生成主鍵的幾種形式
通過JPA的策略生成器來進行生成:JPA hibernate 其他的ORM框架照樣支持
GenerationType.TABLE,SEQUENCE
Hiberante : foriegn uuid
自定義主鍵生成方式
TABLE緩存主鍵生成器
@TableGenerator(name = “pk_gen”,
table=”tb_generator”,
pkColumnName=”gen_name”,
valueColumnName=”gen_value”,
pkColumnValue=”STUDENT_PK”,
allocationSize=1
)
CREATE TABLE tb_generator (
gen_name VARCHAR2(255) NOT NULL,
gen_value NUMBER NOT NULL,
PRIMARY KEY(gen_name)
)
SEQUENCE生成器
@SequenceGenerator(name=”seq_studnet”, sequenceName=”seq_payment”)
IDENTITY主鍵生成器:mysql mmsql
@GeneratedValue(strategy = GenerationType.IDENTITY)
AUTO:主鍵生成器:根據宿主數據庫進行選擇 如果是mysql 自動增長 如果是oracle sequence
@GeneratedValue(strategy = GenerationType.AUTO)
hibernate自己提供的豐富的主鍵生成策略:
assigned手動指派
@GenericGenerator(name = “studentgenerator”, strategy = “assigned”)
sequence
@GenericGenerator(name = “studentgenerator”, strategy = “sequence”,
parameters = { @Parameter(name = “sequence”, value = “seq_Student”) })
native:對於 oracle 採用 Sequence 方式,對於MySQL 和 SQL Server 採用identity(自增主鍵生成機制)
@GeneratedValue(generator = “studentgenerator”)
@GenericGenerator(name = “studentgenerator”, strategy = “native”)
identity:mysql sqlserver Oracle 不支持自增字段
uuid: 採用128位的uuid算法生成主鍵,uuid被編碼爲一個32位16進制數字的字符串。佔用空間大(字符串類型)
@GeneratedValue(generator = “studentgenerator”)
@GenericGenerator(name = “studentgenerator”, strategy = “uuid”)
foreign:外鍵
@GeneratedValue(generator = “idGenerator”)
@GenericGenerator(name = “idGenerator”, strategy = “foreign”,
parameters = { @Parameter(name = “empid”, value = “employee”) })
很少用的算法 :瞭解即可
hilo:使用hilo生成策略,要在數據庫中建立一張額外的表 使用高地位算法
@GeneratedValue(generator = “studentgenerator”)
@GenericGenerator(name = “studentgenerator”, strategy = “hilo”)
increment:hibernate 自增長,多線程不適合
@GeneratedValue(generator = “studentgenerator”)
@GenericGenerator(name = “studentgenerator”, strategy = “increment”)
guid:採用數據庫底層的guid算法機制
1.默認形式
@Entity
@Table(name="TB_STUDENT")
public class Student {
//@Id用來標識主屬性列
@Id
private Long id;
主鍵id未配置默認的,在使用的時候需要手動指定主鍵的值。
public class FirstTest {
public static void main(String[] args) {
Session session = HibernateUtil.getSessionFactory().openSession();
Student s = new Student();
s.setName("lily");
session.save(s);
session.close();
}
}
並未指定id值,看控制檯打印消息:
從控制檯中可以看出,使用默認的主鍵生成策略需要手動去添加主鍵的值。
Student s = new Student();
s.setId(1l);
s.setName("lily");
並未提交事物,所以數據庫方面不會有操作,此時以及沒有了語法錯誤。
TableGenerator
@Entity
@Table(name="TB_STUDENT")
@TableGenerator(name = "pk_gen",
table="tb_generator",
pkColumnName="gen_name",
valueColumnName="gen_value",
pkColumnValue="STUDENT_PK",
allocationSize=1
)
public class StudentTwo {
//@Id用來標識主屬性列
@Id
@GeneratedValue(strategy=GenerationType.TABLE,generator="pk_gen")
private Long id;
private String name;
控制檯打印的消息:
五月 14, 2017 11:32:06 上午 org.hibernate.tool.hbm2ddl.SchemaUpdate execute
INFO: HHH000232: Schema update complete
Hibernate:
select
gen_value
from
tb_generator
where
gen_name = 'STUDENT_PK' for update
Hibernate:
insert
into
tb_generator
(gen_name, gen_value)
values
('STUDENT_PK', ?)
Hibernate:
update
tb_generator
set
gen_value = ?
where
gen_value = ?
and gen_name = 'STUDENT_PK'
Hibernate:
select
gen_value
from
tb_generator
where
gen_name = 'STUDENT_PK' for update
Hibernate:
update
tb_generator
set
gen_value = ?
where
gen_value = ?
and gen_name = 'STUDENT_PK'
數據庫中生成了一張管理主鍵值的表
AUTO:主鍵生成器
@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private Long id;