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一、NTP介绍

NTP：网络时间协议，英文名称：Network Time Protocol（NTP）是用来使计算机时间同步化的一种协议，
它可以使计算机对其服务器或时钟源（如石英钟，GPS等等)做同步化，它可以提供高精准度的时间校正（LAN上与标准间差小于1毫秒，WAN上几十毫秒），
且可介由加密确认的方式来防止恶毒的协议攻击。NTP的目的是在无序的Internet环境中提供精确和健壮的时间服务。

二、NTP对时机制

Android系统使用NTP自动更新系统时间的触发机制：
1、监听数据库字段AUTO_TIME，也就是开关网络时间选项，当这个字段发生变化的时候，会立即触发一次时间同步
2、网络连接发生变化，当网络接通，会触发一次时间检查和同步
3、定时更新机制，当预定的时间到了，会触发一次时间检查和同步

public 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;}
}

三、检查与同步

frameworks/base/services/core/java/com/android/server/NetworkTimeUpdateService.javamPollingIntervalMs = mContext.getResources().getInteger(com.android.internal.R.integer.config_ntpPollingInterval);  // 正常获取网络时间间隔，默认86400000秒，也就是24小时mPollingIntervalShorterMs = mContext.getResources().getInteger( com.android.internal.R.integer.config_ntpPollingIntervalShorter);  // 3次快速尝试获取网络时间，目的是防止网络不好，间隔时间 60000秒，也就是一分钟mTryAgainTimesMax = mContext.getResources().getInteger( com.android.internal.R.integer.config_ntpRetry); // 快速尝试获取网络时间次数，默认三次mTimeErrorThresholdMs = mContext.getResources().getInteger( com.android.internal.R.integer.config_ntpThreshold); // ntp服务器时间与系统时间差阈值，如果大于5秒则更新

以上信息可以通过命令：dumpsys network_time_update_service 查看，例如：
PollingIntervalMs: +1d0h0m0s0ms  
PollingIntervalShorterMs: +1m0s0ms
TryAgainTimesMax: 3
TimeErrorThresholdMs: +5s0ms
TryAgainCounter: 1
LastNtpFetchTime: +1m28s223ms

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) {  // 如果开启NITZ 默认使用nitz更新的时间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) {    // 开机没有获取到网络时间，强制执行连接ntp服务器获取时间mTime.forceRefresh();}// only update when NTP time is freshif (mTime.getCacheAge() < mPollingIntervalMs) {      // 如果已经获取到网络时间，计算时间差并设置系统时间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  // 时间差小于5秒 ，则更新时间|| 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);   // 更新系统时间,LOG为：AlarmManagerService: Setting time of day to sec=1721811969，查看时间是否更新}} else {if (DBG) Log.d(TAG, "Ntp time is close enough = " + ntp);}mLastNtpFetchTime = SystemClock.elapsedRealtime();} else {// Try again shortlymTryAgainCounter++;if (mTryAgainTimesMax < 0 || mTryAgainCounter <= mTryAgainTimesMax) {   // 如果没有获取到网络时间，就间隔一分钟后重试resetAlarm(mPollingIntervalShorterMs);} else {// Try much later       mTryAgainCounter = 0;resetAlarm(mPollingIntervalMs);  // 尝试三次 间隔一分钟获取一次还没有获取到时 ，则24小时后再尝试}return;}}resetAlarm(mPollingIntervalMs);  // 时间已经设置成功，24小时后再获取}

frameworks/base/core/java/android/util/NtpTrustedTime.javafinal String defaultServer = res.getString(com.android.internal.R.string.config_ntpServer); // 默认NTP服务器：2.android.pool.ntp.orgfinal long defaultTimeout = res.getInteger( com.android.internal.R.integer.config_ntpTimeout);  // 连接NTP服务器超时时间默认5秒final String secureServer = Settings.Global.getString( resolver, Settings.Global.NTP_SERVER); // 可以通过命令动态修改ntp服务器，重启后生效 adb shell settings put global ntp_server ntp2.aliyun.comfinal long timeout = Settings.Global.getLong( resolver, Settings.Global.NTP_TIMEOUT, defaultTimeout); // 可以通过命令动态修改ntp服务器超时时间，重启后生效 adb shell settings put global ntp_timeout 50000final String server = secureServer != null ? secureServer : defaultServer; 
sSingleton = new NtpTrustedTime(server, timeout);   // 动态配置npt服务器地址 与超时时间public boolean forceRefresh() {if (LOGD) Log.d(TAG, "forceRefresh() from cache miss");final SntpClient client = new SntpClient();if (client.requestTime(mServer, (int) mTimeout)) {   // 获取网络时间mHasCache = true;mCachedNtpTime = client.getNtpTime();mCachedNtpElapsedRealtime = client.getNtpTimeReference();mCachedNtpCertainty = client.getRoundTripTime() / 2;return true;} else {return false;}}

四、调试

1、确认当前NTP服务器地址，adb shell settings get global ntp_server 为空时 则需要查看代码中默认的服务器。

2、修改NTP服务器：adb shell settings put global ntp_server ntp2.aliyun.com

3、logcat | grep -Ei "NetworkTimeUpdateService|SntpClient|AlarmManagerService"  查看服务及NTP服务器是否异常

4、dumpsys network_time_update_service 查看对时默认配置，比如时间差，快速重试时间，快速重试次数等