CountDownTimer 这个搞 Android 开发的同学，一定不陌生。
在android.os 包下面的一个抽象类。

部分核心源码:

  // handles counting down
    private Handler mHandler = new Handler() {

        @Override
        public void handleMessage(Message msg) {

            synchronized (CountDownTimer.this) {
                if (mCancelled) {
                    return;
                }

                final long millisLeft = mStopTimeInFuture - SystemClock.elapsedRealtime();

                if (millisLeft <= 0) {
                    onFinish();
                } else if (millisLeft < mCountdownInterval) {
                    // no tick, just delay until done
                    sendMessageDelayed(obtainMessage(MSG), millisLeft);
                } else {
                    long lastTickStart = SystemClock.elapsedRealtime();
                    onTick(millisLeft);

                    // take into account user's onTick taking time to execute
                    long delay = lastTickStart + mCountdownInterval - SystemClock.elapsedRealtime();

                    // special case: user's onTick took more than interval to
                    // complete, skip to next interval
                    while (delay < 0) delay += mCountdownInterval;

                    sendMessageDelayed(obtainMessage(MSG), delay);
                }
            }
        }
    };

查看源码可以发现，其实现 是通过 Handler + Message ，呃，那就没什么好说的了。

具体实现示例：

private CountDownTimer countDownTimer；

private void startTimerTask() {
        if (countDownTimer == null) {
            countDownTimer = new CountDownTimer(10000000, 5000) {
                @Override
                public void onTick(long millisUntilFinished) {
                  //todo timer Task
                }

                @Override
                public void onFinish() {

                }
            };
        }
        countDownTimer.start();
    }

private void stopTimerTask() {
        if (countDownTimer != null) {
            countDownTimer.cancel();
            countDownTimer = null;
        }
    }

每隔5秒执行一次操作，共执行 10000000／5000 次。