UserDetailsService implementation in Spring Boot - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to declare the UserDetailsService implementation class.

Spring Boot

public class [1] implements UserDetailsService {
    // implementation
}

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

AUserDetails

BUserService

CMyUserDetailsService

DUserRepository

Attempts:

3 left

💡 Hint

Common Mistakes

Using a class name that does not reflect its purpose.

Not implementing UserDetailsService interface.

The class implementing UserDetailsService should have a clear name like MyUserDetailsService.

2fill in blank

medium

Complete the method signature to override loadUserByUsername.

Spring Boot

@Override
public [1] loadUserByUsername(String username) throws UsernameNotFoundException {
    // method body
}

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

Avoid

BUserDetails

CUser

DString

Attempts:

3 left

💡 Hint

Common Mistakes

Returning void or wrong type.

Missing the @Override annotation.

The method loadUserByUsername must return a UserDetails object.

3fill in blank

hard

Fix the error in fetching user by username from repository.

Spring Boot

User user = userRepository.[1](username);
if (user == null) {
    throw new UsernameNotFoundException("User not found");
}

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

AfindByUsername

BfindById

CgetUser

DloadUser

Attempts:

3 left

💡 Hint

Common Mistakes

Using findById which expects an ID, not username.

Using non-existent methods like getUser or loadUser.

The repository method to find user by username is usually named findByUsername.

4fill in blank

hard

Fill both blanks to return a UserDetails object with username and password.

Spring Boot

return new org.springframework.security.core.userdetails.User([1], [2], new ArrayList<>());

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

Auser.getUsername()

Buser.getPassword()

Cuser.getEmail()

Duser.getRoles()

Attempts:

3 left

💡 Hint

Common Mistakes

Using email instead of username.

Using roles where password is expected.

The UserDetails constructor needs username and password from the user object.

5fill in blank

hard

Fill all three blanks to complete the UserDetailsService implementation with repository injection and method.

Spring Boot

public class MyUserDetailsService implements UserDetailsService {

    private final [1] userRepository;

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

    @Override
    public UserDetails loadUserByUsername(String username) throws UsernameNotFoundException {
        User user = userRepository.[3](username);
        if (user == null) {
            throw new UsernameNotFoundException("User not found");
        }
        return new org.springframework.security.core.userdetails.User(user.getUsername(), user.getPassword(), new ArrayList<>());
    }
}

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

AUserRepository

CfindByUsername

DUserService

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong repository type or service class.

Using incorrect method names for fetching user.

The repository is injected via constructor and used to find user by username.

Practice

1.

What is the main purpose of implementing UserDetailsService in Spring Boot?

easy

A. To configure database connections

B. To load user-specific data during authentication

C. To handle HTTP requests

D. To manage application properties

Solution

1. Step 1: Understand the role of UserDetailsService

   UserDetailsService is used by Spring Security to load user data during login.

2. Step 2: Identify its main function

   It fetches user details like username, password, and roles for authentication.

3. Final Answer:

   To load user-specific data during authentication -> Option B

4. Quick Check:

   UserDetailsService purpose = Load user data [OK]

Hint: UserDetailsService always loads user info for login [OK]

Common Mistakes:

• Confusing UserDetailsService with database config
• Thinking it handles HTTP requests
• Assuming it manages app properties

2.

Which method must be overridden when implementing UserDetailsService?

easy

A. findUserByEmail(String email)

B. getUserRoles(String username)

C. authenticateUser(User user)

D. loadUserByUsername(String username)

Solution

1. Step 1: Recall UserDetailsService interface

   It declares a single method named loadUserByUsername.

2. Step 2: Confirm method signature

   The method takes a username string and returns UserDetails or throws exception.

3. Final Answer:

   loadUserByUsername(String username) -> Option D

4. Quick Check:

   Method to override = loadUserByUsername [OK]

Hint: Only loadUserByUsername is required in UserDetailsService [OK]

Common Mistakes:

• Trying to override non-existent methods
• Confusing with repository methods
• Using wrong method signatures

3.

Given this UserDetailsService implementation snippet, what happens if the user is not found?

public UserDetails loadUserByUsername(String username) {
    Optional<User> user = userRepository.findByUsername(username);
    if (user.isEmpty()) {
        throw new UsernameNotFoundException("User not found");
    }
    return new CustomUserDetails(user.get());
}

medium

A. Throws UsernameNotFoundException stopping login

B. Returns null and allows login

C. Returns empty UserDetails object

D. Logs error but continues login

Solution

1. Step 1: Check user existence condition

   If user is not found, user.isEmpty() is true.

2. Step 2: Analyze exception throwing

   Throws UsernameNotFoundException which stops authentication process.

3. Final Answer:

   Throws UsernameNotFoundException stopping login -> Option A

4. Quick Check:

   User not found = Exception thrown [OK]

Hint: User not found must throw exception to block login [OK]

Common Mistakes:

• Returning null instead of throwing exception
• Ignoring empty user Optional
• Not handling exception properly

4.

Identify the error in this UserDetailsService implementation:

public UserDetails loadUserByUsername(String username) {
    User user = userRepository.findByUsername(username);
    if (user == null) {
        return null;
    }
    return new CustomUserDetails(user);
}

medium

A. Returning null instead of throwing UsernameNotFoundException

B. Using Optional incorrectly

C. Missing @Override annotation

D. Not calling super.loadUserByUsername

Solution

1. Step 1: Check handling of missing user

   Returning null on user not found is incorrect for Spring Security.

2. Step 2: Correct approach for missing user

   Should throw UsernameNotFoundException to stop authentication properly.

3. Final Answer:

   Returning null instead of throwing UsernameNotFoundException -> Option A

4. Quick Check:

   Missing user must throw exception, not return null [OK]

Hint: Never return null; always throw UsernameNotFoundException [OK]

Common Mistakes:

• Returning null causes NullPointerException later
• Ignoring exception requirement
• Assuming @Override is mandatory (optional but recommended)

5.

You want to implement UserDetailsService to load users from a database and assign roles dynamically. Which approach correctly combines fetching user data and setting roles?

public UserDetails loadUserByUsername(String username) {
    User user = userRepository.findByUsername(username)
        .orElseThrow(() -> new UsernameNotFoundException("User not found"));
    List<GrantedAuthority> authorities = user.getRoles().stream()
        .map(role -> new SimpleGrantedAuthority(role.getName()))
        .toList();
    return new org.springframework.security.core.userdetails.User(
        user.getUsername(), user.getPassword(), authorities);
}

hard

A. Returns user without roles assigned

B. Fails to throw exception if user missing

C. Correctly fetches user and maps roles to authorities

D. Uses deprecated method toList() causing error

Solution

1. Step 1: Verify user fetching with exception

   Uses orElseThrow to throw UsernameNotFoundException if user missing, correct behavior.

2. Step 2: Check role mapping to authorities

   Maps user roles to GrantedAuthority list properly using stream and SimpleGrantedAuthority.

3. Step 3: Confirm UserDetails creation

   Creates Spring Security User object with username, password, and authorities as expected.

4. Final Answer:

   Correctly fetches user and maps roles to authorities -> Option C

5. Quick Check:

   User fetch + role mapping = Correct implementation [OK]

Hint: Use orElseThrow and map roles to authorities for UserDetails [OK]

Common Mistakes:

• Not throwing exception on missing user
• Forgetting to map roles to authorities
• Using deprecated or incorrect stream methods