checkNotNull(unit);
    checkState(refreshNanos == UNSET_INT, "refresh was already set to %s ns", refreshNanos);
    checkArgument(duration > 0, "duration must be positive: %s %s", duration, unit);
    this.refreshNanos = unit.toNanos(duration);
    return this;
  }

  @SuppressWarnings("GoodTime") // nanos internally, should be Duration
  long getRefreshNanos() {