Java EE 注解（二）：Hibernate - JPA An

这是个系列教程，看完这个，你对JavaEE注解，包括spring、hibernate，RESTFul web service ，JAXB, 以及junit注解将会有一个全面的了解。
英文原版网址在这里 我们不光是spring网站的搬运工@农夫山泉水，实际上，翻译过程中我保证准确同时，加入了自己的理解。

Hibernate JPA Annotations - Contents:

@Entity
import javax.persistence.Entity;

@Table
import javax.persistence.Table;

@Column
import javax.persistence.Column;

@Id
import javax.persistence.Id;

@GeneratedValue
import javax.persistence.GeneratedValue;

@Version
import javax.persistence.Version;

@OrderBy
import javax.persistence.OrderBy;

@Transient
import javax.persistence.Transient;

@Lob
import javax.persistence.Lob;

Hibernate Association Mapping Annotations

@OneToOne
import javax.persistence.OneToOne;

@ManyToOne
import javax.persistence.ManyToOne;

@OneToMany
import javax.persistence.OneToMany;

@ManyToMany
import javax.persistence.ManyToMany;

@PrimaryKeyJoinColumn
import javax.persistence.PrimaryKeyJoinColumn;

@JoinColumn
import javax.persistence.JoinColumn;

@JoinTable
import javax.persistence.JoinTable;

@MapsId
import javax.persistence.MapsId;

Hibernate Inheritance Mapping Annotations

@Inheritance
import javax.persistence.Inheritance;

@DiscriminatorColumn
import javax.persistence.DiscriminatorColumn;

@DiscriminatorValue
import javax.persistence.DiscriminatorValue;

@Entity,@Table,@Colume,@Id,@GeneratedValue略

@OrderBy（用于One-to-Many等关联表中集合的排序）

@OrderBy("firstName asc")
private Set contacts;

@Transient 字段不会更新到数据库

@Lob：二进制数据

hibernate的注解就这么完了吗？

Paste_Image.png

hibernate注解的<u>重难点</u>在于多表之间的关系：

栗子来了：

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company - companydetail : one-to-one，共享相同主键
contact - contactDetail : 通过外键连接的one-to-one
contact-company : 外键连接的many-to-one, contact作为owner
company-companyStatus : 外键连接的many-to-one，company作为owner

one-to-one tips：

共享相同主键的用@PrimaryKeyJoinColumn
一方持有另一方外键，用 @JoinColumn & @OneToOne和 mappedBy属性
两个Entity通过一个中间表联系，使用@JoinTable and mappedBy
共享键用@MapsId

one-to-one

@Entity
@Table(name = "company")
public class Company implements Serializable {
   
  @Id
  @Column(name = "id")
  @GeneratedValue
  private int id;
   
  @OneToOne(cascade = CascadeType.MERGE)
  @PrimaryKeyJoinColumn
  private CompanyDetail companyDetail;
   
  ...
}
 
@Entity
@Table(name = "companyDetail")
public class CompanyDetail implements Serializable {
 
  @Id
  @Column(name = "id")
  private int id;
   
  ...
}



@Entity
@Table(name = "contactDetail")
public class ContactDetail implements Serializable {
 
  @Id
  @Column(name = "id")
  @GeneratedValue
  private int id;
   
  @OneToOne
  @MapsId
  @JoinColumn(name = "contactId")
  private Contact contact;
   
  ...
}
 
@Entity
@Table(name = "contact")
public class Contact implements Serializable {
 
  @Id
  @Column(name = "ID")
  @GeneratedValue
  private Integer id;
 
  @OneToOne(mappedBy = "contact", cascade = CascadeType.ALL)
  private ContactDetail contactDetail;
 
  ....
}

cascade(级联)级联在编写触发器时经常用到，触发器的作用是当 主控表信息改变时，用来保证其关联表中数据同步更新。若对触发器来修改或删除关联表相记录，必须要删除对应的关联表信息，否则，会存有脏数据。所以，适当的做法是，删除主表的同时，关联表的信息也要同时删除，在hibernate中，只需设置cascade属性值即可。

CascadeType.PERSIST：级联新增（又称级联保存）：对order对象保存时也对items里的对象也会保存。对应EntityManager的presist方法

例子：只有A类新增时，会级联B对象新增。若B对象在数据库存（跟新）在则抛异常（让B变为持久态）

CascadeType.MERGE:级联合并（级联更新）：若items属性修改了那么order对象保存时同时修改items里的对象。对应EntityManager的merge方法

例子：指A类新增或者变化，会级联B对象（新增或者变化）

CascadeType.REMOVE:级联删除：对order对象删除也对items里的对象也会删除。对应EntityManager的remove方法

例子：REMOVE只有A类删除时，会级联删除B类；

CascadeType.REFRESH:级联刷新：获取order对象里也同时也重新获取最新的items时的对象。对应EntityManager的refresh(object)方法有效。即会重新查询数据库里的最新数据 （用的比较少）

CascadeType.ALL:以上四种都是

综上所述：一般的，用CascadeType.MERGE:级联合并（级联更新）就能达到级更新同时又稳定不报错。

@ManyToOne

tips:
Use @JoinColumn when foreign key is held by one of the entities.
Use @JoinTable for entities linked through an association table.

@Entity
@Table(name = "contact")
public class Contact implements Serializable {
 
  @ManyToOne
  @JoinColumn(name = "companyId")
  private Company company;
   
  ...
   
 }
 
@Entity
@Table(name = "company")
public class Company implements Serializable {
 
  @ManyToOne
  @JoinColumn(name = "statusId")
  private CompanyStatus status;
   
  ...
   
 }

@OneToMany

tips:
Use mappedBy attribute for bi-directional associations with ManyToOne being the owner.
OneToMany being the owner or unidirectional with foreign key - try to avoid such associations but can be achieved with @JoinColumn
@JoinTable for Unidirectional with association table

//Please see the many-to-one relationship between Contact and Company above. Company to Contact will be a one-to-many relationship. The owner of this relationship is Contact and hence we will use 'mappedBy' attribute in Company to make it bi-directional relationship.
@Entity
@Table(name = "company")
public class Company implements Serializable {
 
  @OneToMany(mappedBy = "company", fetch = FetchType.EAGER)
  @OrderBy("firstName asc")
  private Set contacts;
     
  ...
   
 }

@ManyToMany

tips:
Use @JoinTable for entities linked through an association table.
Use mappedBy attribute for bi-directional association.