// frameworks/base/services/core/java/com/android/server/NetworkTimeUpdateService.java
public NetworkTimeUpdateService(Context context) {
    mContext = context;
    //NtpTrustedTime用于獲取網(wǎng)絡(luò)時(shí)間
    mTime = NtpTrustedTime.getInstance(context);
    mAlarmManager = mContext.getSystemService(AlarmManager.class);
    mTimeDetector = mContext.getSystemService(TimeDetector.class);
    mCM = mContext.getSystemService(ConnectivityManager.class);

    Intent pollIntent = new Intent(ACTION_POLL, null);
    //創(chuàng)建mPendingPollIntent，用于發(fā)送定時(shí)廣播
    mPendingPollIntent = PendingIntent.getBroadcast(mContext, POLL_REQUEST, pollIntent, 0);
    // 請(qǐng)求服務(wù)器頻率 86400000ms = 24h
    //從配置文件LINUX/android/frameworks/base/core/res/res/values/config.xml中解析獲得
    mPollingIntervalMs = mContext.getResources().getInteger(com.android.internal.R.integer.config_ntpPollingInterval);
    //請(qǐng)求時(shí)間間隔60000ms = 10min
    mPollingIntervalShorterMs = mContext.getResources().getInteger(com.android.internal.R.integer.config_ntpPollingIntervalShorter);
    //最大嘗試次數(shù) 3        
    mTryAgainTimesMax = mContext.getResources().getInteger(com.android.internal.R.integer.config_ntpRetry);

    mWakeLock = context.getSystemService(PowerManager.class).newWakeLock(PowerManager.PARTIAL_WAKE_LOCK, TAG);
}

// frameworks/base/services/core/java/com/android/server/NetworkTimeUpdateService.java
/** Initialize the receivers and initiate the first NTP request */
public void systemRunning() {
    //注冊(cè)廣播
    registerForAlarms();
    //初始化Handler
    HandlerThread thread = new HandlerThread(TAG);
    thread.start();
    mHandler = new MyHandler(thread.getLooper());
    //向ConnectivityManager注冊(cè)網(wǎng)絡(luò)狀況監(jiān)聽
    mNetworkTimeUpdateCallback = new NetworkTimeUpdateCallback();
    mCM.registerDefaultNetworkCallback(mNetworkTimeUpdateCallback, mHandler);
    //使用ContentObsrver監(jiān)聽Settings.Global.AUTO_TIME值的變化
    mAutoTimeSettingObserver = new AutoTimeSettingObserver(mContext, mHandler,
            EVENT_AUTO_TIME_ENABLED);
    mAutoTimeSettingObserver.observe();
}

private void registerForAlarms() {
    mContext.registerReceiver(new BroadcastReceiver() {
        @Override
        public void onReceive(Context context, Intent intent) {
            mHandler.obtainMessage(EVENT_POLL_NETWORK_TIME).sendToTarget();
        }
    }, new IntentFilter(ACTION_POLL));
}

// frameworks/base/services/core/java/com/android/server/NetworkTimeUpdateService.java
// 定義和注冊(cè)監(jiān)聽
mNetworkTimeUpdateCallback = new NetworkTimeUpdateCallback();

mCM.registerDefaultNetworkCallback(mNetworkTimeUpdateCallback, mHandler);

private class NetworkTimeUpdateCallback extends NetworkCallback {
    @Override
    public void onAvailable(Network network) {
        Log.d(TAG, String.format("New default network %s; checking time.", network));
        mDefaultNetwork = network;
        // Running on mHandler so invoke directly.
        onPollNetworkTime(EVENT_NETWORK_CHANGED);
    }

    @Override
    public void onLost(Network network) {
        if (network.equals(mDefaultNetwork)) mDefaultNetwork = null;
    }
}

// frameworks/base/services/core/java/com/android/server/NetworkTimeUpdateService.java
mAutoTimeSettingObserver = new AutoTimeSettingObserver(mContext, mHandler,EVENT_AUTO_TIME_ENABLED);
mAutoTimeSettingObserver.observe();

