[Spring/JPA] 즉시 로딩(Eager Loading)과 지연 로딩(Lazy Loading)

이수진·2022년 11월 10일

즉시 로딩(Eager Loading)과 지연 로딩(Lazy Loading)

즉시 로딩: 엔티티를 조회할 때 자신과 연관되는 엔티티를 조인(join)을 통해 함께 조회하는 방식

엔티티 A 조회시 관련되어 있는 엔티티 B를 같이 가져온다. 실제 엔티티를 맵핑한다. Join을 사용하여 한번에 가져온다.

A join B, 쿼리가 한 번만 나간다

지연 로딩: 자신과 연관된 엔티티를 실제로 사용할 때 연관된 엔티티를 조회(SELECT) 하는 방식

엔티티 A를 조회시 관련(Reference)되어 있는 엔티티 B를 한번에 가져오지 않는다. 프록시를 맵핑하고 실제 B를 조회할때 쿼리가 나간다.

쿼리가 두 번 나간다. A 조회시 한 번, B 조회시 한 번

✔ Fetch Type 이란?

이 FetchType이란, JPA가 하나의 Entity를 조회할 때, 연관관계에 있는 객체들을 어떻게 가져올 것이냐를 나타내는 설정값 입니다

JPA는 ORM기술로, 사용자가 직접 쿼리를 생성하지 않고, JPA에서 JPQL을 이용하여 쿼리문을 생성하기 때문에
객체와 필드를 보고 쿼리를 생성합니다
따라서 다른 객체와 연관관계 매핑이 되어있으면 그 객체들까지 조회하게 되는데, 이때 이 객체를 어떻게 불러올 것인가를 설정할 수 있습니다

✔ 즉시 로딩 (Eager Loading)

즉시 로딩이란, 데이터를 조회할 때 연관된 모든 객체의 데이터까지 한 번에 불러오는 것입니다.

예시

// order.java

package jpabook.jpashop.domain;

import lombok.Getter;
import lombok.Setter;

import javax.persistence.*;
import java.time.LocalDateTime;
import java.util.ArrayList;
import java.util.List;


@Entity
@Table(name = "orders")
@Getter @Setter
public class Order {

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

    @ManyToOne
    @JoinColumn(name = "member_id")
    private Member member;

    @OneToMany(mappedBy = "order", cascade = CascadeType.ALL)
    private List<OrderItem> orderItems = new ArrayList<>();

    @OneToOne(cascade = CascadeType.ALL)
    @JoinColumn(name = "delivery_id")
    private Delivery delivery;

    private LocalDateTime orderDate;

    @Enumerated(EnumType.STRING)
    private OrderStatus status; // 주문상태 [order, cancel]


    //== 연관관계 메서드==//
    public void setMember(Member member) {
        this.member = member;
        member.getOrders().add(this);
    }

    public void addOrderItem(OrderItem orderItem) {
        orderItems.add(orderItem);
        orderItem.setOrder(this);
    }

    public void setDelivery(Delivery delivery) {
        this.delivery = delivery;
        delivery.setOrder(this);
    }
}

해당 예시에서 보면

Order - Member 는 N - 1 매핑관계
Order - Delivery 는 1 - 1 매핑관계

// example.java


package jpabook.jpashop;

import jpabook.jpashop.domain.Order;
import jpabook.jpashop.repository.OrderRepository;
import org.junit.Test;
import org.junit.jupiter.api.DisplayName;
import org.junit.runner.RunWith;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.test.context.junit4.SpringRunner;
import org.springframework.transaction.annotation.Transactional;


@RunWith(SpringRunner.class)
@SpringBootTest
@Transactional
public class example {

    @Autowired OrderRepository orderRepository;

    @Test
    @DisplayName("즉시 로딩은 연관관계를 맺고있는 엔티티는 모두 조회가 된다")
    public void exam1() {

