Android NTP 时间同步机制详解

这篇文章主要为大家介绍了Android NTP时间同步机制实例详解，有需要的朋友可以借鉴参考下，希望能够有所帮助，祝大家多多进步，早日升职加薪

正文

NTP是Android原生通过网络获取时间的机制，其中关键代码逻辑都在NetworkTimeUpdateService，它是Android系统服务，由SystemServer启动。

本篇文章基于Android 10源码分析。

初始化

我们从它的构造方法开始分析：

    public NetworkTimeUpdateService(Context context) { mContext = context; //NtpTrustedTime用于获取网络时间 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); //创建mPendingPollIntent，用于发送定时广播 mPendingPollIntent = PendingIntent.getBroadcast(mContext, POLL_REQUEST, pollIntent, 0); // 请求服务器频率 86400000ms = 24h //从配置文件LINUX/android/frameworks/base/core/res/res/values/config.xml中解析获得 mPollingIntervalMs = mContext.getResources().getInteger( com.android.internal.R.integer.config_ntpPollingInterval); //请求时间间隔 60000ms = 10min mPollingIntervalShorterMs = mContext.getResources().getInteger( com.android.internal.R.integer.config_ntpPollingIntervalShorter); //最大尝试次数 3 mTryAgainTimesMax = mContext.getResources().getInteger( com.android.internal.R.integer.config_ntpRetry); mWakeLock = context.getSystemService(PowerManager.class).newWakeLock( PowerManager.PARTIAL_WAKE_LOCK, TAG); }

构造方法中主要是对各种变量进行初始化操作，接下来分析systemRunning方法，它是个public方法，NetworkTimeUpdateService创建时，被SystemServer调用。

/** Initialize the receivers and initiate the first NTP request */ public void systemRunning() { //注册广播 registerForAlarms(); //初始化Handler HandlerThread thread = new HandlerThread(TAG); thread.start(); mHandler = new MyHandler(thread.getLooper()); //向ConnectivityManager注册网络状况监听 mNetworkTimeUpdateCallback = new NetworkTimeUpdateCallback(); mCM.registerDefaultNetworkCallback(mNetworkTimeUpdateCallback, mHandler); //使用ContentObsrver监听Settings.Global.AUTO_TIME值的变化 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)); }

这个方法中NetworkTimeUpdateService注册了三个监听：

1.调用registerForAlarms，注册一个广播接收者，接收ACTION_POLL广播，收到后向消息队列发送一个EVENT_POLL_NETWORK_TIME的事件；
2.向ConnectivityManager注册网络状况监听；
3.监听Settings.Global.AUTO_TIME值的变化。

咱们对这三个监听以及如何触发挨个的分析。

NetworkTimeUpdateCallback

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; } }

NetworkTimeUpdateCallback实现了NetworkCallback接口，当被回调onAvailable（网络切换/可用）时，赋值mDefaultNetwork并调用onPollNetworkTime(EVENT_NETWORK_CHANGED)方法。

AutoTimeSettingObserver

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; } }

监听Settings.Global.AUTO_TIME的变化，当值改变且AUTO_TIME != 0 时，向消息队列发送一个类型为EVENT_AUTO_TIME_ENABLED的消息。

看到这有三个东西我们还没确认，何时发送ACTION_POLL广播、onPollNetworkTime方法实现、Handler的实现。为了更有条理我打乱一下顺序，先介绍Handler和onPollNetworkTime。

MyHandler

/** 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; } } }

三种类型的消息最终都会调用onPollNetworkTime，只有参数不一样。

onPollNetworkTime

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(); } }

当前无网络情况直接返回，并使用PowerManager.WakeLock（屏幕恒亮）调用onPollNetworkTimeUnderWakeLock(event)。

   private void onPollNetworkTimeUnderWakeLock(int event) { // 使用NtpTrustedTime获取网络时间 NtpTrustedTime.TimeResult cachedNtpResult = mTime.getCachedTimeResult(); //cachedNtpResult.getAgeMillis()是上次请求ntp服务器的时间 //如果大于等于1天，则强制刷新时间 if (cachedNtpResult == null || cachedNtpResult.getAgeMillis() >= mPollingIntervalMs) { if (DBG) Log.d(TAG, "Stale NTP fix; forcing refresh"); //该方法是个阻塞方法 mTime.forceRefresh(); cachedNtpResult = mTime.getCachedTimeResult(); } //cachedNtpResult.getAgeMillis() <1天 if (cachedNtpResult != null && cachedNtpResult.getAgeMillis()  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) {>

注意到上面调用了resetAlarm(),该方法即是定时发送ACTION_POLL广播。

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

总结一下onPollNetworkTimeUnderWakeLock做的事情：