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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.async;
18  
19  import com.lmax.disruptor.util.Util;
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 = 0x3FF;
31      private static CachedClock instance = new CachedClock();
32      private volatile long millis = System.currentTimeMillis();
33      private volatile short count = 0;
34      private final Thread updater = new Thread("Clock Updater Thread") {
35          public void run() {
36              while (true) {
37                  long time = System.currentTimeMillis();
38                  millis = time;
39                  Util.getUnsafe().park(true, time + 1); // abs (millis)
40                  // Util.getUnsafe().park(false, 1000 * 1000);// relative(nanos)
41              }
42          }
43      };
44  
45      private CachedClock() {
46          updater.setDaemon(true);
47          updater.start();
48      }
49  
50      public static CachedClock instance() {
51          return instance;
52      }
53  
54      /**
55       * Returns the value of a private long field that is updated by a background
56       * thread once every millisecond. Timers on most platforms do not
57       * have millisecond granularity, the returned value may "jump" every 10 or
58       * 16 milliseconds. To reduce this problem, this method also updates the
59       * internal time value every 1024 calls.
60       * @return the cached time
61       */
62      @Override
63      public long currentTimeMillis() {
64  
65          // improve granularity: also update time field every 1024 calls.
66          // (the bit fiddling means we don't need to worry about overflows)
67          if ((++count & UPDATE_THRESHOLD) == UPDATE_THRESHOLD) {
68              millis = System.currentTimeMillis();
69          }
70          return millis;
71      }
72  }