Field injection and why to avoid it in Spring Boot - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to inject a dependency using field injection.

Spring Boot

public class UserService {
    @Autowired
    private [1] userRepository;
}

Drag options to blanks, or click blank then click option'

AAutowired

BuserRepository

CUserService

DUserRepository

Attempts:

3 left

💡 Hint

Common Mistakes

Using the variable name instead of the type.

Using the class itself instead of the dependency type.

The field type must be the class or interface to inject, here UserRepository.

2fill in blank

medium

Complete the code to inject a dependency using constructor injection instead of field injection.

Spring Boot

public class UserService {
    private final UserRepository userRepository;

    public UserService([1] userRepository) {
        this.userRepository = userRepository;
    }
}

Drag options to blanks, or click blank then click option'

AUserService

BUserRepository

CAutowired

DService

Attempts:

3 left

💡 Hint

Common Mistakes

Using the class name of the service instead of the dependency.

Forgetting to mark the constructor with @Autowired (optional in Spring Boot).

The constructor parameter type must match the dependency type UserRepository to be injected.

3fill in blank

hard

Fix the error in this field injection example by choosing the correct annotation to avoid issues.

Spring Boot

public class UserService {
    [1]
    private UserRepository userRepository;
}

Drag options to blanks, or click blank then click option'

A@Component

B@Inject

C@Autowired

D@Service

Attempts:

3 left

💡 Hint

Common Mistakes

Using @Component or @Service on fields instead of classes.

Using @Inject without proper configuration.

@Autowired is the Spring annotation for field injection. Using it properly is required for Spring to inject the dependency.

4fill in blank

hard

Fill both blanks to rewrite field injection as constructor injection to improve testability and immutability.

Spring Boot

public class UserService {
    private final UserRepository userRepository;

    public UserService([1] userRepository) {
        this.userRepository = [2];
    }
}

Drag options to blanks, or click blank then click option'

AUserRepository

Bthis.userRepository

CuserRepository

DUserService

Attempts:

3 left

💡 Hint

Common Mistakes

Assigning the field to itself instead of the parameter.

Using wrong parameter type.

The constructor parameter type is UserRepository, and the assignment uses the parameter userRepository.

5fill in blank

hard

Fill all three blanks to create a Spring component with constructor injection and avoid field injection pitfalls.

Spring Boot

@[1]
public class OrderService {
    private final OrderRepository orderRepository;

    public OrderService([2] orderRepository) {
        this.orderRepository = [3];
    }
}

Drag options to blanks, or click blank then click option'

AService

BOrderRepository

CorderRepository

DComponent

Attempts:

3 left

💡 Hint

Common Mistakes

Using @Component instead of @Service for service classes.

Mixing up parameter and field names in assignment.

@Service marks the class as a Spring service component. The constructor parameter type is OrderRepository, and the assignment uses the parameter orderRepository.

Practice

1. What is a main reason to avoid using @Autowired directly on fields in Spring Boot?

easy

A. It disables Spring Boot auto-configuration

B. It causes syntax errors in the code

C. It requires manual object creation

D. It hides dependencies and makes testing harder

Solution

1. Step 1: Understand field injection behavior

   Field injection uses @Autowired on fields, which hides the dependencies inside the class without showing them in the constructor.

2. Step 2: Analyze impact on testing

   Because dependencies are hidden, it becomes harder to create test instances with mocks or stubs, reducing testability.

3. Final Answer:

   It hides dependencies and makes testing harder -> Option D

4. Quick Check:

   Hidden dependencies = avoid field injection [OK]

Hint: Hidden dependencies make testing tough, avoid field injection [OK]

Common Mistakes:

• Thinking field injection causes syntax errors
• Believing field injection disables auto-configuration
• Assuming field injection requires manual object creation

2. Which of the following is the correct way to inject a dependency using constructor injection in Spring Boot?

easy

A. @Autowired private Service service;

B. Service service = new Service();

C. public MyClass(Service service) { this.service = service; }

D. @Autowired public void setService(Service service) { this.service = service; }

Solution

1. Step 1: Identify constructor injection syntax

   Constructor injection requires a constructor that accepts the dependency as a parameter and assigns it to a field.

