Android 时间更新机制之网络更新时间

转载请注明出处：http://blog.csdn.net/droyon/article/details/45701257

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);

        mDefaultServer = ((NtpTrustedTime) mTime).getServer();
        mNtpServers.add(mDefaultServer);
        for (String str : SERVERLIST)
        {
           mNtpServers.add(str);
        }
        mTryAgainCounter = 0;
    }

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) {
                //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();
                }
            }
//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。