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。
一。配置服务器端：
那么比如在英国的话就可以选择下面两个服务器
.uk.pool.ntp.org
.uk.pool.ntp.org
它的一般格式都是number.country.pool.ntp.org
中国的ntp服务器地址:
server 133.100.11.8 prefer
server 210.72.145.44
server 203.117.180.36
server 131.107.1.10
server time.asia.apple.com
server 64.236.96.53
server 130.149.17.21
server 66.92.68.246
server www.freebsd.org
server 18.145.0.30
server clock.via.net
server 137.92.140.80
server 133.100.9.2
server 128.118.46.3
server ntp.nasa.gov
server 129.7.1.66
server ntp-sop.inria.fr server (国家授时中心服务器IP地址)
二。修改默认NTP配置
1.通过SntpClient.java来封装请求。
frameworks/base/core/java/android/net/SntpClient.java
public boolean requestTime(String host, int timeout) {
DatagramSocket socket = null;
try {
socket = new DatagramSocket();
socket.setSoTimeout(timeout);
InetAddress address = InetAddress.getByName(host);
byte[] buffer = new byte[NTP_PACKET_SIZE];
DatagramPacket request = new DatagramPacket(buffer, buffer.length, address, NTP_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[] = NTP_MODE_CLIENT | (NTP_VERSION << );
// get current time and write it to the request packet
long requestTime = System.currentTimeMillis();
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);
long responseTicks = SystemClock.elapsedRealtime();
long responseTime = requestTime + (responseTicks - requestTicks);
// extract the results
long originateTime = readTimeStamp(buffer, ORIGINATE_TIME_OFFSET);
long receiveTime = readTimeStamp(buffer, RECEIVE_TIME_OFFSET);
long transmitTime = readTimeStamp(buffer, TRANSMIT_TIME_OFFSET);
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))/;
// if (false) Log.d(TAG, round trip: + roundTripTime + ms);
// if (false) Log.d(TAG, 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 (false) Log.d(TAG, request time failed: + e);
return false;
} finally {
if (socket != null) {
socket.close();
}
}
return true;
}
2.发起同步的，这个方法的主角为：NtpTrustedTime.java 在该类中通过forceRefresh方法来更新获取服务器时间。
frameworks/base/core/java/android/util/NtpTrustedTime.java
public boolean forceRefresh() {
if (mServer == null) {
// missing server, so no trusted time available
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() / ;
return true;
} else {
return false;
}
}
3.时间同步请求处理逻辑。在onPollNetworkTime方法中进行同步处理。
/frameworks/base/services/core/java/com/android/server/NetworkTimeUpdateService.java
private void onPollNetworkTime(int event) {
//1、是否勾选自动同步时间配置
// If Automatic time is not set, don't bother.
if (!isAutomaticTimeRequested()) return;
//2、mNitzTimeSetTime 来自Moderm，如果当前时间刚通过moderm更新不久，则不进行时间同步。
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;
}
//3、如果机器刚启动，或者机器运行时间大于mPollingIntervalMs，即10天，或者设置等发起的主动更新时间请求，则发起网络时间同步请求。否则，10天后再进行时间同步。
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);
//3.1、是否含有时间缓冲，如无，发起时间同步，
// force refresh NTP cache when outdated
if (mTime.getCacheAge() >= mPollingIntervalMs) {
//LEUI-START [BUG][MOBILEP-6067] [System time sync added
//mTime.forceRefresh();
int index = mTryAgainCounter % mNtpServers.size();
if (DBG) Log.d(TAG, mTryAgainCounter = + mTryAgainCounter + ;mNtpServers.size() = + mNtpServers.size() + ;index = + index + ;mNtpServers = + mNtpServers.get(index));
//3.1.1、遍历时间服务器，发起时间同步
if (mTime instanceof NtpTrustedTime)
{
((NtpTrustedTime) mTime).setServer(mNtpServers.get(index));
mTime.forceRefresh();
((NtpTrustedTime) mTime).setServer(mDefaultServer);
}
else
{
mTime.forceRefresh();
}
//LEUI-END [BUG][MOBILEP-6067] [System time sync added
}
//3.2、获取最新同步的时间缓冲数据，如无，则再次发起时间同步，间隔时间为mPollingIntervalShorterMs，即30秒。
// only update when NTP time is fresh
if (mTime.getCacheAge() < mPollingIntervalMs) {
final long ntp = mTime.currentTimeMillis();
mTryAgainCounter = ;
// 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.
//3.2.1、如果开机第一次同步或者最新时间与当前时间差别超过mTimeErrorThresholdMs即25秒，则进行时间设定。否则认定新同步时间与当前时间差别不大，不覆盖当前时间。
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
//3.2.2、设定同步时间
if (ntp / < 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
//3.3 如果不大于最大同步次数，30秒后进行时间同步，否则，10天后更新。
mTryAgainCounter++;
if (mTryAgainTimesMax < || mTryAgainCounter <= mTryAgainTimesMax) {
resetAlarm(mPollingIntervalShorterMs);
} else {
// Try much later
mTryAgainCounter = ;
resetAlarm(mPollingIntervalMs);
}
return;
}
}
//4、如果刚更新时间不久，则10天后再发起时间同步请求。
resetAlarm(mPollingIntervalMs);
}
4.减小NTP请求的时间，确保开机联网立即同步时间
/frameworks/base/core/res/res/values/config.xml
<integer name="config_ntpPollingIntervalShorter"></integer>