IoC, DI

✅ Inversion of Control

• control flow is inversed from programmer to ➡️ external source(ex: framework)

• Framework will call my code, not the programmer

• 👎🏻 Before IoC
• programmer explicitly codes what class will be used in Service client code
• Service client code knows what concrete class it will use

public class OrderServiceImpl implements OrderService {

//  private final DiscountPolicy discountPolicy = new FixDiscountPolicy();
    private final DiscountPolicy discountPolicy = new RateDiscountPolicy();
}

• 👍🏻 After IoC
• Service client code does not know what concrete class will be used
• controlled by AppConfig
• control flow is inversed to AppConfig

public class OrderServiceImpl implements OrderService {
    private final DiscountPolicy discountPolicy; //no concrete class, only interface
}

public class AppConfig {

    public OrderService orderService(){
        return new OrderServiceImpl(new FixDiscountPolicy());
    }
}

✅ Dependency Injection

Spring automatically gives one bean to another bean that needs it

• 👍🏻 OCP: can refrain from modifying service code, only has to modify config file

• 👍🏻 instead of creating object manually, you let Spring inject it

• ✔️ static DI: can see DI even without running code

  • extends, implements
  • import

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

  • OrderServiceImpl depends on OrderService, MemberRepository, DiscountPolicy
  • MemberRepository, DiscountPolicy is injected into OrderServiceImpl

• ✔️ dynamic DI: check DI when code is run

  public class AppConfig {
  
  public OrderService orderService(){
      return new OrderServiceImpl(new MemoryMemberRepository(), new RateDiscountPolicy());
  }
  

  • when AppConfig is run and orderService is created, return conrete class new OrderServiceImpl
  • when OrderServiceImpl is created, create conrete class MemoryMemberRepository and RateDiscountPolicy

✅ IoC, DI container

• tool that helps create object and handle, connect dependencies
• tool to help IoC and DI
  • ex) AppConfig is an example of IoC, DI container
  • AppConfig handles DI for OrderService

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