正文
NTP是Android原生通过网络获取时间的机制，其中关键代码逻辑都在NetworkTimeUpdateService，它是Android系统服务，由SystemServer启动。
本篇文章基于Android 10源码分析。
初始化
我们从它的构造方法开始分析：
public NetworkTimeUpdateService(Context context) {
mContext = context;
//NtpTrustedTime用于获取网络时间
mTime = NtpTrustedTime.getInstance(context);
mAlarmManager = mContext.getSystemService(AlarmManager.class);
mTimeDetector = mContext.getSystemService(TimeDetector.class);
mCM = mContext.getSystemService(ConnectivityManager.class);
Intent pollIntent = new Intent(ACTION_POLL, null);
//创建mPendingPollIntent，用于发送定时广播
mPendingPollIntent = PendingIntent.getBroadcast(mContext, POLL_REQUEST, pollIntent, 0);
// 请求服务器频率 86400000ms = 24h
//从配置文件LINUX/android/frameworks/base/core/res/res/values/config.xml中解析获得
mPollingIntervalMs = mContext.getResources().getInteger(
com.android.internal.R.integer.config_ntpPollingInterval);
//请求时间间隔 60000ms = 10min
mPollingIntervalShorterMs = mContext.getResources().getInteger(
com.android.internal.R.integer.config_ntpPollingIntervalShorter);
//最大尝试次数 3
mTryAgainTimesMax = mContext.getResources().getInteger(
com.android.internal.R.integer.config_ntpRetry);
mWakeLock = context.getSystemService(PowerManager.class).newWakeLock(
PowerManager.PARTIAL_WAKE_LOCK, TAG);
}
构造方法中主要是对各种变量进行初始化操作，接下来分析systemRunning方法，它是个public方法，NetworkTimeUpdateService创建时，被SystemServer调用。
public void systemRunning() {
//注册广播
registerForAlarms();
//初始化Handler
HandlerThread thread = new HandlerThread(TAG);
thread.start();
mHandler = new MyHandler(thread.getLooper());
//向ConnectivityManager注册网络状况监听
mNetworkTimeUpdateCallback = new NetworkTimeUpdateCallback();
mCM.registerDefaultNetworkCallback(mNetworkTimeUpdateCallback, mHandler);
//使用ContentObsrver监听Settings.Global.AUTO_TIME值的变化
mAutoTimeSettingObserver = new AutoTimeSettingObserver(mContext, mHandler,
EVENT_AUTO_TIME_ENABLED);
mAutoTimeSettingObserver.observe();
}
private void registerForAlarms() {
mContext.registerReceiver(
new BroadcastReceiver() {
@Override
public void onReceive(Context context, Intent intent) {
mHandler.obtainMessage(EVENT_POLL_NETWORK_TIME).sendToTarget();
}
}, new IntentFilter(ACTION_POLL));
}
这个方法中NetworkTimeUpdateService注册了三个监听：
- 1.调用registerForAlarms，注册一个广播接收者，接收ACTION_POLL广播，收到后向消息队列发送一个EVENT_POLL_NETWORK_TIME的事件；
- 2.向ConnectivityManager注册网络状况监听；
- 3.监听Settings.Global.AUTO_TIME值的变化。
咱们对这三个监听以及如何触发挨个的分析。
NetworkTimeUpdateCallback
mNetworkTimeUpdateCallback = new NetworkTimeUpdateCallback();
mCM.registerDefaultNetworkCallback(mNetworkTimeUpdateCallback, mHandler);
private class NetworkTimeUpdateCallback extends NetworkCallback {
@Override
public void onAvailable(Network network) {
Log.d(TAG, String.format("New default network %s; checking time.", network));
mDefaultNetwork = network;
// Running on mHandler so invoke directly.