        Order result = orderRepository.findOne(1L);
    }

}

테스트 실행 결과

2022-11-10 15:11:19.498  INFO 43197 --- [           main] o.s.t.c.transaction.TransactionContext   : Began transaction (1) for test context [DefaultTestContext@5b367418 testClass = example, testInstance = jpabook.jpashop.example@50aa9a91, testMethod = exam1@example, testException = [null], mergedContextConfiguration = [WebMergedContextConfiguration@36060e testClass = example, locations = '{}', classes = '{class jpabook.jpashop.JpashopApplication}', contextInitializerClasses = '[]', activeProfiles = '{}', propertySourceLocations = '{}', propertySourceProperties = '{org.springframework.boot.test.context.SpringBootTestContextBootstrapper=true}', contextCustomizers = set[org.springframework.boot.test.autoconfigure.actuate.metrics.MetricsExportContextCustomizerFactory$DisableMetricExportContextCustomizer@63070bab, org.springframework.boot.test.autoconfigure.properties.PropertyMappingContextCustomizer@0, org.springframework.boot.test.autoconfigure.web.servlet.WebDriverContextCustomizerFactory$Customizer@7c1e2a9e, org.springframework.boot.test.context.filter.ExcludeFilterContextCustomizer@2b4bac49, org.springframework.boot.test.json.DuplicateJsonObjectContextCustomizerFactory$DuplicateJsonObjectContextCustomizer@3e96bacf, org.springframework.boot.test.mock.mockito.MockitoContextCustomizer@0, org.springframework.boot.test.web.client.TestRestTemplateContextCustomizer@236e3f4e, org.springframework.boot.test.context.SpringBootTestArgs@1, org.springframework.boot.test.context.SpringBootTestWebEnvironment@4f83df68], resourceBasePath = 'src/main/webapp', contextLoader = 'org.springframework.boot.test.context.SpringBootContextLoader', parent = [null]], attributes = map['org.springframework.test.context.web.ServletTestExecutionListener.activateListener' -> true, 'org.springframework.test.context.web.ServletTestExecutionListener.populatedRequestContextHolder' -> true, 'org.springframework.test.context.web.ServletTestExecutionListener.resetRequestContextHolder' -> true, 'org.springframework.test.context.event.ApplicationEventsTestExecutionListener.recordApplicationEvents' -> false]]; transaction manager [org.springframework.orm.jpa.JpaTransactionManager@63af52a6]; rollback [true]
2022-11-10 15:11:19.556 DEBUG 43197 --- [           main] org.hibernate.SQL                        :
select
order0_.order_id as order_id1_6_0_,
order0_.delivery_id as delivery4_6_0_,
order0_.member_id as member_i5_6_0_,
order0_.order_date as order_da2_6_0_,
order0_.status as status3_6_0_,
delivery1_.delivery_id as delivery1_2_1_,
delivery1_.city as city2_2_1_,
delivery1_.street as street3_2_1_,
delivery1_.zipcode as zipcode4_2_1_,
delivery1_.status as status5_2_1_,
member2_.member_id as member_i1_4_2_,
member2_.city as city2_4_2_,
member2_.street as street3_4_2_,
member2_.zipcode as zipcode4_4_2_,
member2_.name as name5_4_2_
from
orders order0_
left outer join
delivery delivery1_
on order0_.delivery_id=delivery1_.delivery_id
left outer join
member member2_
on order0_.member_id=member2_.member_id
where
order0_.order_id=?
Hibernate:
select
order0_.order_id as order_id1_6_0_,
order0_.delivery_id as delivery4_6_0_,
order0_.member_id as member_i5_6_0_,
order0_.order_date as order_da2_6_0_,
order0_.status as status3_6_0_,
delivery1_.delivery_id as delivery1_2_1_,
delivery1_.city as city2_2_1_,
delivery1_.street as street3_2_1_,
delivery1_.zipcode as zipcode4_2_1_,
delivery1_.status as status5_2_1_,
member2_.member_id as member_i1_4_2_,
member2_.city as city2_4_2_,
member2_.street as street3_4_2_,
member2_.zipcode as zipcode4_4_2_,
member2_.name as name5_4_2_
from
orders order0_
left outer join
delivery delivery1_
on order0_.delivery_id=delivery1_.delivery_id
left outer join
member member2_
on order0_.member_id=member2_.member_id
where
order0_.order_id=?
2022-11-10 15:11:19.560 TRACE 43197 --- [           main] o.h.type.descriptor.sql.BasicBinder      : binding parameter [1] as [BIGINT] - [1]
2022-11-10 15:11:19.564  INFO 43197 --- [           main] p6spy                                    : #1668060679564 | took 3ms | statement | connection 3| url jdbc:h2:mem:testdb
select order0_.order_id as order_id1_6_0_, order0_.delivery_id as delivery4_6_0_, order0_.member_id as member_i5_6_0_, order0_.order_date as order_da2_6_0_, order0_.status as status3_6_0_, delivery1_.delivery_id as delivery1_2_1_, delivery1_.city as city2_2_1_, delivery1_.street as street3_2_1_, delivery1_.zipcode as zipcode4_2_1_, delivery1_.status as status5_2_1_, member2_.member_id as member_i1_4_2_, member2_.city as city2_4_2_, member2_.street as street3_4_2_, member2_.zipcode as zipcode4_4_2_, member2_.name as name5_4_2_ from orders order0_ left outer join delivery delivery1_ on order0_.delivery_id=delivery1_.delivery_id left outer join member member2_ on order0_.member_id=member2_.member_id where order0_.order_id=?
select order0_.order_id as order_id1_6_0_, order0_.delivery_id as delivery4_6_0_, order0_.member_id as member_i5_6_0_, order0_.order_date as order_da2_6_0_, order0_.status as status3_6_0_, delivery1_.delivery_id as delivery1_2_1_, delivery1_.city as city2_2_1_, delivery1_.street as street3_2_1_, delivery1_.zipcode as zipcode4_2_1_, delivery1_.status as status5_2_1_, member2_.member_id as member_i1_4_2_, member2_.city as city2_4_2_, member2_.street as street3_4_2_, member2_.zipcode as zipcode4_4_2_, member2_.name as name5_4_2_ from orders order0_ left outer join delivery delivery1_ on order0_.delivery_id=delivery1_.delivery_id left outer join member member2_ on order0_.member_id=member2_.member_id where order0_.order_id=1;
2022-11-10 15:11:19.569  INFO 43197 --- [           main] p6spy                                    : #1668060679569 | took 0ms | rollback | connection 3| url jdbc:h2:mem:testdb

