谁负责MySQL和Hibernate之间主键的自动递增？ [英] Who is responsible of the auto-incrementation of a primary key between MySQL and Hibernate?

问题描述

CREATE TABLE`role`（$ b） $ b`id_role` INT（11）unsigned NOT NULL AUTO_INCREMENT，
PRIMARY KEY（`id_role`）
）AUTO_INCREMENT = 1;


Hibernate

< pre class =lang-java prettyprint-override> @Entity
public class Role {

private Integer idRole;
$ b @Column（name =id_role，precision = 10）
@GeneratedValue
@Id
public Integer getIdRole（）{
return idRole;
}

public void setIdRole（Integer idRole）{
this.idRole = idRole;
}

}


鉴于以上背景，负责在创建新的角色时自动增加 id_role 列。换句话说，Hibernate在运行 create SQL语句之前设置了主键值，还是将它设置为 null 并让MySQL自动增加字段，同时获得所选主键？ 使用MySQL AUTO_INCREMENT 列，您应该使用 IDENTITY 策略：

  @Id @GeneratedValue（strategy = GenerationType.IDENTITY）
 private Long id; 
  

使用 AUTO 使用MySQL：

  @Id @GeneratedValue（strategy = GenerationType.AUTO）
 private Long id; 
  

其实相当于

  @Id @GeneratedValue 
私人长ID; 
  

MySQL

CREATE TABLE `role` (
  `id_role` INT(11) unsigned NOT NULL AUTO_INCREMENT,
  PRIMARY KEY (`id_role`)
) AUTO_INCREMENT=1;

Hibernate

@Entity
public class Role {

    private Integer idRole;

    @Column(name = "id_role", precision = 10)
    @GeneratedValue
    @Id
    public Integer getIdRole() {
        return idRole;
    }

    public void setIdRole(Integer idRole) {
        this.idRole = idRole;
    }

}

Given the above background, who is responsible of the auto-incrementation of the id_role column when creating a new role? In other words, does Hibernate set the primary key value before running the create SQL statement, or does it set it to null and let MySQL auto-increments the field while getting back the chosen primary key?

解决方案

To use a MySQL AUTO_INCREMENT column, you are supposed to use an IDENTITY strategy:

@Id @GeneratedValue(strategy=GenerationType.IDENTITY)
private Long id;

Which is what you'd get when using AUTO with MySQL:

@Id @GeneratedValue(strategy=GenerationType.AUTO)
private Long id;

Which is actually equivalent to

@Id @GeneratedValue
private Long id;

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