/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements. See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache license, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License. You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the license for the specific language governing permissions and
 * limitations under the license.
 */
package org.apache.logging.log4j.core.util;

import java.util.concurrent.locks.LockSupport;

Implementation of the Clock interface that tracks the time in a private long field that is updated by a background thread once every millisecond. Timers on most platforms do not have millisecond granularity, so the returned value may "jump" every 10 or 16 milliseconds. To reduce this problem, this class also updates the internal time value every 1024 calls to currentTimeMillis(). /**
 * Implementation of the {@code Clock} interface that tracks the time in a
 * private long field that is updated by a background thread once every
 * millisecond. Timers on most platforms do not have millisecond granularity, so
 * the returned value may "jump" every 10 or 16 milliseconds. To reduce this
 * problem, this class also updates the internal time value every 1024 calls to
 * {@code currentTimeMillis()}.
 */
public final class CachedClock implements Clock {
    private static final int UPDATE_THRESHOLD = 1000;
    private static volatile CachedClock instance;
    private static final Object INSTANCE_LOCK = new Object();
    private volatile long millis = System.currentTimeMillis();
    private short count = 0;

    private CachedClock() {
        final Thread updater = new Log4jThread(new Runnable() {
            @Override
            public void run() {
                while (true) {
                    final long time = System.currentTimeMillis();
                    millis = time;

                    // avoid explicit dependency on sun.misc.Util
                    LockSupport.parkNanos(1000 * 1000);
                }
            }
        }, "CachedClock Updater Thread");
        updater.setDaemon(true);
        updater.start();
    }

    public static CachedClock instance() {
        // LOG4J2-819: use lazy initialization of threads
        CachedClock result = instance;
        if (result == null) {
            synchronized (INSTANCE_LOCK) {
                result = instance;
                if (result == null) {
                    instance = result = new CachedClock();
                }
            }
        }
        return result;
    }

    
Returns the value of a private long field that is updated by a background
thread once every millisecond. Timers on most platforms do not
have millisecond granularity, the returned value may "jump" every 10 or
16 milliseconds. To reduce this problem, this method also updates the
internal time value every 1024 calls.
Returns:
the cached time/**
     * Returns the value of a private long field that is updated by a background
     * thread once every millisecond. Timers on most platforms do not
     * have millisecond granularity, the returned value may "jump" every 10 or
     * 16 milliseconds. To reduce this problem, this method also updates the
     * internal time value every 1024 calls.
     * @return the cached time
     */
    @Override
    public long currentTimeMillis() {

        // The count field is not volatile on purpose to reduce contention on this field.
        // This means that some threads may not see the increments made to this field
        // by other threads. This is not a problem: the timestamp does not need to be
        // updated exactly every 1000 calls.
        if (++count > UPDATE_THRESHOLD) {
            millis = System.currentTimeMillis(); // update volatile field: store-store barrier
            count = 0; // after a memory barrier: this change _is_ visible to other threads
        }
        return millis;
    }
}