    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    }