interactivespaces.time.NtpTimeProvider.java Source code

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Here is the source code for interactivespaces.time.NtpTimeProvider.java

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/*
 * Copyright (C) 2012 Google Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not
 * use this file except in compliance with the License. You may obtain a copy of
 * the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 * License for the specific language governing permissions and limitations under
 * the License.
 */

package interactivespaces.time;

import java.io.IOException;
import java.net.InetAddress;
import java.util.List;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.ScheduledFuture;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicLong;

import org.apache.commons.logging.Log;
import org.apache.commons.net.ntp.NTPUDPClient;
import org.apache.commons.net.ntp.TimeInfo;
import org.ros.math.CollectionMath;

import com.google.common.base.Preconditions;
import com.google.common.collect.Lists;

/**
 * A {@link TimeProvider} which uses NTP.
 *
 * @author Keith M. Hughes
 */
public class NtpTimeProvider implements TimeProvider {

    /**
     * The number of time samples.
     */
    private static final int SAMPLE_SIZE = 11;

    /**
     * The host for the NTP server.
     */
    private final InetAddress host;

    /**
     * The threadpool
     */
    private final ScheduledExecutorService scheduledExecutorService;

    /**
     * A local provider of time.
     */
    private final LocalTimeProvider localTimeProvider;

    /**
     * The NTP client
     */
    private final NTPUDPClient ntpClient;

    /**
     * The time drift offset
     */
    private AtomicLong offset;

    /**
     * Update period for the time provider
     */
    private long updatePeriod;

    /**
     * Time unit for the update period for the time provider.
     */
    private TimeUnit updatePeriodTimeUnit;

    /**
     * The thread reading the NTP client.
     */
    private ScheduledFuture<?> scheduledFuture;

    /**
     * Logging for provider.
     */
    private Log log;

    /**
     * @param host
     *          the NTP host to use
     * @param updatePeriod
     *          how often the time should be updated from NTP
     * @param updatePeriodTimeUnit
     *          time units for the update period
     * @param scheduledExecutorService
     *          thread pool to use
     * @param log
     *          logger for the provider
     */
    public NtpTimeProvider(InetAddress host, long updatePeriod, TimeUnit updatePeriodTimeUnit,
            ScheduledExecutorService scheduledExecutorService, Log log) {
        this.host = host;
        this.scheduledExecutorService = scheduledExecutorService;
        this.log = log;

        localTimeProvider = new LocalTimeProvider();
        ntpClient = new NTPUDPClient();
        offset = new AtomicLong(0);
        scheduledFuture = null;
    }

    /**
     * Update the current time offset from the configured NTP host.
     *
     * @throws IOException
     */
    public void updateTime() throws IOException {
        List<Long> offsets = Lists.newArrayList();
        for (int i = 0; i < SAMPLE_SIZE; i++) {
            offsets.add(computeOffset());
        }
        offset.set(CollectionMath.median(offsets));

        if (log.isDebugEnabled()) {
            log.debug(String.format("NTP time offset: %d ms", offset));
        }
    }

    private long computeOffset() throws IOException {
        log.info("Updating time offset from NTP server: " + host.getHostName());
        try {
            TimeInfo time = ntpClient.getTime(host);
            time.computeDetails();

            return time.getOffset();
        } catch (IOException e) {
            log.error("Failed to read time from NTP server: " + host.getHostName(), e);
            throw e;
        }
    }

    @Override
    public void startup() {
        // Starts periodically updating the current time offset periodically.
        //
        // The first time update happens immediately.
        //
        // Note that errors thrown while periodically updating time will be
        // logged but not rethrown.
        scheduledFuture = scheduledExecutorService.scheduleAtFixedRate(new Runnable() {
            @Override
            public void run() {
                try {
                    updateTime();
                } catch (IOException e) {
                    log.error("Periodic NTP update failed.", e);
                }
            }
        }, 0, updatePeriod, updatePeriodTimeUnit);
    }

    @Override
    public void shutdown() {
        Preconditions.checkNotNull(scheduledFuture);
        scheduledFuture.cancel(true);
        scheduledFuture = null;
    }

    @Override
    public long getCurrentTime() {
        long currentTime = localTimeProvider.getCurrentTime();
        return currentTime + offset.get();
    }
}