의존성 주입 실습
애플리케이션 내 의존성을 관리하기 위해 `@Autowired`와 생성자 주입을 사용하는 방법을 학습합니다.
의존성 주입 실습은(는) CoddyKit의 무료 Spring Boot 4 Complete Guide 강의입니다. 이것은 4개 중 2번째 강의입니다. 아래에서 전체 강의를 무료로 읽을 수 있으며, 내장 코드 에디터와 24/7 AI 튜터와 함께 브라우저에서 직접 실습할 수 있습니다. 이 강의는 Spring Boot 4 Complete Guide 학습 경로의 일부이며, 진행 상황이 웹과 CoddyKit 앱에 동기화됩니다. Spring Boot 4 Complete Guide 강의에는 총 4개의 강의가 포함되어 있습니다.
이 강의의 일부는 아직 번역되지 않았으며 영어로 표시됩니다.
DI in Practice: Intro
Welcome to Dependency Injection in Practice! In Spring Boot, we don't manually create objects. Instead, Spring handles this for us, 'injecting' what an object needs.
This lesson focuses on the two primary ways to ask Spring to inject dependencies into your components: Field Injection and Constructor Injection.
What's a Dependency?
Think of dependencies as the 'ingredients' an object needs to do its job. For example, a CoffeeMaker might need a WaterHeater and a CoffeeBeanGrinder.
- Dependency: An object that another object relies on.
- Injection: Spring providing these required objects automatically.
This keeps your code cleaner and easier to test!
Field Injection with @Autowired
Field injection is the simplest way to tell Spring to inject a dependency. You place the @Autowired annotation directly above the field that needs the dependency.
Spring will then find a suitable bean (an object managed by Spring's IoC container) and assign it to that field.
Field Injection Example
Let's see field injection in action. Here, MyService needs an instance of MyComponent.
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;
@Component
class MyComponent {
public String greet() {
return "Hello from Component!";
}
}
@Component
class MyService {
@Autowired
private MyComponent component;
public void performAction() {
System.out.println(component.greet());
}
}
public class Main {
public static void main(String[] args) {
// In a real Spring app, Spring manages this.
// For this example, we simulate it.
MyComponent comp = new MyComponent();
MyService service = new MyService();
// Simulate injection (Spring does this automatically)
service.component = comp;
service.performAction();
}
}Field Injection: The Downsides
While easy, field injection has some drawbacks:
- Testing: It's harder to unit test components without a Spring context because you can't easily pass mock dependencies.
- Immutability: Fields can't be declared
final, making objects mutable after creation. - Tight Coupling: It obscures the object's dependencies, making it less clear what it needs.
Most Spring developers prefer an alternative...
Constructor Injection: The Preferred Way
Constructor injection is generally considered the best practice for dependency injection. Here, dependencies are provided as arguments to the class's constructor.
This makes dependencies explicit and ensures that an object is fully initialized and valid upon creation.
Constructor Injection Example
Notice how MyService now requires MyComponent as a constructor argument. Spring automatically provides it!
import org.springframework.stereotype.Component;
@Component
class MyComponent {
public String greet() {
return "Hello from Component!";
}
}
@Component
class MyService {
private final MyComponent component;
// Spring automatically injects MyComponent here
public MyService(MyComponent component) {
this.component = component;
}
public void performAction() {
System.out.println(component.greet());
}
}
public class Main {
public static void main(String[] args) {
// In a real Spring app, Spring manages this.
// For this example, we simulate it.
MyComponent comp = new MyComponent();
MyService service = new MyService(comp); // Manual injection for demo
service.performAction();
}
}@Autowired on Constructor?
When a class has only one constructor, Spring 4.3+ automatically treats it as a constructor for dependency injection. You don't need to add @Autowired explicitly.
If there are multiple constructors, you would use @Autowired on the specific constructor you want Spring to use for injection.
Which Injection Type to Choose?
Here's a quick guide:
- Constructor Injection: Recommended for most cases. Promotes immutability, easier testing, and clear dependencies.
- Field Injection: Avoid if possible. Can be used for optional dependencies or in very specific legacy scenarios.
- Setter Injection: Useful for optional dependencies that might change during the object's lifecycle. Less common than constructor injection.
Test Your Knowledge!
Which statement best describes the primary benefit of Constructor Injection over Field Injection in Spring Boot?
Recap: DI in Practice
You've learned about practical dependency injection in Spring Boot!
- Field Injection uses
@Autowireddirectly on fields, but has drawbacks. - Constructor Injection is the preferred method, making dependencies explicit and promoting immutability.
- Spring automatically handles constructor injection for single constructors.
Using the right injection type leads to more robust and testable applications. Next, we'll look at externalizing application properties!
자주 묻는 질문
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“의존성 주입 실습”에서 뭘 배우나요?
애플리케이션 내 의존성을 관리하기 위해 `@Autowired`와 생성자 주입을 사용하는 방법을 학습합니다. 브라우저에서 직접 실행하는 실습 코드로 Spring Boot 4 Complete Guide을(를) 배우며, 24/7 AI 튜터가 강의를 진행하면서 질문에 답변해줍니다.
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