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 */
017package org.apache.logging.log4j.core.util;
018
019import 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 */
025public 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}