[Spring] IoC 가 그래서 뭐지?

March 16, 2023 2 분 소요

What is IOC?

Well, it’s simply the design process of externalizing the construction and management of your objects.

So in a nutshell, it basically says that your application is gonna outsource the creation and management of the objects, and that outsourcing will be handled by a object factory.

And that’s the big idea of Inversion of Control.

요약 하자면 IoC 는 디자인 프로세서고 변경이 자유롭다.

Spring Container

Primary functions
- Create and manage objects (Inversion of Control)
- Inject object dependencies(Dependency Injection)

Cofiguring Spring Container

XML configuration file (legacy)
Java Annotations (modern)
Java Source Code (modern)

Dependency Injection

The dependency inversion principle
The client delegates to another object
- The responsibility of providing its dependencies

Dependency inversion Injection

그림으로 설명하면 이런 개념이다. 내가 직접 자동차를 생산하지는 않는다.

You just outsorcethe construction and injection of your object to an external entity. In this case, That’s the car factory. They inject the engine, tires and so on for you. You don’t have to build the car.

Demo Example

Coach will provide daily workouts
The DemoController wants to use a Coach
- New helper: Coach
- This is a dependency
Need to inject this dependency

Injection Types

There are multiple types of injection with Spring
You will cover the two recommended types of injection
- Constructor Injection
- Setter Injection

Which one to use?

Constructor Injection
- Use this when you have required dependencies
- Generally recommended by the spring.io development team as first choice
Setter Injection
- Use this when you have optional dependencies
- If dependency is not provided, your app can provide reasonable default logic

What is Spring AutoWiring

For dependency injection, Spring can use autowiring
Spring will look for a class that matches
- matches by type : class or interface
Spring will inject it automatically .. hence it is autowired

Autowiring Example

Injecting a Coach implementation
Spring will scan for @Components
Any one implements the Coach interface??
If so, let’s inject them. For example: Cricket Coach

Example Application

Development Process - Constructor Injection

Define the dependency interface and class
Create Demo REST Controller
Create a constructor in your class for injections
Add @GetMapping for /dailyworkout

Step 1: Define the dependency interface and class

File: Coach.java

package com.luv2code.springcoredemo;

public interface Coach {
    String getDailyWorkout();
}

File: CricketCoach.java

package com.luv2code.springcoredemo;

import org.springframework.sterotype.Component;

@Component // <- marks as a Spring Bean
public class CricketCoach implements Coach {

    @Override
    public String getDailyWorkout() {
        return "Practice fast bowling for 15 min";
}
}

@Component annotation

@Component annotation marks the class as Spring Bean
- A Spring bean is just a regular Java class that is managed by Spring
@Component also makes the bean available for dependency injection

Step 2: Create Demo REST Controller

File: DemoController.java

package com.luv2code.springcoredemo;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class DemoController {

}

Step 3: Create a constructor in your class for injections

File: DemoController.java

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class DemoController {
    private Coach myCoach;

    @Autowired
    public DemoController (Coach theCoach) {
        myCoach = theCoach;
}
}

@Autowired annotation tells Spring to inject a dependency
if you only have one constructor then @Autowired on constructor is optional

Step 4: Add @GetMapping for /dailyworkout

File: DemoController.java

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.RestController;
import org.springframework.web.bind.annotation.GetMapping;

@RestController
public class DemoController {
    private Coach myCoach;

    @Autowired
    public DemoController (Coach theCoach) {
        myCoach = theCoach;
}

    @GetMapping
    public String getDailyWorkout() {
        return myCoach.getDailyWorkout();
}

}

참고로 no usages 는 무시해도 된다.

왜냐하면 스프링 내부에서 알아서 돌아가는데 IDE는 인식하지 못하는 경우가 있다.

이런 경우에는 무시하면 된다.

출처 유데미 & https://www.baeldung.com/inversion-control-and-dependency-injection-in-spring

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