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DWR is easy Ajax for Java. It makes it simple to call Java code directly from Javascript. It gets rid of almost all the boilerplate code between the web browser and your Java code. This version 4.0.2 works with Jakarta Servlet 4.0.2.

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package org.directwebremoting.util;

/**
 * We need a way to record how heavily used the server is, and adjust our
 * behavior to reduce the load on the server.
 * @author Joe Walker [joe at getahead dot ltd dot uk]
 */
public class HitMonitor
{
    /**
     * Create a HitMonitor that records the number of hits in the last n
     * milliseconds.
     * @param seconds The number of seconds to record hits for
     */
    public HitMonitor(int seconds)
    {
        hitLog = new long[seconds];
    }

    /**
     * A hit has happened, record some load on the server
     */
    public void recordHit()
    {
        synchronized (hitLog)
        {
            trimHitLog();
            hitLog[0]++;
        }
    }

    /**
     * How to detect the number of hits in the time period specified in the
     * constructor.
     * @return The hit count
     */
    public int getHitsInLastPeriod()
    {
        synchronized (hitLog)
        {
            trimHitLog();

            int count = 0;
            for (long hit : hitLog)
            {
                count += hit;
            }

            return count;
        }
    }

    /**
     * Remove all the hits that are no longer relevant.
     * PERFORMANCE: There is probably a faster way to do this
     */
    private void trimHitLog()
    {
        long now = getCurrentTimestamp();
        long secondsPassedSinceLastHit = now - zeroTimestamp;
        zeroTimestamp = now;

        if (secondsPassedSinceLastHit > 0)
        {
            // Move the counts down
            for (int i = hitLog.length - 1; i >= 0; i--)
            {
                if (i >= secondsPassedSinceLastHit)
                {
                    hitLog[i] = hitLog[i - (int) secondsPassedSinceLastHit];
                }
                else
                {
                    hitLog[i] = 0;
                }
            }
        }
    }

    /**
     * A timestamp is {@link System#currentTimeMillis()} divided by 1000
     * @return The current timestamp
     */
    private static long getCurrentTimestamp()
    {
        return System.currentTimeMillis() / 1000;
    }

    /**
     * What is the timestamp of the first element of the hitLog?
     */
    private long zeroTimestamp = getCurrentTimestamp();

    /**
     * Our log of hits
     */
    private final long[] hitLog;
}