;
2022-11-10 15:11:19.570  INFO 43197 --- [           main] o.s.t.c.transaction.TransactionContext   : Rolled back transaction for test: [DefaultTestContext@5b367418 testClass = example, testInstance = jpabook.jpashop.example@50aa9a91, testMethod = exam1@example, testException = [null], mergedContextConfiguration = [WebMergedContextConfiguration@36060e testClass = example, locations = '{}', classes = '{class jpabook.jpashop.JpashopApplication}', contextInitializerClasses = '[]', activeProfiles = '{}', propertySourceLocations = '{}', propertySourceProperties = '{org.springframework.boot.test.context.SpringBootTestContextBootstrapper=true}', contextCustomizers = set[org.springframework.boot.test.autoconfigure.actuate.metrics.MetricsExportContextCustomizerFactory$DisableMetricExportContextCustomizer@63070bab, org.springframework.boot.test.autoconfigure.properties.PropertyMappingContextCustomizer@0, org.springframework.boot.test.autoconfigure.web.servlet.WebDriverContextCustomizerFactory$Customizer@7c1e2a9e, org.springframework.boot.test.context.filter.ExcludeFilterContextCustomizer@2b4bac49, org.springframework.boot.test.json.DuplicateJsonObjectContextCustomizerFactory$DuplicateJsonObjectContextCustomizer@3e96bacf, org.springframework.boot.test.mock.mockito.MockitoContextCustomizer@0, org.springframework.boot.test.web.client.TestRestTemplateContextCustomizer@236e3f4e, org.springframework.boot.test.context.SpringBootTestArgs@1, org.springframework.boot.test.context.SpringBootTestWebEnvironment@4f83df68], resourceBasePath = 'src/main/webapp', contextLoader = 'org.springframework.boot.test.context.SpringBootContextLoader', parent = [null]], attributes = map['org.springframework.test.context.web.ServletTestExecutionListener.activateListener' -> true, 'org.springframework.test.context.web.ServletTestExecutionListener.populatedRequestContextHolder' -> true, 'org.springframework.test.context.web.ServletTestExecutionListener.resetRequestContextHolder' -> true, 'org.springframework.test.context.event.ApplicationEventsTestExecutionListener.recordApplicationEvents' -> false]]

