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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 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 {

  /**
   * 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() {
    final long NANOSECONDS_PER_MILLISECOND = 1000000;
    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();
  }
}