Android 时间更新NITZ和NTP详解
1、NTP和NITZ简介
最近在项目中遇到手机首次插上移动卡时不能自动更新时间的问题，就特意跟了下Android系统中手机时间更新有两种方式NTP和NITZ，下面先来看看NTP和NITZ的简介
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，需要运营商支持，可从运营商获取时间和时区具体信息。
NTP：Network Time Protocol（网络时间协议），用来同步网络中各个计算机的时间的协议。在手机中，NTP更新时间的方式是通过GPRS或wifi向特定服务器获取时间信息(不包含时区信息)。
接着我们来看看两种方式具体更新流程
2、NITZ更新时间流程
NITZ更新时间依赖运营商，当运营商基站发出更新时间的消息，基站附近的手机接收到对应消息后，会通过RIL层上报UNSOL_NITZ_TIME_RECEIVED事件，此时ServiceStateTracker便会处理相关时间更新流程，
相关时序图如下：
由于NITZ主要依赖于运营商，但在国内移动和联通貌似不怎么好用，在这里就不在详细说了，简单总结下如下：
1、在ServiceStateTracker构造方法里调用setOnNITZTime注册RIL事件RIL_UNSOL_NITZ_TIME_RECEIVED
2、RIL层上报RIL_UNSOL_NITZ_TIME_RECEIVED，在ServiceStateTracker的handleMessage里处理
3、调用ServiceStateTracker的setTimeFromNITZString设置时间和时区，在setAndBroadcastNetworkSetTime里调用setCurrentTimeMillis设置系统时间，并发送广播通知NetworkTimeUpdateService
3、NTP时间更新流程
NTP时间更新主要依赖于GPRS和wifi，即通过网络的方式去获取时间，在NetworkTimeUpdateService中调用onPollNetworkTime访问NtpServer获取网络时间，我们先来看看整体流程
当SystemServer启动，会调用networkTimeUpdaterF.systemRunning()初始化各种NTP request监听
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 thread
mHandler.obtainMessage(EVENT_POLL_NETWORK_TIME).sendToTarget();
mSettingsObserver = new SettingsObserver(mHandler, EVENT_AUTO_TIME_CHANGED);
mSettingsObserver.observe(mContext);
}
在registerForTelephonyIntents中主要是监听ACTION_NETWORK_SET_TIME和ACTION_NETWORK_SET_TIMEZONE的广播，registerForAlarms中监听"com.android.server.NetworkTimeUpdateService.action.POLL"广播，registerForConnectivityIntents监听网络状态改变的广播，SettingsObserver里监听Settings.Global.AUTO_TIME值的改变
//frameworks/base/services/core/java/com/android/server/NetworkTimeUpdateService.java
private class MyHandler extends Handler {
public MyHandler(Looper l) {
super(l);
}
@Override
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;
}
}
}
在这个MyHandler中可以看到，当上面某种监听触发时都会调用onPollNetworkTime，而这个方法里主要调用了onPollNetworkTimeUnderWakeLock，接下来看看这个方法
//frameworks/base/services/core/java/com/android/server/NetworkTimeUpdateService.java
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 time
if (mLastNtpFetchTime == NOT_SET || refTime >= mLastNtpFetchTime + mPollingIntervalMs
|| event == EVENT_AUTO_TIME_CHANGED) {
if (DBG) Log.d(TAG, "Before Ntp fetch");
// force refresh NTP cache when outdated
if (mTime.getCacheAge() >= mPollingIntervalMs) {
mTime.forceRefresh();
}
// only update when NTP time is fresh
if (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 time
if (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 int
if (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 shortly
mTryAgainCounter++;
if (mTryAgainTimesMax < 0 || mTryAgainCounter <= mTryAgainTimesMax) {
resetAlarm(mPollingIntervalShorterMs);
} else {
// Try much later
mTryAgainCounter = 0;
resetAlarm(mPollingIntervalMs);
}
return;
}
}
resetAlarm(mPollingIntervalMs);
}
这个方法中主要调用了TrustedTime实例的forceRefresh方法去获取时间，获取之后通过mTime.currentTimeMillis获得获取成功之后的时间ntp，最后调用 SystemClock.setCurrentTimeMillis(ntp)设置系统时间有几个参数需要特别说下
public NetworkTimeUpdateService(Context context) {
mContext = context;
mTime = NtpTrustedTime.getInstance(context);
...
//正常的轮询频率
mPollingIntervalMs = mContext.getResources().getInteger(
com.android.internal.R.integer.config_ntpPollingInterval);
//重试轮询间隔，以防网络请求失败
mPollingIntervalShorterMs = mContext.getResources().getInteger(
com.android.internal.R.integer.config_ntpPollingIntervalShorter);
//再次尝试次数
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。
mTimeErrorThresholdMs = mContext.getResources().getInteger(
com.android.internal.R.integer.config_ntpThreshold);
...
