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Spring Security 6 & JWT Authentication · Lesson

Custom UserDetailsService Implementation

Create a custom `UserDetailsService` to load user-specific data from your application's data store for authentication.

Custom UserDetailsService Implementation is a free Spring Security 6 & JWT Authentication lesson on CoddyKit — lesson 1 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Spring Security 6 & JWT Authentication learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Beyond In-Memory Users

In previous lessons, you might have used in-memory users for simple authentication. This means usernames and passwords are hardcoded directly in your application's configuration.

While easy for testing, real-world applications need to load user data from a persistent source like a database, LDAP, or another service. This is where a custom UserDetailsService comes in!

The UserDetailsService Interface

Spring Security uses the UserDetailsService interface to retrieve user-specific data during authentication. It has just one method you need to implement:

  • UserDetails loadUserByUsername(String username)

This method is crucial. When a user tries to log in, Spring Security calls this method, passing the username provided by the user.

What is UserDetails?

The loadUserByUsername method must return a UserDetails object. This interface represents the authenticated user's information, including:

  • Username
  • Password
  • Authorities (roles/permissions)
  • Account status (e.g., enabled, locked, expired)

Spring Security provides a default implementation called org.springframework.security.core.userdetails.User that you'll often use.

Creating Your Custom Service

To create a custom user service, you simply create a class that implements UserDetailsService. Inside, you'll override the loadUserByUsername method.

This method is where you'll write the logic to fetch user data from your chosen data store. For now, we'll use some hardcoded examples.

Implementing loadUserByUsername

Inside loadUserByUsername, you'll perform these steps:

  1. Receive the username.
  2. Look up the user in your data source.
  3. If found, create a UserDetails object with their details (username, password, roles).
  4. If not found, throw a UsernameNotFoundException.

Remember, password encoding is vital for security, but we'll cover that in a later lesson. For now, we'll use a plain text password prefix: {noop}.

Code: Basic UserDetailsService

Let's see a simple implementation. This example hardcodes users, simulating fetching from a data source. Run it to see how it works!

import org.springframework.security.core.userdetails.User;
import org.springframework.security.core.userdetails.UserDetails;
import org.springframework.security.core.userdetails.UserDetailsService;
import org.springframework.security.core.userdetails.UsernameNotFoundException;
import java.util.Arrays;
import java.util.Collections;

class MyUserDetailsService implements UserDetailsService {
    @Override
    public UserDetails loadUserByUsername(String username) throws UsernameNotFoundException {
        // In a real app, you'd fetch user from a database
        if ("user".equals(username)) {
            return User.withUsername("user")
                       .password("{noop}password") // {noop} for plain text
                       .roles("USER")
                       .build();
        }
        if ("admin".equals(username)) {
            return User.withUsername("admin")
                       .password("{noop}adminpass")
                       .roles("ADMIN", "USER")
                       .build();
        }
        throw new UsernameNotFoundException("User not found: " + username);
    }
}

public class Main {
    public static void main(String[] args) {
        MyUserDetailsService service = new MyUserDetailsService();

        System.out.println("Attempting to load 'user'...");
        try {
            UserDetails user = service.loadUserByUsername("user");
            System.out.println("Loaded User: " + user.getUsername());
            System.out.println("Authorities: " + user.getAuthorities());
        } catch (UsernameNotFoundException e) {
            System.out.println(e.getMessage());
        }

        System.out.println("\nAttempting to load 'admin'...");
        try {
            UserDetails admin = service.loadUserByUsername("admin");
            System.out.println("Loaded Admin: " + admin.getUsername());
            System.out.println("Authorities: " + admin.getAuthorities());
        } catch (UsernameNotFoundException e) {
            System.out.println(e.getMessage());
        }

        System.out.println("\nAttempting to load 'unknown'...");
        try {
            service.loadUserByUsername("unknown");
        } catch (UsernameNotFoundException e) {
            System.out.println(e.getMessage());
        }
    }
}

Adding Roles and Authorities

Notice in the example, we used .roles("USER") and .roles("ADMIN", "USER").

  • Roles are high-level permissions, like 'ADMIN' or 'USER'.
  • These roles are converted into GrantedAuthority objects by Spring Security.
  • When building the UserDetails object, you specify the roles/authorities the user possesses.

These authorities are later used by Spring Security for authorization (determining what a user can access).

Registering Your Service

Once you've created your custom UserDetailsService, Spring Security needs to know about it. In a Spring Boot application, you typically register it as a Spring bean.

By simply defining your custom service as a @Bean, Spring Security's auto-configuration will usually pick it up and use it for authentication.

The Custom Authentication Flow

Here's how custom authentication typically works with your service:

  1. User submits login credentials (username, password).
  2. Spring Security receives the request.
  3. It calls your custom UserDetailsService's loadUserByUsername() method with the provided username.
  4. Your method fetches user data and returns a UserDetails object.
  5. Spring Security then compares the provided password with the password from UserDetails (after encoding/decoding).
  6. If they match, authentication succeeds!

Check Your Understanding

Consider the core purpose and components of implementing a custom UserDetailsService.

Recap: Custom UserDetailsService

You've learned how to implement a custom UserDetailsService, a fundamental component for advanced user authentication in Spring Security:

  • It allows loading user data from any source.
  • You implement the loadUserByUsername method.
  • This method returns a UserDetails object, containing user credentials and authorities.
  • It's essential for moving beyond in-memory user management.

Next, we'll dive into securing those passwords with proper encoding!

Frequently asked questions

Is the “Custom UserDetailsService Implementation” lesson free?

Yes — the full text of “Custom UserDetailsService Implementation” is free to read here on the web, and the Spring Security 6 & JWT Authentication course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Spring Security 6 & JWT Authentication course, upgrade to CoddyKit PRO.

What will I learn in “Custom UserDetailsService Implementation”?

Create a custom `UserDetailsService` to load user-specific data from your application's data store for authentication. You practise Spring Security 6 & JWT Authentication with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Spring Security 6 & JWT Authentication?

No prior experience is required. Spring Security 6 & JWT Authentication on CoddyKit is structured for beginners through advanced learners; this is — lesson 1 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Custom UserDetailsService Implementation” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Spring Security 6 & JWT Authentication lesson?

Yes. Every Spring Security 6 & JWT Authentication lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

  1. Custom UserDetailsService Implementation
  2. Understanding Password Encoders
  3. Database User Management Integration
  4. Role-Based Authorization with Granted Authorities
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