Contao\CoreBundle\Security\User\ContaoUserProvider::loadUserByUsername PHP Method

    public function loadUserByUsername($username)
    {
        if ($this->isBackendUsername($username)) {
            $this->framework->initialize();
            return BackendUser::getInstance();
        }
        if ($this->isFrontendUsername($username)) {
            $this->framework->initialize();
            return FrontendUser::getInstance();
        }
        throw new UsernameNotFoundException('Can only load user "frontend" or "backend".');
    }

 /**
  * Tests an unsupported username.
  *
  * @expectedException \Symfony\Component\Security\Core\Exception\UsernameNotFoundException
  */
 public function testLoadUnsupportedUsername()
 {
     $container = new Container();
     $container->addScope(new Scope(ContaoCoreBundle::SCOPE_FRONTEND));
     $container->enterScope(ContaoCoreBundle::SCOPE_FRONTEND);
     $provider = new ContaoUserProvider($container, $this->framework);
     $provider->loadUserByUsername('foo');
 }