Hibernate:为什么绑定字段类似天气显式@JoinColum 注释是否使用? [英] Hibernate: Why is binding for a field similar weather explicit @JoinColum annotation is used or not?

问题描述

@JoinColum 是否在我们使用任何关联类型注释(如@OneToOne、@OneToMany 等)时隐式指定.

Is @JoinColum implicitly specified when we use any association type annotation like @OneToOne, @OneToMany, etc.

这是来自 Student 实体与 Laptop 实体的片段

Here is snippet from Student entity in association with Laptop entity

案例 1) 不使用显式 @JoinColum

Case 1) Without using explicit @JoinColum

@OneToMany(cascade=CascadeType.ALL)
private List<Laptop> laptops=new ArrayList<Laptop>();  

案例 2) 使用显式 @JoinColum

Case 2) Using explicit @JoinColum

@OneToMany(cascade=CascadeType.ALL)
@JoinColumn
private List<Laptop> laptops=new ArrayList<Laptop>(); 

当我检查 DEBUG 时，两种情况都有几乎"相似的 Binding，即如下所示.

When I check DEBUG both cases have "almost" similar Binding, i.e something like below.

1) 那么我可以假设 @JoinColum 是在指定 @OneToMany 时隐式指定的吗?但是等等，我还注意到 Hibernate 试图引用一些我没有在任何地方创建的完全奇怪的表student_laptops"(可能是连接表).

1) So could I assume @JoinColum is implicitly specified when @OneToManyis specified? But wait I also noticed Hibernate is trying to refer to some totally strange table 'student_laptops' which I have not created anywhere (probably a join table).

那么有人可以描述一下这里的流程是什么吗?

So can someone describe whats exactly the flow going here?

Debug: private Listnotebooks=new ArrayList();

//You can totally ignore this. Just specifying for any reference.

DEBUG - Binding column: Ejb3JoinColumn{logicalColumnName='null', referencedColumn='null', mappedBy=''}
DEBUG - Binding column: Ejb3Column{table=org.hibernate.mapping.Table(STUDENT), mappingColumn=laptops, insertable=true, updatable=true, unique=false}
DEBUG - Binding column: Ejb3Column{table=org.hibernate.mapping.Table(STUDENT), mappingColumn=null, insertable=true, updatable=true, unique=false}
DEBUG - Binding column: Ejb3Column{table=org.hibernate.mapping.Table(STUDENT), mappingColumn=element, insertable=true, updatable=true, unique=false}
DEBUG - Binding column: Ejb3Column{table=org.hibernate.mapping.Table(STUDENT), mappingColumn=laptops_KEY, insertable=true, updatable=true, unique=false}
DEBUG - Binding column: Ejb3JoinColumn{logicalColumnName='laptops_KEY', referencedColumn='null', mappedBy='null'}
DEBUG - Binding column: Ejb3JoinColumn{logicalColumnName='null', referencedColumn='null', mappedBy=''}
DEBUG - Binding column: Ejb3JoinColumn{logicalColumnName='null', referencedColumn='null', mappedBy=''}
DEBUG - Collection role: com.infiniteskills.data.entities.Student.laptops
DEBUG - Building property laptops

推荐答案

当我们使用@OneToOne、@OneToMany 等任何关联类型注解时，@JoinColum 是否隐式指定

Is @JoinColum implicitly specified when we use any association type annotation like @OneToOne, @OneToMany, etc.

答案是:否.使用 mappedBy 的情况除外.

The answer is: NO. Except the case when mappedBy is used.

当您指定 @JoinColumn 或 mappedBy(即使没有 @JoinColumn)Hibernate 意味着您希望通过外键使用关联(没有连接表).

When you specify @JoinColumn or mappedBy (even without @JoinColumn) Hibernate implies that you want to use an association via a foreign key (without a join table).

当您不指定 @JoinColumn 时，Hibernate 使用连接表并为您创建它 - student_laptops 在您的情况下.

When you don't specify @JoinColumn Hibernate uses a join table and creates it for you — student_laptops in your case.

您可以添加 @JoinTable 注释以强调您使用的是 jon 表这一事实.

You can add a @JoinTable annotation to stress the fact that you are using a jon table.

@OneToMany(cascade = CascadeType.ALL)
@JoinTable
private List<Laptop> laptops = new ArrayList<Laptop>();  

这篇关于Hibernate:为什么绑定字段类似天气显式@JoinColum 注释是否使用?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！