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
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:
我们期望的直观sql是:
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Car
的sequence_name
是seq_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
.