싱글톤 컨테이너

public class SingletonTest {
@Test
@DisplayName("스프링 없는 순수한 DI 컨테이너")
  
    void pureContainer() {
      
        AppConfig appConfig = new AppConfig();
        //1. 조회: 호출할 때 마다 객체를 생성
        MemberService memberService1 = appConfig.memberService();
      
        //2. 조회: 호출할 때 마다 객체를 생성
        MemberService memberService2 = appConfig.memberService();
      
        //참조값이 다른 것을 확인
        System.out.println("memberService1 = " + memberService1);
        System.out.println("memberService2 = " + memberService2);
      
        //memberService1 != memberService2
        assertThat(memberService1).isNotSameAs(memberService2);
    }
}

public class SingletonService {
  
    //1. static 영역에 객체를 딱 1개만 생성해둔다.
    private static final SingletonService instance = new SingletonService();
  
    //2. public으로 열어서 객체 인스터스가 필요하면 이 static 메서드를 통해서만 조회하도록 허용한다.
    public static SingletonService getInstance() {
      return instance;
    }
  
    //3. 생성자를 private으로 선언해서 외부에서 new 키워드를 사용한 객체 생성을 못하게 막는다.
    private SingletonService() {
    }
  
    public void logic() {
        System.out.println("싱글톤 객체 로직 호출");
    }
}

@Test
@DisplayName("싱글톤 패턴을 적용한 객체 사용")
public void singletonServiceTest() {
  
    //private으로 생성자를 막아두었다. 컴파일 오류가 발생한다.
    //new SingletonService();
  
    //1. 조회: 호출할 때 마다 같은 객체를 반환
    SingletonService singletonService1 = SingletonService.getInstance();
    //2. 조회: 호출할 때 마다 같은 객체를 반환
    SingletonService singletonService2 = SingletonService.getInstance();
  
    //참조값이 같은 것을 확인
    System.out.println("singletonService1 = " + singletonService1);
    System.out.println("singletonService2 = " + singletonService2);
  
    // singletonService1 == singletonService2
    assertThat(singletonService1).isSameAs(singletonService2);
    singletonService1.logic();
}

@Test
@DisplayName("스프링 컨테이너와 싱글톤")
void springContainer() {
  
    ApplicationContext ac = new AnnotationConfigApplicationContext(AppConfig.class);
  
    //1. 조회: 호출할 때 마다 같은 객체를 반환
    MemberService memberService1 = ac.getBean("memberService", MemberService.class);
  
    //2. 조회: 호출할 때 마다 같은 객체를 반환
    MemberService memberService2 = ac.getBean("memberService", MemberService.class);
  
    //참조값이 같은 것을 확인
    System.out.println("memberService1 = " + memberService1);
    System.out.println("memberService2 = " + memberService2);
  
    //memberService1 == memberService2
    assertThat(memberService1).isSameAs(memberService2);
}

public class StatefulService {
  
    private int price; //상태를 유지하는 필드
  
    public void order(String name, int price) {
        System.out.println("name = " + name + " price = " + price);
        this.price = price; //여기가 문제!
    }
  
    public int getPrice() {
        return price;
    }
}

public class StatefulServiceTest {
  
    @Test
    void statefulServiceSingleton() {
        ApplicationContext ac = new AnnotationConfigApplicationContext(TestConfig.class);
        StatefulService statefulService1 = ac.getBean("statefulService", StatefulService.class);
        StatefulService statefulService2 = ac.getBean("statefulService", StatefulService.class);
      
        //ThreadA: A사용자 10000원 주문
        statefulService1.order("userA", 10000);
        //ThreadB: B사용자 20000원 주문
        statefulService2.order("userB", 20000);
      
        //ThreadA: 사용자A 주문 금액 조회
        int price = statefulService1.getPrice();
        //ThreadA: 사용자A는 10000원을 기대했지만, 기대와 다르게 20000원 출력
        System.out.println("price = " + price);
          Assertions.assertThat(statefulService1.getPrice()).isEqualTo(20000);
    }
  
    static class TestConfig {
      
        @Bean
        public StatefulService statefulService() {
            return new StatefulService();
        }
    }
}

@Configuration
public class AppConfig {
  
    @Bean
    public MemberService memberService() {
        return new MemberServiceImpl(memberRepository());
    }
  
    @Bean
    public OrderService orderService() {
        return new OrderServiceImpl(memberRepository(), discountPolicy());
    }
  
    @Bean
    public MemberRepository memberRepository() {
        return new MemoryMemberRepository();
    }
    ...
}

public class MemberServiceImpl implements MemberService {
  
    private final MemberRepository memberRepository;
  
    //테스트 용도
    public MemberRepository getMemberRepository() {
        return memberRepository;
    }
}

public class OrderServiceImpl implements OrderService {
    private final MemberRepository memberRepository;
  
    //테스트 용도
    public MemberRepository getMemberRepository() {
        return memberRepository;
    }
}

public class ConfigurationSingletonTest {
  
    @Test
    void configurationTest() {
        ApplicationContext ac = new AnnotationConfigApplicationContext(AppConfig.class);
        MemberServiceImpl memberService = ac.getBean("memberService", MemberServiceImpl.class);
        OrderServiceImpl orderService = ac.getBean("orderService", OrderServiceImpl.class);
        MemberRepository memberRepository = ac.getBean("memberRepository", MemberRepository.class);
      
        //모두 같은 인스턴스를 참고하고 있다.
        System.out.println("memberService -> memberRepository = " + memberService.getMemberRepository());
        System.out.println("orderService -> memberRepository = " + orderService.getMemberRepository());
        System.out.println("memberRepository = " + memberRepository);
      
        //모두 같은 인스턴스를 참고하고 있다.
        assertThat(memberService.getMemberRepository()).isSameAs(memberRepository);
        assertThat(orderService.getMemberRepository()).isSameAs(memberRepository);
    }
}

@Configuration
public class AppConfig {
  
    @Bean
    public MemberService memberService() {
        //1번
        System.out.println("call AppConfig.memberService");
        return new MemberServiceImpl(memberRepository());
    }
  
    @Bean
    public OrderService orderService() {
        //1번
        System.out.println("call AppConfig.orderService");
        return new OrderServiceImpl(memberRepository(), discountPolicy());
    }
  
    @Bean
    public MemberRepository memberRepository() {
        //2번? 3번?
        System.out.println("call AppConfig.memberRepository");
        return new MemoryMemberRepository();
    }
  
    @Bean
    public DiscountPolicy discountPolicy() {
        return new RateDiscountPolicy();
    }
}

@Test
void configurationDeep() {
    ApplicationContext ac = new AnnotationConfigApplicationContext(AppConfig.class);
  
    //AppConfig도 스프링 빈으로 등록된다.
    AppConfig bean = ac.getBean(AppConfig.class);
  
    System.out.println("bean = " + bean.getClass());
    //출력: bean = class hello.core.AppConfig$$EnhancerBySpringCGLIB$$bd479d70
}

@Bean
public MemberRepository memberRepository() {
  
    if (memoryMemberRepository가 이미 스프링 컨테이너에 등록되어 있으면?) {
        return 스프링 컨테이너에서 찾아서 반환;
    } else { //스프링 컨테이너에 없으면
        기존 로직을 호출해서 MemoryMemberRepository를 생성하고 스프링 컨테이너에 등록
        return 반환
    }
}

//@Configuration 삭제
public class AppConfig {
}