(스프링의 핵심 원리 – 기본) 싱글톤

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 다른것을 확인
        System.out.println("memberService1 = " + memberService1);
        System.out.println("memberService2 = " + memberService2);

        Assertions.assertThat(memberService1).isNotSameAs(memberService2);
    }
}

public class SingletonService {

    private static final SingletonService instance = new SingletonService();

    public static SingletonService getInstance(){
        return instance;
    }
    private  SingletonService(){
        
    }
    
    public void logic(){
        System.out.println("싱글톤 객체");
    }
}

@Test
@DisplayName("싱글톤 패턴을 사용")
void singletonServiceTest(){
    SingletonService instance1 = SingletonService.getInstance();
    SingletonService instance2 = SingletonService.getInstance();

    System.out.println("instance1 = " + instance1);
    System.out.println("instance2 = " + instance2);
    Assertions.assertThat(instance1).isSameAs(instance2);
    //same ==
    //equal equals메서드
}