2. Step 2: Match the correct code

   public MyClass(Service service) { this.service = service; } shows a constructor receiving Service and assigning it, which is the correct pattern.

3. Final Answer:

   public MyClass(Service service) { this.service = service; } -> Option C

4. Quick Check:

   Constructor injection = constructor with parameters [OK]

Hint: Constructor injection uses constructor parameters, not field annotations [OK]

Common Mistakes:

• Confusing field injection with constructor injection
• Using manual object creation instead of injection
• Using setter injection when constructor injection is asked

3. Given this Spring Boot class using field injection:

public class MyService {
  @Autowired
  private Repository repo;

  public String getData() {
    return repo.fetch();
  }
}

What is a likely problem when writing unit tests for MyService?

medium

A. The getData() method will not compile

B. You cannot replace repo with a mock easily

C. The repo field will always be null at runtime

D. Spring Boot will throw a runtime exception on startup

Solution

1. Step 1: Understand field injection impact on testing

   Field injection hides the dependency inside the private field, making it hard to inject a mock instance during unit tests.

2. Step 2: Analyze testability of MyService

   Since repo is private and injected by Spring, test code cannot easily replace it with a mock, limiting test control.

3. Final Answer:

   You cannot replace repo with a mock easily -> Option B

4. Quick Check:

   Field injection blocks mock replacement [OK]

Hint: Field injection hides fields, blocking mock injection in tests [OK]

Common Mistakes:

• Assuming compilation errors occur
• Thinking Spring throws exceptions on field injection
• Believing the field is always null at runtime

4. Identify the problem in this Spring Boot class using field injection:

public class UserService {
  @Autowired
  private UserRepository userRepository;

  public UserService() {}

  public User findUser(Long id) {
    return userRepository.findById(id).orElse(null);
  }
}

medium

A. Field injection hides dependencies making testing harder

B. Constructor is missing the @Autowired annotation

C. The userRepository field should be static

D. The findUser method should be static

Solution

1. Step 1: Review injection style

   The class uses field injection with @Autowired on a private field, which hides dependencies.

2. Step 2: Understand testing impact

   Because dependencies are hidden, it is harder to inject mocks or stubs for unit testing, reducing testability.

3. Final Answer:

   Field injection hides dependencies making testing harder -> Option A

4. Quick Check:

   Field injection = hidden dependencies = testing difficulty [OK]

Hint: Field injection hides dependencies, causing test problems [OK]

Common Mistakes:

• Thinking constructor needs @Autowired always
• Believing fields must be static for injection
• Assuming methods must be static for injection

5. You have a Spring Boot service class that uses field injection:

public class OrderService {
  @Autowired
  private PaymentService paymentService;

  public boolean processOrder(Order order) {
    return paymentService.charge(order.getAmount());
  }
}

How would switching to constructor injection improve this class?

hard

A. It makes dependencies explicit and improves testability

B. It removes the need to annotate fields with @Autowired

C. It automatically creates new instances of dependencies

D. It allows Spring to inject dependencies without a constructor

Solution

1. Step 1: Understand constructor injection benefits

   Constructor injection requires dependencies to be passed in explicitly, making them visible and clear in the class design.

2. Step 2: Analyze testability improvements

   Explicit dependencies allow easy replacement with mocks or stubs during testing, improving testability and code clarity.

3. Final Answer:

   It makes dependencies explicit and improves testability -> Option A

4. Quick Check:

   Constructor injection = explicit dependencies + better tests [OK]

Hint: Constructor injection shows dependencies clearly, aiding tests [OK]

Common Mistakes:

• Thinking constructor injection removes @Autowired completely
• Believing constructor injection creates new instances automatically
• Assuming Spring injects without constructor or annotations