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
    <!-- Normal polling frequency in milliseconds -->
    <integer name="config_ntpPollingInterval">86400000</integer>
    <!-- Try-again polling interval in milliseconds, in case the network request failed -->
    <integer name="config_ntpPollingIntervalShorter">60000</integer>
    <!-- Number of times to try again with the shorter interval, before backing
         off until the normal polling interval. A value < 0 indicates infinite. -->
    <integer name="config_ntpRetry">3</integer>
    <!-- If the time difference is greater than this threshold in milliseconds,
         then update the time. -->
    <integer name="config_ntpThreshold">5000</integer>
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
    <!-- Remote server that can provide NTP responses. -->
    <string translatable="false" name="config_ntpServer">2.android.pool.ntp.org</string>
    <!-- Timeout to wait for NTP server response in milliseconds. -->
    <integer name="config_ntpTimeout">5000</integer>
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。