스프링 컨테이너란?

#스프링 컨테이너의 역할
스프링 컨테이너는 자바 객체의 생명 주기를 관리하며, 생성된 자바 객체들에게 추가적인 기능을 제공하는 역할을 합니다.
여기서 말하는 자바 객체를 스프링에서는 빈(Bean)이라고 부릅니다.
스프링 컨테이너로 인해서 IoC와 DI의 원리를 적용할 수 있습니다.

개발자는 new 연산자, 인터페이스 호출, 팩토리 호출 방식으로 객체를 생성하고 소멸시킬 수 있는데,
스프링 컨테이너가 이 역할을 대신해 줍니다. 즉, 제어 흐름을 외부에서 관리 할 수 있게 합니다.
또한, 객체들 간의 의존 관계를 스프링 컨테이너가 런타임 과정에서 알아서 만들어 줍니다.

#스프링 컨테이너란?
ApplicationContext 를 스프링 컨테이너라고 한다.
더 정확하게는 BeanFactory , ApplicationContext 로 구분해서 이야기 할 수 있지만,
일반적으로 ApplicationContext를 스프링 컨테이너 라고 합니다.
둘중 어떠한 클래스를 호출하더라도, 동일한 결과를 가지고 오게 됩니다.

ApplicationContext

public static void main(String[] args) {
        ApplicationContext applicationContext = new AnnotationConfigApplicationContext(AppConfig.class);
        MemberService memberService = applicationContext.getBean("memberService", MemberService.class);
        OrderService orderService = applicationContext.getBean("orderService", OrderService.class);

        Long memberId = 1L;
        Member member = new Member(memberId, "memberA", Grade.VIP);
        memberService.join(member);

        Order order = orderService.createOrder(memberId, "itemA", 10000);

        System.out.println("order = " + order.toString());
        System.out.println("order.calculatePrice = " + order.calculatePrice());
    }

