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综述：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。
SystemServer.java
try {
startBootstrapServices();
startCoreServices();
startOtherServices();
} catch (Throwable ex) {
Slog.e("System", "******************************************");
Slog.e("System", "************ Failure starting system services", ex);
throw ex;
}
startOtherServices方法中，会初始化该类实例：
networkTimeUpdater = new NetworkTimeUpdateService(context);
在ActivityManagerService的systemReady方法中，初始化时间更新环境。
mActivityManagerService.systemReady(new Runnable() {
@Override
public void run() {
try {
if (networkManagementF != null) networkManagementF.systemReady();
} catch (Throwable e) {
reportWtf("making Network Managment Service ready", e);
}
}
}
涉及代码路径如下：
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
一、NetworkTimeUpdateService实例
public NetworkTimeUpdateService(Context context) {
mContext = context;
mTime = NtpTrustedTime.getInstance(context);
mAlarmManager = (AlarmManager) mContext.getSystemService(Context.ALARM_SERVICE);
Intent pollIntent = new Intent(ACTION_POLL, null);
mPendingPollIntent = PendingIntent.getBroadcast(mContext, POLL_REQUEST, pollIntent, 0);//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。
mPollingIntervalMs = mContext.getResources().getInteger(
com.android.internal.R.integer.config_ntpPollingInterval);//10天
mPollingIntervalShorterMs = mContext.getResources().getInteger(
com.android.internal.R.integer.config_ntpPollingIntervalShorter);//30秒
mTryAgainTimesMax = mContext.getResources().getInteger(
com.android.internal.R.integer.config_ntpRetry);
mTimeErrorThresholdMs = mContext.getResources().getInteger(
com.android.internal.R.integer.config_ntpThreshold);
//LEUI-START [BUG][MOBILEP-6067] [System time sync added
mDefaultServer = ((NtpTrustedTime) mTime).getServer();
mNtpServers.add(mDefaultServer);
for (String str : SERVERLIST)
{
mNtpServers.add(str);
}
mTryAgainCounter = 0;
//LEUI-END [BUG][MOBILEP-6067] [System time sync added
}
在该构造上，有几个重要的变量：
1、mPollingIntervalMs：多次尝试同步时间无果，10天会再次发起时间同步请求
2、mPollingIntervalShorterMs ：时间同步超时，再次发起时间同步请求。
3、SERVERLIST：时间同步服务器。此处建议多增加几个时间同步服务器，大陆、美国、台湾等多梯度配置。
4、初始化NtpTrustedTime对象。
mTime = NtpTrustedTime.getInstance(context);
一、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：
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);
}
1、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);
}
2、registerForAlarms此方法，是配合第“一”中介绍的mPendingPollIntent 来工作的，主要作用是构造handler Message并再次发起时间同步请求。
3、registerForConnectivityIntents此方法监听移动数据连接，移动网络连接后，收到信息，发起时间同步请求。此部分会在moderm相关同步时间中介绍。
private void registerForConnectivityIntents() {
IntentFilter intentFilter = new IntentFilter();
intentFilter.addAction(ConnectivityManager.CONNECTIVITY_ACTION);
mContext.registerReceiver(mConnectivityReceiver, intentFilter);
}
4、构建Message，发起时间同步请求。
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();
5、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);
我们的第二部分，很多地方都会主动或者被动发送Handler Message请求，在我们Handler中，我们是如何处理的那？
三、时间同步请求处理逻辑。
在第二部分，我们讲到了接收的来自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：
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_CONNECTED:
onPollNetworkTime(msg.what);
break;
}
}
}
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。
1、onPollNetworkTime：
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 = 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.
//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 / 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
//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 < 0 || mTryAgainCounter <= mTryAgainTimesMax) {
resetAlarm(mPollingIntervalShorterMs);
} else {
// Try much later
mTryAgainCounter = 0;
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);
}
四、三中介绍了时间获取的相关逻辑，我们接下来介绍下时间是如何发起同步的，这个方法的主角为：NtpTrustedTime
在该类中通过forceRefresh方法来更新获取服务器时间。
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() / 2;
return true;
} else {
return false;
}
}
在该方法逻辑中，通过SntpClient来封装请求。
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[0] = NTP_MODE_CLIENT | (NTP_VERSION << 3);
// 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))/2;
// 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;
}
我们传入在NetworkTimeUpdateService传入的服务器地址以及请求超时时间，向host服务器发起请求，并将相应结果按照编解码规则封装进二进制数组。
总结：NetworkTimeUpdateService时间同步，一旦发起成功的时间同步，时间数据会存在内存中，并根据当前机器运行时间来设定最新的时间。