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Android 4.2 network time update

2013年09月21日 ⁄ 综合 ⁄ 共 6214字 ⁄ 字号 评论关闭

frameworks/base/services/java/com/android/server/NetworkTimeUpdateService.java:

private void onPollNetworkTime(int event) {
        // If Automatic time is not set, don't bother.
        if (!isAutomaticTimeRequested()) return;

        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            // force refresh NTP cache when outdated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);
    }

 /**
     * Cancel old alarm and starts a new one for the specified interval.
     *
     * @param interval when to trigger the alarm, starting from now.
     */
    private void resetAlarm(long interval) {
        mAlarmManager.cancel(mPendingPollIntent);
        long now = SystemClock.elapsedRealtime();
        long next = now + interval;
        mAlarmManager.set(AlarmManager.ELAPSED_REALTIME, next, mPendingPollIntent);
    }

frameworks/base/core/java/android/util/NtpTrustedTime.java:

 private NtpTrustedTime(String server, long timeout) {
        if (LOGD) Log.d(TAG, "creating NtpTrustedTime using " + server);
        mServer = server;
        mTimeout = timeout;
    }

    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);
        }

        return sSingleton;
    }

    @Override
    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;
        }
    }

    @Override
    public boolean hasCache() {
        return mHasCache;
    }

    @Override
    public long getCacheAge() {
        if (mHasCache) {
            return SystemClock.elapsedRealtime() - mCachedNtpElapsedRealtime;
        } else {
            return Long.MAX_VALUE;
        }
    }

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>

frameworks/base/core/java/android/net/SntpClient.java:

/**
     * Sends an SNTP request to the given host and processes the response.
     *
     * @param host host name of the server.
     * @param timeout network timeout in milliseconds.
     * @return true if the transaction was successful.
     */
    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;
    }

    /**
     * Returns the time computed from the NTP transaction.
     *
     * @return time value computed from NTP server response.
     */
    public long getNtpTime() {
        return mNtpTime;
    }

reference:

http://blog.sina.com.cn/s/blog_85e826230101bqc1.html

http://blog.sina.com.cn/s/blog_85e826230101bp35.html

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