两种时间更新机制
NITZ
NITZ(Network Identity and Time Zone，网络标识和时区），是一种用于自动配置本地的时间和日期的机制，同时也通过无线网向移动设备提供运营商信息。NITZ是自从PHASE 2+ RELEASE 96 的GSM中的可选功能，经常被用来自动更新移动电话的系统时钟。NITZ需要运营商网络支持（通过CS网络），目前国内电信、移动都支持NITZ方式更新时间日期，而联通目前不支持。
参考：/wiki/NITZ
NTP
NTP：NTP(Network Time Protocol)提供准确时间，首先要有准确的时间来源，这一时间应该是国际标准时间UTC。 NTP获得UTC的时间来源可以是原子钟、天文台、卫星，也可以从Internet上获取。这样就有了准确而可靠的时间源。时间按NTP服务器的等级传播。与NITZ不同的是，NTP需要从专门的NTP服务器来获取时间，只要手机连接上网络，都可以实现时间的更新。
参考：/wiki/Network_Time_Protocol
Android如何更新系统时间
Android有一个专门的系统服务NetworkTimeUpdateServcie来负责更新系统时间，该服务在系统启动时在SystemServer.java中被创建：
if (!disableNetwork && !disableNetworkTime) {try {Slog.i(TAG, "NetworkTimeUpdateService");networkTimeUpdater = new NetworkTimeUpdateService(context);} catch (Throwable e) {reportWtf("starting NetworkTimeUpdate service", e);}}......try {if (networkTimeUpdaterF != null) networkTimeUpdaterF.systemRunning();} catch (Throwable e) {reportWtf("Notifying NetworkTimeService running", e);}
源码：
/frameworks/base/services/core/java/com/android/server/NetworkTimeUpdateService.java
服务初始化
NetworkTimeUpdateService初始时会：
注册RIL的ACTION_NETWORK_SET_TIME以及ACTION_NETWORK_SET_TIMEZONE事件，以接受来自Telephony FW的NITZ时间更新；监听 ACTION_POLL事件(定时更新时间)以及手机网络连接状态；发送消息同步NTP时间监听 Settings中“自动更新时间”选项的变化
/** Initialize the receivers and initiate the first NTP request */public void systemRunning() {registerForTelephonyIntents();registerForAlarms();registerForConnectivityIntents();HandlerThread thread = new HandlerThread(TAG);thread.start();mHandler = new MyHandler(thread.getLooper());// Check the network time on the new threadmHandler.obtainMessage(EVENT_POLL_NETWORK_TIME).sendToTarget();mSettingsObserver = new SettingsObserver(mHandler, EVENT_AUTO_TIME_CHANGED);mSettingsObserver.observe(mContext);}
同步NTP时间
如果没有收到NITZ时间的更新并且NTP超过一定间隔没有更新时间，服务会主动去同步NTP时间：
// force refresh NTP cache when outdatedif (mTime.getCacheAge() >= mPollingIntervalMs) {mTime.forceRefresh();}
这里可能会出现问题，时区选对了，但是时间日期都不准确，这里原因NTPServer服务器配置不对
修改配置位置：frameworks\base\core\res\res\values\config.xml
关键参数config_ntpServer
可以参考一下
<!-- Remote server that can provide NTP responses. -->
<string translatable="false" name="config_ntpServer">2.android.</string>
<!-- Normal polling frequency in milliseconds -->
<integer name="config_ntpPollingInterval">864000000</integer>
<!-- Try-again polling interval in milliseconds, in case the network request failed -->
<integer name="config_ntpPollingIntervalShorter">60000</integer>
<!-- Number of times to try again with the shorter interval, before backing
off until the normal polling interval. A value < 0 indicates infinite. -->
<integer name="config_ntpRetry">3</integer>
<!-- If the time difference is greater than this threshold in milliseconds,
then update the time. -->
<integer name="config_ntpThreshold">5000</integer>
<!-- Timeout to wait for NTP server response. -->
<integer name="config_ntpTimeout">20000</integer>
NTP从服务器获取时间：
@Overridepublic boolean forceRefresh() {...// We can't do this at initialization time: ConnectivityService might not be running yet.synchronized (this) {if (mCM == null) {mCM = (ConnectivityManager) sContext.getSystemService(Context.CONNECTIVITY_SERVICE);}}...if (LOGD) Log.d(TAG, "forceRefresh() from cache miss");final SntpClient client = new SntpClient();if (client.requestTime(mServer, (int) mTimeout)) {mHasCache = true;mCachedNtpTime = client.getNtpTime();mCachedNtpElapsedRealtime = client.getNtpTimeReference();mCachedNtpCertainty = client.getRoundTripTime() / 2;return true;} else {return false;}}
源码: /frameworks/base/core/java/android/util/NtpTrustedTime.java
接收NITZ时间
Telephony Framework层在接收到最新的NITZ时间后，会主动发送广播请求更新系统时间，NetworkTimeUpateService接收到广播后，保存相应的NITZ时间,下一次poll请求时，就会将该事件更新为系统时间。
private BroadcastReceiver mNitzReceiver = new BroadcastReceiver() {@Overridepublic void onReceive(Context context, Intent intent) {String action = intent.getAction();if (TelephonyIntents.ACTION_NETWORK_SET_TIME.equals(action)) {mNitzTimeSetTime = SystemClock.elapsedRealtime();} else if (TelephonyIntents.ACTION_NETWORK_SET_TIMEZONE.equals(action)) {mNitzZoneSetTime = SystemClock.elapsedRealtime();}}};
监听Settings中 “自动确定时间和日期”的变化
在setting中勾选“自动确定时间和日期”、“自动确定时区”后对key值为AUTO_TIME和AUTO_TIME_ZONE的Preference进行了赋值.
源码路径：packages/apps/Settings/src/com/android/settings/DateTimeSettings.java
void observe(Context context) {ContentResolver resolver = context.getContentResolver();resolver.registerContentObserver(Settings.Global.getUriFor(Settings.Global.AUTO_TIME),false, this);}@Overridepublic void onChange(boolean selfChange) {mHandler.obtainMessage(mMsg).sendToTarget();}
NetworkTimeUpdateService在检测到key值改变的时，就会发送一个消息EVENT_AUTO_TIME_CHANGED;handler接到消息后进行消息处理调用onPollNetworkTime(msg.what):
public void handleMessage(Message msg) {switch (msg.what) {case EVENT_AUTO_TIME_CHANGED:case EVENT_POLL_NETWORK_TIME:case EVENT_NETWORK_CONNECTED:onPollNetworkTime(msg.what);break;}}
在onPollNetworkTime方法中先判断是否勾选“自动更新时间”，如果没勾选直接退出，如果勾选了再看。如果NITZ已经更新了（不为NOT_SET(-1))，且更新间隔小于mPollingIntervalMs(mPollingIntervalMs=24*60*60*1000)，则直接用NITZ更新系统时间，否则用NTP同步时间。
// 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 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 time......if (ntp / 1000 < Integer.MAX_VALUE) {SystemClock.setCurrentTimeMillis(ntp);}
当从NTP服务器上获取的时间和当前时间之差的绝对值大于一个阀值，则认为当前时间错误，需要更新时间。
总结：
时间自动同步选项未勾选，不主动更新时间，直接返回；NITZ已同步且上次NITZ同步未超过24小时，则设置定时器24小时后再触发同步，即广播NetworkTimeUpdateService.ACTION_POLL；NTP上次成功同步超过24小时或用户勾选自动同步选项，则进行下面的NTP同步，否则同上设置定时器24小时后再触发同步
