UserDetailsService with a Database

ShopEase stores customers in a users table (JPA mapping details are in the Hibernate & JPA material):

package com.shopease.user;

import jakarta.persistence.*;

@Entity
@Table(name = "users")              // "user" is a reserved word in PostgreSQL
public class User {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @Column(nullable = false, unique = true)
    private String email;

    @Column(nullable = false)
    private String passwordHash;     // BCrypt hash, never the raw password

    @Column(nullable = false)
    private String role;             // "CUSTOMER" or "ADMIN"

    private boolean enabled = true;

    protected User() {}

    public User(String email, String passwordHash, String role) {
        this.email = email;
        this.passwordHash = passwordHash;
        this.role = role;
    }

    public Long getId() { return id; }
    public String getEmail() { return email; }
    public String getPasswordHash() { return passwordHash; }
    public String getRole() { return role; }
    public boolean isEnabled() { return enabled; }
}

public interface UserRepository extends JpaRepository<User, Long> {
    Optional<User> findByEmail(String email);
    boolean existsByEmail(String email);
}

package com.shopease.security;

import com.shopease.user.UserRepository;
import org.springframework.security.core.userdetails.UserDetails;
import org.springframework.security.core.userdetails.UserDetailsService;
import org.springframework.security.core.userdetails.UsernameNotFoundException;
import org.springframework.stereotype.Service;

@Service
public class ShopEaseUserDetailsService implements UserDetailsService {

    private final UserRepository users;

    public ShopEaseUserDetailsService(UserRepository users) {
        this.users = users;
    }

    @Override
    public UserDetails loadUserByUsername(String email) throws UsernameNotFoundException {
        com.shopease.user.User user = users.findByEmail(email.toLowerCase())
                .orElseThrow(() -> new UsernameNotFoundException("User not found: " + email));

        return org.springframework.security.core.userdetails.User
                .withUsername(user.getEmail())
                .password(user.getPasswordHash())
                .roles(user.getRole())
                .disabled(!user.isEnabled())
                .build();
    }
}

• Annotating it with @Service is enough: with one UserDetailsService bean and one PasswordEncoder bean, Spring wires the DaoAuthenticationProvider for you.
• Our entity is also called User, so the Spring builder is written with its full name.
• The "username" can be anything unique: here it is the email address.