Why Constructor injection (preferred) in Spring Boot? - Purpose & Use Cases

The Big Idea

Discover how a simple constructor can save you hours of debugging and messy code!

The Scenario

Imagine building a Spring Boot app where you manually create and connect all your service objects inside your code.

You have to write extra code to create each dependency and pass it around everywhere.

The Problem

Manually creating and wiring dependencies is slow and error-prone.

You might forget to create a needed object or pass the wrong one, causing bugs that are hard to find.

It also makes your code messy and hard to test.

The Solution

Constructor injection lets Spring automatically provide the needed objects when creating your classes.

This means your classes clearly state what they need, and Spring handles the rest.

It makes your code cleaner, safer, and easier to test.

Before vs After

✗ Before

public class UserService {
  private EmailService emailService;
  public UserService() {
    this.emailService = new EmailService();
  }
}

✓ After

public class UserService {
  private final EmailService emailService;
  public UserService(EmailService emailService) {
    this.emailService = emailService;
  }
}

What It Enables

It enables clear, reliable, and testable code where dependencies are managed automatically.

Real Life Example

When building a web app, constructor injection ensures your UserService always gets the right EmailService without extra setup code.

Key Takeaways

Manual dependency setup is error-prone and messy.

Constructor injection lets Spring provide dependencies automatically.

This leads to cleaner, safer, and easier-to-test code.

Practice

1. What is the main advantage of using constructor injection in Spring Boot?

easy

A. It automatically creates objects without any code.

B. It requires less code than field injection.

C. It allows changing dependencies after the object is created.

D. It makes dependencies explicit and helps with testing.

Solution

1. Step 1: Understand constructor injection purpose

   Constructor injection passes dependencies when creating the object, making them clear and fixed.

2. Step 2: Benefits of explicit dependencies

   This clarity helps testing because dependencies can be easily replaced with mocks.

3. Final Answer:

   It makes dependencies explicit and helps with testing. -> Option D

4. Quick Check:

   Constructor injection = explicit dependencies [OK]

Hint: Constructor injection shows dependencies clearly [OK]

Common Mistakes:

• Thinking constructor injection creates objects automatically
• Confusing constructor injection with field injection
• Believing dependencies can be changed after creation

2. Which of the following is the correct syntax for constructor injection in a Spring Boot component?

easy

A. public class Service { private final Repo repo; public Service(Repo repo) { this.repo = repo; } }

B. public class Service { @Autowired private Repo repo; }

C. public class Service { private Repo repo; public void setRepo(Repo repo) { this.repo = repo; } }

D. public class Service { private Repo repo = new Repo(); }

Solution

1. Step 1: Identify constructor injection pattern

   Constructor injection requires a constructor that takes dependencies as parameters and assigns them.

2. Step 2: Check each option

   public class Service { private final Repo repo; public Service(Repo repo) { this.repo = repo; } } shows a constructor with a parameter and assigns it to a final field, which is correct.

3. Final Answer:

   public class Service { private final Repo repo; public Service(Repo repo) { this.repo = repo; } } -> Option A

4. Quick Check:

   Constructor with parameters = constructor injection [OK]

Hint: Constructor injection needs constructor with parameters [OK]

Common Mistakes:

• Using @Autowired on fields instead of constructor
• Using setter methods instead of constructor
• Creating new dependency instances inside the class

3. Given the code below, what will be the output when the printMessage method is called?

\@Component
public class Printer {
  private final MessageService service;
  public Printer(MessageService service) {
    this.service = service;
  }
  public void printMessage() {
    System.out.println(service.getMessage());
  }
}

\@Component
public class MessageService {
  public String getMessage() {
    return "Hello Spring!";
  }
}

medium

A. null

B. Hello Spring!

C. Compilation error

D. Runtime NullPointerException

Solution

1. Step 1: Understand constructor injection wiring

   Spring injects MessageService into Printer via constructor, so service is not null.

2. Step 2: Check method output

   Calling service.getMessage() returns "Hello Spring!", which is printed.

3. Final Answer:

   Hello Spring! -> Option B

4. Quick Check:

   Injected service returns message [OK]

Hint: Constructor injection ensures dependencies are not null [OK]

Common Mistakes:

• Assuming service is null without injection
• Confusing field injection with constructor injection
• Expecting compilation error due to missing annotations

4. What is wrong with the following Spring Boot component using constructor injection?

\@Component
public class UserService {
  private final UserRepository repo;
  public UserService() {
    this.repo = null;
  }
}

medium

A. Constructor does not accept UserRepository, so dependency is not injected.

B. Field repo should not be final.

C. Missing @Autowired annotation on the constructor.

D. UserRepository should be instantiated inside the constructor.

Solution

1. Step 1: Check constructor parameters for injection

   The constructor has no parameters, so Spring cannot inject UserRepository.

2. Step 2: Understand consequence of no injection

   repo is assigned null manually, so dependency is missing and will cause errors later.

3. Final Answer:

   Constructor does not accept UserRepository, so dependency is not injected. -> Option A

4. Quick Check:

   Constructor must have dependency parameters [OK]

Hint: Constructor must accept dependencies as parameters [OK]

Common Mistakes:

• Forgetting to add parameters to constructor
• Thinking @Autowired is required on constructor in Spring Boot
• Instantiating dependencies manually inside constructor

5. You have a Spring Boot service that depends on two other services: PaymentService and NotificationService. How should you write the constructor to follow best practices for constructor injection?

hard

A. public OrderService(PaymentService paymentService) { this.paymentService = paymentService; }

B. public OrderService() { this.paymentService = new PaymentService(); this.notificationService = new NotificationService(); }

C. public OrderService(PaymentService paymentService, NotificationService notificationService) { this.paymentService = paymentService; this.notificationService = notificationService; }

D. public OrderService(@Autowired PaymentService paymentService, NotificationService notificationService) { this.paymentService = paymentService; this.notificationService = notificationService; }

Solution

1. Step 1: Identify correct constructor injection with multiple dependencies

   The constructor should accept all required dependencies as parameters and assign them.

2. Step 2: Check options for best practice

   public OrderService(PaymentService paymentService, NotificationService notificationService) { this.paymentService = paymentService; this.notificationService = notificationService; } correctly accepts both services and assigns them, following best practice.

3. Final Answer:

   public OrderService(PaymentService paymentService, NotificationService notificationService) { this.paymentService = paymentService; this.notificationService = notificationService; } -> Option C

4. Quick Check:

   Constructor with all dependencies = best practice [OK]

Hint: Include all dependencies as constructor parameters [OK]

Common Mistakes:

• Instantiating dependencies inside constructor
• Missing one of the dependencies in constructor
• Using @Autowired on constructor parameters unnecessarily