Join）告诉我们是否希望我们的实体被加载额外的select或者带有join或subselect的查询。

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

regardsjayendra

解决方案

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
}

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 selectet unless you call it explicity in your code child.getParent().

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

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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