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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.substitution;

/*
 * 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 org.apache.commons.digester3.Substitutor;

import org.xml.sax.Attributes;

/**
 * Substitutor implementation that support variable replacement for both attributes and body text. The actual expansion
 * of variables into text is delegated to {@link VariableExpander} implementations. Supports setting an expander just
 * for body text or just for attributes. Also supported is setting no expanders for body text and for attributes.
 * 
 * @since 1.6
 */
public class VariableSubstitutor
    extends Substitutor
{

    /**
     * The expander to be used to expand variables in the attributes. Null when no expansion should be performed.
     */
    private final VariableExpander attributesExpander;

    /**
     * Attributes implementation that (lazily) performs variable substitution. Will be lazily created when needed then
     * reused.
     */
    private final VariableAttributes variableAttributes;

    /**
     * The expander to be used to expand variables in the body text. Null when no expansion should be performed.
     */
    private final VariableExpander bodyTextExpander;

    /**
     * Constructs a Substitutor which uses the same VariableExpander for both body text and attibutes.
     * 
     * @param expander VariableExpander implementation, null if no substitutions are to be performed
     */
    public VariableSubstitutor( VariableExpander expander )
    {
        this( expander, expander );
    }

    /**
     * Constructs a Substitutor.
     * 
     * @param attributesExpander VariableExpander implementation to be used for attributes, null if no attribute
     *            substitutions are to be performed
     * @param bodyTextExpander VariableExpander implementation to be used for bodyTextExpander, null if no attribute
     *            substitutions are to be performed
     */
    public VariableSubstitutor( VariableExpander attributesExpander, VariableExpander bodyTextExpander )
    {
        this.attributesExpander = attributesExpander;
        this.bodyTextExpander = bodyTextExpander;
        variableAttributes = new VariableAttributes();
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public Attributes substitute( Attributes attributes )
    {
        Attributes results = attributes;
        if ( attributesExpander != null )
        {
            variableAttributes.init( attributes, attributesExpander );
            results = variableAttributes;
        }
        return results;
    }

    /**
     * {@inheritDoc}
     */
    @Override
    public String substitute( String bodyText )
    {
        String result = bodyText;
        if ( bodyTextExpander != null )
        {
            result = bodyTextExpander.expand( bodyText );
        }
        return result;
    }

}