4 ways to inject dependency and Lombok

✅ 4 Ways to inject dependency

• constructor
• setter
• field
• method

💡Constructor dependency injection

• 👍🏻 recommended
• Injected at the moment when object is created with constuctor
• 👍🏻 only called once, never can be changed again
• dependency cannot be changed at runtime
• 👍🏻 used for final, immutable
• dependency is required
• @Autowired can be ommited if there is only one constuctor

@Component
public class OrderServiceImpl implements OrderService {
    private final MemberRepository repository; //final, immutable
    private final DiscountPolicy discountPolicy;

    @Autowired //constuctor dependency injection
    public OrderServiceImpl(MemberRepository repository, DiscountPolicy  discountPolicy) {
        this.repository = repository;
        this.discountPolicy = discountPolicy;
    }

💡 Setter dependency injection

• when injecting dependency should be changeable
• used for mutable, more flexible
• dependency is not necessarily required

@Component
public class OrderServiceImpl implements OrderService {
    private  MemberRepository repository; //mutable
    private  DiscountPolicy discountPolicy;

    @Autowired
    public void setRepository(MemberRepository repository) {
        this.repository = repository;
    }

    @Autowired(required = false) //dependency is not necessarily required
    public void setDiscountPolicy(DiscountPolicy discountPolicy) {
        this.discountPolicy = discountPolicy;
    }

💡 Field dependency injection

• ⚠️ not recommended
• dependency injected directed into class field
• code looks very simple
• 👎🏻 however, field cannot be changed from outside this class
• 👎🏻 difficult to write test code
• 👎🏻 cannot use final

@Component
public class OrderServiceImpl implements OrderService {

    @Autowired private  MemberRepository repository;
    @Autowired private  DiscountPolicy discountPolicy;
}

💡 Method dependency injection

• ⚠️ not used frequently
• dependency is passed as parameter
• used when dependency is only needed temporarily

✅ Lombok

• annotation을 통해서 자동으로 만들어준다.

• constructor, getter, setter, toString 등를 자동으로 만들어준다

• 👎🏻 DI with constructor before using Lombok

@Component
public class OrderServiceImpl implements OrderService {
    private final MemberRepository repository;
    private final DiscountPolicy discountPolicy;

    //needed to code constructor
    public OrderServiceImpl(MemberRepository repository, DiscountPolicy discountPolicy) {
        this.repository = repository;
        this.discountPolicy = discountPolicy;
    }
}

• 👍🏻 DI with constructor with Lombok
• @RequiredArgsConstructor: create constructor with final fields

@Component
@RequiredArgsConstructor //lombok creates constructor
public class OrderServiceImpl implements OrderService {
    private final MemberRepository repository;
    private final DiscountPolicy discountPolicy;
}

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