Inversion of Control (IoC) & Dependency Injection

🔁 Inversion of Control (IoC)

Inversion of Control means giving control to the framework (like Spring) to manage object creation and dependencies — instead of writing that logic yourself.

📦 Without IoC:

Service service = new Service();
Controller controller = new Controller(service);

☝️ You're manually creating objects and passing dependencies.

✅ With IoC (Spring does this):

@Autowired
Controller controller;

🔄 Spring automatically:

  • Creates the object
  • Resolves and injects dependencies

🎯 This inversion of "who controls what" is called Inversion of Control.

🧩 Dependency Injection (DI)

Dependency Injection is a design pattern used to implement IoC.
It means: instead of a class creating its own dependencies, the framework injects them.

📖 Types of Dependency Injection in Spring:

Type Example
Constructor Preferred way

 

@Component
public class Controller {
    private final Service service;

    public Controller(Service service) {
        this.service = service;
    }
}

| Field Injection | Quick and easy (not recommended for testing) |

@Autowired
private Service service;

| Setter Injection | Used when dependency is optional |

@Autowired
public void setService(Service service) {
    this.service = service;
}

🧠 Summary

Concept Meaning
IoC Spring controls object creation and wiring
Dependency Injection Spring injects required objects into your class automatically

✅ Why It's Useful

  • 🔄 Reduces tight coupling
  • 🔧 Improves testability
  • 🧼 Cleaner code with less boilerplate
  • ♻️ Easy to swap components (e.g., switch DB layer)
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