Spring Security : Multiple HTTP Config not working

Look at the Spring Security Reference:

@EnableWebSecurity
public class MultiHttpSecurityConfig {
  @Autowired
  public void configureGlobal(AuthenticationManagerBuilder auth) { 1
      auth
          .inMemoryAuthentication()
              .withUser("user").password("password").roles("USER").and()
              .withUser("admin").password("password").roles("USER", "ADMIN");
  }

  @Configuration
  @Order(1)                                                        2
  public static class ApiWebSecurityConfigurationAdapter extends WebSecurityConfigurerAdapter {
      protected void configure(HttpSecurity http) throws Exception {
          http
              .antMatcher("/api/**")                               3
              .authorizeRequests()
                  .anyRequest().hasRole("ADMIN")
                  .and()
              .httpBasic();
      }
  }    

  @Configuration                                                   4
  public static class FormLoginWebSecurityConfigurerAdapter extends WebSecurityConfigurerAdapter {

      @Override
      protected void configure(HttpSecurity http) throws Exception {
          http
              .authorizeRequests()
                  .anyRequest().authenticated()
                  .and()
              .formLogin();
      }
  }
}

1 Configure Authentication as normal

2 Create an instance of WebSecurityConfigurerAdapter that contains @Order to specify which WebSecurityConfigurerAdapter should be considered first.

3 The http.antMatcher states that this HttpSecurity will only be applicable to URLs that start with /api/

4 Create another instance of WebSecurityConfigurerAdapter. If the URL does not start with /api/ this configuration will be used. This configuration is considered after ApiWebSecurityConfigurationAdapter since it has an @Order value after 1 (no @Order defaults to last).

Your second configuration is not used, because your first configuration matches /** (no antMatcher configured). And your first configuration restricts only /admin/**, all other URLs are permitted by default.

Your first WebSecurityConfigurerAdapter's

http
            .authorizeRequests()

matches all the URLs, limit it to only URLs start with /admin by using antMatcher:

@Configuration
@Order(1)
public static class ProviderSecurity extends WebSecurityConfigurerAdapter{
    @Override
    protected void configure(HttpSecurity http) throws Exception {
        http
            .antMatcher("/admin/**")
                .authorizeRequests()
                .antMatchers("/admin/login").permitAll()
                .antMatchers("/admin/**").access("hasRole('BASE_USER')")
                .and()

                ...