View Javadoc

    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   * Implementation of the {@code Clock} interface that tracks the time in a
    23   * private long field that is updated by a background thread once every
    24   * millisecond. Timers on most platforms do not have millisecond granularity, so
    25   * the returned value may "jump" every 10 or 16 milliseconds. To reduce this
    26   * problem, this class also updates the internal time value every 1024 calls to
    27   * {@code currentTimeMillis()}.
    28   */
    29  public final class CachedClock implements Clock {
    30      private static final int UPDATE_THRESHOLD = 1000;
    31      private static volatile CachedClock instance;
    32      private static final Object INSTANCE_LOCK = new Object();
    33      private volatile long millis = System.currentTimeMillis();
    34      private short count = 0;
    35
    36      private CachedClock() {
    37          final Thread updater = new Thread(new Runnable() {
    38              @Override
    39              public void run() {
    40                  while (true) {
    41                      final long time = System.currentTimeMillis();
    42                      millis = time;
    43
    44                      // avoid explicit dependency on sun.misc.Util
    45                      LockSupport.parkNanos(1000 * 1000);
    46                  }
    47              }
    48          }, "Clock Updater Thread");
    49          updater.setDaemon(true);
    50          updater.start();
    51      }
    52
    53      public static CachedClock instance() {
    54          // LOG4J2-819: use lazy initialization of threads
    55          CachedClock result = instance;
    56          if (result == null) {
    57              synchronized (INSTANCE_LOCK) {
    58                  result = instance;
    59                  if (result == null) {
    60                      instance = result = new CachedClock();
    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. Timers on most platforms do not
    70       * have millisecond granularity, the returned value may "jump" every 10 or
    71       * 16 milliseconds. To reduce this problem, this method also updates the
    72       * internal time value every 1024 calls.
    73       * @return the cached time
    74       */
    75      @Override
    76      public long currentTimeMillis() {
    77
    78          // The count field is not volatile on purpose to reduce contention on this field.
    79          // This means that some threads may not see the increments made to this field
    80          // by other threads. This is not a problem: the timestamp does not need to be
    81          // updated exactly every 1000 calls.
    82          if (++count > UPDATE_THRESHOLD) {
    83              millis = System.currentTimeMillis(); // update volatile field: store-store barrier
    84              count = 0; // after a memory barrier: this change _is_ visible to other threads
    85          }
    86          return millis;
    87      }
    88  }