생성자 주입을 선택해라!

과거에는 수정자 주입, 필드 주입을 많이 사용했다.
최근에는 스프링을 포함한 DI 프레임워크 대부분이 생성자 주입을 권장한다. 3가지 이유가 있다.


1. 불변
· 대부분의 의존관계 주입은 1번 일어나면 애플리케이션 종료시점까지 의존관계를 변경할 일이 없다.
오히려 종료시점까지 변하면 안됨
· 수정자 주입 사용시, setXxx 메서드를 public 으로 열어서 실수로 변경할 위험이 있다.
변경하면 안되는 메서드를 열어두는 것은 좋은 설계 방법 아님
· 생성자 주입 사용시, 객체 생성시 딱 1번만 호출되므로 이후에 노출되는 일이 없기 때문에 불변하게 설계가 가능하다.


2. 누락
프레임워크 없이 순수 자바 코드를 단위 테스트할 때 수정자 의존관계인 경우)
@Autowired 가 프레임워크 안에서 동작할 때 의존관계 없으면 오류가 발생하지만, 프레임워크 없이 순수 자바 코드로만 단위 테스트를 수행한다.

- src/main/java/hello/core/order/OrderServiceImpl.java

@Component
public class OrderServiceImpl implements OrderService {

    private MemberRepository memberRepository;
    private DiscountPolicy discountPolicy;

    @Autowired
    public void setMemberRepository(MemberRepository memberRepository) {
        this.memberRepository = memberRepository;
    }

    @Autowired
    public void setDiscountPolicy(DiscountPolicy discountPolicy) {
        this.discountPolicy = discountPolicy;
    }

}

- src/test/java/hello/core/order/OrderServiceImplTest.java

package hello.core.order;

import org.junit.jupiter.api.Test;

public class OrderServiceImplTest {

    @Test
    void createOrder() {
        OrderServiceImpl orderService = new OrderServiceImpl();
        orderService.createOrder(1L, "itemA", 10000);
    }

}

- 콘솔

Cannot invoke "hello.core.member.MemberRepository.findById(java.lang.Long)" because "this.memberRepository" is null
java.lang.NullPointerException: Cannot invoke "hello.core.member.MemberRepository.findById(java.lang.Long)" because "this.memberRepository" is null
	at hello.core.order.OrderServiceImpl.createOrder(OrderServiceImpl.java:30)
	at hello.core.order.OrderServiceImplTest.createOrder(OrderServiceImplTest.java:10)

* 순수 자바 코드로 하는 단위 테스트가 좋은 테스트이다.

테스트가 실행은 되지만, 실행 결과는 NPE(Null Point Exception)가 발생한다. memberRepository, discountPolicy 모두 의존관계 주입이 누락되었기 때문이다.

생성자 주입 사용시, 주입 데이터를 누락했을 때 컴파일 오류가 발생한다.
테스트 코드에서 컴파일 오류가 발생하기 때문에, IDE에서 어떤 값을 필수로 주입해야 하는지 알 수 있다.

- src/test/java/hello/core/order/OrderServiceImplTest.java

package hello.core.order;

import hello.core.discount.FixDiscountPolicy;
import hello.core.member.Grade;
import hello.core.member.Member;
import hello.core.member.MemoryMemberRepository;
import org.junit.jupiter.api.Test;

import static org.assertj.core.api.Assertions.*;

public class OrderServiceImplTest {

    @Test
    void createOrder() {
        MemoryMemberRepository memberRepository = new MemoryMemberRepository();
        memberRepository.save(new Member(1L, "name", Grade.VIP));

        OrderServiceImpl orderService = new OrderServiceImpl(memberRepository, new FixDiscountPolicy());
        Order order = orderService.createOrder(1L, "itemA", 10000);

        assertThat(order.getDiscountPrice()).isEqualTo(1000);
    }

}

3. final 키워드
생성자 주입 사용시, 필드에 final 키워드를 사용할 수 있다. 따라서 생성자에서 혹시라도 값이 설정되지 않는 오류를 컴파일 시점에 막아준다.

* 참고
생성자 주입 이외 나머지 주입 방식은 모두 생성자 이후에 호출되므로, 필드에 final 키워드 사용이 불가능하다.

- src/main/java/hello/core/order/OrderServiceImpl.java

@Component
public class OrderServiceImpl implements OrderService {

    private final MemberRepository memberRepository;
    private final DiscountPolicy discountPolicy;

    @Autowired
    public OrderServiceImpl(MemberRepository memberRepository, DiscountPolicy discountPolicy) {
        this.memberRepository = memberRepository;
        // this.discountPolicy = discountPolicy;
    }

}

생성자 내부에 discountPolicy 값을 설정하는 부분을 누락시키면, 자바는 컴파일 시점에 오류를 발생시킨다.
오류 내용 : error: variable discountPolicy might not have been initialized

* 컴파일 오류는 세상에서 가장 빠르고 좋은 오류다!


4. 정리
· 생성자 주입 방식은 프레임워크에 의존하지 않고 순수 자바 언어의 특징을 잘 살리는 방법
· 항상 기본으로 생성자 주입을 사용하고, 필수 값이 아닌 경우 수정자 주입 방식을 옵션으로 사용한다. 둘이 동시에 사용 가능하다.
필드 주입은 사용하지 않는 것을 권장한다.


참고링크 : https://www.inflearn.com/course/%EC%8A%A4%ED%94%84%EB%A7%81-%ED%95%B5%EC%8B%AC-%EC%9B%90%EB%A6%AC-%EA%B8%B0%EB%B3%B8%ED%8E%B8?cid=325969

스프링 핵심 원리 - 기본편| 김영한 - 인프런 강의