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Provides common utility functions
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/*
 * Copyright (c) CQSE GmbH
 *
 * 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.conqat.lib.commons.xml;

import java.io.ByteArrayInputStream;
import java.io.IOException;

import org.xml.sax.InputSource;
import org.xml.sax.SAXException;
import org.xml.sax.helpers.DefaultHandler;

/**
 * Base class for SAX parser handlers. This class has a default implementation of
 * {@link #resolveEntity(String, String)} that works around a bug in the Java SAX parser
 * implementation:
 * 
 * If {@link #resolveEntity(String, String)} returns null for a given DTD (the default
 * behavior of {@link DefaultHandler}), the Java SAX parser will try to download that DTD. This will
 * fail on machines that have no Internet connection or require a proxy to access the Internet or
 * for DTDs where the URI is not a valid URL. This class' default implementation instead returns an
 * empty {@link InputSource}, which effectively causes the parser to skip DTD validation.
 * 
 * Please note that downloading the DTD from the Internet is against the DTD specification (as the
 * DTD URI is not required to be a valid URL) and is highly error-prone, resulting in a hard to
 * interpret error message. It is thus recommended you use this base class and never return
 * null from {@link #resolveEntity(String, String)}.
 */
public class OfflineSaxHandlerBase extends DefaultHandler {

	/**
	 * {@inheritDoc}
	 * 
	 * Default implementation that can be overwritten by subclasses. This implementation returns an
	 * empty document for any entity that should be resolved. This effectively causes the parser to skip
	 * DTD validation.
	 * 
	 * @throws IOException
	 *             may be thrown by subclasses.
	 * @throws SAXException
	 *             may be thrown by subclasses.
	 */
	@Override
	public InputSource resolveEntity(String publicId, String systemId) throws IOException, SAXException {
		return new InputSource(new ByteArrayInputStream(new byte[0]));
	}
}