@Qualifier for ambiguous beans in Spring Boot - Mini Project: Build & Apply

Using @Qualifier to Resolve Ambiguous Beans in Spring Boot

📖 Scenario: You are building a simple Spring Boot application that sends notifications. You have two different notification services: one for email and one for SMS. Both implement the same interface, so Spring does not know which one to use automatically.

🎯 Goal: Learn how to use the @Qualifier annotation to tell Spring Boot exactly which bean to use when there are multiple candidates.

📋 What You'll Learn

Create two service classes implementing the same interface

Create a configuration or component class to inject the correct service using @Qualifier

Use @Qualifier annotation to specify which bean to inject

Run the application to verify the correct service is used

💡 Why This Matters

🌍 Real World

In real applications, you often have multiple implementations of the same interface, like different payment gateways or notification channels. Using @Qualifier helps you pick the right one.

💼 Career

Understanding @Qualifier is essential for Spring developers to manage dependency injection clearly and avoid errors when multiple beans exist.

1

Create NotificationService interface and two implementations

Create an interface called NotificationService with a method sendNotification(String message). Then create two classes: EmailNotificationService and SmsNotificationService that implement NotificationService. Each class should have the @Service annotation and implement the sendNotification method with a simple print statement indicating which service is sending the message.

Spring Boot

// Your code here to create NotificationService interface and two @Service classes EmailNotificationService and SmsNotificationService

Need a hint?

Remember to add @Service annotation on both implementation classes so Spring can detect them as beans.

2

Create a NotificationController with a NotificationService field

Create a class called NotificationController annotated with @RestController. Add a private field of type NotificationService called notificationService. Add a constructor that takes a NotificationService parameter and assigns it to the field.

Spring Boot

import org.springframework.stereotype.Service;

public interface NotificationService {
    void sendNotification(String message);
}

@Service
public class EmailNotificationService implements NotificationService {
    @Override
    public void sendNotification(String message) {
        System.out.println("Email sent: " + message);
    }
}

@Service
public class SmsNotificationService implements NotificationService {
    @Override
    public void sendNotification(String message) {
        System.out.println("SMS sent: " + message);
    }
}

// Create NotificationController class with NotificationService field and constructor

Need a hint?

Use constructor injection to assign the notificationService field.

3

Use @Qualifier to specify which NotificationService to inject

Modify the constructor of NotificationController to use @Qualifier("emailNotificationService") on the NotificationService parameter. This tells Spring to inject the EmailNotificationService bean specifically.

Spring Boot

import org.springframework.stereotype.Service;

public interface NotificationService {
    void sendNotification(String message);
}

@Service
public class EmailNotificationService implements NotificationService {
    @Override
    public void sendNotification(String message) {
        System.out.println("Email sent: " + message);
    }
}

@Service
public class SmsNotificationService implements NotificationService {
    @Override
    public void sendNotification(String message) {
        System.out.println("SMS sent: " + message);
    }
}

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

@RestController
public class NotificationController {
    private final NotificationService notificationService;

    // Add @Qualifier annotation to constructor parameter
    public NotificationController(@Qualifier("emailNotificationService") NotificationService notificationService) {
        this.notificationService = notificationService;
    }
}

Need a hint?

Use the exact bean name emailNotificationService in the @Qualifier annotation.

4

Add a REST endpoint to send a notification

In NotificationController, add a method sendEmailNotification annotated with @GetMapping("/send"). This method should call notificationService.sendNotification("Hello from Spring Boot!") and return a simple string like "Notification sent".

Spring Boot

import org.springframework.stereotype.Service;

public interface NotificationService {
    void sendNotification(String message);
}

@Service
public class EmailNotificationService implements NotificationService {
    @Override
    public void sendNotification(String message) {
        System.out.println("Email sent: " + message);
    }
}

@Service
public class SmsNotificationService implements NotificationService {
    @Override
    public void sendNotification(String message) {
        System.out.println("SMS sent: " + message);
    }
}

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

@RestController
public class NotificationController {
    private final NotificationService notificationService;

    public NotificationController(@Qualifier("emailNotificationService") NotificationService notificationService) {
        this.notificationService = notificationService;
    }

    // Add a GET endpoint /send that calls sendNotification and returns a string
}

Need a hint?

Use @GetMapping("/send") to create a simple GET endpoint.