To customize the login form in Spring Security to use a custom database, you can follow these steps:

Create a Custom UserDetailsService:

Implement the UserDetailsService interface to load user details from your custom database.

Override the loadUserByUsername method to query your database for the user details.

Configure Spring Security:

In your Spring Security configuration, define the UserDetailsService bean.

Configure the AuthenticationManager to use your custom UserDetailsService.

Customize the login form by specifying the login page URL and the login processing URL.

Implement the Custom Login Form:

Create a JSP or HTML file for the custom login form.

Include input fields for the username and password, and a submit button.

Use the login processing URL specified in the Spring Security configuration to submit the form.

Here's an example implementation:

1. Create a Custom UserDetailsService

public class CustomUserDetailsService implements UserDetailsService {
@Autowired
private JdbcTemplate jdbcTemplate;

@Override
public UserDetails loadUserByUsername(String username) throws UsernameNotFoundException {
    String query = "SELECT * FROM users WHERE username = ?";
    User user = jdbcTemplate.queryForObject(query, new Object[]{username}, new UserRowMapper());
    if (user == null) {
        throw new UsernameNotFoundException("User not found");
    }
    return user;
}

}

1. Configure Spring Security

@Configuration
@EnableWebSecurity
public class SecurityConfig extends WebSecurityConfigurerAdapter {
@Autowired
private CustomUserDetailsService customUserDetailsService;

@Autowired
private PasswordEncoder passwordEncoder;

@Override
protected void configure(AuthenticationManagerBuilder auth) throws Exception {
    auth.userDetailsService(customUserDetailsService)
        .passwordEncoder(passwordEncoder);
}

@Override
protected void configure(HttpSecurity http) throws Exception {
    http
        .authorizeRequests()
            .antMatchers("/login").permitAll()
            .anyRequest().authenticated()
        .and()
        .formLogin()
            .loginPage("/login")
            .loginProcessingUrl("/login")
            .defaultSuccessUrl("/welcome")
            .failureUrl("/login?error")
            .permitAll();
}

}

1. Implement the Custom Login Form

Create a login.jsp (or login.html) file in your src/main/webapp/WEB-INF/views directory (or equivalent location):

Login

In this example, the login form is submitted to the /login URL, which is the login processing URL specified in the Spring Security configuration.

By following these steps, you can customize the login form in Spring Security to use a custom database for user authentication.