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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.impl;

import java.io.InputStream;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.directwebremoting.extend.DwrConstants;
import org.directwebremoting.util.LocalUtil;
import org.xml.sax.EntityResolver;
import org.xml.sax.InputSource;
import org.xml.sax.SAXException;


/**
 * We want to read the config files in validating mode, and keep the DTD within
 * the dwr.jar file so we need to be able to help the parser find the DTD.
 * @author Joe Walker [joe at getahead dot ltd dot uk]
 */
public final class DTDEntityResolver implements EntityResolver
{
    /* (non-Javadoc)
     * @see org.xml.sax.EntityResolver#resolveEntity(java.lang.String, java.lang.String)
     */
    public InputSource resolveEntity(String publicId, String systemId) throws SAXException
    {
        for (int i = 0; i < MAPPINGS.length; i++)
        {
            if (publicId.equals(MAPPINGS[i][0]))
            {
                if (i != MAPPINGS.length - 1)
                {
                    String doctype = "";

                    log.warn("Deprecated public id in dwr.xml. Use: " + doctype);
                }

                String dtdname = DwrConstants.PACKAGE_PATH + MAPPINGS[i][1];
                InputStream raw = LocalUtil.getInternalResourceAsStream(dtdname);
                return new InputSource(raw);
            }
        }

        throw new SAXException("Failed to resolve: publicId=" + publicId + " systemId=" + systemId);
    }

    /**
     * An array of mappings from public ids to files to read from.
     * The last one of these is the "only true DTD" the others are deprecated
     * so be careful if you re-order them.
     */
    private static final String[][] MAPPINGS =
    {
        { "-//GetAhead Limited//DTD Direct Web Remoting 0.4//EN", "/dwr10.dtd"},
        { "-//GetAhead Limited//DTD Direct Web Remoting 1.0//EN", "/dwr10.dtd"},
        { "-//GetAhead Limited//DTD Direct Web Remoting 2.0//EN", "/dwr20.dtd"},
        { "-//GetAhead Limited//DTD Direct Web Remoting 3.0//EN", "/dwr30.dtd"},
    };

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