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HIbernate Annotations @GeneratedValue策略类型之间的区别

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我是Hibernate Framework的新手 . 我正在使用Hibernate 5和MySQL 5.7.14-log .

我正在学习主键@Id @GeneratedValue . 我尝试过生成类型策略,例如AUTO,SEQUENCE,IDENTITY,TABLE .

以下是我对AUTO,SEQUENCE,TABLE的观察结果创建结果如下

@Id @GeneratedValue
Hibernate: drop table if exists hibernate_sequence
Hibernate: drop table if exists USER_DETAILS
Hibernate: create table hibernate_sequence (next_val bigint)
Hibernate: insert into hibernate_sequence values ( 1 )
Hibernate: create table USER_DETAILS (userId integer not null, address varchar(255), joiningDate date, userName varchar(255), primary key (userId))
Hibernate: select next_val as id_val from hibernate_sequence for update
Hibernate: update hibernate_sequence set next_val= ? where next_val=?
Hibernate: select next_val as id_val from hibernate_sequence for update
Hibernate: update hibernate_sequence set next_val= ? where next_val=?
Hibernate: insert into USER_DETAILS (address, joiningDate, userName, userId) values (?, ?, ?, ?)
Hibernate: insert into USER_DETAILS (address, joiningDate, userName, userId) values (?, ?, ?, ?)
Hibernate: select userdetail0_.userId as userId1_0_0_, userdetail0_.address as address2_0_0_, userdetail0_.joiningDate as joiningD3_0_0_, userdetail0_.userName as userName4_0_0_ from USER_DETAILS userdetail0_ where userdetail0_.userId=?
UserDetails [userId=1, userName=First user, address=Adddress, joiningDate=2016-12-04]

    @Id @GeneratedValue(strategy=GenerationType.AUTO)
Hibernate: drop table if exists hibernate_sequence
Hibernate: drop table if exists USER_DETAILS
Hibernate: create table hibernate_sequence (next_val bigint)
Hibernate: insert into hibernate_sequence values ( 1 )
Hibernate: create table USER_DETAILS (userId integer not null, address varchar(255), joiningDate date, userName varchar(255), primary key (userId))
Hibernate: select next_val as id_val from hibernate_sequence for update
Hibernate: update hibernate_sequence set next_val= ? where next_val=?
Hibernate: select next_val as id_val from hibernate_sequence for update
Hibernate: update hibernate_sequence set next_val= ? where next_val=?
Hibernate: insert into USER_DETAILS (address, joiningDate, userName, userId) values (?, ?, ?, ?)
Hibernate: insert into USER_DETAILS (address, joiningDate, userName, userId) values (?, ?, ?, ?)
Hibernate: select userdetail0_.userId as userId1_0_0_, userdetail0_.address as address2_0_0_, userdetail0_.joiningDate as joiningD3_0_0_, userdetail0_.userName as userName4_0_0_ from USER_DETAILS userdetail0_ where userdetail0_.userId=?
UserDetails [userId=1, userName=First user, address=Adddress, joiningDate=2016-12-04]

    @Id @GeneratedValue(strategy=GenerationType.SEQUENCE)
Hibernate: drop table if exists hibernate_sequence
Hibernate: drop table if exists USER_DETAILS
Hibernate: create table hibernate_sequence (next_val bigint)
Hibernate: insert into hibernate_sequence values ( 1 )
Hibernate: create table USER_DETAILS (userId integer not null, address varchar(255), joiningDate date, userName varchar(255), primary key (userId))
Hibernate: select next_val as id_val from hibernate_sequence for update
Hibernate: update hibernate_sequence set next_val= ? where next_val=?
Hibernate: select next_val as id_val from hibernate_sequence for update
Hibernate: update hibernate_sequence set next_val= ? where next_val=?
Hibernate: insert into USER_DETAILS (address, joiningDate, userName, userId) values (?, ?, ?, ?)
Hibernate: insert into USER_DETAILS (address, joiningDate, userName, userId) values (?, ?, ?, ?)
Hibernate: select userdetail0_.userId as userId1_0_0_, userdetail0_.address as address2_0_0_, userdetail0_.joiningDate as joiningD3_0_0_, userdetail0_.userName as userName4_0_0_ from USER_DETAILS userdetail0_ where userdetail0_.userId=?
UserDetails [userId=1, userName=First user, address=Adddress, joiningDate=2016-12-04]

GenerationType.AUTO/SEQUENCE给出相同的结果(即创建一个新的表hibernate_sequence) . 而TABLE创建一个表hibernate_sequences

我有以下疑问 .

  • AUTO和SEQUENCE之间有什么区别吗?如果是,那么差异是什么以及它们具体针对哪些数据库?

  • MySQL的默认策略是什么?

  • 对于GenerationType.TABLE,hibernate_sequences表创建了2列,sequence_name和next_val . sequence_name列指示什么 . [当前值是默认值]

这是我的POJO课程

@Entity (name="USER_DETAILS")
public class UserDetails
{
    @Id @GeneratedValue
    private int     userId;
    private String  userName;   
    private String address;

    @Temporal (TemporalType.DATE)
    private Date joiningDate;

    public String getAddress()
    {
        return address;
    }

    public void setAddress(String address)
    {
        this.address = address;
    }

    public Date getJoiningDate()
    {
        return joiningDate;
    }

    public void setJoiningDate(Date joiningDate)
    {
        this.joiningDate = joiningDate;
    }

    public int getUserId()
    {
        return userId;
    }

    public void setUserId(int userId)
    {
        this.userId = userId;
    }

    public String getUserName()
    {
        return userName + " from getter";
    }

    public void setUserName(String userName)
    {
        this.userName = userName;
    }
}

我在网上搜索过,但我更加困惑 . 链接见过

The differences between GeneratedValue strategies

http://docs.oracle.com/javaee/5/api/javax/persistence/GenerationType.html

1 回答

  • 1

    令人困惑,因为它是一种解决方法 . 解决方法处理在将行(也称为新的hibernate实体)插入数据库时出现的问题 .

    有什么问题?

    问题是insert-statement不返回新的id!

    有这个java代码:

    Car car = new Car();
    car.setName("Hot car");
    System.out.println(car.getId()); // returns null
    entityManager.persist(car));
    System.out.println(car.getId()); // returns 367
    

    我们期望的直观sql是:

    INSERT INTO car (id, name) VALUES (null, "Hot car");
    

    正如您所看到的,对于完全插入的id( 367 )行,没有像 SELECT MAX(ID) FROM car; 这样的语句 . 我们无法选择 MAX(ID) ,因为最大ID可能会被另一个交易同时更改!

    相信我,没有数据库通用解决方案 .

    Q1

    Auto尝试找出最佳实践,它查找hibernate_sequences-table,如果找不到,则查找内置序列 .

    Q2 MySQL

    如果您使用的是InnoDB,则应使用TABLE,因为您可以为 hibernate_sequences 表创建一个foregin键 . 否则,它支持本机内置序列,这些序列应该是首选,并且该表的 pk 绑定到此序列 .

    Q3

    sequence_name 是使用hibernate处理的每个表的主键(也是组合pk)的名称 . 如果您有三个表,则 hibernate_sequences 中有三行 . 例如 Carsequence_nameseq_car .

    next_val 给出了上述问题的解决方案,它保存了在 sequence_name 中具有pk序列的表中尚未使用的下一个原始键 . 如果数据库中有200个UserDetails, next_val 可能是 201 .

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