Spring 핵심 원리 3 - 싱글톤 패턴과 스프링 컨테이너

스프링은 필요한 메소드들을 스프링 컨테이너에 빈 형태로 관리하게 되는데, 그 동안 DIP(Dependency Inversion Principle)를 지키기위해 AppConfig이라는 Config 파일을 만들어두어, 스프링 빈으로 등록해서 사용해왔다.

 

코드만 보면, memberRepository() 메소드가 총 3번 실행된다.

1. memberService()에서

2. memberRepository()를 Bean에 등록 할 때

3. orderService()

하지만, 실제로 테스트를 돌려보면 각각의 메소드는 한 번만 실행된다.

package hello.core;

import hello.core.discount.DiscountPolicy;
import hello.core.discount.RateDiscountPolicy;
import hello.core.member.*;
import hello.core.order.OrderService;
import hello.core.order.OrderServiceImpl;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

@Configuration
public class AppConfig {

    @Bean
    public MemberService memberService() {
        System.out.println("call AppConfig.memberService");
        return new MemberServiceImpl(memberRepository());
    }

    @Bean
    public MemberRepository memberRepository() {
        System.out.println("call AppConfig.memberRepository");
        return new MemoryMemberRepository();
    }

    @Bean
    public DiscountPolicy discountPolicy() {
        return new RateDiscountPolicy();
    }

    @Bean
    public OrderService orderService() {
        System.out.println("call AppConfig.orderService");
        return new OrderServiceImpl(memberRepository(), discountPolicy());
    }
}

 

/Users/jeyoung/Library/Java/JavaVirtualMachines/liberica-11.0.14/bin/java -ea -Didea.test.cyclic.buffer.size=1048576 -javaagent:/Applications/IntelliJ IDEA.app/Contents/lib/idea_rt.jar=58199:/Applications/IntelliJ IDEA.app/Contents/bin -Dfile.encoding=UTF-8 -classpath /Users/jeyoung/.m2/repository/org/junit/platform/junit-platform-launcher/1.8.2/junit-platform-launcher-1.8.2.jar:/Users/jeyoung/.m2/repository/org/junit/platform/junit-platform-engine/1.8.2/junit-platform-engine-1.8.2.jar:/Users/jeyoung/.m2/repository/org/opentest4j/opentest4j/1.2.0/opentest4j-1.2.0.jar:/Users/jeyoung/.m2/repository/org/junit/platform/junit-platform-commons/1.8.2/junit-platform-commons-1.8.2.jar:/Users/jeyoung/.m2/repository/org/apiguardian/apiguardian-api/1.1.2/apiguardian-api-1.1.2.jar:/Applications/IntelliJ IDEA.app/Contents/lib/idea_rt.jar:/Applications/IntelliJ IDEA.app/Contents/plugins/junit/lib/junit5-rt.jar:/Applications/IntelliJ 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com.intellij.rt.junit.JUnitStarter -ideVersion5 -junit5 hello.core.singleton.ConfigurationSingletonTest,configurationDeep
19:19:45.322 [main] DEBUG org.springframework.context.annotation.AnnotationConfigApplicationContext - Refreshing org.springframework.context.annotation.AnnotationConfigApplicationContext@428640fa
19:19:45.330 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'org.springframework.context.annotation.internalConfigurationAnnotationProcessor'
19:19:45.390 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'org.springframework.context.event.internalEventListenerProcessor'
19:19:45.391 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'org.springframework.context.event.internalEventListenerFactory'
19:19:45.392 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'org.springframework.context.annotation.internalAutowiredAnnotationProcessor'
19:19:45.393 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'org.springframework.context.annotation.internalCommonAnnotationProcessor'
19:19:45.396 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'appConfig'
19:19:45.398 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'memberService'
call AppConfig.memberService
19:19:45.406 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'memberRepository'
call AppConfig.memberRepository
19:19:45.406 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'discountPolicy'
19:19:45.407 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'orderService'
call AppConfig.orderService
bean = hello.core.AppConfig$$EnhancerBySpringCGLIB$$9c1df86@4233e892

Process finished with exit code 0

call 출력문이 한 번씩만 출력 된 것을 확인 할 수 있다.

 

이는 Spring이 @Configuration 어노테이션이 포함된 설정 클래스를, 내부적으로 CGLIB라는 바이트 변환 라이브러리를 사용해 내가 만든 AppConfig라는 클래스를 상속한 AppConfigCGLIB라는 클래스를 스프링 컨테이너에 등록해두기 때문이다.

 

이렇게 @Configuration 어노테이션이 포함된 설정 파일에 스프링 빈 들을 등록해두면, 스프링이 자체적으로 각각의 메소드를 한 번만 실행해 싱글톤 패턴을 지킬 수 있게 해준다.

 

@Configuration 어노테이션이 없다면, 위의 코드에 적힌 대로 new MemoryMemberRepository()가 3번 실행되어, 싱글톤 패턴이 깨지게 된다. 없어도 동작은 하게 된다.

 

++ 추가적으로 배운 부분은 인터페이스를 구현해 구현체를 만들 때, 필드를 만들지 말라는 것이다. 클래스의 필드에 값이 있게 된다면, 싱글톤 패턴으로 운용되기 때문에 여러 사용자가 요청을 했을 때 같은 값을 받게 될 수도 있다.

(ex : UserA가 10000원짜리 상품 주문, 이후 UserB가 20000원짜리 상품 주문. 해당 클래스에 int price와 같이 값을 필드 형태로 저장하게 되면, 이후 UserA가 price를 가져오려고 호출 했을 때 20000이라는 잘못된 값을 리턴 할 수 있다.)