global $user deprecated in favor of current_user service

In previous versions of Drupal, the current user was available as the global $user variable. This approach was often unstable, and in some cases resulted in a potential security risk.

In Drupal 8, global $user has been deprecated in favor of a current_user service.

Drupal 7:

public function behave() {
  global $user;
  if ($user->uid == 1) {
    return "Hiya, boss!";
  }
  else {
    return "You are not the site administrator.";
  }
}

Drupal 8:

public function behave() {
  $account = \Drupal::currentUser();
  if ($account->id() == 1) {
    return "Hiya, boss!";
  }
  else {
    return "You are not the site administrator.";
  }
}

Note that in neither case is the value an actual User object. Rather, as of Drupal 8 it is a UserSession object, which duplicates some, but not all, of the information of a user. It may also represent the Anonymous User rather than a saved User entity. See https://drupal.org/node/2017231 for more information.

NOTE: The global $user variable still exists as it is necessary for certain portions of the installer and simpletest, but its use by modules is not supported and should be considered deprecated.