Starting JPA

Hibernate library

1. hibernate-core : hibernate library
2. hibernate-entitymanager : library that implemented JPA standard. Let hibernate operate as implemented JPA
3. hibernate-jpa-X.X-api : collection of JPA X.X standard API

or simply by spring-boot-starter-data-jpa

• spring-boot-starter-data-jpa
  • Abstract library for Spring Boot
  • Manages version of JPA-related libraries by version of Spring Boot

Object Mapping

CREATE TABLE MEMBER (
    ID LONG AUTO_INCREMENT NOT NULL,   -- 아이디(기본키)
    NAME VARCHAR(255),                 -- 이름
    AGE INTEGER NOT NULL,              -- 나이
    PRIMARY KEY (ID)
)

package jpabook.start;

import javax.persistence.*;

@Entity
@Table(name="MEMBER")
public class Member {

    @Id
    @Column(name = "ID")
    private Long id;

    @Column(name = "NAME")
    private String username;

    private Integer age;

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public Integer getAge() {
        return age;
    }

    public void setAge(Integer age) {
        this.age = age;
    }
}

MEMBER TABLE Member Class

ID	id
NAME	username
AGE	age

• @Entity
  • Class will be mapped to Table
  • Class with @Entity is called Entity Class
• @Table
  • Tells which Table to map
  • Specify Table by name property : @Table(name="MEMBER")
    • map Class to Table named CLASS_NAME if name is omitted
• @Id
  • Filed with @Id is mapped to Primary Key of Table
  • Filed is called Identifier Field
• @Column
  • map filed to column of table
  • can specify column name by name property : @Column(name="NAME")
  • Filed without mapping data
    • Filed is mapped to column named FILED_NAME
    • if DB is case sensitive, mapping must be done explicitly by name property

Hibernate ddl-auto configuration

• create : delete existing schema & re-create schema when Session factory is executed
• create-drop : same as create but delete shcema when ssesion factory shuts
• update : compare domain & schema, and do extra works that are required. Data is not deleted
• validate : check whether schema is valid when Session factory is executed. Throws Exception when schema is not valid

Entity Manager

Creating EntityManager Procedure

1. Create EntityManagerFactory by configuration data of persistence.xml
2. Persistence.createEntityManagerFactory(PERSISTENCE_UNIT_NAME)
   • generally, regist persistence unit per DB to connect
   • Entity Manager Factory costs a lot
     • configurations, Object for JPA operation, DB Connection pool, …
     • should be created ONLY ONCE & be shared for entire application
     • also shared for different threads & safe for simultaneous accesses
3. Create EntityManager
   • entityManagerFactory.createEntityManager()
   • EntityManager has close relationship with DB connection, shouldn’t be shared for threads
   • can CRUD entity to DB by using Entity Manager
4. Closing
   • Entity Manager must be closed after usage
   • Entity Manager Factory must be close when application exits

Transaction Management

• JPA always modifies data in Transaction.
  • modifying without transaction throws exception
• Must get Transaction API from EntityManager before starting transaction

EntityTransaction tx = entityManager.Transaction();

try {
	tx.begin();
	// BIZ LOGIC
	tx.commit(); // commit transaction
} catch (Exception e) {
	tx.rollback(); // rollback transaction
}

• JPA modification : find → MODIFY → persist
  • JPA tracks which Entity has been modifed

JPQL(Java Persistence Query Language)

• When searching multiple items, Searching Entity Object requires all the data of DB to be loaded & transformed to Entity Object.
• JPA resolves it by JPQL
• Unlike SQL, JPQL is Query about Entity Object
  • JPQL got no idea about DB table, JPA anlyze JPQL & create adequate SQL to read DB
  • JPQL is case-sensitive