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 * 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 = 1000;
031    private static volatile CachedClock instance;
032    private static final Object INSTANCE_LOCK = new Object();
033    private volatile long millis = System.currentTimeMillis();
034    private short count = 0;
035
036    private CachedClock() {
037        final Thread updater = new Thread(new Runnable() {
038            @Override
039            public void run() {
040                while (true) {
041                    final long time = System.currentTimeMillis();
042                    millis = time;
043
044                    // avoid explicit dependency on sun.misc.Util
045                    LockSupport.parkNanos(1000 * 1000);
046                }
047            }
048        }, "Clock Updater Thread");
049        updater.setDaemon(true);
050        updater.start();
051    }
052
053    public static CachedClock instance() {
054        // LOG4J2-819: use lazy initialization of threads
055        CachedClock result = instance;
056        if (result == null) {
057            synchronized (INSTANCE_LOCK) {
058                result = instance;
059                if (result == null) {
060                    instance = result = new CachedClock();
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. Timers on most platforms do not
070     * have millisecond granularity, the returned value may "jump" every 10 or
071     * 16 milliseconds. To reduce this problem, this method also updates the
072     * internal time value every 1024 calls.
073     * @return the cached time
074     */
075    @Override
076    public long currentTimeMillis() {
077
078        // The count field is not volatile on purpose to reduce contention on this field.
079        // This means that some threads may not see the increments made to this field
080        // by other threads. This is not a problem: the timestamp does not need to be
081        // updated exactly every 1000 calls.
082        if (++count > UPDATE_THRESHOLD) {
083            millis = System.currentTimeMillis(); // update volatile field: store-store barrier
084            count = 0; // after a memory barrier: this change _is_ visible to other threads
085        }
086        return millis;
087    }
088}