}
对应参数的配置如下，可能存在overlay替换，此处是framework里默认配置的值
//frameworks/base/core/res/res/values/config.xml
上面讲到主要通过TrustedTime实例的forceRefresh获取时间，下面就来跟下这个方法
//frameworks/base/core/java/android/util/NtpTrustedTime.java
public boolean forceRefresh() {
if (TextUtils.isEmpty(mServer)) {
// missing server, so no trusted time available
return false;
}
// 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);
}
}
final NetworkInfo ni = mCM == null ? null : mCM.getActiveNetworkInfo();
if (ni == null || !ni.isConnected()) {
if (LOGD) Log.d(TAG, "forceRefresh: no connectivity");
return false;
}
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;
}
}
上面方法中主要是通过SntpClient的requestTime根据传入的mServer获取时间，mServer是在调用NtpTrustedTime的getInstance中初始化的，具体如下
public static synchronized NtpTrustedTime getInstance(Context context) {
if (sSingleton == null) {
final Resources res = context.getResources();
final ContentResolver resolver = context.getContentResolver();
final String defaultServer = res.getString(
com.android.internal.R.string.config_ntpServer);
final long defaultTimeout = res.getInteger(
com.android.internal.R.integer.config_ntpTimeout);
final String secureServer = Settings.Global.getString(
resolver, Settings.Global.NTP_SERVER);
final long timeout = Settings.Global.getLong(
resolver, Settings.Global.NTP_TIMEOUT, defaultTimeout);
final String server = secureServer != null ? secureServer : defaultServer;
sSingleton = new NtpTrustedTime(server, timeout);
sContext = context;
}
return sSingleton;
}
config_ntpServer和config_ntpTimeout也是在framework下res中配置的，Settings.Global.NTP_SERVER和Settings.Global.NTP_TIMEOUT是配置在SettingProvider中的，就不在具体说明了。从上面代码可看出server是由secureServer和defaultServer决定的
//frameworks/base/core/res/res/values/config.xml
其中SntpClient主要是提供访问Ntp server的一个类，在requestTime中主要通过DatagramSocket访问传入的server，来获取时间,具体实现如下：
//frameworks/base/core/java/android/net/SntpClient.java
public boolean requestTime(InetAddress address, int port, int timeout) {
DatagramSocket socket = null;
try {
socket = new DatagramSocket();
socket.setSoTimeout(timeout);
byte[] buffer = new byte[NTP_PACKET_SIZE];
DatagramPacket request = new DatagramPacket(buffer, buffer.length, address, port);
// set mode = 3 (client) and version = 3
// mode is in low 3 bits of first byte
// version is in bits 3-5 of first byte
buffer[0] = NTP_MODE_CLIENT | (NTP_VERSION << 3);
// get current time and write it to the request packet
final long requestTime = System.currentTimeMillis();
final long requestTicks = SystemClock.elapsedRealtime();
writeTimeStamp(buffer, TRANSMIT_TIME_OFFSET, requestTime);
socket.send(request);
// read the response
DatagramPacket response = new DatagramPacket(buffer, buffer.length);
socket.receive(response);
final long responseTicks = SystemClock.elapsedRealtime();
final long responseTime = requestTime + (responseTicks - requestTicks);
// extract the results
final byte leap = (byte) ((buffer[0] >> 6) & 0x3);
final byte mode = (byte) (buffer[0] & 0x7);
final int stratum = (int) (buffer[1] & 0xff);
final long originateTime = readTimeStamp(buffer, ORIGINATE_TIME_OFFSET);
final long receiveTime = readTimeStamp(buffer, RECEIVE_TIME_OFFSET);
final long transmitTime = readTimeStamp(buffer, TRANSMIT_TIME_OFFSET);
/* do sanity check according to RFC */
// TODO: validate originateTime == requestTime.
checkValidServerReply(leap, mode, stratum, transmitTime);
long roundTripTime = responseTicks - requestTicks - (transmitTime - receiveTime);
// receiveTime = originateTime + transit + skew
// responseTime = transmitTime + transit - skew
// clockOffset = ((receiveTime - originateTime) + (transmitTime - responseTime))/2
// = ((originateTime + transit + skew - originateTime) +
// (transmitTime - (transmitTime + transit - skew)))/2
// = ((transit + skew) + (transmitTime - transmitTime - transit + skew))/2
// = (transit + skew - transit + skew)/2
// = (2 * skew)/2 = skew
long clockOffset = ((receiveTime - originateTime) + (transmitTime - responseTime))/2;
if (DBG) {
Log.d(TAG, "round trip: " + roundTripTime + "ms, " +
"clock offset: " + clockOffset + "ms");
}
// save our results - use the times on this side of the network latency
// (response rather than request time)
mNtpTime = responseTime + clockOffset;
mNtpTimeReference = responseTicks;
mRoundTripTime = roundTripTime;
} catch (Exception e) {
if (DBG) Log.d(TAG, "request time failed: " + e);
return false;
} finally {
if (socket != null) {
socket.close();
}
}
return true;
}
其实Ntp说的明白点，就是通过网络去获取时间然后更新系统时间，具体流程上面也简单说了下，当遇到手机不能更新时间时，先要看看网络是否可用，网络可用的情况下，就得看看对应的NtpServer是否能够访问。曾经就遇到过移动的数据业务不能访问NtpServer，导致手机不能更新时间。一般可以通过ntp等关键字在log中搜索，一般是SocketException或unknown host的错误。也可以直接adb shell命令进去手机，然后利用ping NtpServer 查看当前服务器是否可访问。在这里需要说的一点是，NtpServer一般都是对应的网址，访问网络时会根据当前的运营商网络，找到对应的IP地址，再去访问，有可能存在同一个NtpServer联通网络可以访问而移动网络不行的情况。
4、NITZ和NTP的总结
①NITZ的优先级要高于NTP的优先级，当NITZ更新系统时间后，NTP即使触发更新条件，也会检查NITZ更新时间距今是否超过864000000毫秒(10天，config_ntpPollingInterval)，若不满10天，则重设Alarm并取消此次NTP更新请求。
②NITZ主要依赖于运营商上报，NTP则主要依赖于网络环境，NITZ通过被动接收获取时间，NTP通过访问NtpServer获取网络时间，最后都是通过调用SystemClock.setCurrentTimeMillis更新手机时间。