org.apache.hadoop.util.Time.java Source code

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/**
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you 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 org.apache.hadoop.util;

import java.text.SimpleDateFormat;
import java.util.Calendar;
import java.util.TimeZone;

import org.apache.hadoop.classification.InterfaceAudience;
import org.apache.hadoop.classification.InterfaceStability;

/**
 * Utility methods for getting the time and computing intervals.
 */
@InterfaceAudience.LimitedPrivate({ "HDFS", "MapReduce" })
@InterfaceStability.Unstable
public final class Time {

    /**
     * number of nano seconds in 1 millisecond
     */
    private static final long NANOSECONDS_PER_MILLISECOND = 1000000;

    private static final TimeZone UTC_ZONE = TimeZone.getTimeZone("UTC");

    private static final ThreadLocal<SimpleDateFormat> DATE_FORMAT = new ThreadLocal<SimpleDateFormat>() {
        @Override
        protected SimpleDateFormat initialValue() {
            return new SimpleDateFormat("yyyy-MM-dd HH:mm:ss,SSSZ");
        }
    };

    /**
     * Current system time.  Do not use this to calculate a duration or interval
     * to sleep, because it will be broken by settimeofday.  Instead, use
     * monotonicNow.
     * @return current time in msec.
     */
    public static long now() {
        return System.currentTimeMillis();
    }

    /**
     * Current time from some arbitrary time base in the past, counting in
     * milliseconds, and not affected by settimeofday or similar system clock
     * changes.  This is appropriate to use when computing how much longer to
     * wait for an interval to expire.
     * This function can return a negative value and it must be handled correctly
     * by callers. See the documentation of System#nanoTime for caveats.
     * @return a monotonic clock that counts in milliseconds.
     */
    public static long monotonicNow() {
        return System.nanoTime() / NANOSECONDS_PER_MILLISECOND;
    }

    /**
     * Same as {@link #monotonicNow()} but returns its result in nanoseconds.
     * Note that this is subject to the same resolution constraints as
     * {@link System#nanoTime()}.
     * @return a monotonic clock that counts in nanoseconds.
     */
    public static long monotonicNowNanos() {
        return System.nanoTime();
    }

    /**
     * Convert time in millisecond to human readable format.
     * @return a human readable string for the input time
     */
    public static String formatTime(long millis) {
        return DATE_FORMAT.get().format(millis);
    }

    /**
     * Get the current UTC time in milliseconds.
     * @return the current UTC time in milliseconds.
     */
    public static long getUtcTime() {
        return Calendar.getInstance(UTC_ZONE).getTimeInMillis();
    }
}