Field injection and why to avoid it in Spring Boot - Mini Project: Build & Apply

Field Injection and Why to Avoid It in Spring Boot

📖 Scenario: You are building a simple Spring Boot service that sends notifications. You want to learn how to inject dependencies properly and understand why field injection is discouraged.

🎯 Goal: Create a Spring Boot service class that uses constructor injection instead of field injection to get a NotificationSender bean.

📋 What You'll Learn

Create a NotificationSender interface with a method send(String message).

Create a class EmailNotificationSender that implements NotificationSender.

Create a service class NotificationService that uses constructor injection to get NotificationSender.

Avoid using field injection (@Autowired on fields).

💡 Why This Matters

🌍 Real World

In real Spring Boot applications, managing dependencies clearly helps maintain and test code effectively.

💼 Career

Understanding dependency injection patterns is essential for writing clean, testable, and maintainable Spring Boot applications.

1

Create NotificationSender interface and EmailNotificationSender class

Create an interface called NotificationSender with a method void send(String message). Then create a class called EmailNotificationSender that implements NotificationSender and overrides the send method with an empty body.

Spring Boot

// Your code here

Need a hint?

Use public interface NotificationSender and public class EmailNotificationSender implements NotificationSender. Annotate EmailNotificationSender with @Component so Spring can find it.

2

Add NotificationService class with constructor injection

Create a class called NotificationService annotated with @Service. Add a private final field called notificationSender of type NotificationSender. Create a constructor that takes a NotificationSender parameter and assigns it to the field.

Spring Boot

public interface NotificationSender {
    void send(String message);
}

import org.springframework.stereotype.Component;

@Component
public class EmailNotificationSender implements NotificationSender {
    @Override
    public void send(String message) {
        // Sending email logic here
    }
}

// Your code here

Need a hint?

Use @Service annotation and create a constructor that takes NotificationSender and assigns it to the private final field.

3

Explain why to avoid field injection

Add a comment inside NotificationService above the field notificationSender explaining why field injection (using @Autowired on fields) is discouraged. Mention issues like difficulty in testing and hidden dependencies.

Spring Boot

public interface NotificationSender {
    void send(String message);
}

import org.springframework.stereotype.Component;

@Component
public class EmailNotificationSender implements NotificationSender {
    @Override
    public void send(String message) {
        // Sending email logic here
    }
}

import org.springframework.stereotype.Service;

@Service
public class NotificationService {
    // Add comment here explaining why to avoid field injection
    private final NotificationSender notificationSender;

    public NotificationService(NotificationSender notificationSender) {
        this.notificationSender = notificationSender;
    }
}

Need a hint?

Write a comment above the field explaining that field injection hides dependencies and makes testing harder, while constructor injection is better.

4

Use NotificationSender in NotificationService method

Add a public method notifyUser(String message) in NotificationService that calls send(message) on the notificationSender field.

Spring Boot

public interface NotificationSender {
    void send(String message);
}

import org.springframework.stereotype.Component;

@Component
public class EmailNotificationSender implements NotificationSender {
    @Override
    public void send(String message) {
        // Sending email logic here
    }
}

import org.springframework.stereotype.Service;

@Service
public class NotificationService {
    // Field injection is discouraged because it hides dependencies and makes testing harder.
    // Constructor injection makes dependencies explicit and easier to manage.
    private final NotificationSender notificationSender;

    public NotificationService(NotificationSender notificationSender) {
        this.notificationSender = notificationSender;
    }

    // Your code here
}

Need a hint?

Create a method notifyUser that calls send on the injected notificationSender.