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

/**
 * An Alarm that goes off 'manually', that is to say by the calling of
 * {@link #raiseAlarm()}.
 * @author Joe Walker [joe at getahead dot ltd dot uk]
 */
public class BasicAlarm implements Alarm
{
    /**
     * It's all gone wrong - inform the Sleeper
     */
    public void raiseAlarm()
    {
        synchronized (sleeperLock)
        {
            if (sleeper != null)
            {
                sleeper.wakeUp();
            }
        }
    }

    /* (non-Javadoc)
     * @see org.directwebremoting.dwrp.Alarm#cancel()
     */
    public void cancel()
    {
        synchronized (sleeperLock)
        {
            sleeper = null;
        }
    }

    /* (non-Javadoc)
     * @see org.directwebremoting.dwrp.Alarm#setAlarmAction(org.directwebremoting.dwrp.Sleeper)
     */
    public void setAlarmAction(Sleeper sleeper)
    {
        // I think the life-cycle dictates that we don't need to synchronize here
        // however it's probably simpler to synchronize and be sure
        synchronized (sleeperLock)
        {
            this.sleeper = sleeper;
        }
    }

    /**
     * The protection for the sleeper
     */
    private Object sleeperLock = new Object();

    /**
     * The thread that needs to know about shutdown
     * @protectedBy(sleeperLock)
     */
    private Sleeper sleeper;
}