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Android NTP时间同步源码分析

• Android系统设置自动时间后，如果连接了可用的网络。会同步网络时间。这个处理是 NetworkTimeUpdateService完成的。
• 某些定制化的系统，需要禁止网络时间同步。比如仅仅使用GPS时间。基于Android9，分析一下 Android NTP时间的同步流程。

时序图：

• 服务启动：NetworkTimeUpdateService在SystemServer的startOtherServices中启动（frameworks/base/services/java/com/android/server/SystemServer.java）

if (!isWatch) {traceBeginAndSlog("StartNetworkTimeUpdateService");try {networkTimeUpdater = new NetworkTimeUpdateService(context);ServiceManager.addService("network_time_update_service", networkTimeUpdater);} catch (Throwable e) {reportWtf("starting NetworkTimeUpdate service", e);}traceEnd();
}
// 省略
try {if (networkTimeUpdaterF != null) networkTimeUpdaterF.systemRunning();
} catch (Throwable e) {reportWtf("Notifying NetworkTimeService running", e);
}

• NetworkTimeUpdateService启动过程中，会调用NTP、Conectivity服务，注册监听NITZ、监听自动时间设定项(frameworks/base/services/core/java/com/android/server/NetworkTimeUpdateService.java)

public NetworkTimeUpdateService(Context context) {mContext = context;mTime = NtpTrustedTime.getInstance(context);mAlarmManager = mContext.getSystemService(AlarmManager.class);mCM = mContext.getSystemService(ConnectivityManager.class);Intent pollIntent = new Intent(ACTION_POLL, null);mPendingPollIntent = PendingIntent.getBroadcast(mContext, POLL_REQUEST, pollIntent, 0);mPollingIntervalMs = mContext.getResources().getInteger(com.android.internal.R.integer.config_ntpPollingInterval);mPollingIntervalShorterMs = mContext.getResources().getInteger(com.android.internal.R.integer.config_ntpPollingIntervalShorter);mTryAgainTimesMax = mContext.getResources().getInteger(com.android.internal.R.integer.config_ntpRetry);mTimeErrorThresholdMs = mContext.getResources().getInteger(com.android.internal.R.integer.config_ntpThreshold);mWakeLock = context.getSystemService(PowerManager.class).newWakeLock(PowerManager.PARTIAL_WAKE_LOCK, TAG);
}/** Initialize the receivers and initiate the first NTP request */
public void systemRunning() {registerForTelephonyIntents();registerForAlarms();HandlerThread thread = new HandlerThread(TAG);thread.start();mHandler = new MyHandler(thread.getLooper());mNetworkTimeUpdateCallback = new NetworkTimeUpdateCallback();mCM.registerDefaultNetworkCallback(mNetworkTimeUpdateCallback, mHandler);mSettingsObserver = new SettingsObserver(mHandler, EVENT_AUTO_TIME_CHANGED);mSettingsObserver.observe(mContext);
}

• 当自动时间设定变更、网络状态变更、更新周期达到时，会触发NetworkTimeUpdateService更新系统时间。通过获取NTP时间，以及进行各种判断（比如近期是否更新过NITZ时间），最终判断是否使用NTP时间更新（参考上面的时序图）

/** Handler to do the network accesses on */
private class MyHandler extends Handler {public MyHandler(Looper l) {super(l);}@Overridepublic void handleMessage(Message msg) {switch (msg.what) {case EVENT_AUTO_TIME_CHANGED:case EVENT_POLL_NETWORK_TIME:case EVENT_NETWORK_CHANGED:onPollNetworkTime(msg.what);break;}}
}private void onPollNetworkTime(int event) {// If Automatic time is not set, don't bother. Similarly, 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) {// Force an NTP fix when outdatedif (mTime.getCacheAge() >= mPollingIntervalMs) {if (DBG) Log.d(TAG, "Stale NTP fix; forcing refresh");mTime.forceRefresh();}if (mTime.getCacheAge() < mPollingIntervalMs) {// Obtained fresh fix; schedule next normal updateresetAlarm(mPollingIntervalMs);if (isAutomaticTimeRequested()) {updateSystemClock(event);}} else {// No fresh fix; schedule retrymTryAgainCounter++;if (mTryAgainTimesMax < 0 || mTryAgainCounter <= mTryAgainTimesMax) {resetAlarm(mPollingIntervalShorterMs);} else {// Try much latermTryAgainCounter = 0;resetAlarm(mPollingIntervalMs);}}
}private void updateSystemClock(int event) {final boolean forceUpdate = (event == EVENT_AUTO_TIME_CHANGED);if (!forceUpdate) {if (getNitzAge() < mPollingIntervalMs) {if (DBG) Log.d(TAG, "Ignoring NTP update due to recent NITZ");return;}final long skew = Math.abs(mTime.currentTimeMillis() - System.currentTimeMillis());if (skew < mTimeErrorThresholdMs) {if (DBG) Log.d(TAG, "Ignoring NTP update due to low skew");return;}}SystemClock.setCurrentTimeMillis(mTime.currentTimeMillis());
}

• 上面的代码，是基于Android9的。在Android12中引入了 TimeDetect服务，通过配置frameworks/base/core/res/res/values/config.xml中的 “config_autoTimeSourcesPriority”这个设定项，指定优先的时间源。所以在Android12中，NetworkTimeUpdateService会将时间更新请求发送给 TimeDetect服务，而不是直接使用SystemClock更新时间。关于Android12的NetworkTimeUpdateService详细流程，这里不进行分析。