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.helpers;
018
019import java.util.concurrent.locks.LockSupport;
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 */
029public 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        @Override
036        public void run() {
037            while (true) {
038                final long time = System.currentTimeMillis();
039                millis = time;
040
041                // avoid explicit dependency on sun.misc.Util
042                LockSupport.parkNanos(1000 * 1000);
043            }
044        }
045    };
046
047    private CachedClock() {
048        updater.setDaemon(true);
049        updater.start();
050    }
051
052    public static CachedClock 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. Timers on most platforms do not
059     * have millisecond granularity, the returned value may "jump" every 10 or
060     * 16 milliseconds. To reduce this problem, this method also updates the
061     * internal time value every 1024 calls.
062     * @return the cached time
063     */
064    @Override
065    public long currentTimeMillis() {
066
067        // improve granularity: also update time field every 1024 calls.
068        // (the bit fiddling means we don't need to worry about overflows)
069        if ((++count & UPDATE_THRESHOLD) == UPDATE_THRESHOLD) {
070            millis = System.currentTimeMillis();
071        }
072        return millis;
073    }
074}