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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.convert;

import java.lang.reflect.Modifier;
import java.util.ArrayList;
import java.util.Collection;
import java.util.HashSet;
import java.util.Iterator;
import java.util.List;
import java.util.Set;
import java.util.SortedSet;
import java.util.StringTokenizer;
import java.util.TreeSet;

import org.directwebremoting.dwrp.ArrayOutboundVariable;
import org.directwebremoting.dwrp.ErrorOutboundVariable;
import org.directwebremoting.dwrp.ParseUtil;
import org.directwebremoting.dwrp.ProtocolConstants;
import org.directwebremoting.extend.Converter;
import org.directwebremoting.extend.ConverterManager;
import org.directwebremoting.extend.InboundContext;
import org.directwebremoting.extend.InboundVariable;
import org.directwebremoting.extend.MarshallException;
import org.directwebremoting.extend.OutboundContext;
import org.directwebremoting.extend.OutboundVariable;
import org.directwebremoting.extend.TypeHintContext;
import org.directwebremoting.util.LocalUtil;
import org.directwebremoting.util.Logger;
import org.directwebremoting.util.Messages;

/**
 * An implementation of Converter for Collections of Strings.
 * @author Joe Walker [joe at eireneh dot com]
 * @version $Id: StringConverter.java,v 1.2 2004/11/04 15:54:07 joe_walker Exp $
 */
public class CollectionConverter extends BaseV20Converter implements Converter
{
    /* (non-Javadoc)
     * @see org.directwebremoting.convert.BaseV20Converter#setConverterManager(org.directwebremoting.ConverterManager)
     */
    public void setConverterManager(ConverterManager newConfig)
    {
        this.config = newConfig;
    }

    /* (non-Javadoc)
     * @see org.directwebremoting.Converter#convertInbound(java.lang.Class, org.directwebremoting.InboundVariable, org.directwebremoting.InboundContext)
     */
    public Object convertInbound(Class paramType, InboundVariable iv, InboundContext inctx) throws MarshallException
    {
        String value = iv.getValue();

        // If the text is null then the whole bean is null
        if (value.trim().equals(ProtocolConstants.INBOUND_NULL))
        {
            return null;
        }

        if (!value.startsWith(ProtocolConstants.INBOUND_ARRAY_START))
        {
            throw new MarshallException(paramType, Messages.getString("CollectionConverter.FormatError", ProtocolConstants.INBOUND_ARRAY_START));
        }

        if (!value.endsWith(ProtocolConstants.INBOUND_ARRAY_END))
        {
            throw new MarshallException(paramType, Messages.getString("CollectionConverter.FormatError", ProtocolConstants.INBOUND_ARRAY_END));
        }

        value = value.substring(1, value.length() - 1);

        try
        {
            TypeHintContext icc = inctx.getCurrentTypeHintContext();

            TypeHintContext subthc = icc.createChildContext(0);
            Class subtype = subthc.getExtraTypeInfo();

            // subtype.getMethod("h", null).getTypeParameters();
            Collection col;

            // If they want an iterator then just use an array list and fudge
            // at the end.
            if (Iterator.class.isAssignableFrom(paramType))
            {
                col = new ArrayList();
            }
            // If paramType is concrete then just use whatever we've got.
            else if (!paramType.isInterface() && !Modifier.isAbstract(paramType.getModifiers()))
            {
                // If there is a problem creating the type then we have no way
                // of completing this - they asked for a specific type and we
                // can't create that type. I don't know of a way of finding
                // subclasses that might be instaniable so we accept failure.
                col = (Collection) paramType.newInstance();
            }
            // If they want a SortedSet then use TreeSet
            else if (SortedSet.class.isAssignableFrom(paramType))
            {
                col = new TreeSet();
            }
            // If they want a Set then use HashSet
            else if (Set.class.isAssignableFrom(paramType))
            {
                col = new HashSet();
            }
            // If they want a List then use an ArrayList
            else if (List.class.isAssignableFrom(paramType))
            {
                col = new ArrayList();
            }
            // If they just want a Collection then just use an ArrayList
            else if (Collection.class.isAssignableFrom(paramType))
            {
                col = new ArrayList();
            }
            else
            {
                throw new MarshallException(paramType);
            }

            // We should put the new object into the working map in case it
            // is referenced later nested down in the conversion process.
            inctx.addConverted(iv, paramType, col);

            StringTokenizer st = new StringTokenizer(value, ProtocolConstants.INBOUND_ARRAY_SEPARATOR);
            int size = st.countTokens();
            for (int i = 0; i < size; i++)
            {
                String token = st.nextToken();

                String[] split = ParseUtil.splitInbound(token);
                String splitType = split[LocalUtil.INBOUND_INDEX_TYPE];
                String splitValue = split[LocalUtil.INBOUND_INDEX_VALUE];

                InboundVariable nested = new InboundVariable(iv.getLookup(), null, splitType, splitValue);

                Object output = config.convertInbound(subtype, nested, inctx, subthc);
                col.add(output);
            }

            // If they wanted an Iterator then give them one otherwise use
            // the type we created
            if (Iterator.class.isAssignableFrom(paramType))
            {
                return col.iterator();
            }
            else
            {
                return col;
            }
        }
        catch (Exception ex)
        {
            throw new MarshallException(paramType, ex);
        }
    }

    /* (non-Javadoc)
     * @see org.directwebremoting.Converter#convertOutbound(java.lang.Object, org.directwebremoting.OutboundContext)
     */
    public OutboundVariable convertOutbound(Object data, OutboundContext outctx) throws MarshallException
    {
        // First we need to get ourselves the collection data
        Iterator it;
        if (data instanceof Collection)
        {
            Collection col = (Collection) data;
            it = col.iterator();
        }
        else if (data instanceof Iterator)
        {
            it = (Iterator) data;
        }
        else
        {
            throw new MarshallException(data.getClass());
        }

        // Stash this bit of data to cope with recursion
        ArrayOutboundVariable ov = new ArrayOutboundVariable(outctx);
        outctx.put(data, ov);

        // Convert all the data members
        List ovs = new ArrayList();
        while (it.hasNext())
        {
            Object member = it.next();
            OutboundVariable nested;

            try
            {
                nested = config.convertOutbound(member, outctx);
            }
            catch (Exception ex)
            {
                String errorMessage = "Conversion error for " + data.getClass().getName() + ".";
                log.warn(errorMessage, ex);

                nested = new ErrorOutboundVariable(outctx, errorMessage, true);
            }

            ovs.add(nested);
        }

        // Group the list of converted objects into this OutboundVariable
        ov.init(ovs);

        return ov;
    }

    /**
     * For nested conversions
     */
    private ConverterManager config = null;

    /**
     * The log stream
     */
    private static final Logger log = Logger.getLogger(CollectionConverter.class);
}