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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.event;

import org.directwebremoting.ConversionException;
import org.directwebremoting.Hub;
import org.directwebremoting.extend.ConverterManager;
import org.directwebremoting.extend.RealRawData;

/**
 * An MessageEvent is fired to a set of {@link MessageListener}s by the DWR
 * {@link Hub}.
 * @author Joe Walker [joe at getahead dot ltd dot uk]
 */
public class DefaultMessageEvent implements MessageEvent
{
    /**
     * Constructor for use with server-side originated messages
     * @param hub The hub used to send the data
     * @param data The data to publish
     */
    public DefaultMessageEvent(Hub hub, Object data)
    {
        this.hub = hub;
        this.data = data;

        source = Source.SERVER;
    }

    /**
     * Constructor for use with client-side originated messages
     * @param hub The hub used to send the data
     * @param converterManager
     * @param rawData
     */
    public DefaultMessageEvent(Hub hub, ConverterManager converterManager, RealRawData rawData)
    {
        this.hub = hub;
        this.converterManager = converterManager;
        this.rawData = rawData;

        source = Source.INTERNET;
    }

    /* (non-Javadoc)
     * @see org.directwebremoting.event.MessageEvent#getHub()
     */
    public Hub getHub()
    {
        return hub;
    }

    /* (non-Javadoc)
     * @see org.directwebremoting.event.MessageEvent#getData(java.lang.Class)
     */
    @SuppressWarnings("unchecked")
    public  T getData(Class asType)
    {
        if (source == Source.SERVER)
        {
            try
            {
                return (T) data;
            }
            catch (ClassCastException ex)
            {
                throw new ConversionException(asType, ex);
            }
        }
        else
        {
            return converterManager.convertInbound(asType, rawData);
        }
    }

    /**
     * WARNING: This method is for internal use only. It may well disappear at
     * some stage in the future
     * Sometimes we just want to get at whatever the data was originally without
     * any conversion.
     * @return The original data probably of type RawData
     */
    public Object getRawData()
    {
        if (source == Source.SERVER)
        {
            return data;
        }
        else
        {
            return rawData;
        }
    }

    /**
     * Did the data come from the web or from the server?
     */
    private enum Source
    {
        SERVER, INTERNET
    }

    private Hub hub;
    private Object data;
    private RealRawData rawData;
    private Source source;
    private ConverterManager converterManager;
}