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 }