/**
 * Observer to watch for changes to the AUTO_TIME setting. It only triggers when the setting
 * is enabled.
 */
private static class AutoTimeSettingObserver extends ContentObserver {

    private final Context mContext;
    private final int mMsg;
    private final Handler mHandler;

    AutoTimeSettingObserver(Context context, Handler handler, int msg) {}

    void observe() {
        ContentResolver resolver = mContext.getContentResolver();
        resolver.registerContentObserver(Settings.Global.getUriFor(Settings.Global.AUTO_TIME),false, this);
    }

    @Override
    public void onChange(boolean selfChange) {
        if (isAutomaticTimeEnabled()) {
            mHandler.obtainMessage(mMsg).sendToTarget();
        }
    }

    /**
     * Checks if the user prefers to automatically set the time.
     */
    private boolean isAutomaticTimeEnabled() {
        ContentResolver resolver = mContext.getContentResolver();
        return Settings.Global.getInt(resolver, Settings.Global.AUTO_TIME, 0) != 0;
    }
}

/** Handler to do the network accesses on */
private class MyHandler extends Handler {

    MyHandler(Looper l) {
        super(l);
    }

    @Override
    public void handleMessage(Message msg) {
        switch (msg.what) {
            case EVENT_AUTO_TIME_ENABLED:
            case EVENT_POLL_NETWORK_TIME:
            case EVENT_NETWORK_CHANGED:
                onPollNetworkTime(msg.what);
                break;
        }
    }
}

private void onPollNetworkTime(int event) {
    // If we don't have any default network, don't bother.
    if (mDefaultNetwork == null) return;
    mWakeLock.acquire();
    try {
        onPollNetworkTimeUnderWakeLock(event);
    } finally {
        mWakeLock.release();
    }
}

private void onPollNetworkTimeUnderWakeLock(int event) {
    // 使用NtpTrustedTime獲取網(wǎng)絡(luò)時(shí)間
    NtpTrustedTime.TimeResult cachedNtpResult = mTime.getCachedTimeResult();

    //cachedNtpResult.getAgeMillis()是上次請(qǐng)求ntp服務(wù)器的時(shí)間
    //如果大于等于1天，則強(qiáng)制刷新時(shí)間
    if (cachedNtpResult == null || cachedNtpResult.getAgeMillis() >= mPollingIntervalMs) {
        if (DBG) Log.d(TAG, "Stale NTP fix; forcing refresh");
        //該方法是個(gè)阻塞方法
        mTime.forceRefresh();
        cachedNtpResult = mTime.getCachedTimeResult();
    }

    //cachedNtpResult.getAgeMillis() < 1天
    if (cachedNtpResult != null && cachedNtpResult.getAgeMillis() < mPollingIntervalMs) {
        //設(shè)置定時(shí)廣播，1天后觸發(fā)
        resetAlarm(mPollingIntervalMs);

        // Suggest the time to the time detector. It may choose use it to set the system clock.
      // 設(shè)置系統(tǒng)時(shí)間
        TimestampedValue<Long> timeSignal = new TimestampedValue<>(
                cachedNtpResult.getElapsedRealtimeMillis(), cachedNtpResult.getTimeMillis());
        NetworkTimeSuggestion timeSuggestion = new NetworkTimeSuggestion(timeSignal);
        timeSuggestion.addDebugInfo("Origin: NetworkTimeUpdateService. event=" + event);
        mTimeDetector.suggestNetworkTime(timeSuggestion);
    } else {
        mTryAgainCounter++;
        if (mTryAgainTimesMax < 0 || mTryAgainCounter <= mTryAgainTimesMax) {
            //設(shè)置定時(shí)廣播，10分鐘后觸發(fā)
            resetAlarm(mPollingIntervalShorterMs);
        } else {
            //設(shè)置定時(shí)廣播，1天后觸發(fā)
            mTryAgainCounter = 0;
            resetAlarm(mPollingIntervalMs);
        }
    }
}

private void resetAlarm(long interval) {
    mAlarmManager.cancel(mPendingPollIntent);
    long now = SystemClock.elapsedRealtime();
    long next = now + interval;
    mAlarmManager.set(AlarmManager.ELAPSED_REALTIME, next, mPendingPollIntent);
}