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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.io.IOException;
import java.io.StringReader;
import java.io.StringWriter;

import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.transform.Source;
import javax.xml.transform.Transformer;
import javax.xml.transform.TransformerFactory;
import javax.xml.transform.dom.DOMSource;
import javax.xml.transform.stream.StreamResult;

import org.directwebremoting.ConversionException;
import org.directwebremoting.extend.AbstractConverter;
import org.directwebremoting.extend.EnginePrivate;
import org.directwebremoting.extend.InboundVariable;
import org.directwebremoting.extend.NonNestedOutboundVariable;
import org.directwebremoting.extend.OutboundContext;
import org.directwebremoting.extend.OutboundVariable;
import org.w3c.dom.Document;
import org.w3c.dom.Element;
import org.w3c.dom.Node;
import org.xml.sax.EntityResolver;
import org.xml.sax.InputSource;
import org.xml.sax.SAXException;

/**
 * An implementation of Converter for DOM objects.
 * @author Joe Walker [joe at getahead dot ltd dot uk]
 */
public class DOMConverter extends AbstractConverter
{
    /* (non-Javadoc)
     * @see org.directwebremoting.Converter#convertInbound(java.lang.Class, org.directwebremoting.InboundVariable, org.directwebremoting.InboundContext)
     */
    public Object convertInbound(Class paramType, InboundVariable data) throws ConversionException
    {
        if (data.isNull())
        {
            return null;
        }

        String value = data.urlDecode();

        try
        {
            if (buildFactory == null)
            {
                buildFactory = DocumentBuilderFactory.newInstance();

                // Protect us from hackers, see:
                // https://www.owasp.org/index.php/XML_External_Entity_%28XXE%29_Processing
                buildFactory.setFeature("http://xml.org/sax/features/external-general-entities", false);
                buildFactory.setFeature("http://xml.org/sax/features/external-parameter-entities", false);
                try {
                    buildFactory.setFeature("http://apache.org/xml/features/disallow-doctype-decl", true);
                } catch(Exception ex) {
                    // XML parser doesn't have this setting, never mind
                }
            }

            DocumentBuilder builder = buildFactory.newDocumentBuilder();

            // Extra protection from external entity hacking
            builder.setEntityResolver(new EntityResolver()
            {
                public InputSource resolveEntity(String publicId, String systemId) throws SAXException, IOException
                {
                    return new InputSource(); // no lookup, just return empty
                }
            });

            InputSource is = new InputSource(new StringReader(value));
            Document doc = builder.parse(is);

            if (paramType == Document.class)
            {
                return doc;
            }
            else if (paramType == Element.class)
            {
                return doc.getDocumentElement();
            }

            throw new ConversionException(paramType);
        }
        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)
     */
    public OutboundVariable convertOutbound(Object data, OutboundContext outctx) throws ConversionException
    {
        try
        {
            Transformer transformer = xslFact.newTransformer();

            // Using XSLT to convert to a stream. Setup the source
            Source source;
            if (data instanceof Node)
            {
                Node node = (Node) data;
                source = new DOMSource(node);
            }
            else
            {
                throw new ConversionException(data.getClass());
            }

            // Setup the destination
            StringWriter xml = new StringWriter();
            StreamResult result = new StreamResult(xml);

            transformer.transform(source, result);

            xml.flush();

            String script;
            if (data instanceof Element)
            {
                script = EnginePrivate.xmlStringToJavascriptDomElement(xml.toString());
            }
            else
            {
                script = EnginePrivate.xmlStringToJavascriptDomDocument(xml.toString());
            }

            OutboundVariable ov = new NonNestedOutboundVariable(script);
            outctx.put(data, ov);

            return ov;
        }
        catch (ConversionException ex)
        {
            throw ex;
        }
        catch (Exception ex)
        {
            throw new ConversionException(data.getClass(), ex);
        }
    }

    /**
     * How we create new transformers
     */
    private final TransformerFactory xslFact = TransformerFactory.newInstance();

    /**
     * How we create new documents
     */
    private DocumentBuilderFactory buildFactory = null;
}