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[표준] JPA (Java Persistence API)란?
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자바 표준 ORM (Object-Relational Mapping) 기술을 정의한 API
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 [구현체] Hibernate 란?
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 - 객체-관계 매핑(Object-Relational Mapping, ORM)을 구현한 대표적 프레임워크
 - JPA의 구현체 중 하나로, JPA 표준을 따르면서도 자체적으로 추가 기능 제공
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 Hibernate 특징
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- 객체-관계 매핑(ORM)해 SQL 쿼리 자동 생성
- HQL (Hibernate Query Language): 객체 지향 쿼리 언어로, SQL과 유사하지만 객체를 대상으로 쿼리 작성
- 자동 데이터베이스 스키마 생성 및 업데이트 : hibernate.hbm2ddl.auto 속성 지정
- 캐싱 (Caching): : 1차캐시(Session 캐시)와 2차캐시 지원 
  자주 사용하는 데이터를 메모리에 저장하여 데이터베이스 접근 횟수를 줄임으로써 성능 향상
- 지연 로딩 (Lazy Loading) : 메모리 절약(필요할 때까지 데이터를 로드하지 않음)
- 자동 Dirty Checking     : 객체 상태 변화를 자동으로 감지하여 데이터베이스에 반영

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 Hibernate 기본 구성
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 - 설정 파일 (hibernate.cfg.xml) 
 - Entity 파일(UserVO.java)

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 Hibernate 설정파일 예시
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  <!DOCTYPE hibernate-configuration PUBLIC
        "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
        "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
    <session-factory>
        <!-- DB연결정보 Database connection settings -->
        <property name="hibernate.connection.driver_class">org.h2.Driver</property>
         <property name="hibernate.connection.url">jdbc:h2:mem:test;DB_CLOSE_DELAY=-1</property>
         <property name="hibernate.connection.username">sa</property>
         <property name="hibernate.connection.password"></property>

        <!-- JDBC connection pool settings ... using built-in test pool -->
        <property name="hibernate.c3p0.min_size">5</property>
        <property name="hibernate.c3p0.max_size">20</property>
        <property name="hibernate.c3p0.timeout">300</property>
        <property name="hibernate.c3p0.max_statements">50</property>
        <property name="hibernate.c3p0.idle_test_period">3000</property>

<!-- Hibernate 설정 =============================================== -->
        <!-- SQL방언(문법,기능집합) Select our SQL dialect -->
        <property name="hibernate.dialect">org.hibernate.dialect.H2Dialect</property>

        <!-- 로그출력 Echo all executed SQL to stdout -->
        <property name="hibernate.show_sql">true</property>

        <!-- 스키마처리방식 Drop and re-create the database schema on startup -->
        <property name="hibernate.hbm2ddl.auto">update</property>
<!-- ============================================================= -->

        <!-- 엔디티스캔범위 Names the annotated entity class -->
        <mapping class="com.example.MyEntity"/>
    </session-factory>
</hibernate-configuration>



* 주요 Hibernate 방언 목록
H2 Database: org.hibernate.dialect.H2Dialect
MySQL: org.hibernate.dialect.MySQLDialect
MySQL 5: org.hibernate.dialect.MySQL5Dialect
MySQL 8: org.hibernate.dialect.MySQL8Dialect
Oracle 10g: org.hibernate.dialect.Oracle10gDialect
Oracle 12c: org.hibernate.dialect.Oracle12cDialect
PostgreSQL: org.hibernate.dialect.PostgreSQLDialect
SQL Server 2008: org.hibernate.dialect.SQLServer2008Dialect
SQL Server 2012: org.hibernate.dialect.SQLServer2012Dialect

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 Entity.java 파일 예시
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import javax.persistence.Entity;
import javax.persistence.Id;

@Entity
public class MyEntity {
    @Id
    private Long id;
    private String name;

    // getters and setters
}


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 엔티티 스캔 방법
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1) VO클래스에 @Entity 어노테이션 사용

@Data
@ToString
@Entity
@Table(name = "users3")
public class UserVO {
    
    @Id
    @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "users3_seq_gen")
    @SequenceGenerator(name = "users3_seq_gen", sequenceName = "users3_seq", allocationSize = 1)
    @Column(name = "user_seq")
    private Long userSeq;
    ~~
}

2) hibernate.cfg.xml 설정 파일에 엔티티 클래스가 위치한 패키지 지정 
        <mapping package="cohttp://m.lec11.orm.jpa"/>
        
3) spring-context.xml 설정 파일에 packagesToScan 속성 사용
 <bean id="sessionFactory" class="org.springframework.orm.hibernate5.LocalSessionFactoryBean">
<property name="packagesToScan" value="cohttp://m.lec11.orm.jpa"/>
~~~
 </bean>




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 pom.xml
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<!-- Hibernate Core -->
    <dependency>
    <groupId>org.hibernate</groupId>
    <artifactId>hibernate-core</artifactId>
    <version>5.4.33.Final</version>
</dependency>
    <!-- Hibernate EntityManager -->
<dependency>
    <groupId>org.hibernate</groupId>
    <artifactId>hibernate-entitymanager</artifactId>
    <version>5.4.33.Final</version>
</dependency>
    <!-- Hibernate JPA API
    <dependency>
        <groupId>org.hibernate.javax.persistence</groupId>
        <artifactId>hibernate-jpa-2.1-api</artifactId>
        <version>1.0.0.Final</version>
    </dependency>  -->
    <!-- JPA API -->
    <dependency>
    <groupId>javax.persistence</groupId>
    <artifactId>javax.persistence-api</artifactId>
    <version>2.2</version>
</dependency>