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I know that using Dependency Injection/Service locator design pattern/etc. we are achieving Inversion of control. but where does the Dependency Inversion principle fit into? or it is completely separate than these 2?

  1. Do we achieve IOC using the DIP as well?
  2. Does DIP somehow relate to IOC or DI?
Girish
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1 Answers1

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For example, if Class A has a dependency on Class B, refactoring this via dependency inversion would result in Class A depending on Interface B which is implemented by Class B.

  • Before refactoring: Class A ---> Class B
  • After refactoring: Class A ---> Interface B <--- Class B

In a nutshell, the dependency inversion principle states that both your high level abstractions (in example, Class A) and your low level abstractions (in example, Class B) should not depend on something concrete (as Class A depended on Class B before refactoring), but be decoupled by a middle abstract layer (the Interface B abstraction).

Dependency inversion is used as a means of implementing dependency injection. For example, in Spring, 

interface AbstractB {

}

@Component
class ConcreteB implements AbstractB {

}

@Component 
class ConcreteA {

    @Autowired
    // note reference type is AbstractB
    private AbstractB concreteB;

}
  • By depending on a reference of type AbstractB (instead of ConcreteB) in ConcreteA, we are applying dependency inversion.
  • By using @Autowired and letting Spring inject that dependency for us (instead of instantiating it ourselves with the 'new' keyword) we are using dependency injection.
  • Inversion of control is the principle of allowing an external framework (in this example, the Spring IoC container) to instantiate your dependencies and inject them into your code.

Regarding 'Do we achieve IOC using the DIP as well?', in my opinion, not necessarily, since we could autowire with a Reference of type ConcreteB, achieving IoC and DI but violating DIP.

Does this answer your question?

  • Could you also talk about IOC in your example? – Girish Sep 17 '18 at 16:43
  • https://www.codeproject.com/Articles/615139/An-Absolute-Beginners-Tutorial-on-Dependency-Inver This article describes IOC is a way to implement DIP. Please share your view. – Girish Sep 17 '18 at 16:53
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    Indeed, IoC is a way to implement DIP, but IoC by itself is neither sufficient nor necessary to enforce DIP (you can implement IoC and violate DIP, and you could also implement DIP without relying on IoC). –  Sep 17 '18 at 17:28
  • How would you implement DIP without using IoC? – Al Xx Dec 21 '22 at 13:34