How to Use Dependency Injection in Spring Boot: Simple Guide

In Spring Boot, use @Component or @Service to mark classes as beans and @Autowired to inject dependencies automatically. Spring Boot manages object creation and wiring, so you just declare dependencies in your classes.

Syntax

Dependency injection in Spring Boot uses annotations to declare beans and inject them where needed.

• @Component or @Service: Marks a class as a Spring-managed bean.
• @Autowired: Injects the required bean into a class field, constructor, or setter.
• Constructor injection is preferred for mandatory dependencies.

java

import org.springframework.stereotype.Component;
import org.springframework.beans.factory.annotation.Autowired;

@Component
public class MyService {
    public String serve() {
        return "Service is running";
    }
}

@Component
public class MyController {
    private final MyService myService;

    @Autowired
    public MyController(MyService myService) {
        this.myService = myService;
    }

    public String process() {
        return myService.serve();
    }
}

Example

This example shows a Spring Boot application where MyService is injected into MyController using constructor injection. Spring Boot automatically creates and wires these beans.

java

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.stereotype.Component;
import org.springframework.beans.factory.annotation.Autowired;

@SpringBootApplication
public class DemoApplication {
    public static void main(String[] args) {
        var context = SpringApplication.run(DemoApplication.class, args);
        MyController controller = context.getBean(MyController.class);
        System.out.println(controller.process());
    }
}

@Component
class MyService {
    public String serve() {
        return "Service is running";
    }
}

@Component
class MyController {
    private final MyService myService;

    @Autowired
    public MyController(MyService myService) {
        this.myService = myService;
    }

    public String process() {
        return myService.serve();
    }
}

Output

Service is running

Common Pitfalls

Common mistakes include:

• Not annotating classes with @Component or similar, so Spring does not manage them.
• Using field injection instead of constructor injection, which is less testable and harder to maintain.
• Forgetting to enable component scanning or placing classes outside the scanned packages.

java

/* Wrong: Field injection (not recommended) */
@Component
class WrongController {
    @Autowired
    private MyService myService; // Harder to test and maintain
}

/* Right: Constructor injection (recommended) */
@Component
class RightController {
    private final MyService myService;

    @Autowired
    public RightController(MyService myService) {
        this.myService = myService;
    }
}

Quick Reference

Remember these key points for dependency injection in Spring Boot:

• Use @Component, @Service, or @Repository to declare beans.
• Prefer constructor injection with @Autowired for mandatory dependencies.
• Spring Boot auto-scans packages under the main application class by default.
• Use @Qualifier if multiple beans of the same type exist.

Key Takeaways

Annotate classes with @Component or @Service to make them Spring beans.

Use constructor injection with @Autowired for better testability and clarity.

Spring Boot automatically scans and wires beans in the main package and subpackages.

Avoid field injection as it reduces code maintainability and testability.

Use @Qualifier to resolve conflicts when multiple beans of the same type exist.