Database and app orchestration in Spring Boot - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to define a Spring Boot application entry point.

Spring Boot

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class Application {
    public static void main(String[] args) {
        SpringApplication.[1](Application.class, args);
    }
}

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

Astart

Blaunch

Crun

Dexecute

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'start' or 'launch' instead of 'run' causes errors.

Forgetting to pass the application class and args.

The SpringApplication.run() method starts the Spring Boot application.

2fill in blank

medium

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

Spring Boot

import org.springframework.stereotype.Service;
import org.springframework.beans.factory.annotation.[1];

@Service
public class UserService {

    private final UserRepository userRepository;

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

    // service methods
}

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

AAutowired

BInject

CResource

DQualifier

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Inject' without proper imports.

Confusing 'Qualifier' with injection annotation.

The @Autowired annotation tells Spring to inject the repository automatically.

3fill in blank

hard

Fix the error in the repository interface declaration.

Spring Boot

import org.springframework.data.jpa.repository.JpaRepository;

public interface UserRepository extends JpaRepository<[1], Long> {
}

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

AUserDTO

BUserEntity

CUserController

DUserService

Attempts:

3 left

💡 Hint

Common Mistakes

Using service or controller classes instead of entity.

Using DTO classes which are not entities.

The repository must extend JpaRepository with the entity class, here UserEntity.

4fill in blank

hard

Fill both blanks to create a REST controller method that fetches a user by ID.

Spring Boot

import org.springframework.web.bind.annotation.*;

@RestController
@RequestMapping("/users")
public class UserController {

    private final UserService userService;

    public UserController(UserService userService) {
        this.userService = userService;
    }

    @GetMapping("/[1]")
    public UserEntity getUserById(@PathVariable [2] id) {
        return userService.findById(id);
    }
}

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

Aid

BuserId

CLong

DString

Attempts:

3 left

💡 Hint

Common Mistakes

Mismatching path variable name and parameter name.

Using wrong data type like String for ID.

The path variable name should match the placeholder in the URL, here 'id'. The parameter type is Long for the user ID.

5fill in blank

hard

Fill all three blanks to define a Spring Data JPA query method that finds users by their email.

Spring Boot

import org.springframework.data.jpa.repository.JpaRepository;
import java.util.Optional;

public interface UserRepository extends JpaRepository<UserEntity, Long> {
    Optional<UserEntity> findBy[1]([2] [3]);
}

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

AEmail

BString

Cemail

DId

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong method name like findById instead of findByEmail.

Using wrong parameter type or name.

The method name must be findByEmail. The parameter type is String and the parameter name is email.

Practice

1. What is the main role of a Spring Boot repository in database orchestration?

easy

A. To provide methods for saving, reading, updating, and deleting data

B. To define the database schema manually

C. To handle user interface rendering

D. To manage application security settings

Solution

1. Step 1: Understand repository purpose

   Spring Boot repositories act as a bridge between the app and the database, offering easy data operations.

2. Step 2: Identify repository capabilities

   They provide built-in methods like save(), findById(), delete(), which handle data safely without manual SQL.

3. Final Answer:

   To provide methods for saving, reading, updating, and deleting data -> Option A

4. Quick Check:

   Repository = Data operations [OK]

Hint: Repositories manage data actions like save and find [OK]

Common Mistakes:

• Confusing repositories with UI components
• Thinking repositories define database schema
• Assuming repositories handle security

2. Which of the following is the correct way to declare a Spring Data JPA repository interface for an entity User with primary key type Long?

easy

A. public interface UserRepository extends Repository<User> {}

B. public interface UserRepository extends JpaRepository {}

C. public class UserRepository implements JpaRepository {}

D. public interface UserRepository extends CrudRepository {}

Solution

1. Step 1: Recall correct interface extension

   Spring Data JPA repositories extend JpaRepository to get full CRUD support.

2. Step 2: Check syntax correctness

   public interface UserRepository extends JpaRepository {} correctly extends JpaRepository with User entity and Long ID type; others misuse implements or wrong generics order.

3. Final Answer:

   public interface UserRepository extends JpaRepository {} -> Option B

4. Quick Check:

   JpaRepository syntax [OK]

Hint: Use extends JpaRepository for repositories [OK]

Common Mistakes:

• Using implements instead of extends
• Swapping generic type order
• Extending incomplete Repository interface

3. Given the following Spring Boot repository method call:

Optional<User> user = userRepository.findById(5L);

What will user.isPresent() return if no user with ID 5 exists in the database?

medium

A. true

B. Throws an exception

C. null

D. false

Solution

1. Step 1: Understand findById return type

   findById returns an Optional which is empty if no matching record is found.

2. Step 2: Check isPresent behavior

   isPresent() returns false when Optional is empty, meaning no user found.

3. Final Answer:

   false -> Option D

4. Quick Check:

   Optional empty means isPresent() = false [OK]

Hint: Optional.isPresent() false means no data found [OK]

Common Mistakes:

• Assuming isPresent() returns true always
• Expecting null instead of Optional
• Thinking it throws exception if not found

4. Consider this Spring Boot repository interface:

public interface ProductRepository extends JpaRepository {
    List<Product> findByName(String name);
}

Which of the following issues will cause a runtime error when calling findByName(null)?

medium

A. No error; it returns an empty list

B. Method signature is invalid and won't compile

C. Calling findByName with null returns all products

D. Passing null causes a NullPointerException in query generation

Solution

1. Step 1: Analyze null parameter in derived queries

   Spring Data JPA derived query methods accept null parameters and generate SQL like WHERE name = null, which matches no rows (since NULL = NULL is false).

2. Step 2: Confirm behavior

   No exception is thrown at runtime or compile time; it safely returns an empty list.

3. Final Answer:

   No error; it returns an empty list -> Option A

4. Quick Check:

   findByName(null) = empty list [OK]

Hint: Null args in derived queries return empty lists safely [OK]

Common Mistakes:

• Thinking null causes NullPointerException
• Believing signature won't compile
• Assuming null returns all products

5. You want to update a user's email in the database using Spring Boot. Which sequence correctly orchestrates this update using repository methods?

1. Retrieve user by ID
2. Change user's email
3. Save updated user back to database

Which code snippet correctly implements this?

hard

A. userRepository.save(new User(id, newEmail));

B. userRepository.updateEmailById(id, newEmail);

C. var user = userRepository.findById(id).orElseThrow(); user.setEmail(newEmail); userRepository.save(user);

D. userRepository.findById(id).getEmail(newEmail); userRepository.save();

Solution

1. Step 1: Retrieve user safely

   Use findById(id).orElseThrow() to get user or throw if missing.

2. Step 2: Modify user object

   Set new email on retrieved user object.

3. Step 3: Save updated user

   Call save(user) to persist changes back to database.

4. Final Answer:

   var user = userRepository.findById(id).orElseThrow(); user.setEmail(newEmail); userRepository.save(user); -> Option C

5. Quick Check:

   Retrieve, modify, save = update pattern [OK]

Hint: Retrieve entity, change fields, then save [OK]

Common Mistakes:

• Trying to save new object without ID
• Using non-existent updateEmailById method
• Calling getEmail() instead of setEmail()