src/main/java/org/apache/commons/configuration2/HierarchicalConfigurationConverter.java

/*
 * 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.
 */

package org.apache.commons.configuration2;

import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.HashSet;
import java.util.Iterator;
import java.util.List;
import java.util.Set;

import org.apache.commons.configuration2.tree.DefaultConfigurationKey;
import org.apache.commons.configuration2.tree.DefaultExpressionEngine;

/**
 * <p>A base class for converters that transform a normal configuration
 * object into a hierarchical configuration.</p>
 * <p>This class provides a default mechanism for iterating over the keys in a
 * configuration and to throw corresponding element start and end events. By
 * handling these events a hierarchy can be constructed that is equivalent to
 * the keys in the original configuration.</p>
 * <p>Concrete sub classes will implement event handlers that generate SAX
 * events for XML processing or construct a
 * {@code HierarchicalConfiguration} root node. All in all with this class
 * it is possible to treat a default configuration as if it was a hierarchical
 * configuration, which can be sometimes useful.</p>
 * @see HierarchicalConfiguration
 *
 * @author <a
 * href="http://commons.apache.org/configuration/team-list.html">Commons Configuration team</a>
 * @version $Id$
 */
abstract class HierarchicalConfigurationConverter
{
    /**
     * Processes the specified configuration object. This method implements
     * the iteration over the configuration's keys. All defined keys are
     * translated into a set of element start and end events represented by
     * calls to the {@code elementStart()} and
     * {@code elementEnd()} methods.
     *
     * @param config the configuration to be processed
     */
    public void process(Configuration config)
    {
        if (config != null)
        {
            DefaultExpressionEngine exprEngine = DefaultExpressionEngine.INSTANCE;
            DefaultConfigurationKey keyEmpty =
                    new DefaultConfigurationKey(exprEngine);
            DefaultConfigurationKey keyLast = keyEmpty;
            Set<String> keySet = new HashSet<String>();

            for (Iterator<String> it = config.getKeys(); it.hasNext();)
            {
                String key = it.next();
                if (keySet.contains(key))
                {
                    // this key has already been processed by openElements
                    continue;
                }
                DefaultConfigurationKey keyAct =
                        new DefaultConfigurationKey(exprEngine, key);
                closeElements(keyLast, keyAct);
                String elem = openElements(keyLast, keyAct, config, keySet);
                fireValue(elem, config.getProperty(key));
                keyLast = keyAct;
            }

            // close all open
            closeElements(keyLast, keyEmpty);
        }
    }

    /**
     * An event handler method that is called when an element starts.
     * Concrete sub classes must implement it to perform a proper event
     * handling.
     *
     * @param name the name of the new element
     * @param value the element's value; can be <b>null</b> if the element
     * does not have any value
     */
    protected abstract void elementStart(String name, Object value);

    /**
     * An event handler method that is called when an element ends. For each
     * call of {@code elementStart()} there will be a corresponding call
     * of this method. Concrete sub classes must implement it to perform a
     * proper event handling.
     *
     * @param name the name of the ending element
     */
    protected abstract void elementEnd(String name);

    /**
     * Fires all necessary element end events for the specified keys. This
     * method is called for each key obtained from the configuration to be
     * converted. It calculates the common part of the actual and the last
     * processed key and thus determines how many elements must be
     * closed.
     *
     * @param keyLast the last processed key
     * @param keyAct the actual key
     */
    protected void closeElements(DefaultConfigurationKey keyLast, DefaultConfigurationKey keyAct)
    {
        DefaultConfigurationKey keyDiff = keyAct.differenceKey(keyLast);
        Iterator<String> it = reverseIterator(keyDiff);
        if (it.hasNext())
        {
            // Skip first because it has already been closed by fireValue()
            it.next();
        }

        while (it.hasNext())
        {
            elementEnd(it.next());
        }
    }

    /**
     * Helper method for determining a reverse iterator for the specified key.
     * This implementation returns an iterator that returns the parts of the
     * given key in reverse order, ignoring indices.
     *
     * @param key the key
     * @return a reverse iterator for the parts of this key
     */
    protected Iterator<String> reverseIterator(DefaultConfigurationKey key)
    {
        List<String> list = new ArrayList<String>();
        for (DefaultConfigurationKey.KeyIterator it = key.iterator(); it.hasNext();)
        {
            list.add(it.nextKey());
        }

        Collections.reverse(list);
        return list.iterator();
    }

    /**
     * Fires all necessary element start events for the specified key. This
     * method is called for each key obtained from the configuration to be
     * converted. It ensures that all elements "between" the last key and the
     * actual key are opened and their values are set.
     *
     * @param keyLast the last processed key
     * @param keyAct the actual key
     * @param config the configuration to process
     * @param keySet the set with the processed keys
     * @return the name of the last element on the path
     */
    protected String openElements(DefaultConfigurationKey keyLast, DefaultConfigurationKey keyAct,
            Configuration config, Set<String> keySet)
    {
        DefaultConfigurationKey.KeyIterator it = keyLast.differenceKey(keyAct).iterator();
        DefaultConfigurationKey k = keyLast.commonKey(keyAct);
        for (it.nextKey(); it.hasNext(); it.nextKey())
        {
            k.append(it.currentKey(true));
            elementStart(it.currentKey(true), config.getProperty(k.toString()));
            keySet.add(k.toString());
        }
        return it.currentKey(true);
    }

    /**
     * Fires all necessary element start events with the actual element values.
     * This method is called for each key obtained from the configuration to be
     * processed with the last part of the key as argument. The value can be
     * either a single value or a collection.
     *
     * @param name the name of the actual element
     * @param value the element's value
     */
    protected void fireValue(String name, Object value)
    {
        if (value instanceof Collection)
        {
            Collection<?> valueCol = (Collection<?>) value;
            for (Object v : valueCol)
            {
                fireValue(name, v);
            }
        }
        else
        {
            elementStart(name, value);
            elementEnd(name);
        }
    }
}