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。我尝试过生成类型策略，例如AUTO，SEQUENCE，IDENTITY，TABLE。

以下是我对AUTO，SEQUENCE，TABLE的观察创建的结果如下

    @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创建了一个表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，使用2列sequence_name和next_val创建hibernate_sequences表。 sequence_name列指示什么。 [当前值为默认值]

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

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不会返回新的id！

拥有此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id（ 367 ）完全插入的行，没有像SELECT MAX(ID) FROM car;这样的声明。我们无法选择MAX(ID)，因为可以通过其他交易同时更改最大ID！

相信我，没有数据库通用解决方案。

Q1

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

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密钥。否则，它支持本机内置序列，这些序列应该是首选，并且表的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，sequence_name的{{1}}为Car。

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

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

1 个答案:

Q1

Q2 MySQL

Q3