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     * Implementation of the {@code Clock} interface that tracks the time in a
023     * private long field that is updated by a background thread once every
024     * millisecond. Timers on most platforms do not have millisecond granularity, so
025     * the returned value may "jump" every 10 or 16 milliseconds. To reduce this
026     * problem, this class also updates the internal time value every 1024 calls to
027     * {@code currentTimeMillis()}.
028     */
029    public final class CachedClock implements Clock {
030        private static final int UPDATE_THRESHOLD = 0x3FF;
031        private static CachedClock instance = new CachedClock();
032        private volatile long millis = System.currentTimeMillis();
033        private volatile short count = 0;
034        private final Thread updater = new Thread("Clock Updater Thread") {
035            public void run() {
036                while (true) {
037                    long time = System.currentTimeMillis();
038                    millis = time;
039                    Util.getUnsafe().park(true, time + 1); // abs (millis)
040                    // Util.getUnsafe().park(false, 1000 * 1000);// relative(nanos)
041                }
042            }
043        };
044    
045        private CachedClock() {
046            updater.setDaemon(true);
047            updater.start();
048        }
049    
050        public static CachedClock 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. Timers on most platforms do not
057         * have millisecond granularity, the returned value may "jump" every 10 or
058         * 16 milliseconds. To reduce this problem, this method also updates the
059         * internal time value every 1024 calls.
060         * @return the cached time
061         */
062        @Override
063        public long currentTimeMillis() {
064    
065            // improve granularity: also update time field every 1024 calls.
066            // (the bit fiddling means we don't need to worry about overflows)
067            if ((++count & UPDATE_THRESHOLD) == UPDATE_THRESHOLD) {
068                millis = System.currentTimeMillis();
069            }
070            return millis;
071        }
072    }