JPA 설정하기

1. 라이브러리 추가 - pom.xml

-자바 8

<?xml version="1.0" encoding="UTF-8"?> 
<project xmlns="http://maven.apache.org/POM/4.0.0" 
 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
 xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"> 
 <modelVersion>4.0.0</modelVersion> 
 <groupId>jpa-basic</groupId> 
 <artifactId>ex1-hello-jpa</artifactId> 
 <version>1.0.0</version> 
 <dependencies> 
 <!-- JPA 하이버네이트 --> 
 <dependency> 
 <groupId>org.hibernate</groupId> 
 <artifactId>hibernate-entitymanager</artifactId> 
 <version>5.3.10.Final</version> 
 </dependency> 
 <!-- H2 데이터베이스 --> 
 <dependency> 
 <groupId>com.h2database</groupId> 
 <artifactId>h2</artifactId> 
 <version>1.4.199</version> 
 </dependency> 
 </dependencies> 
</project>

- 자바 11일 때 추가

<dependency>
            <groupId>javax.xml.bind</groupId>
            <artifactId>jaxb-api</artifactId>
            <version>2.3.0</version>
        </dependency>

 

2. JPA 설정하기 - persistence.xml

• JPA 설정 파일
• /META-INF/persistence.xml 위치
• persistence-unit name으로 이름 지정
• javax.persistence로 시작: JPA 표준 속성
• hibernate로 시작: 하이버네이트 전용 속성

<?xml version="1.0" encoding="UTF-8"?> 
<persistence version="2.2" 
 xmlns="http://xmlns.jcp.org/xml/ns/persistence" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
 xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/persistence http://xmlns.jcp.org/xml/ns/persistence/persistence_2_2.xsd"> 
 <persistence-unit name="hello"> 
 <properties> 
 <!-- 필수 속성 --> 
 <property name="javax.persistence.jdbc.driver" value="org.h2.Driver"/> 
 <property name="javax.persistence.jdbc.user" value="sa"/> 
 <property name="javax.persistence.jdbc.password" value=""/> 
 <property name="javax.persistence.jdbc.url" value="jdbc:h2:tcp://localhost/~/test"/> 
 <property name="hibernate.dialect" value="org.hibernate.dialect.H2Dialect"/> 
 
 <!-- 옵션 --> 
 <property name="hibernate.show_sql" value="true"/> 
 <property name="hibernate.format_sql" value="true"/> 
 <property name="hibernate.use_sql_comments" value="true"/> 
 <!--<property name="hibernate.hbm2ddl.auto" value="create" />--> 
 </properties> 
 </persistence-unit> 
</persistence>

 

3. 데이터베이스 방언

• JPA는 특정 데이터베이스에 종속 X 
• 각각의 데이터베이스가 제공하는 SQL 문법과 함수는 조금씩 다름
• 가변 문자: MySQL은 VARCHAR, Oracle은 VARCHAR2 
• 문자열을 자르는 함수: SQL 표준은 SUBSTRING(), Oracle은
SUBSTR() 
• 페이징: MySQL은 LIMIT , Oracle은 ROWNUM 
• 방언: SQL 표준을 지키지 않는 특정 데이터베이스만의 고유한 기능

• hibernate.dialect 속성에 지정
• H2 : org.hibernate.dialect.H2Dialect 
• Oracle 10g : org.hibernate.dialect.Oracle10gDialect 
• MySQL : org.hibernate.dialect.MySQL5InnoDBDialect 
• 하이버네이트는 40가지 이상의 데이터베이스 방언 지원

 

4. JPA 구동 방식

5. 객체와 테이블을 생성하고 매핑하기

• @Entity: JPA가 관리할 객체
• @Id: 데이터베이스 PK와 매핑

package hellojpa; 
import javax.persistence.Entity; 
import javax.persistence.Id; 
@Entity 
public class Member { 
 @Id 
 private Long id; 
 private String name; 
 //Getter, Setter … 
}

 

create table Member ( 
 id bigint not null, 
 name varchar(255), 
 primary key (id) 
);

• 엔티티 매니저 팩토리는 하나만 생성해서 애플리케이션 전체에서 공유
• 엔티티 매니저는 쓰레드간에 공유X (사용하고 버려야 한다). 
• JPA의 모든 데이터 변경은 트랜잭션 안에서 실행

 

6. JPQL

• JPA를 사용하면 엔티티 객체를 중심으로 개발
• 문제는 검색 쿼리
• 검색을 할 때도 테이블이 아닌 엔티티 객체를 대상으로 검색
• 모든 DB 데이터를 객체로 변환해서 검색하는 것은 불가능
• 애플리케이션이 필요한 데이터만 DB에서 불러오려면 결국 검색 조건이 포함된 SQL이 필요

• JPA는 SQL을 추상화한 JPQL이라는 객체 지향 쿼리 언어 제공
• SQL과 문법 유사, SELECT, FROM, WHERE, GROUP BY, HAVING, JOIN 지원
• JPQL은 엔티티 객체를 대상으로 쿼리
• SQL은 데이터베이스 테이블을 대상으로 쿼리

• 테이블이 아닌 객체를 대상으로 검색하는 객체 지향 쿼리
• SQL을 추상화해서 특정 데이터베이스 SQL에 의존X 
• JPQL을 한마디로 정의하면 객체 지향 SQL 
• JPQL은 뒤에서 아주 자세히 다룸

 

 

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