ApplicationContext

/Users/gwnam/Library/Java/JavaVirtualMachines/openjdk-17.0.2/Contents/Home/bin/java -javaagent:/Applications/IntelliJ IDEA.app/Contents/lib/idea_rt.jar=59327:/Applications/IntelliJ IDEA.app/Contents/bin -Dfile.encoding=UTF-8 -classpath /Users/gwnam/Desktop/dev/IDE/core/out/production/classes:/Users/gwnam/Desktop/dev/IDE/core/out/production/resources:/Users/gwnam/.gradle/caches/modules-2/files-2.1/org.springframework.boot/spring-boot-starter/2.6.6/b34b4f0c1e99edc181ea40a6b1b79fa72658ee04/spring-boot-starter-2.6.6.jar:/Users/gwnam/.gradle/caches/modules-2/files-2.1/org.springframework.boot/spring-boot-autoconfigure/2.6.6/e5b575d304ec5c6746fef3cc594bbe89721d64c2/spring-boot-autoconfigure-2.6.6.jar:/Users/gwnam/.gradle/caches/modules-2/files-2.1/org.springframework.boot/spring-boot/2.6.6/286137a0c57d159f845faa1c2e8d0b7548391085/spring-boot-2.6.6.jar:/Users/gwnam/.gradle/caches/modules-2/files-2.1/org.springframework.boot/spring-boot-starter-logging/2.6.6/e37b388e2407a1b03f920a7fd0722e41745c92c6/spring-boot-starter-logging-2.6.6.jar:/Users/gwnam/.gradle/caches/modules-2/files-2.1/jakarta.annotation/jakarta.annotation-api/1.3.5/59eb84ee0d616332ff44aba065f3888cf002cd2d/jakarta.annotation-api-1.3.5.jar:/Users/gwnam/.gradle/caches/modules-2/files-2.1/org.springframework/spring-core/5.3.18/7ff3000f3342989cb011b6095a0e86f2e5176cef/spring-core-5.3.18.jar:/Users/gwnam/.gradle/caches/modules-2/files-2.1/org.yaml/snakeyaml/1.29/6d0cdafb2010f1297e574656551d7145240f6e25/snakeyaml-1.29.jar:/Users/gwnam/.gradle/caches/modules-2/files-2.1/org.springframework/spring-context/5.3.18/34f6683d9dbe6edb02ad9393df3d3211b5484622/spring-context-5.3.18.jar:/Users/gwnam/.gradle/caches/modules-2/files-2.1/ch.qos.logback/logback-classic/1.2.11/4741689214e9d1e8408b206506cbe76d1c6a7d60/logback-classic-1.2.11.jar:/Users/gwnam/.gradle/caches/modules-2/files-2.1/org.apache.logging.log4j/log4j-to-slf4j/2.17.2/17dd0fae2747d9a28c67bc9534108823d2376b46/log4j-to-slf4j-2.17.2.jar:/Users/gwnam/.gradle/caches/modules-2/files-2.1/org.slf4j/jul-to-slf4j/1.7.36/ed46d81cef9c412a88caef405b58f93a678ff2ca/jul-to-slf4j-1.7.36.jar:/Users/gwnam/.gradle/caches/modules-2/files-2.1/org.springframework/spring-jcl/5.3.18/7e5d8cd447981bc20d4c397a2ba0a1c65ff2267a/spring-jcl-5.3.18.jar:/Users/gwnam/.gradle/caches/modules-2/files-2.1/org.springframework/spring-aop/5.3.18/8e9cce60c60257ae1d5b3cd675ec3a8286ed1955/spring-aop-5.3.18.jar:/Users/gwnam/.gradle/caches/modules-2/files-2.1/org.springframework/spring-beans/5.3.18/3f0ea6598a5a1eae0a672f025a33a0b7e0d6dfd3/spring-beans-5.3.18.jar:/Users/gwnam/.gradle/caches/modules-2/files-2.1/org.springframework/spring-expression/5.3.18/61c51831e49a85fc5bf925253ca63f46fec6f013/spring-expression-5.3.18.jar:/Users/gwnam/.gradle/caches/modules-2/files-2.1/ch.qos.logback/logback-core/1.2.11/a01230df5ca5c34540cdaa3ad5efb012f1f1f792/logback-core-1.2.11.jar:/Users/gwnam/.gradle/caches/modules-2/files-2.1/org.slf4j/slf4j-api/1.7.36/6c62681a2f655b49963a5983b8b0950a6120ae14/slf4j-api-1.7.36.jar:/Users/gwnam/.gradle/caches/modules-2/files-2.1/org.apache.logging.log4j/log4j-api/2.17.2/f42d6afa111b4dec5d2aea0fe2197240749a4ea6/log4j-api-2.17.2.jar hello.core.OrderApp
23:22:48.573 [main] DEBUG org.springframework.context.annotation.AnnotationConfigApplicationContext - Refreshing org.springframework.context.annotation.AnnotationConfigApplicationContext@33e5ccce
23:22:48.580 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'org.springframework.context.annotation.internalConfigurationAnnotationProcessor'
23:22:48.633 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'org.springframework.context.event.internalEventListenerProcessor'
23:22:48.634 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'org.springframework.context.event.internalEventListenerFactory'
23:22:48.634 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'org.springframework.context.annotation.internalAutowiredAnnotationProcessor'
23:22:48.635 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'org.springframework.context.annotation.internalCommonAnnotationProcessor'
23:22:48.638 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'appConfig'
23:22:48.641 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'memberService'
23:22:48.649 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'memberRepository'
23:22:48.650 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'orderService'
23:22:48.650 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'discountPolicy'
order = Order{memberId=1, itemName='itemA', itemPrice=10000, discountPrice=1000}
order.calculatePrice = 9000

Process finished with exit code 0

BeanFactory

public static void main(String[] args) { 
    BeanFactory applicationContext = new AnnotationConfigApplicationContext(AppConfig.class); MemberService         memberService = applicationContext.getBean("memberService", MemberService.class); 
    OrderService orderService = applicationContext.getBean("orderService", OrderService.class);

