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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.io.IOException;
import java.util.TimerTask;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.directwebremoting.ScriptBuffer;
import org.directwebremoting.ScriptSession;
import org.directwebremoting.extend.RealScriptSession;
import org.directwebremoting.extend.ScriptConduit;
import org.directwebremoting.util.StaticTimer;

/**
 * An Alarm that goes off whenever output happens on a {@link ScriptSession}.
 * @author Joe Walker [joe at getahead dot ltd dot uk]
 */
public class OutputAlarm extends BasicAlarm implements Alarm
{
    /**
     * @param scriptSession The script session to monitor
     * @param maxWaitAfterWrite How long do we wait after output
     */
    public OutputAlarm(RealScriptSession scriptSession, int maxWaitAfterWrite)
    {
        this.maxWaitAfterWrite = maxWaitAfterWrite;
        this.scriptSession = scriptSession;
    }

    /* (non-Javadoc)
     * @see org.directwebremoting.dwrp.Alarm#setAlarmAction(org.directwebremoting.dwrp.Sleeper)
     */
    public void setAlarmAction(Sleeper sleeper)
    {
        try
        {
            scriptSession.addScriptConduit(conduit);
        }
        catch (IOException ex)
        {
            log.warn("Error adding monitor to script session", ex);
        }

        super.setAlarmAction(sleeper);
    }

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

    /**
     * @author Joe Walker [joe at getahead dot ltd dot uk]
     */
    protected class AlarmScriptConduit extends ScriptConduit
    {
        /**
         * Create an AlarmScriptConduit
         */
        protected AlarmScriptConduit()
        {
            super(RANK_PROCEDURAL);
        }

        /* (non-Javadoc)
         * @see org.directwebremoting.extend.ScriptConduit#addScript(org.directwebremoting.ScriptBuffer)
         */
        public boolean addScript(ScriptBuffer script)
        {
            if (maxWaitAfterWrite == 0)
            {
                raiseAlarm();
            }
            else
            {
                task = new TimerTask()
                {
                    public void run()
                    {
                        try
                        {
                            raiseAlarm();
                        }
                        catch (Exception ex)
                        {
                            log.warn("Unexpected error raising alarm", ex);
                        }
                    }
                };

                StaticTimer.schedule(task, maxWaitAfterWrite);
            }

            return false;
        }
    }

    /**
     * A conduit to alert us if there is output
     */
    protected ScriptConduit conduit = new AlarmScriptConduit();

    /**
     * How long do we wait after output happens in case there is more output
     */
    protected int maxWaitAfterWrite;

    /**
     * The script session to monitor for output
     */
    protected RealScriptSession scriptSession;

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

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