计算Meter，每个一个UPDATE_INTERVAL_SECONDS调用一次update方法

MeterView.java

package org.apache.flink.metrics;

public class MeterView implements Meter, View {
	/** The underlying counter maintaining the count */
	private final Counter counter;
	/** The time-span over which the average is calculated */
	private final int timeSpanInSeconds;
	/** Circular array containing the history of values */
	private final long[] values;
	/** The index in the array for the current time */
	private int time = 0;
	/** The last rate we computed */
	private double currentRate = 0;

	public MeterView(int timeSpanInSeconds) {
		this(new SimpleCounter(), timeSpanInSeconds);
	}

	public MeterView(Counter counter, int timeSpanInSeconds) {
		this.counter = counter;
		this.timeSpanInSeconds = timeSpanInSeconds - (timeSpanInSeconds % UPDATE_INTERVAL_SECONDS);
		this.values = new long[this.timeSpanInSeconds / UPDATE_INTERVAL_SECONDS + 1];
	}
	
	@Override
	public void markEvent() {
		this.counter.inc();
	}

	@Override
	public void markEvent(long n) {
		this.counter.inc(n);
	}

	@Override
	public long getCount() {
		return counter.getCount();
	}

	@Override
	public double getRate() {
		return currentRate;
	}

	@Override
	//这个方法很有趣，
	public void update() {
		time = (time + 1) % values.length;
		values[time] = counter.getCount();
		currentRate =  ((double) (values[time] - values[(time + 1) % values.length]) / timeSpanInSeconds);
	}
}

update方法写的很有意思