001    /*
002     * Licensed to the Apache Software Foundation (ASF) under one or more
003     * contributor license agreements. See the NOTICE file distributed with
004     * this work for additional information regarding copyright ownership.
005     * The ASF licenses this file to You under the Apache license, Version 2.0
006     * (the "License"); you may not use this file except in compliance with
007     * the License. You may obtain a copy of the License at
008     *
009     *      http://www.apache.org/licenses/LICENSE-2.0
010     *
011     * Unless required by applicable law or agreed to in writing, software
012     * distributed under the License is distributed on an "AS IS" BASIS,
013     * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014     * See the license for the specific language governing permissions and
015     * limitations under the license.
016     */
017    package org.apache.logging.log4j.core.util;
018    
019    import java.util.concurrent.locks.LockSupport;
020    
021    /**
022     * This Clock implementation is similar to CachedClock. It is slightly faster at
023     * the cost of some accuracy.
024     */
025    public final class CoarseCachedClock implements Clock {
026        private static volatile CoarseCachedClock instance;
027        private static final Object INSTANCE_LOCK = new Object();
028        // ignore IDE complaints; volatile long is fine
029        private volatile long millis = System.currentTimeMillis();
030    
031        private final Thread updater = new Thread("Clock Updater Thread") {
032            @Override
033            public void run() {
034                while (true) {
035                    millis = System.currentTimeMillis();
036    
037                    // avoid explicit dependency on sun.misc.Util
038                    LockSupport.parkNanos(1000 * 1000);
039                }
040            }
041        };
042    
043        private CoarseCachedClock() {
044            updater.setDaemon(true);
045            updater.start();
046        }
047    
048        /**
049         * Returns the singleton instance.
050         *
051         * @return the singleton instance
052         */
053        public static CoarseCachedClock instance() {
054            // LOG4J2-819: use lazy initialization of threads
055            CoarseCachedClock result = instance;
056            if (result == null) {
057                synchronized (INSTANCE_LOCK) {
058                    result = instance;
059                    if (result == null) {
060                        instance = result = new CoarseCachedClock();
061                    }
062                }
063            }
064            return result;
065        }
066    
067        /**
068         * Returns the value of a private long field that is updated by a background
069         * thread once every millisecond. Because timers on most platforms do not
070         * have millisecond granularity, the returned value may "jump" every 10 or
071         * 16 milliseconds.
072         * @return the cached time
073         */
074        @Override
075        public long currentTimeMillis() {
076            return millis;
077        }
078    }