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.