•AUTO_ INCREMENT는 데이터베이스에 INSERT SQL을 실행한 이후에 ID 값을 알 수 있음
•IDENTITY 전략은 em.persist() 시점에 즉시 INSERT SQL 실행하고 DB에서 식별자를 조회
@Entity
public class Member {
@Id
@GeneratedValue(strategy = GenerationType.IDENTITY)
private Long id;
@Column(name = "name", nullable = false)
private String username;
}
//Main Class
public class JpaMain {
public static void main(String[] args) {
//데이터베이스 하나당 하나씩 묶여서 돌아감
EntityManagerFactory emf = Persistence.createEntityManagerFactory("hello");
//고객의 요청이 올때 EntityManager 생성해서 실행
EntityManager em = emf.createEntityManager();
//Transaction 안에서 진행되어야 한다.
EntityTransaction tx = em.getTransaction();
tx.begin(); //Transaction 시작
try {
//영속
Member member = new Member();
member.setUsername("chris-2");
em.persist(member);
tx.commit();//inset query가 나가는 시점
} catch ( Exception e) {
tx.rollback();
} finally {
em.close();
}
emf.close();
}
}
//로그
Hibernate:
/* insert hellojpa.Member
*/ insert
into
Member
(id, name)
values
(null, ?)
member Id = 1 chris-2
*em.persist(member); 시점에 insert 진행됨
SEQUENCE 전략 - 특징
•데이터베이스시퀀스는 유일한 값을 순서대 로생성하는 특별한 데이터베이스오브젝트(예: 오라클시퀀스)
•오라클, PostgreSQL, DB2, H2 데이터베이스에서 사용
@Entity
@SequenceGenerator(
name = "MEMBER_SEQ_GENERATOR",
sequenceName = "MEMBER_SEQ",
initialValue = 1, allocationSize = 1)
public class Member {
@Id
@GeneratedValue(strategy = GenerationType.SEQUENCE,
generator = "MEMBER_SEQ_GENERATOR"
)
private Long id;
@Column(name = "name")
private String username;
}
drop sequence if exists MEMBER_SEQ
Hibernate: create sequence MEMBER_SEQ start with 1 increment by 1
Hibernate:
create table Member (
id bigint not null,
name varchar(255),
primary key (id)
)
신규 멤버 생성시 MEMBER_SEQ 호출하는 것을 볼 수 있음
INFO: HHH000400: Using dialect: org.hibernate.dialect.H2Dialect
Hibernate:
call next value for MEMBER_SEQ //em.persist 전에 DB에서 값을 먼저 얻어오고 cmmit 시점에 저장한다.
Hibernate:
/* insert hellojpa.Member
*/ insert
into
Member
(name, id)
values
(?, ?)
**allocationSize: DB에 숫자만큼 먼저 올려놓고 메모리에 가져와서 하나씩 사용한다.
Table 전략
•키생성전용테이블을하나만들어서데이터베이스시퀀스를흉내내는전략
•장점: 모든데이터베이스에적용가능
•단점: 성능
@Entity
@TableGenerator(
name = "MEMBER_SEQ_GENERATOR",
table = "MY_SEQUENCES",
pkColumnValue = "MEMBER_SEQ", allocationSize = 1)
public class Member {
@Id
@GeneratedValue(strategy = GenerationType.TABLE,
generator = "MEMBER_SEQ_GENERATOR"
)
private Long id;
@Column(name = "name")
private String username;
}
Hibernate:
drop table Member if exists
Hibernate:
drop table MY_SEQUENCES if exists
Hibernate:
create table Member (
id bigint not null,
name varchar(255),
primary key (id)
)
Hibernate:
create table MY_SEQUENCES (
sequence_name varchar(255) not null,
next_val bigint,
primary key (sequence_name)
)
Hibernate:
insert into MY_SEQUENCES(sequence_name, next_val) values ('MEMBER_SEQ',0)
Hibernate:
select
tbl.next_val
from
MY_SEQUENCES tbl
where
tbl.sequence_name=? for update
Hibernate:
update
MY_SEQUENCES
set
next_val=?
where
next_val=?
and sequence_name=?
Hibernate:
/* insert hellojpa.Member
*/ insert
into
Member
(name, id)
values
(?, ?)
