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The Apache Commons Digester package lets you configure an XML to Java object mapping module which triggers certain actions called rules whenever a particular pattern of nested XML elements is recognized.

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package org.apache.commons.digester3.plugins.strategies;

/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you 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.
 */

import static org.apache.commons.digester3.binder.DigesterLoader.newLoader;

import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.InputStream;

import org.apache.commons.digester3.Digester;
import org.apache.commons.digester3.plugins.PluginException;
import org.apache.commons.digester3.plugins.RuleLoader;
import org.apache.commons.digester3.xmlrules.FromXmlRulesModule;
import org.apache.commons.logging.Log;
import org.xml.sax.InputSource;

/**
 * A rule-finding algorithm which loads an xmlplugins-format file.
 * 
 * Note that the "include" feature of xmlrules is not supported.
 * 
 * @since 1.6
 */
public class LoaderFromStream
    extends RuleLoader
{

    private final byte[] input;

    /**
     * The contents of the input stream are loaded into memory, and cached for later use.
     * 
     * The caller is responsible for closing the input stream after this method has returned.
     *
     * @param s the input stream has to be loaded into memory
     * @throws Exception if any error occurs while reading the input stream
     */
    public LoaderFromStream( InputStream s )
        throws Exception
    {
        try
        {
            ByteArrayOutputStream baos = new ByteArrayOutputStream();
            byte[] buf = new byte[256];
            for ( ;; )
            {
                int i = s.read( buf );
                if ( i == -1 )
                {
                    break;
                }
                baos.write( buf, 0, i );
            }
            input = baos.toByteArray();
        }
        finally
        {
            try
            {
                if ( s != null )
                {
                    s.close();
                }
            }
            catch ( IOException e )
            {
                // close quietly
            }
        }
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public void addRules( final Digester d, final String path )
        throws PluginException
    {
        Log log = d.getLogger();
        boolean debug = log.isDebugEnabled();
        if ( debug )
        {
            log.debug( "LoaderFromStream: loading rules for plugin at path [" + path + "]" );
        }

        // Note that this input-source doesn't have any idea of its
        // system id, so it has no way of resolving relative URLs
        // such as the "include" feature of xmlrules. This is ok,
        // because that doesn't work well with our approach of
        // caching the input data in memory anyway.

        final InputSource source = new InputSource( new ByteArrayInputStream( input ) );
        newLoader( new FromXmlRulesModule()
        {

            @Override
            protected void loadRules()
            {
                useRootPath( path );
                loadXMLRules( source );
            }

        } ).createRuleSet().addRuleInstances( d );
    }

}