一、Android网络时间更新
Android网络时间更新，大体分两类。1、moderm相关更新，2、网络更新。本次主要介绍网路更新时间，主要涉及到NetworkTimeUpdateService，该类运行在SystemServer（ActivityManagerService）进程中。它有点特殊，从名字来看，其实Service，其实它和WifiService、ConnectivityManagerService等系统Service不同。
ZygoteInit.java的startSystemServer函数中启动了SyetemServer进程,代码路径如下所示：\frameworks\base\services\java\com\android\server\SystemServer.java

 /*** The main entry point from zygote.*/public static void main(String[] args) {new SystemServer().run();}

startOtherServices方法中，会初始化该类实例：

  if (!disableNetwork && !disableNetworkTime) {traceBeginAndSlog("StartNetworkTimeUpdateService");try {networkTimeUpdater = new NetworkTimeUpdateService(context);ServiceManager.addService("network_time_update_service", networkTimeUpdater);} catch (Throwable e) {reportWtf("starting NetworkTimeUpdate service", e);}traceEnd();}

网络更新的相关代码：

frameworks/base/services/core/java/com/android/server/NetworkTimeUpdateService.java
frameworks/base/core/java/android/util/NtpTrustedTime.java
frameworks/base/core/java/android/net/SntpClient.java

NetworkTimeUpdateService构造函数初始化

  public NetworkTimeUpdateService(Context context) {Log.d(TAG, "NetworkTimeUpdateService()");mContext = context;mTime = NtpTrustedTime.getInstance(context);mAlarmManager = (AlarmManager) mContext.getSystemService(Context.ALARM_SERVICE);mCM = (ConnectivityManager) mContext.getSystemService(Context.CONNECTIVITY_SERVICE);//时间同步有可能超时，使用该PendingIntent进行（间隔再次发起）时间同步。Intent pollIntent = new Intent(ACTION_POLL, null);mPendingPollIntent = PendingIntent.getBroadcast(mContext, POLL_REQUEST, pollIntent, 0);//正常轮询频率 24小时mPollingIntervalMs = mContext.getResources().getInteger(com.android.internal.R.integer.config_ntpPollingInterval);//再次尝试轮询间隔，以防网络请求失败 60秒mPollingIntervalShorterMs = mContext.getResources().getInteger(com.android.internal.R.integer.config_ntpPollingIntervalShorter);//重试次数 3次mTryAgainTimesMax = mContext.getResources().getInteger(com.android.internal.R.integer.config_ntpRetry);//如果时间差大于此阈值，则更新时间。 5秒mTimeErrorThresholdMs = mContext.getResources().getInteger(com.android.internal.R.integer.config_ntpThreshold);mWakeLock = ((PowerManager) context.getSystemService(Context.POWER_SERVICE)).newWakeLock(PowerManager.PARTIAL_WAKE_LOCK, TAG);Log.d(TAG, "mPollingIntervalMs  "+mPollingIntervalMs);Log.d(TAG, "mPollingIntervalShorterMs  "+mPollingIntervalShorterMs);Log.d(TAG, "mTryAgainTimesMax  "+mTryAgainTimesMax);Log.d(TAG, "mTimeErrorThresholdMs  "+mTimeErrorThresholdMs);}

在SystemServer.java中调用systemRunning(),初始化接收方并启动第一个NTP请求

//在SystemServer.java中调用systemRunning()traceBeginAndSlog("MakeCountryDetectionServiceReady");try {if (countryDetectorF != null) countryDetectorF.systemRunning();} catch (Throwable e) {reportWtf("Notifying CountryDetectorService running", e);}traceEnd();traceBeginAndSlog("MakeNetworkTimeUpdateReady");

NetworkTimeUpdateService初始化时间同步环境开机后，会调用该类的systemRunning方法，在该方法中中实现systemRunning();

 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);}

registerForTelephonyIntents该方法，注册监听来自Telephony Ril相关的广播。此部分会在moderm相关同步时间中介绍

  private void registerForTelephonyIntents() {IntentFilter intentFilter = new IntentFilter();intentFilter.addAction(TelephonyIntents.ACTION_NETWORK_SET_TIME);intentFilter.addAction(TelephonyIntents.ACTION_NETWORK_SET_TIMEZONE);mContext.registerReceiver(mNitzReceiver, intentFilter);}

系统启动后，会收到一个Telephony的广播android.intent.action.NETWORK_SET_TIMEZONE，获取系统深度睡眠时间

//尼兹时间事件的接收者private BroadcastReceiver mNitzReceiver = new BroadcastReceiver() {@Overridepublic void onReceive(Context context, Intent intent) {String action = intent.getAction();if (DBG) Log.d(TAG, "Received " + action);if (TelephonyIntents.ACTION_NETWORK_SET_TIME.equals(action)) {mNitzTimeSetTime = SystemClock.elapsedRealtime();//从设备开机到现在的时间，单位毫秒，含系统深度睡眠时间} else if (TelephonyIntents.ACTION_NETWORK_SET_TIMEZONE.equals(action)) {mNitzZoneSetTime = SystemClock.elapsedRealtime();}}};

