UserDetailsService implementation in Spring Boot - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output when loading a user by username?

Given this UserDetailsService implementation, what will be returned when calling loadUserByUsername("alice")?

Spring Boot

public class MyUserDetailsService implements UserDetailsService {
    private Map<String, UserDetails> users = Map.of(
        "alice", User.withUsername("alice").password("pass").roles("USER").build(),
        "bob", User.withUsername("bob").password("pass").roles("ADMIN").build()
    );

    @Override
    public UserDetails loadUserByUsername(String username) throws UsernameNotFoundException {
        if (!users.containsKey(username)) {
            throw new UsernameNotFoundException("User not found");
        }
        return users.get(username);
    }
}

A Throws UsernameNotFoundException

B Returns a UserDetails object for alice with role USER

C Returns null

D Returns a UserDetails object for bob with role ADMIN

💡 Hint

Check if the username exists in the users map before returning.

The method checks if the username exists in the map. For "alice", it finds the user and returns the UserDetails with role USER.

📝 Syntax

intermediate

Identify the syntax error in this UserDetailsService code

Which option contains a syntax error in the UserDetailsService implementation?

Spring Boot

public class CustomUserDetailsService implements UserDetailsService {
    @Override
    public UserDetails loadUserByUsername(String username) throws UsernameNotFoundException {
        UserDetails user = User.withUsername(username).password("pass").roles("USER").build()
        return user;
    }
}

A Incorrect method name loadUserByUser

B Missing override annotation

C Missing semicolon after build() method call

D UserDetails is not imported

💡 Hint

Look carefully at the end of the line with build()

The line calling build() is missing a semicolon at the end, causing a syntax error.

🔧 Debug

advanced

Why does this UserDetailsService throw UsernameNotFoundException for existing user?

Given this code, why does loadUserByUsername("John") throw UsernameNotFoundException even though john exists?

Spring Boot

public class MyUserDetailsService implements UserDetailsService {
    private Map<String, UserDetails> users = new HashMap<>();

    public MyUserDetailsService() {
        users.put("john", User.withUsername("john").password("pass").roles("USER").build());
    }

    @Override
    public UserDetails loadUserByUsername(String username) throws UsernameNotFoundException {
        if (users.get(username) == null) {
            throw new UsernameNotFoundException("User not found");
        }
        return users.get(username);
    }
}

A The username passed has different case than stored key

B The users map is immutable and cannot be modified

C The UserDetails object is null due to missing build() call

D The users map is empty because it is not initialized properly

💡 Hint

Check if the username key matches exactly in case.

The map key is "john" lowercase, but if the input username has different case like "John", the get returns null causing the exception.

❓ state_output

advanced

What roles does the returned UserDetails have?

In this UserDetailsService, what roles will the UserDetails returned by loadUserByUsername("admin") have?

Spring Boot

public class AdminUserDetailsService implements UserDetailsService {
    @Override
    public UserDetails loadUserByUsername(String username) throws UsernameNotFoundException {
        if (!"admin".equals(username)) {
            throw new UsernameNotFoundException("User not found");
        }
        return User.withUsername("admin").password("adminpass").roles("ADMIN", "USER").build();
    }
}

A ["ADMIN", "USER"]

B ["ADMIN"]

C Empty list

D ["USER"]

💡 Hint

Look at the roles method call in the return statement.

The roles method is called with "ADMIN" and "USER", so both roles are assigned.

🧠 Conceptual

expert

What happens if UserDetailsService returns null?

In Spring Security, what is the effect if a UserDetailsService implementation returns null instead of throwing UsernameNotFoundException when a user is not found?

A Authentication succeeds with an anonymous user

B The application crashes with NullPointerException

C Spring Security ignores and continues authentication

D Spring Security throws an InternalAuthenticationServiceException causing authentication failure

💡 Hint

Think about how Spring Security handles null UserDetails.

Returning null causes Spring Security to throw InternalAuthenticationServiceException, failing authentication cleanly.

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