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 final CoarseCachedClock instance = new CoarseCachedClock();
027        // ignore IDE complaints; volatile long is fine
028        private volatile long millis = System.currentTimeMillis();
029    
030        private final Thread updater = new Thread("Clock Updater Thread") {
031            @Override
032            public void run() {
033                while (true) {
034                    millis = System.currentTimeMillis();
035    
036                    // avoid explicit dependency on sun.misc.Util
037                    LockSupport.parkNanos(1000 * 1000);
038                }
039            }
040        };
041    
042        private CoarseCachedClock() {
043            updater.setDaemon(true);
044            updater.start();
045        }
046    
047        /**
048         * Returns the singleton instance.
049         *
050         * @return the singleton instance
051         */
052        public static CoarseCachedClock instance() {
053            return instance;
054        }
055    
056        /**
057         * Returns the value of a private long field that is updated by a background
058         * thread once every millisecond. Because timers on most platforms do not
059         * have millisecond granularity, the returned value may "jump" every 10 or
060         * 16 milliseconds.
061         * @return the cached time
062         */
063        @Override
064        public long currentTimeMillis() {
065            return millis;
066        }
067    }