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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.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;
    }

    public Hub getHub()
    {
        return hub;
    }

    @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;
}