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This guide explains how to set up Spring Security authentication with H2 database in a Spring Boot application.

1. Add Dependencies

In pom.xml: 
<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-security</artifactId>
</dependency>
<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-data-jpa</artifactId>
</dependency>
<dependency>
  <groupId>com.h2database</groupId>
  <artifactId>h2</artifactId>
</dependency>

2. Configure H2

In application.properties: 
spring.datasource.url=jdbc:h2:mem:authdb
spring.datasource.driver-class-name=org.h2.Driver
spring.datasource.username=sa
spring.datasource.password=
spring.jpa.hibernate.ddl-auto=update
spring.h2.console.enabled=true

3. User Entity

import jakarta.persistence.*;

@Entity
public class AppUser {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    private String username;
    private String password;
    private String role;
}

4. Repository

import org.springframework.data.jpa.repository.JpaRepository;

public interface UserRepository extends JpaRepository<AppUser, Long> {
    AppUser findByUsername(String username);
}

5. UserDetails Service

import org.springframework.security.core.userdetails.*;
import org.springframework.stereotype.Service;

@Service
public class CustomUserDetailsService implements UserDetailsService {
    private final UserRepository repo;

    public CustomUserDetailsService(UserRepository repo) {
        this.repo = repo;
    }

    @Override
    public UserDetails loadUserByUsername(String username) {
        AppUser user = repo.findByUsername(username);
        return User.withUsername(user.getUsername())
                   .password(user.getPassword())
                   .roles(user.getRole())
                   .build();
    }
}

6. Security Configuration

import org.springframework.context.annotation.Bean;
import org.springframework.security.config.annotation.web.builders.HttpSecurity;
import org.springframework.security.crypto.bcrypt.BCryptPasswordEncoder;
import org.springframework.security.crypto.password.PasswordEncoder;
import org.springframework.security.web.SecurityFilterChain;
import org.springframework.context.annotation.Configuration;

@Configuration
public class SecurityConfig {

    @Bean
    public SecurityFilterChain filterChain(HttpSecurity http) throws Exception {
        http.authorizeHttpRequests(auth -> auth
                .requestMatchers("/public/**").permitAll()
                .anyRequest().authenticated())
            .formLogin();
        return http.build();
    }

    @Bean
    public PasswordEncoder passwordEncoder() {
        return new BCryptPasswordEncoder();
    }
}

7. Insert Test Data

In data.sql: 
INSERT INTO app_user (username, password, role) 
VALUES ('admin', '{bcrypt}$2a$10$Dow1l4.kN9w3hF9T6Gn4QOR9LfxA3KmhFzYstXgqE4vQ5/MLAzA/6', 'ADMIN');
Password is "password" encoded with BCrypt.

8. Run Application

Start the app, go to /login, and use admin/password for authentication.

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