결과 분석
- 주문과 N:1 매핑관계인 회원 관계까지 모두 조회됨
- 주문과 1:1 매핑관계인 배송 관계까지 모두 조회됨

즉시 로딩(Eager Loading) 을 방식을 사용하면 Order를 조회하는 시점에 바로 Member와 Delivery까지 불러오는 쿼리를 날려
한꺼번에 데이터를 불러오는 것을 확인할 수 있습니다

✔ 지연 로딩 (Lazy Loading)

지연 로딩이란, 필요한 시점에 연관된 객체의 대이터를 불러오는 것입니다

위의 예제에서 FetchType을 Lazy로 바꿔서 실행해보면,

// order.java
package jpabook.jpashop.domain;

import lombok.Getter;
import lombok.Setter;

import javax.persistence.*;
import java.time.LocalDateTime;
import java.util.ArrayList;
import java.util.List;


@Entity
@Table(name = "orders")
@Getter @Setter
public class Order {

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

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "member_id")
    private Member member;

    @OneToMany(mappedBy = "order", cascade = CascadeType.ALL)
    private List<OrderItem> orderItems = new ArrayList<>();

    @OneToOne(fetch = FetchType.LAZY, cascade = CascadeType.ALL)
    @JoinColumn(name = "delivery_id")
    private Delivery delivery;

    private LocalDateTime orderDate;

    @Enumerated(EnumType.STRING)
    private OrderStatus status; // 주문상태 [order, cancel]


    //== 연관관계 메서드==//
    public void setMember(Member member) {
        this.member = member;
        member.getOrders().add(this);
    }

    public void addOrderItem(OrderItem orderItem) {
        orderItems.add(orderItem);
        orderItem.setOrder(this);
    }

    public void setDelivery(Delivery delivery) {
        this.delivery = delivery;
        delivery.setOrder(this);
    }
}

// example.java

package jpabook.jpashop;

import jpabook.jpashop.domain.Order;
import jpabook.jpashop.repository.OrderRepository;
import org.junit.Test;
import org.junit.jupiter.api.DisplayName;
import org.junit.runner.RunWith;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.test.context.junit4.SpringRunner;
import org.springframework.transaction.annotation.Transactional;


@RunWith(SpringRunner.class)
@SpringBootTest
@Transactional
public class example {

    @Autowired OrderRepository orderRepository;

