Service calling repository in Spring Boot - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when the service calls the repository method?

Consider a Spring Boot service class calling a repository method that returns an Optional. What will be the output if the repository returns Optional.empty() and the service calls getUserNameById() which returns the user's name or 'Unknown' if not found?

Spring Boot

public class UserService {
    private final UserRepository userRepository;

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

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

public interface UserRepository {
    Optional<User> findById(Long id);
}

public record User(Long id, String name) {}

A "Unknown"

B null

C Throws NoSuchElementException

D "" (empty string)

💡 Hint

Think about how Optional.orElse() works when the Optional is empty.

The repository returns Optional.empty(), so map() is skipped and orElse("Unknown") returns the default string 'Unknown'.

📝 Syntax

intermediate

Which repository method signature is correct for Spring Data JPA?

You want to define a repository interface method to find a User by email. Which method signature is correct to let Spring Data JPA generate the query automatically?

Spring Boot

public interface UserRepository extends JpaRepository<User, Long> {
    // Choose the correct method signature
}

A Optional<User> findByEmail(String email);

B Optional<User> findUserByEmail(String email);

C User findByEmail(String email);

D User getUserByEmail(String email);

💡 Hint

Spring Data JPA supports Optional return types for nullable results.

The correct signature is Optional findByEmail(String email); because it follows Spring Data naming conventions and handles absent results safely.

🔧 Debug

advanced

Why does the service fail to autowire the repository?

Given the service class below, why does Spring fail to inject the UserRepository bean?

Spring Boot

import org.springframework.stereotype.Service;

@Service
public class UserService {

    private final UserRepository userRepository;

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

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

A Because the service field userRepository is private

B Because the service class is missing @Component annotation

C Because the service has no constructor with UserRepository parameter for Spring to inject

D Because UserRepository is not annotated with @Repository

💡 Hint

Spring injects dependencies via constructor or field injection. Check the constructor.

The service defines a no-args constructor that sets userRepository to null, so Spring cannot inject the repository. A constructor with UserRepository parameter is needed.

❓ state_output

advanced

What is the state of the user list after service method call?

The service calls repository.saveAll() with a list of new users. What will be the size of the list returned by the service method?

Spring Boot

public class UserService {
    private final UserRepository userRepository;

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

    public List<User> saveUsers(List<User> users) {
        return userRepository.saveAll(users);
    }
}

public interface UserRepository {
    List<User> saveAll(List<User> users);
}

A The returned list is always empty

B The returned list size equals the input list size

C The returned list size is zero if any user is null

D The returned list size is double the input list size

💡 Hint

saveAll returns the saved entities, usually matching the input size.

The repository's saveAll method returns a list of saved users matching the input list size unless an error occurs.

🧠 Conceptual

expert

What happens if a transactional service method calls a repository and an exception occurs?

In a Spring Boot application, a service method annotated with @Transactional calls a repository save method. If a runtime exception is thrown after the save call but before the method ends, what is the state of the database?

Spring Boot

@Service
@Transactional
public class UserService {
    private final UserRepository userRepository;

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

    public void saveUserWithError(User user) {
        userRepository.save(user);
        if (true) {
            throw new RuntimeException("Error after save");
        }
    }
}

A The transaction commits partially with warnings

B The user is saved despite the exception

C The database state is undefined and may be corrupted

D The save is rolled back; no user is persisted

💡 Hint

Think about how @Transactional handles runtime exceptions and rollbacks.

Spring rolls back the entire transaction if a runtime exception occurs, so the save is undone and no user is persisted.