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// Copyright 2016 Yahoo Inc. Licensed under the terms of the Apache 2.0 license. See LICENSE in the project root.
package com.yahoo.time;
import com.google.common.annotations.Beta;
/**
* A source for high-resolution timestamps.
*
* @author arnej27959
*/
@Beta
public class WallClockSource {
private volatile long offset;
/**
* Obtain the current time in nanoseconds.
* The epoch is January 1, 1970 UTC just as for System.currentTimeMillis(),
* but with greater resolution. Note that the absolute accuracy may be
* no better than currentTimeMills().
* @return nanoseconds since the epoch.
**/
public final long currentTimeNanos() {
return System.nanoTime() + offset;
}
/**
* Create a source with 1 millisecond accuracy at start.
**/
WallClockSource() {
long actual = System.currentTimeMillis();
actual *= 1000000;
this.offset = actual - System.nanoTime();
initialAdjust();
}
/** adjust the clock source from currentTimeMillis()
* to ensure that it is no more than 1 milliseconds off.
* @return true if we want adjust called again soon
**/
boolean adjust() {
long nanosB = System.nanoTime();
long actual = System.currentTimeMillis();
long nanosA = System.nanoTime();
if (nanosA - nanosB > 100000) {
return true; // not a good time to adjust, try again soon
}
return adjustOffset(nanosB, actual, nanosA);
}
private boolean adjustOffset(long before, long actual, long after) {
actual *= 1000000; // convert millis to nanos
if (actual > after + offset) {
// System.out.println("WallClockSource adjust UP "+(actual-after-offset));
offset = actual - after;
return true;
}
if (actual + 999999 < before + offset) {
// System.out.println("WallClockSource adjust DOWN "+(before+offset-actual-999999));
offset = actual + 999999 - before;
return true;
}
return false;
}
private void initialAdjust() {
for (int i = 0; i < 100; i++) {
long nanosB = System.nanoTime();
long actual = System.currentTimeMillis();
long nanosA = System.nanoTime();
adjustOffset(nanosB, actual, nanosA);
}
}
static private WallClockSource autoAdjustingInstance = new WallClockSource();
/**
* Get a WallClockSource which auto adjusts to wall clock time.
**/
static public WallClockSource get() {
autoAdjustingInstance.startAdjuster();
return autoAdjustingInstance;
}
private Thread adjuster;
private synchronized void startAdjuster() {
if (adjuster == null) {
adjuster = new AdjustThread();
adjuster.setDaemon(true);
adjuster.start();
}
}
private class AdjustThread extends Thread {
public void run() {
int millis = 0;
int nanos = 313373; // random number
while (true) {
try {
sleep(millis, nanos);
if (++millis > 4321) {
millis = 1000; // do not sleep too long
}
} catch (InterruptedException e) {
return;
}
if (adjust()) {
// adjust more often in case clock jumped
millis = 0;
}
}
}
}
}
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