    Long memberId = 1L;
    Member member = new Member(memberId, "memberA", Grade.VIP);
    memberService.join(member);

    Order order = orderService.createOrder(memberId, "itemA", 10000);

    System.out.println("order = " + order.toString());
    System.out.println("order.calculatePrice = " + order.calculatePrice());
}

BeanFactory

/Users/gwnam/Library/Java/JavaVirtualMachines/openjdk-17.0.2/Contents/Home/bin/java -javaagent:/Applications/IntelliJ IDEA.app/Contents/lib/idea_rt.jar=59257:/Applications/IntelliJ IDEA.app/Contents/bin -Dfile.encoding=UTF-8 -classpath /Users/gwnam/Desktop/dev/IDE/core/out/production/classes:/Users/gwnam/Desktop/dev/IDE/core/out/production/resources:/Users/gwnam/.gradle/caches/modules-2/files-2.1/org.springframework.boot/spring-boot-starter/2.6.6/b34b4f0c1e99edc181ea40a6b1b79fa72658ee04/spring-boot-starter-2.6.6.jar:/Users/gwnam/.gradle/caches/modules-2/files-2.1/org.springframework.boot/spring-boot-autoconfigure/2.6.6/e5b575d304ec5c6746fef3cc594bbe89721d64c2/spring-boot-autoconfigure-2.6.6.jar:/Users/gwnam/.gradle/caches/modules-2/files-2.1/org.springframework.boot/spring-boot/2.6.6/286137a0c57d159f845faa1c2e8d0b7548391085/spring-boot-2.6.6.jar:/Users/gwnam/.gradle/caches/modules-2/files-2.1/org.springframework.boot/spring-boot-starter-logging/2.6.6/e37b388e2407a1b03f920a7fd0722e41745c92c6/spring-boot-starter-logging-2.6.6.jar:/Users/gwnam/.gradle/caches/modules-2/files-2.1/jakarta.annotation/jakarta.annotation-api/1.3.5/59eb84ee0d616332ff44aba065f3888cf002cd2d/jakarta.annotation-api-1.3.5.jar:/Users/gwnam/.gradle/caches/modules-2/files-2.1/org.springframework/spring-core/5.3.18/7ff3000f3342989cb011b6095a0e86f2e5176cef/spring-core-5.3.18.jar:/Users/gwnam/.gradle/caches/modules-2/files-2.1/org.yaml/snakeyaml/1.29/6d0cdafb2010f1297e574656551d7145240f6e25/snakeyaml-1.29.jar:/Users/gwnam/.gradle/caches/modules-2/files-2.1/org.springframework/spring-context/5.3.18/34f6683d9dbe6edb02ad9393df3d3211b5484622/spring-context-5.3.18.jar:/Users/gwnam/.gradle/caches/modules-2/files-2.1/ch.qos.logback/logback-classic/1.2.11/4741689214e9d1e8408b206506cbe76d1c6a7d60/logback-classic-1.2.11.jar:/Users/gwnam/.gradle/caches/modules-2/files-2.1/org.apache.logging.log4j/log4j-to-slf4j/2.17.2/17dd0fae2747d9a28c67bc9534108823d2376b46/log4j-to-slf4j-2.17.2.jar:/Users/gwnam/.gradle/caches/modules-2/files-2.1/org.slf4j/jul-to-slf4j/1.7.36/ed46d81cef9c412a88caef405b58f93a678ff2ca/jul-to-slf4j-1.7.36.jar:/Users/gwnam/.gradle/caches/modules-2/files-2.1/org.springframework/spring-jcl/5.3.18/7e5d8cd447981bc20d4c397a2ba0a1c65ff2267a/spring-jcl-5.3.18.jar:/Users/gwnam/.gradle/caches/modules-2/files-2.1/org.springframework/spring-aop/5.3.18/8e9cce60c60257ae1d5b3cd675ec3a8286ed1955/spring-aop-5.3.18.jar:/Users/gwnam/.gradle/caches/modules-2/files-2.1/org.springframework/spring-beans/5.3.18/3f0ea6598a5a1eae0a672f025a33a0b7e0d6dfd3/spring-beans-5.3.18.jar:/Users/gwnam/.gradle/caches/modules-2/files-2.1/org.springframework/spring-expression/5.3.18/61c51831e49a85fc5bf925253ca63f46fec6f013/spring-expression-5.3.18.jar:/Users/gwnam/.gradle/caches/modules-2/files-2.1/ch.qos.logback/logback-core/1.2.11/a01230df5ca5c34540cdaa3ad5efb012f1f1f792/logback-core-1.2.11.jar:/Users/gwnam/.gradle/caches/modules-2/files-2.1/org.slf4j/slf4j-api/1.7.36/6c62681a2f655b49963a5983b8b0950a6120ae14/slf4j-api-1.7.36.jar:/Users/gwnam/.gradle/caches/modules-2/files-2.1/org.apache.logging.log4j/log4j-api/2.17.2/f42d6afa111b4dec5d2aea0fe2197240749a4ea6/log4j-api-2.17.2.jar hello.core.OrderApp
23:05:23.832 [main] DEBUG org.springframework.context.annotation.AnnotationConfigApplicationContext - Refreshing org.springframework.context.annotation.AnnotationConfigApplicationContext@33e5ccce
23:05:23.840 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'org.springframework.context.annotation.internalConfigurationAnnotationProcessor'
23:05:23.890 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'org.springframework.context.event.internalEventListenerProcessor'
23:05:23.891 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'org.springframework.context.event.internalEventListenerFactory'
23:05:23.891 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'org.springframework.context.annotation.internalAutowiredAnnotationProcessor'
23:05:23.892 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'org.springframework.context.annotation.internalCommonAnnotationProcessor'
23:05:23.895 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'appConfig'
23:05:23.898 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'memberService'
23:05:23.905 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'memberRepository'
23:05:23.905 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'orderService'
23:05:23.906 [main] DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'discountPolicy'
order = Order{memberId=1, itemName='itemA', itemPrice=10000, discountPrice=1000}
order.calculatePrice = 9000

