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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;

/*
 * 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 java.util.ArrayList;
import java.util.HashMap;
import java.util.List;

import org.xml.sax.Attributes;

/**
 * 
 * Default implementation of the Rules interface that supports the standard rule matching behavior. This
 * class can also be used as a base class for specialized Rules implementations.
 * 
 * 
 * The matching policies implemented by this class support two different types of pattern matching rules:
 * 
 * 
 * Exact Match - A pattern "a/b/c" exactly matches a <c> element, nested inside a
 * <b> element, which is nested inside an <a> element.
 * Tail Match - A pattern "*/a/b" matches a <b> element, nested inside an
 * <a> element, no matter how deeply the pair is nested.
 * 
 * 
 * Note that wildcard patterns are ignored if an explicit match can be found (and when multiple wildcard patterns match,
 * only the longest, ie most explicit, pattern is considered a match).
 * 
 * 
 * See the package documentation for package org.apache.commons.digester3 for more information.
 * 
 */

public class RulesBase
    extends AbstractRulesImpl
{

    // ----------------------------------------------------- Instance Variables

    /**
     * The set of registered Rule instances, keyed by the matching pattern. Each value is a List containing the Rules
     * for that pattern, in the order that they were orginally registered.
     */
    protected HashMap> cache = new HashMap>();

    /**
     * The set of registered Rule instances, in the order that they were originally registered.
     */
    protected ArrayList rules = new ArrayList();

    // ------------------------------------------------------------- Properties

    /**
     * {@inheritDoc}
     */
    @Override
    public void setDigester( Digester digester )
    {
        super.setDigester( digester );
        for ( Rule rule : rules )
        {
            rule.setDigester( digester );
        }
    }

    // --------------------------------------------------------- Public Methods

    /**
     * {@inheritDoc}
     */
    @Override
    protected void registerRule( String pattern, Rule rule )
    {
        // to help users who accidently add '/' to the end of their patterns
        int patternLength = pattern.length();
        if ( patternLength > 1 && pattern.endsWith( "/" ) )
        {
            pattern = pattern.substring( 0, patternLength - 1 );
        }

        List list = cache.get( pattern );
        if ( list == null )
        {
            list = new ArrayList();
            cache.put( pattern, list );
        }
        list.add( rule );
        rules.add( rule );
    }

    /**
     * {@inheritDoc}
     */
    public void clear()
    {
        cache.clear();
        rules.clear();
    }

    /**
     * {@inheritDoc}
     */
    public List match( String namespaceURI, String pattern, String name, Attributes attributes )
    {
        // List rulesList = (List) this.cache.get(pattern);
        List rulesList = lookup( namespaceURI, pattern );
        if ( ( rulesList == null ) || ( rulesList.size() < 1 ) )
        {
            // Find the longest key, ie more discriminant
            String longKey = "";
            for ( String key : cache.keySet() )
            {
                if ( key.startsWith( "*/" )
                                && ( pattern.equals( key.substring( 2 ) ) || pattern.endsWith( key.substring( 1 ) )
                                && key.length() > longKey.length() ) )
                {
                    // rulesList = (List) this.cache.get(key);
                    rulesList = lookup( namespaceURI, key );
                    longKey = key;
                }
            }
        }
        if ( rulesList == null )
        {
            rulesList = new ArrayList();
        }
        return ( rulesList );
    }

    /**
     * {@inheritDoc}
     */
    public List rules()
    {
        return ( this.rules );
    }

    // ------------------------------------------------------ Protected Methods

    /**
     * Return a List of Rule instances for the specified pattern that also match the specified namespace URI (if any).
     * If there are no such rules, return null.
     *
     * @param namespaceURI Namespace URI to match, or null to select matching rules regardless of namespace
     *            URI
     * @param pattern Pattern to be matched
     * @return a List of Rule instances for the specified pattern that also match the specified namespace URI (if any)
     */
    protected List lookup( String namespaceURI, String pattern )
    {
        // Optimize when no namespace URI is specified
        List list = this.cache.get( pattern );
        if ( list == null )
        {
            return ( null );
        }
        if ( ( namespaceURI == null ) || ( namespaceURI.length() == 0 ) )
        {
            return ( list );
        }

        // Select only Rules that match on the specified namespace URI
        ArrayList results = new ArrayList();
        for ( Rule item : list )
        {
            if ( ( namespaceURI.equals( item.getNamespaceURI() ) ) || ( item.getNamespaceURI() == null ) )
            {
                results.add( item );
            }
        }
        return ( results );
    }

}