    @Test
    @DisplayName("지연 로딩")
    public void exam1() {

        Order result = orderRepository.findOne(1L);
    }

}

테스트 결과

2022-11-10 15:21:24.046  INFO 45242 --- [           main] o.s.t.c.transaction.TransactionContext   : Began transaction (1) for test context [DefaultTestContext@5b367418 testClass = example, testInstance = jpabook.jpashop.example@782ed745, testMethod = exam1@example, testException = [null], mergedContextConfiguration = [WebMergedContextConfiguration@36060e testClass = example, locations = '{}', classes = '{class jpabook.jpashop.JpashopApplication}', contextInitializerClasses = '[]', activeProfiles = '{}', propertySourceLocations = '{}', propertySourceProperties = '{org.springframework.boot.test.context.SpringBootTestContextBootstrapper=true}', contextCustomizers = set[org.springframework.boot.test.autoconfigure.actuate.metrics.MetricsExportContextCustomizerFactory$DisableMetricExportContextCustomizer@63070bab, org.springframework.boot.test.autoconfigure.properties.PropertyMappingContextCustomizer@0, org.springframework.boot.test.autoconfigure.web.servlet.WebDriverContextCustomizerFactory$Customizer@7c1e2a9e, org.springframework.boot.test.context.filter.ExcludeFilterContextCustomizer@2b4bac49, org.springframework.boot.test.json.DuplicateJsonObjectContextCustomizerFactory$DuplicateJsonObjectContextCustomizer@3e96bacf, org.springframework.boot.test.mock.mockito.MockitoContextCustomizer@0, org.springframework.boot.test.web.client.TestRestTemplateContextCustomizer@236e3f4e, org.springframework.boot.test.context.SpringBootTestArgs@1, org.springframework.boot.test.context.SpringBootTestWebEnvironment@4f83df68], resourceBasePath = 'src/main/webapp', contextLoader = 'org.springframework.boot.test.context.SpringBootContextLoader', parent = [null]], attributes = map['org.springframework.test.context.web.ServletTestExecutionListener.activateListener' -> true, 'org.springframework.test.context.web.ServletTestExecutionListener.populatedRequestContextHolder' -> true, 'org.springframework.test.context.web.ServletTestExecutionListener.resetRequestContextHolder' -> true, 'org.springframework.test.context.event.ApplicationEventsTestExecutionListener.recordApplicationEvents' -> false]]; transaction manager [org.springframework.orm.jpa.JpaTransactionManager@7c6fc278]; rollback [true]
2022-11-10 15:21:24.112 DEBUG 45242 --- [           main] org.hibernate.SQL                        : 
    select
        order0_.order_id as order_id1_6_0_,
        order0_.delivery_id as delivery4_6_0_,
        order0_.member_id as member_i5_6_0_,
        order0_.order_date as order_da2_6_0_,
        order0_.status as status3_6_0_ 
    from
        orders order0_ 
    where
        order0_.order_id=?
Hibernate: 
    select
        order0_.order_id as order_id1_6_0_,
        order0_.delivery_id as delivery4_6_0_,
        order0_.member_id as member_i5_6_0_,
        order0_.order_date as order_da2_6_0_,
        order0_.status as status3_6_0_ 
    from
        orders order0_ 
    where
        order0_.order_id=?
2022-11-10 15:21:24.115 TRACE 45242 --- [           main] o.h.type.descriptor.sql.BasicBinder      : binding parameter [1] as [BIGINT] - [1]
2022-11-10 15:21:24.119  INFO 45242 --- [           main] p6spy                                    : #1668061284119 | took 3ms | statement | connection 3| url jdbc:h2:mem:testdb
select order0_.order_id as order_id1_6_0_, order0_.delivery_id as delivery4_6_0_, order0_.member_id as member_i5_6_0_, order0_.order_date as order_da2_6_0_, order0_.status as status3_6_0_ from orders order0_ where order0_.order_id=?
select order0_.order_id as order_id1_6_0_, order0_.delivery_id as delivery4_6_0_, order0_.member_id as member_i5_6_0_, order0_.order_date as order_da2_6_0_, order0_.status as status3_6_0_ from orders order0_ where order0_.order_id=1;
2022-11-10 15:21:24.125  INFO 45242 --- [           main] p6spy                                    : #1668061284125 | took 0ms | rollback | connection 3| url jdbc:h2:mem:testdb

;
2022-11-10 15:21:24.127  INFO 45242 --- [           main] o.s.t.c.transaction.TransactionContext   : Rolled back transaction for test: [DefaultTestContext@5b367418 testClass = example, testInstance = jpabook.jpashop.example@782ed745, testMethod = exam1@example, testException = [null], mergedContextConfiguration = [WebMergedContextConfiguration@36060e testClass = example, locations = '{}', classes = '{class jpabook.jpashop.JpashopApplication}', contextInitializerClasses = '[]', activeProfiles = '{}', propertySourceLocations = '{}', propertySourceProperties = '{org.springframework.boot.test.context.SpringBootTestContextBootstrapper=true}', contextCustomizers = set[org.springframework.boot.test.autoconfigure.actuate.metrics.MetricsExportContextCustomizerFactory$DisableMetricExportContextCustomizer@63070bab, org.springframework.boot.test.autoconfigure.properties.PropertyMappingContextCustomizer@0, org.springframework.boot.test.autoconfigure.web.servlet.WebDriverContextCustomizerFactory$Customizer@7c1e2a9e, org.springframework.boot.test.context.filter.ExcludeFilterContextCustomizer@2b4bac49, org.springframework.boot.test.json.DuplicateJsonObjectContextCustomizerFactory$DuplicateJsonObjectContextCustomizer@3e96bacf, org.springframework.boot.test.mock.mockito.MockitoContextCustomizer@0, org.springframework.boot.test.web.client.TestRestTemplateContextCustomizer@236e3f4e, org.springframework.boot.test.context.SpringBootTestArgs@1, org.springframework.boot.test.context.SpringBootTestWebEnvironment@4f83df68], resourceBasePath = 'src/main/webapp', contextLoader = 'org.springframework.boot.test.context.SpringBootContextLoader', parent = [null]], attributes = map['org.springframework.test.context.web.ServletTestExecutionListener.activateListener' -> true, 'org.springframework.test.context.web.ServletTestExecutionListener.populatedRequestContextHolder' -> true, 'org.springframework.test.context.web.ServletTestExecutionListener.resetRequestContextHolder' -> true, 'org.springframework.test.context.event.ApplicationEventsTestExecutionListener.recordApplicationEvents' -> false]]

