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 }