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.async;
018    
019    import com.lmax.disruptor.util.Util;
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 CoarseCachedClock instance = new CoarseCachedClock();
027        private volatile long millis = System.currentTimeMillis();
028    
029        private final Thread updater = new Thread("Clock Updater Thread") {
030            public void run() {
031                while (true) {
032                    long time = System.currentTimeMillis();
033                    millis = time;
034                    Util.getUnsafe().park(true, time + 1); // abs (millis)
035                    // Util.getUnsafe().park(false, 1000 * 1000);// relative(nanos)
036                }
037            }
038        };
039    
040        private CoarseCachedClock() {
041            updater.setDaemon(true);
042            updater.start();
043        }
044    
045        /**
046         * Returns the singleton instance.
047         * 
048         * @return the singleton instance
049         */
050        public static CoarseCachedClock instance() {
051            return instance;
052        }
053    
054        /**
055         * Returns the value of a private long field that is updated by a background
056         * thread once every millisecond. Because timers on most platforms do not
057         * have millisecond granularity, the returned value may "jump" every 10 or
058         * 16 milliseconds.
059         * @return the cached time
060         */
061        @Override
062        public long currentTimeMillis() {
063            return millis;
064        }
065    }