java - HIbernate注解@GeneratedValue策略类型的区别

Question

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。我尝试了生成类型策略，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

我有以下问题。

1. AUTO 和 SEQUENCE 之间有什么区别吗？如果是，那么有什么区别，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，创建的 hibernate_sequences 表包含 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，但我更糊涂了。看到的链接

The differences between GeneratedValue strategies

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

Answer 1

它令人困惑，因为它是一种变通方法。变通方法处理在向数据库中插入行(又名新的 hibernate 实体)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!

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");

如您所见，没有像 SELECT MAX(ID) FROM car; 这样的语句用于 ID (367) 现在插入的行。我们不能选择 MAX(ID) 因为最大 id 可能同时被另一个事务更改!

相信我，没有通用的数据库解决方案。

Q1

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

Q2 MySQL

如果您正在使用 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 native 内置序列，应该首选将表的 pk 绑定(bind)到此序列。

Q3

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，它保存了在 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，则 next_val 可能是 201。