当我们点击设置中的自动确定日期和时间时，然后通过观察者模式会执行如下代码，构建监听数据库的Observer，监听来自设置等发起的时间同步请求。在SettingsObserver中构建handler Message请求，发起时间同步。

 mSettingsObserver = new SettingsObserver(mHandler, EVENT_AUTO_TIME_CHANGED);mSettingsObserver.observe(mContext);private static class SettingsObserver extends ContentObserver {private int mMsg;private Handler mHandler;SettingsObserver(Handler handler, int msg) {super(handler);mHandler = handler;mMsg = msg;}void observe(Context context) {ContentResolver resolver = context.getContentResolver();resolver.registerContentObserver(Settings.Global.getUriFor(Settings.Global.AUTO_TIME),false, this);}@Overridepublic void onChange(boolean selfChange) {Log.d(TAG, "onChange()  selfChange "+selfChange);mHandler.obtainMessage(mMsg).sendToTarget();}}

接收的来自Telephony相关的广播，或者数据库变化，我们都会发送Message给Handler，我们的handler是如下处理这些请求的：

 /** 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;}}}

当我们打开移动数据或者wifi打开连接网络后是就会执行如下代码

  private class NetworkTimeUpdateCallback extends NetworkCallback {@Overridepublic 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);}@Overridepublic void onLost(Network network) {if (network.equals(mDefaultNetwork)) mDefaultNetwork = null;}}

下面是时间同步请求处理逻辑
接收请求类型：EVENT_AUTO_TIME_CHANGED、EVENT_POLL_NETWORK_TIME、
EVENT_NETWORK_CONNECTED，这些请求逻辑，我们都会发起onPollNetworkTime来进行相关逻辑处理。也就是说，onPollNetworkTime方法就是我们时间同步的主要关注对象。

 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.Log.d(TAG, "isAutomaticTimeRequested()  "+isAutomaticTimeRequested());Log.d(TAG, "mDefaultNetwork()  "+mDefaultNetwork);//自动使用网络提供时间sAutomaticTimeRequested() 就是trueif (!isAutomaticTimeRequested() || mDefaultNetwork == null) return;mWakeLock.acquire();try {onPollNetworkTimeUnderWakeLock(event);} finally {mWakeLock.release();}}private void onPollNetworkTimeUnderWakeLock(int event) {final long refTime = SystemClock.elapsedRealtime();// If NITZ time was received less than mPollingIntervalMs time ago,// no need to sync to NTP.if (mNitzTimeSetTime != NOT_SET && refTime - mNitzTimeSetTime < mPollingIntervalMs) {resetAlarm(mPollingIntervalMs);return;}final long currentTime = System.currentTimeMillis();if (DBG) Log.d(TAG, "System time = " + currentTime);// Get the NTP timeif (mLastNtpFetchTime == NOT_SET || refTime >= mLastNtpFetchTime + mPollingIntervalMs|| event == EVENT_AUTO_TIME_CHANGED) {if (DBG) Log.d(TAG, "Before Ntp fetch");// force refresh NTP cache when outdatedif (mTime.getCacheAge() >= mPollingIntervalMs) {mTime.forceRefresh();}// only update when NTP time is freshif (mTime.getCacheAge() < mPollingIntervalMs) {final long ntp = mTime.currentTimeMillis();mTryAgainCounter = 0;// If the clock is more than N seconds off or this is the first time it's been// fetched since boot, set the current time.if (Math.abs(ntp - currentTime) > mTimeErrorThresholdMs|| mLastNtpFetchTime == NOT_SET) {// Set the system timeif (DBG && mLastNtpFetchTime == NOT_SET&& Math.abs(ntp - currentTime) <= mTimeErrorThresholdMs) {Log.d(TAG, "For initial setup, rtc = " + currentTime);}if (DBG) Log.d(TAG, "Ntp time to be set = " + ntp);// Make sure we don't overflow, since it's going to be converted to an intif (ntp / 1000 < Integer.MAX_VALUE) {SystemClock.setCurrentTimeMillis(ntp);}} else {if (DBG) Log.d(TAG, "Ntp time is close enough = " + ntp);}mLastNtpFetchTime = SystemClock.elapsedRealtime();} else {// Try again shortlymTryAgainCounter++;if (mTryAgainTimesMax < 0 || mTryAgainCounter <= mTryAgainTimesMax) {resetAlarm(mPollingIntervalShorterMs);} else {// Try much latermTryAgainCounter = 0;resetAlarm(mPollingIntervalMs);}return;}}resetAlarm(mPollingIntervalMs);}

日志流程

05-29 11:14:38.284   901   901 D NetworkTimeUpdateService: NetworkTimeUpdateService()
05-29 11:14:38.285   901   901 D NetworkTimeUpdateService: mPollingIntervalMs  86400000
05-29 11:14:38.285   901   901 D NetworkTimeUpdateService: mPollingIntervalShorterMs  60000
05-29 11:14:38.285   901   901 D NetworkTimeUpdateService: mTryAgainTimesMax  3
05-29 11:14:38.285   901   901 D NetworkTimeUpdateService: mTimeErrorThresholdMs  5000
05-29 11:14:44.805   901   901 D NetworkTimeUpdateService: Received android.intent.action.NETWORK_SET_TIMEZONE
05-29 11:20:12.155   901  1303 D NetworkTimeUpdateService: New default network 100; checking time.
05-29 11:20:12.158   901  1303 D NetworkTimeUpdateService: System time = 1590722412158
05-29 11:20:12.158   901  1303 D NetworkTimeUpdateService: Before Ntp fetch