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.helpers;
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 = 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          @Override
36          public void run() {
37              while (true) {
38                  final long time = System.currentTimeMillis();
39                  millis = time;
40  
41                  // avoid explicit dependency on sun.misc.Util
42                  LockSupport.parkNanos(1000 * 1000);
43              }
44          }
45      };
46  
47      private CachedClock() {
48          updater.setDaemon(true);
49          updater.start();
50      }
51  
52      public static CachedClock instance() {
53          return instance;
54      }
55  
56      /**
57       * Returns the value of a private long field that is updated by a background
58       * thread once every millisecond. Timers on most platforms do not
59       * have millisecond granularity, the returned value may "jump" every 10 or
60       * 16 milliseconds. To reduce this problem, this method also updates the
61       * internal time value every 1024 calls.
62       * @return the cached time
63       */
64      @Override
65      public long currentTimeMillis() {
66  
67          // improve granularity: also update time field every 1024 calls.
68          // (the bit fiddling means we don't need to worry about overflows)
69          if ((++count & UPDATE_THRESHOLD) == UPDATE_THRESHOLD) {
70              millis = System.currentTimeMillis();
71          }
72          return millis;
73      }
74  }