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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.impl;

import org.directwebremoting.extend.Alarm;
import org.directwebremoting.extend.ServerLoadMonitor;
import org.directwebremoting.extend.Sleeper;
import org.directwebremoting.extend.WaitController;

/**
 * An Alarm that allows the system to close all connections when it is shutting
 * down.
 * WARNING: This code has a non-obvious side effect - The server load
 * monitor (which hands out shutdown messages) also monitors usage by looking
 * at the number of connected alarms.
 * @author Joe Walker [joe at getahead dot ltd dot uk]
 */
public class ShutdownAlarm implements Alarm
{
    /**
     * Register ourselves with the ServerLoadMonitor so we can raise an
     * Alarm if we get shutdown
     * @param sleeper ...
     * @param serverLoadMonitor ..
     */
    public ShutdownAlarm(Sleeper sleeper, ServerLoadMonitor serverLoadMonitor)
    {
        this.sleeper = sleeper;
        this.serverLoadMonitor = serverLoadMonitor;
        this.serverLoadMonitor.threadWaitStarting(waitController);
    }

    public void cancel()
    {
        serverLoadMonitor.threadWaitEnding(waitController);
    }

    /**
     * The listener to allow the ServerLoadMonitor to shut us down
     */
    private final WaitController waitController = new WaitController()
    {
        public void shutdown()
        {
            sleeper.wakeUpToClose();
            shutdown = true;
        }

        public boolean isShutdown()
        {
            return shutdown;
        }

        /**
         * We need to be able to say if we have been shutdown
         */
        boolean shutdown = false;
    };

    /**
     * The thread that needs to know about shutdown
     */
    private final Sleeper sleeper;

    /**
     * The source of shutdown messages
     */
    protected final ServerLoadMonitor serverLoadMonitor;
}