DropwizardMeanBug.java

import com.codahale.metrics.ExponentiallyDecayingReservoir;
import com.codahale.metrics.Timer;
import com.codahale.metrics.Timer.Context;

public class DropwizardMeanBug {

	public static void main (String[] args) throws InterruptedException {
		MyClock clock= new MyClock(System.currentTimeMillis());
		ExponentiallyDecayingReservoir reservoir= new ExponentiallyDecayingReservoir(1028, 0.015, clock);
		Timer timer= new Timer(reservoir, clock);

		Context context= timer.time();
		clock.progressInTime(100);
		context.stop();

		for (int i= 1; i < 24; i++) {
			System.out.println("Time: " + clock.getTime() + " Mean: " + timer.getSnapshot().getMean());
			clock.progressInTime(i * 60 * 60 * 1000);
		}
	}
}

MyClock.java

import com.codahale.metrics.Clock;

public class MyClock extends Clock {

	private long millis;

	public MyClock (long millis) {
		this.millis= millis;
	}

	@Override
	public long getTick () {
		return this.millis * 1000000;
	}

	@Override
	public long getTime () {
		return this.millis;
	}

	public void progressInTime (long millis) {
		this.millis+= millis;
	}
}

The output of this example is:

Time: 1502354348353 Mean: 1.0E8
Time: 1502357948353 Mean: 1.0E8
Time: 1502365148353 Mean: 1.0E8
Time: 1502375948353 Mean: 1.0E8
Time: 1502390348353 Mean: 1.0E8
Time: 1502408348353 Mean: NaN
Time: 1502429948353 Mean: NaN
Time: 1502455148353 Mean: NaN
Time: 1502483948353 Mean: NaN
Time: 1502516348353 Mean: NaN
Time: 1502552348353 Mean: NaN
Time: 1502591948353 Mean: NaN
Time: 1502635148353 Mean: NaN
Time: 1502681948353 Mean: NaN
Time: 1502732348353 Mean: 0.0
Time: 1502786348353 Mean: 0.0
Time: 1502843948353 Mean: 0.0
Time: 1502905148353 Mean: 0.0
Time: 1502969948353 Mean: 0.0
Time: 1503038348353 Mean: 0.0
Time: 1503110348353 Mean: 0.0
Time: 1503185948353 Mean: 0.0
Time: 1503265148353 Mean: 0.0