code: protected void schedule() {
      try (LockResource r = new LockResource(mLock)) {
        Preconditions.checkState(!mScheduled, "Called schedule twice without waiting for any ticks");
        mScheduled = true;
        mTickCondition.signal();
        HeartbeatScheduler.removeTimer(this);
      }
    }
docs: Schedules execution of the heartbeat.