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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.dwrp;

import java.util.TimerTask;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.directwebremoting.util.StaticTimer;

/**
 * An Alarm that goes off after a certain length of time.
 * @author Joe Walker [joe at getahead dot ltd dot uk]
 */
public class TimedAlarm extends BasicAlarm implements Alarm
{
    /**
     * @param waitTime How long we wait before the Alarm goes off
     */
    public TimedAlarm(long waitTime)
    {
        this.waitTime = waitTime;
    }

    /* (non-Javadoc)
     * @see org.directwebremoting.dwrp.PollHandler.Alarm#cancel()
     */
    public void cancel()
    {
        if (task != null)
        {
            task.cancel();
        }

        super.cancel();
    }

    /* (non-Javadoc)
     * @see org.directwebremoting.dwrp.PollHandler.Alarm#setAlarmAction(org.directwebremoting.dwrp.PollHandler.Sleeper)
     */
    public void setAlarmAction(Sleeper sleeper)
    {
        if (task != null)
        {
            throw new IllegalStateException("An alarm action has already been set.");
        }

        super.setAlarmAction(sleeper);

        if (waitTime == 0)
        {
            raiseAlarm();
        }
        else
        {
            task = new TimerTask()
            {
                public void run()
                {
                    try
                    {
                        raiseAlarm();
                    }
                    catch (Exception ex)
                    {
                        log.warn("Unexpected error raising alarm", ex);
                    }
                }
            };
    
            StaticTimer.schedule(task, waitTime);
        }
    }

    /**
     * The task that causes the alarm to go off
     */
    private TimerTask task;

    /**
     * How long do we wait for?
     */
    protected long waitTime;

    /**
     * The log stream
     */
    protected static final Log log = LogFactory.getLog(TimedAlarm.class);
}