UserDetailsService implementation in Spring Boot - Step-by-Step Execution

Concept Flow - UserDetailsService implementation

Start Authentication

↓

Call loadUserByUsername(username)

↓

Search user in database

↓

Create UserDetails

↓

Return UserDetails

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Spring Security uses UserDetails for auth

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End

The flow starts with Spring Security calling loadUserByUsername. The method searches the database for the user. If found, it creates and returns a UserDetails object. If not, it throws an exception.

Execution Sample

Spring Boot

public class MyUserDetailsService implements UserDetailsService {
  @Override
  public UserDetails loadUserByUsername(String username) throws UsernameNotFoundException {
    User user = userRepository.findByUsername(username);
    if (user == null) throw new UsernameNotFoundException(username);
    return new MyUserDetails(user);
  }
}

This code looks up a user by username, throws if not found, and returns a UserDetails object for Spring Security.

Execution Table

Step	Action	Input username	Database Result	Exception Thrown	Return Value
1	Call loadUserByUsername	alice	Search for 'alice'	No	N/A
2	User found in DB	alice	User object with username 'alice'	No	N/A
3	Create UserDetails	alice	User object	No	UserDetails object for 'alice'
4	Return UserDetails	alice	User object	No	UserDetails object for 'alice'
5	Call loadUserByUsername	bob	Search for 'bob'	No	N/A
6	User not found	bob	null	Yes: UsernameNotFoundException	N/A

💡 Execution stops after returning UserDetails or throwing UsernameNotFoundException.

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
username	N/A	alice	alice	alice
user	null	User('alice')	User('alice')	User('alice')
userDetails	null	null	UserDetails('alice')	UserDetails('alice')

Key Moments - 2 Insights

Why do we throw UsernameNotFoundException when the user is not found?

Spring Security expects this exception to know authentication failed. See execution_table row 6 where 'bob' is not found and exception is thrown.

What does the UserDetails object represent?

It holds user info like username, password, and roles for Spring Security to check. See execution_table rows 3 and 4 where UserDetails is created and returned.

Visual Quiz - 3 Questions

Look at the execution_table, what is returned at step 4 when username is 'alice'?

A UserDetails object for 'alice'

B null

C UsernameNotFoundException

D User object for 'alice'

💡 Hint

Check the 'Return Value' column at step 4 in execution_table.

At which step does the exception get thrown for a missing user?

A Step 2

B Step 4

C Step 6

D Step 3

💡 Hint

Look for 'Exception Thrown' column in execution_table.

If the database returned null for username 'alice', what would happen?

A UserDetails object is returned

B UsernameNotFoundException is thrown

C NullPointerException occurs

D Authentication succeeds anyway

💡 Hint

See execution_table row 6 for behavior when user is not found.

Concept Snapshot

UserDetailsService is a Spring Security interface.
Implement loadUserByUsername to find user by username.
Return UserDetails if found, else throw UsernameNotFoundException.
Spring Security uses UserDetails to authenticate users.
This method is called during login to verify credentials.

Full Transcript

This visual trace shows how Spring Security calls the loadUserByUsername method in a UserDetailsService implementation. The method receives a username, searches the database for a matching user, and either returns a UserDetails object or throws a UsernameNotFoundException if the user is not found. The execution table tracks each step, showing the input username, database search result, exceptions thrown, and return values. Variables like username, user, and userDetails are tracked to show their values at each step. Key moments clarify why exceptions are thrown and what UserDetails represents. The quiz tests understanding of return values and exception points. This helps beginners see exactly how user lookup works in Spring Security authentication.