Constructor injection (preferred) in Spring Boot - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is constructor injection in Spring Boot?

Constructor injection is a way to provide dependencies to a class by passing them through its constructor. Spring Boot automatically calls the constructor and injects the required beans.

intermediate

Why is constructor injection preferred over field injection in Spring Boot?

Constructor injection is preferred because it makes dependencies explicit, helps with immutability, supports easier testing, and avoids issues with uninitialized fields.

intermediate

How does Spring Boot know which constructor to use for injection?

Spring Boot uses the constructor with the most parameters that can be satisfied by beans in the context. If there is only one constructor, it uses that by default.

beginner

Show a simple example of constructor injection in a Spring Boot service class.

import org.springframework.stereotype.Service;

@Service
public class MyService {
  private final MyRepository repo;

  public MyService(MyRepository repo) {
    this.repo = repo;
  }

  // service methods
}

intermediate

What happens if a required dependency is missing when using constructor injection?

Spring Boot will fail to start and throw an error because it cannot find a bean to inject into the constructor parameter.

Which of these is a benefit of constructor injection in Spring Boot?

A Automatically injects private fields without constructor

B Allows setting dependencies after object creation

C Requires less code than field injection

D Dependencies are clearly required and immutable

Constructor injection makes dependencies explicit and supports immutability, unlike field injection.

How does Spring Boot choose which constructor to use for injection?

A The constructor with the most parameters that can be injected

B The constructor with no parameters

C The constructor annotated with @Autowired only

D It randomly picks one constructor

Spring Boot uses the constructor with the most parameters that can be satisfied by beans.

What happens if a dependency is missing for constructor injection?

A Spring Boot throws an error and fails to start

B Spring Boot ignores the missing dependency

C Spring Boot uses a default value

D Spring Boot logs a warning but continues

Missing dependencies cause Spring Boot to fail startup with an error.

Which annotation is required on the constructor for injection in Spring Boot 2.6+?

A @Component is required on constructor

B No annotation needed if only one constructor

C @Inject is mandatory

D @Autowired is always required

If there is only one constructor, Spring Boot injects dependencies without needing @Autowired.

Constructor injection helps with which of the following?

A Reducing constructor parameters

B Allowing circular dependencies easily

C Making dependencies immutable

D Injecting dependencies after object creation

Constructor injection supports immutability by setting dependencies once at creation.

Explain constructor injection in Spring Boot and why it is preferred over other injection methods.

Think about how dependencies are passed and why that helps code quality.

Describe what happens if Spring Boot cannot find a bean to inject into a constructor parameter.

Consider how Spring Boot handles required dependencies.

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