Service calling repository in Spring Boot - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to inject the repository into the service class.

Spring Boot

public class UserService {
    private final UserRepository [1];

    public UserService(UserRepository userRepository) {
        this.userRepository = userRepository;
    }
}

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

AuserRepo

BuserRepository

Crepository

Drepo

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different variable name than the constructor parameter

Forgetting to declare the field

The service class should declare a field named userRepository to match the constructor parameter and follow common naming conventions.

2fill in blank

medium

Complete the code to annotate the service class for Spring to detect it as a service.

Spring Boot

[1]
public class UserService {
    // service methods
}

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

A@Repository

B@Component

C@Controller

D@Service

Attempts:

3 left

💡 Hint

Common Mistakes

Using @Repository instead of @Service

Forgetting to annotate the class

The @Service annotation marks the class as a service component in Spring.

3fill in blank

hard

Fix the error in the service method to call the repository's findById method correctly.

Spring Boot

public User getUserById(Long id) {
    return userRepository.[1](id).orElse(null);
}

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

AgetOne

BgetById

CfindById

DfindOne

Attempts:

3 left

💡 Hint

Common Mistakes

Using deprecated methods like getOne

Using methods that do not return Optional

The correct repository method to find by ID returning Optional is findById.

4fill in blank

hard

Fill both blanks to complete the service method that saves a user using the repository.

Spring Boot

public User saveUser(User [1]) {
    return userRepository.[2]([1]);
}

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

Auser

Bsave

Cadd

Dentity

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect parameter names

Using wrong repository method names like add

The method parameter should be named user and the repository method to save is save.

5fill in blank

hard

Fill all three blanks to complete the service method that deletes a user by ID using the repository.

Spring Boot

public void deleteUserById([1] [2]) {
    userRepository.[3]([2]);
}

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

ALong

Bid

CdeleteById

DuserId

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong parameter types

Using wrong method names like delete

The method parameter type is Long, the parameter name is id, and the repository method to delete by ID is deleteById.

Practice

1. What is the main role of a Service class in Spring Boot when it calls a Repository?

easy

A. To configure the database settings

B. To directly manage database connections

C. To replace the repository and perform SQL queries

D. To handle business logic and use the repository for data access

Solution

1. Step 1: Understand the role of Service

   The Service layer contains business logic and does not directly access the database.

2. Step 2: Understand the role of Repository

   The Repository handles data access and database operations.

3. Final Answer:

   To handle business logic and use the repository for data access -> Option D

4. Quick Check:

   Service handles logic, Repository handles data [OK]

Hint: Service = logic, Repository = data access [OK]

Common Mistakes:

• Thinking Service manages database connections
• Confusing Repository with Service responsibilities
• Assuming Service runs SQL queries directly

2. Which is the correct way to inject a repository into a service class in Spring Boot?

easy

A. Use @Autowired on a constructor parameter

B. Create a new repository instance inside the service method

C. Use new keyword to instantiate repository in service constructor

D. Declare repository as a static variable in the service

Solution

1. Step 1: Understand dependency injection in Spring Boot

   Spring Boot recommends constructor injection with @Autowired for better testability and immutability.

2. Step 2: Check options for repository injection

   Creating new instances manually or static variables break Spring's management and are not recommended.

3. Final Answer:

   Use @Autowired on a constructor parameter -> Option A

4. Quick Check:

   Constructor injection with @Autowired [OK]

Hint: Use @Autowired constructor injection for repositories [OK]

Common Mistakes:

• Manually creating repository instances
• Using static variables for repository
• Not using Spring's dependency injection

3. Given this service code snippet, what will getUserName(1) return if the repository finds a user with name "Alice"?

public class UserService {
  private final UserRepository userRepository;

  public UserService(UserRepository userRepository) {
    this.userRepository = userRepository;
  }

  public String getUserName(int id) {
    return userRepository.findById(id).map(User::getName).orElse("Unknown");
  }
}

medium

A. null

B. "Unknown"

C. "Alice"

D. Throws NullPointerException

Solution

1. Step 1: Understand repository method behavior

   findById(id) returns an Optional containing the User if found.

2. Step 2: Analyze service method logic

   The method maps the User to its name or returns "Unknown" if no user is found.

3. Final Answer:

   "Alice" -> Option C

4. Quick Check:

   User found returns name, else "Unknown" [OK]

Hint: Optional.map returns value or default [OK]

Common Mistakes:

• Assuming null is returned instead of default
• Expecting exception when user not found
• Confusing Optional usage

4. What is wrong with this service code that calls a repository?

@Service
public class ProductService {
  private ProductRepository productRepository;

  public void saveProduct(Product product) {
    productRepository.save(product);
  }
}

medium

A. The save method does not exist in repositories

B. The repository is not injected, so it will cause a NullPointerException

C. The service class must be abstract

D. The Product parameter should be annotated with @Entity

Solution

1. Step 1: Check repository injection

   The repository field is declared but not injected or initialized.

2. Step 2: Understand consequence of missing injection

   Calling save on a null repository causes NullPointerException at runtime.

3. Final Answer:

   The repository is not injected, so it will cause a NullPointerException -> Option B

4. Quick Check:

   Missing injection causes null pointer error [OK]

Hint: Always inject repository before use [OK]

Common Mistakes:

• Forgetting @Autowired or constructor injection
• Assuming repository auto-initializes
• Confusing entity annotation with parameter

5. You want to create a service method that returns all active users from the database. The repository has a method List<User> findByActiveTrue(). How should the service method call the repository and return the list?

hard

A. public List<User> getActiveUsers() { return userRepository.findByActiveTrue(); }

B. public List<User> getActiveUsers() { return userRepository.findAll(); }

C. public List<User> getActiveUsers() { return userRepository.findByActiveFalse(); }

D. public List<User> getActiveUsers() { return null; }

Solution

1. Step 1: Identify repository method for active users

   The repository method findByActiveTrue() returns users with active = true.

2. Step 2: Use repository method in service

   The service should call this method and return its result directly.

3. Final Answer:

   public List<User> getActiveUsers() { return userRepository.findByActiveTrue(); } -> Option A

4. Quick Check:

   Call matching repository method for active users [OK]

Hint: Call repository method matching your filter [OK]

Common Mistakes:

• Calling findAll() returns all users, not filtered
• Using findByActiveFalse() returns inactive users
• Returning null instead of data