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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 boiler plate code between the web browser and your Java code.

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/*
 * Copyright 2005 Joe Walker
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package org.directwebremoting.impl;

import java.util.Random;

import org.directwebremoting.extend.ServerLoadMonitor;
import org.directwebremoting.extend.WaitController;
import org.directwebremoting.util.HitMonitor;

/**
 * A smart implementation of ServerLoadMonitor, customized for Jetty.
 * The ThreadDroppingServerLoadMonitor attempts to keep the hit rate down by increasing
 * the disconnected time as usage increases.
 * @author Joe Walker [joe at getahead dot org]
 */
public class ThreadDroppingServerLoadMonitor extends AbstractServerLoadMonitor implements ServerLoadMonitor
{
    /* (non-Javadoc)
     * @see org.directwebremoting.extend.ServerLoadMonitor#getConnectedTime()
     */
    public long getConnectedTime()
    {
        return connectedTime;
    }

    /* (non-Javadoc)
     * @see org.directwebremoting.ServerLoadMonitor#timeToNextPoll()
     */
    public int getDisconnectedTime()
    {
        return disconnectedTime;
    }

    /* (non-Javadoc)
     * @see org.directwebremoting.impl.AbstractServerLoadMonitor#threadWaitStarting(org.directwebremoting.extend.WaitController)
     */
    public void threadWaitStarting(WaitController controller)
    {
        hitMonitor.recordHit();
        super.threadWaitStarting(controller);

        checkLoading();
    }

    /* (non-Javadoc)
     * @see org.directwebremoting.impl.AbstractServerLoadMonitor#threadWaitEnding(org.directwebremoting.extend.WaitController)
     */
    public void threadWaitEnding(WaitController controller)
    {
        super.threadWaitEnding(controller);
    }

    /**
     * Check that we are setting the time to next poll correctly.
     */
    private void checkLoading()
    {
        float hitsPerSecond = (float) hitMonitor.getHitsInLastPeriod() / SECONDS_MONITORED;

        // If we're getting close to the upper bound then slow down
        float load = hitsPerSecond / maxHitsPerSecond;

        disconnectedTime = (int) (disconnectedTime * load);
        if (disconnectedTime == 0)
        {
            disconnectedTime = 1;
        }
    }

    /**
     * @param maxHitsPerSecond the maxHitsPerSecond to set
     */
    public void setMaxHitsPerSecond(int maxHitsPerSecond)
    {
        this.maxHitsPerSecond = maxHitsPerSecond;
    }

    /**
     * Static configuration data: The max number of hits per second.
     * We increase the poll time to compensate and reduce the load. If this
     * number is not at least half maxWaitingThreads then the USAGE_HIGH mode
     * will not exist and the system will sublime from USAGE_LOW to USAGE_DIGG
     */
    protected int maxHitsPerSecond = 100;

    /**
     * We ask clients to wait a random number of millis before they come
     * back to avoid killing the server
     */
    protected Random random = new Random();

    /**
     * The time we are currently waiting before sending a browser away and
     * asking it to reconnect.
     */
    protected int connectedTime = 60000;

    /**
     * How long are we telling users to wait before they come back next
     */
    protected int disconnectedTime = 1;

    /**
     * We are recording the number of hits in the last 5 seconds.
     * Maybe we should think about making this configurable.
     */
    protected static final int SECONDS_MONITORED = 10;

    /**
     * Our record of the server loading
     */
    protected HitMonitor hitMonitor = new HitMonitor(SECONDS_MONITORED);
}