UserDetailsService implementation in Spring Boot - Mini Project: Build & Apply

UserDetailsService Implementation in Spring Boot

📖 Scenario: You are building a simple Spring Boot application that needs to authenticate users. To do this, you will implement the UserDetailsService interface, which loads user data for authentication.

🎯 Goal: Create a class that implements UserDetailsService to load user details from a predefined list of users.

📋 What You'll Learn

Create a list of users with username and password

Define a variable for the default user role

Implement the loadUserByUsername method to find a user by username

Return a UserDetails object with username, password, and roles

💡 Why This Matters

🌍 Real World

UserDetailsService is used in Spring Boot applications to load user information for authentication and authorization.

💼 Career

Understanding UserDetailsService is essential for backend developers working with Spring Security to implement custom authentication logic.

1

Create a list of users

Create a private final list called users of type UserDetails with two users: username alice with password password1, and username bob with password password2. Use User.withUsername(...).password(...).roles(...).build() to create each user with role USER.

Spring Boot

import org.springframework.security.core.userdetails.User;
import org.springframework.security.core.userdetails.UserDetails;
import java.util.List;

public class MyUserDetailsService implements org.springframework.security.core.userdetails.UserDetailsService {
    // Create a private final List called users with alice and bob
    // Your code here
}

Hint

Use List.of(...) to create an immutable list of UserDetails objects.

2

Add a role variable

Add a private final string variable called defaultRole and set it to USER.

Spring Boot

import org.springframework.security.core.userdetails.User;
import org.springframework.security.core.userdetails.UserDetails;
import java.util.List;

public class MyUserDetailsService implements org.springframework.security.core.userdetails.UserDetailsService {
    private final List users = List.of(
        User.withUsername("alice").password("password1").roles("USER").build(),
        User.withUsername("bob").password("password2").roles("USER").build()
    );
    // Add private final String defaultRole = "USER";
    // Your code here
}

Hint

This variable will hold the role name for users.

3

Implement loadUserByUsername method

Override the loadUserByUsername method with parameter username of type String. Use a for loop with variable user to iterate over users. If user.getUsername().equals(username), return user. If no user is found, throw new UsernameNotFoundException("User not found").

Spring Boot

import org.springframework.security.core.userdetails.User;
import org.springframework.security.core.userdetails.UserDetails;
import org.springframework.security.core.userdetails.UsernameNotFoundException;
import java.util.List;

public class MyUserDetailsService implements org.springframework.security.core.userdetails.UserDetailsService {
    private final List users = List.of(
        User.withUsername("alice").password("password1").roles("USER").build(),
        User.withUsername("bob").password("password2").roles("USER").build()
    );
    private final String defaultRole = "USER";

    @Override
    public UserDetails loadUserByUsername(String username) throws UsernameNotFoundException {
        // Your code here
    }
}

Hint

Use a simple for loop to find the user by username and throw an exception if not found.

4

Complete the UserDetailsService class

Add the class declaration to implement UserDetailsService interface and import UsernameNotFoundException. Ensure the class is public and named MyUserDetailsService.

Spring Boot

// Ensure the class declaration and imports are correct
// Your code here

Hint

Make sure the class implements UserDetailsService and imports are complete.

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