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

Question

我是 Hibernate 框架的新手。我正在使用带有 MySQL 5.7.14-log 的 Hibernate 5。

我正在学习主键@Id @GeneratedValue。我已经尝试过生成类型策略，即AUTO、SEQUENCE、IDENTITY、TABLE。

以下是我对AUTO、SEQUENCE、TABLE的观察 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（即创建一个新表hibernate_sequence）。而 TABLE 创建一个表 hibernate_sequences

我有以下疑问。

1. AUTO 和 SEQUENCE 之间有什么区别。如果是，那么有什么区别以及它们特定于哪些数据库？

2. MySQL 的默认策略是什么？

3. 对于 GenerationType.TABLE，hibernate_sequences 表创建有 2 列，sequence_name 和 next_val。 sequence_name 列表示什么。 [当前值为默认值]

这是我的 POJO 类

@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。 看到的链接

The differences between GeneratedValue strategies

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

Answer 1

这很令人困惑，因为它是一种解决方法。该解决方法可以处理在将行（也称为新的休眠实体）插入数据库时​​出现的问题。

问题是什么？

问题是插入语句没有返回新的id！

拥有这个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

我们期待的直观sql是：

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

如您所见，对于现在插入的行的 id (367)，没有像 SELECT MAX(ID) FROM car; 这样的语句。我们不能选择MAX(ID)，因为max id可能同时被另一个事务改变！

相信我，没有数据库通用解决方案。

第一季度

Auto 尝试找出最佳实践，它会查找 hibernate_sequences-table，如果找不到，它会查找内置序列。

第二季度 MySQL

如果您使用 InnoDB，您应该使用 TABLE，因为您可以为 hibernate_sequences 表创建一个外键。否则，它支持本机内置序列，应该首选将表的pk 绑定到此序列。

第三季度

sequence_name 是使用 hibernate 处理的每个表的主键（也组合 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，它拥有在sequence_name 中具有 pk 序列的表中尚未使用的下一个主键。如果数据库中有 200 个 UserDetails，next_val 可能是 201。