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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.convert;

import java.lang.reflect.Modifier;
import java.util.HashMap;
import java.util.Map;
import java.util.Map.Entry;
import java.util.StringTokenizer;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.directwebremoting.ConversionException;
import org.directwebremoting.extend.ConvertUtil;
import org.directwebremoting.extend.Converter;
import org.directwebremoting.extend.ConverterManager;
import org.directwebremoting.extend.InboundContext;
import org.directwebremoting.extend.InboundVariable;
import org.directwebremoting.extend.ObjectOutboundVariable;
import org.directwebremoting.extend.OutboundContext;
import org.directwebremoting.extend.OutboundVariable;
import org.directwebremoting.extend.Property;
import org.directwebremoting.extend.ProtocolConstants;
import org.directwebremoting.util.JavascriptUtil;
import org.directwebremoting.util.LocalUtil;

/**
 * An implementation of Converter for Maps.
 * @author Joe Walker [joe at getahead dot ltd dot uk]
 */
public class MapConverter implements Converter
{
    /* (non-Javadoc)
     * @see org.directwebremoting.Converter#setConverterManager(org.directwebremoting.ConverterManager)
     */
    public void setConverterManager(ConverterManager converterManager)
    {
        this.converterManager = converterManager;
    }

    /* (non-Javadoc)
     * @see org.directwebremoting.Converter#convertInbound(java.lang.Class, org.directwebremoting.InboundVariable, org.directwebremoting.InboundContext)
     */
    @SuppressWarnings("unchecked")
    public Object convertInbound(Class paramType, InboundVariable data) throws ConversionException
    {
        if (data.isNull())
        {
            return null;
        }

        String value = data.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_MAP_START) || !value.endsWith(ProtocolConstants.INBOUND_MAP_END))
        {
            log.warn("Expected object while converting data for " + paramType.getName() + " in " + data.getContext().getCurrentProperty() + ". Passed: " + value);
            throw new ConversionException(paramType, "Data conversion error. See logs for more details.");
        }

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

        try
        {
            // Maybe we ought to check that the paramType isn't expecting a more
            // distinct type of Map and attempt to create that?
            Map map;

            // If paramType is concrete then just use whatever we've got.
            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 instantiable so we accept failure.
                map = (Map) paramType.getDeclaredConstructor().newInstance();
            }
            else
            {
                map = new HashMap();
            }

            // Get the extra type info
            Property parent = data.getContext().getCurrentProperty();

            Property keyProp = parent.createChild(0);
            keyProp = converterManager.checkOverride(keyProp);
            Class keyType = keyProp.getPropertyType();

            Property valProp = parent.createChild(1);
            valProp = converterManager.checkOverride(valProp);
            Class valType = valProp.getPropertyType();

            // We should put the new object into the working map in case it
            // is referenced later nested down in the conversion process.
            data.getContext().addConverted(data, paramType, map);
            InboundContext incx = data.getContext();

            // Loop through the property declarations
            StringTokenizer st = new StringTokenizer(value, ",");
            int size = st.countTokens();
            for (int i = 0; i < size; i++)
            {
                String token = st.nextToken();
                if (token.trim().length() == 0)
                {
                    continue;
                }

                int colonpos = token.indexOf(ProtocolConstants.INBOUND_MAP_ENTRY);
                if (colonpos == -1)
                {
                    throw new ConversionException(paramType, "Missing " + ProtocolConstants.INBOUND_MAP_ENTRY + " in object description: {1}" + token);
                }

                // Convert the value part of the token by splitting it into the
                // type and value (as passed in by Javascript)
                String valStr = token.substring(colonpos + 1).trim();
                String[] splitIv = ConvertUtil.splitInbound(valStr);
                String splitIvValue = splitIv[ConvertUtil.INBOUND_INDEX_VALUE];
                String splitIvType = splitIv[ConvertUtil.INBOUND_INDEX_TYPE];
                InboundVariable valIv = new InboundVariable(incx, null, splitIvType, splitIvValue);
                valIv.dereference();
                Object val = converterManager.convertInbound(valType, valIv, valProp);

                // Keys (unlike values) do not have type info passed with them
                // Could we have recursive key? - I don't think so because keys
                // must be strings in Javascript
                String keyStr = token.substring(0, colonpos).trim();
                //String[] keySplit = LocalUtil.splitInbound(keyStr);
                //InboundVariable keyIv = new InboundVariable(incx, splitIv[LocalUtil.INBOUND_INDEX_TYPE], splitIv[LocalUtil.INBOUND_INDEX_VALUE]);
                InboundVariable keyIv = new InboundVariable(incx, null, ProtocolConstants.TYPE_STRING, keyStr);
                keyIv.dereference();

                Object key = converterManager.convertInbound(keyType, keyIv, keyProp);

                map.put(key, val);
            }

            return map;
        }
        catch (ConversionException ex)
        {
            throw ex;
        }
        catch (Exception ex)
        {
            throw new ConversionException(paramType, ex);
        }
    }

    /* (non-Javadoc)
     * @see org.directwebremoting.Converter#convertOutbound(java.lang.Object, org.directwebremoting.OutboundContext)
     */
    @SuppressWarnings("unchecked")
    public OutboundVariable convertOutbound(Object data, OutboundContext outctx) throws ConversionException
    {
        // First we just collect our converted children
        //noinspection unchecked
        Map ovs = LocalUtil.classNewInstance("OrderedConvertOutbound", "java.util.LinkedHashMap", Map.class);
        if (ovs == null)
        {
            ovs = new HashMap();
        }

        ObjectOutboundVariable ov = new ObjectOutboundVariable(outctx);
        outctx.put(data, ov);

        // Loop through the map outputting any init code and collecting
        // converted outbound variables into the ovs map
        Map map = (Map) data;
        for (Entry entry : map.entrySet())
        {
            Object key = entry.getKey();
            Object value = entry.getValue();

            // If the key is null, retrieve the configured null key value from the configuration of this converter.
            if (null == key && null != nullKey) {
                log.debug("MapConverter: A null key was encountered DWR is using the configured nullKey string: " + nullKey);
                key = nullKey;
            }

            // It would be nice to check for Enums here
            if (!(key instanceof String) && !sentNonStringWarning)
            {
                log.warn("--Javascript does not support non string keys. Converting '" + key.getClass().getName() + "' using toString()");
                sentNonStringWarning = true;
            }

            String outkey = JavascriptUtil.escapeJavaScript(key.toString());

            /*
            OutboundVariable ovkey = converterManager.convertOutbound(key, outctx);
            buffer.append(ovkey.getInitCode());
            outkey = ovkey.getAssignCode();
            */

            OutboundVariable nested = converterManager.convertOutbound(value, outctx);

            ovs.put(outkey, nested);
        }

        ov.setChildren(ovs);

        return ov;
    }

    /**
     * @return the nullKey
     */
    public String getNullKey()
    {
        return nullKey;
    }

    /**
     * @param nullKey the nullKey to set
     */
    public void setNullKey(String nullKey)
    {
        this.nullKey = nullKey;
    }

    /**
     * We don't want to give the non-string warning too many times.
     */
    private static boolean sentNonStringWarning = false;

    /**
     * To forward marshalling requests
     */
    private ConverterManager converterManager = null;

    /**
     * Used by DWR if a null key is encountered.  Configure in the init section when the map converter is declared.
     * 
     *     
     * 
     */
    private String nullKey = null;

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