결과 분석
- order를 조회해도 order만 조회하는 쿼리만 생성이 되고
- 나머지 연관 객체들을 조회하는 쿼리는 생성되지 않음

✔ ️즉시 로딩(Eager Loading)과 지연 로딩(Lazy Loading)의 장단점 분석

즉시 로딩(Eager Loading)
- 장점
  - 연관된 엔티티를 모두 가져올 수 있음
- 단점
  - 엔티티간의 관계가 복잡해질수록 조인으로 인한 성능 저하 가 나타날 수 있음
    - 불필요한 조인까지 포함해서 처리하는 경우가 많음
    - 예를 들어 Order 연관된 객체가 Member가 N개라면, Order 1개 조회 시 필요하지 않은 Member 객체를 조회하는 쿼리 N개가 생성됨
  - JPQL에서 N+1 문제를 일으킴

지연 로딩(Lazy Loading)
- 장점
  - 다른 접근 방식보다 훨씬 적은 초기의 로딩 시간
  - 다른 접근 방식에 비해 메모리 소비량 감소
- 단점
  - 초기화가 지연되면 원하지 않는 순간 성능에 영향을 줄 수 있음

모든 연관관계는 지연로딩으로 설정하자!

즉시 로딩(Eager)는 예측이 어렵고, 어떤 SQL이 실행될지 추적하기 어렵다. 특히 JPQL을 실행할 때 N+1 문제가 자주 발생한다

실무에서 모든 연관관계는 지연로딩(LAZY)으로 설정해야 한다.

연관된 엔티티를 함께 DB에서 조회해야 하면, fetch join 또는 엔티티 그래프 기능을 사용한다.

@XToOne(OneToOne, ManyToOne) 관계는 기본이 즉시로딩이므로 직접 지연로딩으로 설정해야 한다.

추가)

✔ ️지연 로딩은 어떻게 이루어질 수 있는가? -> "프록시 객체"

객체 조회 시 항상 연관된 객체까지 함께 조회하는 것은 효율적이지 않다.
그래서 JPA는 엔티티가 실제로 사용되기 전까지 데이터베이스 조회를 지연할 수 있도록 제공하는데 이를 "지연 로딩" 이라고 한다.

"실제로 사용하는 시점에 데이터베이스에서 필요한 데이터를 가져오는 것"

하지만 지연 로딩을 사용하면 실제 엔티티 객체 대신 가짜 객체가 필요한데, 이것이 바로 "프록시 객체"

ManyToOne 관계인 Order(N) - Member(1) 를 예로 들어 보자.

엔티티 Order 조회 시 관련 되어 있는 엔티티 Member를 한번에 가져오지 않는다.
프록시를 맵핑하고 실제 Member를 조회할 때 쿼리가 나간다.

지연로딩을 설정해놓으면, Order find 메소드 호출 시 Member는 Proxy 상태이다.
order.getMember(); 메소드를 호출하는 순간 JPA는 DB를 조회하여 Proxy에 값을 채운다. (바꾸는 게 아니라 값을 채워넣는 것!)
프록시는 실제 엔티티를 상속받은 오브젝트이다. 따라서 Member 엔티티와 겉모양이 같고 JPA는 이 프록시(Target)에 값을 채워넣는 것이다.

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1개의 댓글

hexator

2023년 3월 28일

잘봤습니다! 가장 마지막 사진에서 em 은 무엇인가용?

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