code: public @CheckForNull Authentication authenticate(Queue.Item item) {
          if (Util.isOverridden(QueueItemAuthenticator.class, getClass(), "authenticate", Queue.Task.class)) {
              return authenticate(item.task);
          } else {
              throw new AbstractMethodError("you must override at least one of the QueueItemAuthenticator.authenticate methods");
          }
      }
docs: Determines the identity in which the {@link hudson.model.Queue.Executable} will run as.
  The default implementation delegates to {@link #authenticate(hudson.model.Queue.Task)}.
  @param item
  The contextual information to assist the authentication.
  The primary interest is likely {@link hudson.model.Queue.Item#task}, which is often {@link AbstractProject}.
  {@link Action}s associated with the item is also likely of interest, such as {@link CauseAction}.
  
  @return
  returning non-null will determine the identity. If null is returned, the next
  configured {@link QueueItemAuthenticator} will be given a chance to authenticate
  the executor. If everything fails, fall back to {@link Task#getDefaultAuthentication()}.