onPollNetworkTime(EVENT_NETWORK_CHANGED);
}
@Override
public void onLost(Network network) {
if (network.equals(mDefaultNetwork)) mDefaultNetwork = null;
}
}
NetworkTimeUpdateCallback实现了NetworkCallback接口，当被回调onAvailable（网络切换/可用）时， 赋值mDefaultNetwork并调用onPollNetworkTime(EVENT_NETWORK_CHANGED)方法。
AutoTimeSettingObserver
mAutoTimeSettingObserver = new AutoTimeSettingObserver(mContext, mHandler,
EVENT_AUTO_TIME_ENABLED);
mAutoTimeSettingObserver.observe();
private static class AutoTimeSettingObserver extends ContentObserver {
private final Context mContext;
private final int mMsg;
private final Handler mHandler;
AutoTimeSettingObserver(Context context, Handler handler, int msg) {...}
void observe() {
ContentResolver resolver = mContext.getContentResolver();
resolver.registerContentObserver(Settings.Global.getUriFor(Settings.Global.AUTO_TIME),
false, this);
}
@Override
public void onChange(boolean selfChange) {
if (isAutomaticTimeEnabled()) {
mHandler.obtainMessage(mMsg).sendToTarget();
}
}
private boolean isAutomaticTimeEnabled() {
ContentResolver resolver = mContext.getContentResolver();
return Settings.Global.getInt(resolver, Settings.Global.AUTO_TIME, 0) != 0;
}
}
监听Settings.Global.AUTO_TIME的变化，当值改变且AUTO_TIME != 0 时，向消息队列发送一个类型为EVENT_AUTO_TIME_ENABLED的消息。
看到这有三个东西我们还没确认，何时发送ACTION_POLL广播、onPollNetworkTime方法实现、Handler的实现。为了更有条理我打乱一下顺序，先介绍Handler和onPollNetworkTime。
MyHandler
private class MyHandler extends Handler {
MyHandler(Looper l) {
super(l);
}
@Override
public void handleMessage(Message msg) {
switch (msg.what) {
case EVENT_AUTO_TIME_ENABLED:
case EVENT_POLL_NETWORK_TIME:
case EVENT_NETWORK_CHANGED:
onPollNetworkTime(msg.what);
break;
}
}
}
三种类型的消息最终都会调用onPollNetworkTime，只有参数不一样。
onPollNetworkTime
private void onPollNetworkTime(int event) {
// If we don't have any default network, don't bother.
if (mDefaultNetwork == null) return;
mWakeLock.acquire();
try {
onPollNetworkTimeUnderWakeLock(event);
} finally {
mWakeLock.release();
}
}
当前无网络情况直接返回，并使用PowerManager.WakeLock（屏幕恒亮）调用onPollNetworkTimeUnderWakeLock(event)。
private void onPollNetworkTimeUnderWakeLock(int event) {
// 使用NtpTrustedTime获取网络时间
NtpTrustedTime.TimeResult cachedNtpResult = mTime.getCachedTimeResult();
//cachedNtpResult.getAgeMillis()是上次请求ntp服务器的时间
//如果大于等于1天，则强制刷新时间
if (cachedNtpResult == null || cachedNtpResult.getAgeMillis() >= mPollingIntervalMs) {
if (DBG) Log.d(TAG, "Stale NTP fix; forcing refresh");
//该方法是个阻塞方法
mTime.forceRefresh();
cachedNtpResult = mTime.getCachedTimeResult();
}
//cachedNtpResult.getAgeMillis() < 1天
if (cachedNtpResult != null && cachedNtpResult.getAgeMillis() < mPollingIntervalMs) {
//设置定时广播，1天后触发
resetAlarm(mPollingIntervalMs);
// Suggest the time to the time detector. It may choose use it to set the system clock.
// 设置系统时间
TimestampedValue<Long> timeSignal = new TimestampedValue<>(
cachedNtpResult.getElapsedRealtimeMillis(), cachedNtpResult.getTimeMillis());
NetworkTimeSuggestion timeSuggestion = new NetworkTimeSuggestion(timeSignal);
timeSuggestion.addDebugInfo("Origin: NetworkTimeUpdateService. event=" + event);
mTimeDetector.suggestNetworkTime(timeSuggestion);
} else {
mTryAgainCounter++;
if (mTryAgainTimesMax < 0 || mTryAgainCounter <= mTryAgainTimesMax) {
//设置定时广播，10分钟后触发
resetAlarm(mPollingIntervalShorterMs);
} else {
//设置定时广播，1天后触发
mTryAgainCounter = 0;
resetAlarm(mPollingIntervalMs);
}
}
}
注意到上面调用了resetAlarm(),该方法即是定时发送ACTION_POLL广播。
private void resetAlarm(long interval) {
mAlarmManager.cancel(mPendingPollIntent);
long now = SystemClock.elapsedRealtime();
long next = now + interval;
mAlarmManager.set(AlarmManager.ELAPSED_REALTIME, next, mPendingPollIntent);
}
总结一下onPollNetworkTimeUnderWakeLock做的事情：
总结
NetworkTimeUpdateService启动后有两种方式触发时间更新。
1.网络发生变化；
2.Settings中的AUTO_TIME开关变化；
最终都会调用onPollNetworkTimeUnderWakeLock去设置时间，根据获取的cachedNtpResult情况设置不同时间的定时广播再去尝试更新时间。
以上就是Android NTP 时间同步机制详解的详细内容，更多关于Android NTP时间同步的资料请关注编程网其它相关文章！
