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    1   /*
    2    * Licensed to the Apache Software Foundation (ASF) under one or more
    3    * contributor license agreements. See the NOTICE file distributed with
    4    * this work for additional information regarding copyright ownership.
    5    * The ASF licenses this file to You under the Apache license, Version 2.0
    6    * (the "License"); you may not use this file except in compliance with
    7    * the License. You may obtain a copy of the License at
    8    *
    9    *      http://www.apache.org/licenses/LICENSE-2.0
    10   *
    11   * Unless required by applicable law or agreed to in writing, software
    12   * distributed under the License is distributed on an "AS IS" BASIS,
    13   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    14   * See the license for the specific language governing permissions and
    15   * limitations under the license.
    16   */
    17  package org.apache.logging.log4j.core.util;
    18
    19  import java.util.concurrent.locks.LockSupport;
    20
    21  /**
    22   * This Clock implementation is similar to CachedClock. It is slightly faster at
    23   * the cost of some accuracy.
    24   */
    25  public final class CoarseCachedClock implements Clock {
    26      private static volatile CoarseCachedClock instance;
    27      private static final Object INSTANCE_LOCK = new Object();
    28      // ignore IDE complaints; volatile long is fine
    29      private volatile long millis = System.currentTimeMillis();
    30
    31      private final Thread updater = new Thread("Clock Updater Thread") {
    32          @Override
    33          public void run() {
    34              while (true) {
    35                  millis = System.currentTimeMillis();
    36
    37                  // avoid explicit dependency on sun.misc.Util
    38                  LockSupport.parkNanos(1000 * 1000);
    39              }
    40          }
    41      };
    42
    43      private CoarseCachedClock() {
    44          updater.setDaemon(true);
    45          updater.start();
    46      }
    47
    48      /**
    49       * Returns the singleton instance.
    50       *
    51       * @return the singleton instance
    52       */
    53      public static CoarseCachedClock instance() {
    54          // LOG4J2-819: use lazy initialization of threads
    55          CoarseCachedClock result = instance;
    56          if (result == null) {
    57              synchronized (INSTANCE_LOCK) {
    58                  result = instance;
    59                  if (result == null) {
    60                      instance = result = new CoarseCachedClock();
    61                  }
    62              }
    63          }
    64          return result;
    65      }
    66
    67      /**
    68       * Returns the value of a private long field that is updated by a background
    69       * thread once every millisecond. Because timers on most platforms do not
    70       * have millisecond granularity, the returned value may "jump" every 10 or
    71       * 16 milliseconds.
    72       * @return the cached time
    73       */
    74      @Override
    75      public long currentTimeMillis() {
    76          return millis;
    77      }
    78  }