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 }