Process finished with exit code 0

그럼 여기서 의문점이 생기게 될 것입니다.

두가지 경우의 결과가 같은 이유는 무엇일까?

그 이유는 ApplicationContext 가 BeanFactory 를 상속 받았기 때문입니다.

public interface ApplicationContext extends EnvironmentCapable,  
ListableBeanFactory, HierarchicalBeanFactory, MessageSource, 
ApplicationEventPublisher, ResourcePatternResolver 
{ ... }

그렇다면 두 경우의 차이점은 무엇일까?

어플리케이션을 개발할때는 빈을 관리하고 조회하는 기능은 물론, 다른 추가적인 기능이 필요합니다.

BeanFactory에서 빈을 관리하는 기능을 물려받은 것은 맞습니다.
그리고 ApplicationContext는 국제화가 지원되는 텍스트 메시지 관리, 이미지같은 파일 자원을 로드, 리너스로 등록된 빈에게 이벤트 발생 알림 등 부가적인 기능을 갖고 있습니다.
그래서 스프링 컨테이너하면 주로 이 ApplicationContext를 뜻합니다.

하지만 빈 관리에서도 두 컨테이너가 동일한 기능을 하지는 않습니다.

BeanFactory는 처음으로 getBean() 메소드가 호출된 시점에서야 해당 빈을 생성하고,
ApplicationContext는 Context 초기화 시점에 모든 싱글톤 빈을 미리 로드한 후 애플리케이션 가동 후에는 빈을 지연 없이 받을 수 있습니다.

해당 이점 때문에 실제 개발에서는 ApplicationContext를 주로 사용합니다.