    001    /*
    002     * Licensed to the Apache Software Foundation (ASF) under one or more
    003     * contributor license agreements.  See the NOTICE file distributed with
    004     * this work for additional information regarding copyright ownership.
    005     * The ASF licenses this file to You under the Apache License, Version 2.0
    006     * (the "License"); you may not use this file except in compliance with
    007     * the License.  You may obtain a copy of the License at
    008     *
    009     *     http://www.apache.org/licenses/LICENSE-2.0
    010     *
    011     * Unless required by applicable law or agreed to in writing, software
    012     * distributed under the License is distributed on an "AS IS" BASIS,
    013     * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    014     * See the License for the specific language governing permissions and
    015     * limitations under the License.
    016     */
    017    package org.apache.commons.configuration.tree;
    018
    019    import java.util.Iterator;
    020
    021    /**
    022     * <p>
    023     * A concrete combiner implementation that is able to construct an override
    024     * combination.
    025     * </p>
    026     * <p>
    027     * An <em>override combination</em> means that nodes in the first node
    028     * structure take precedence over nodes in the second, or - in other words -
    029     * nodes of the second structure are only added to the resulting structure if
    030     * they do not occure in the first one. This is especially suitable for dealing
    031     * with the properties of configurations that are defined in an
    032     * <code>override</code> section of a configuration definition file (hence the
    033     * name).
    034     * </p>
    035     * <p>
    036     * This combiner will iterate over the second node hierarchy and find all nodes
    037     * that are not contained in the first hierarchy; these are added to the result.
    038     * If a node can be found in both structures, it is checked whether a
    039     * combination (in a recursive way) can be constructed for the two, which will
    040     * then be added. Per default, nodes are combined, which occur only once in both
    041     * structures. This test is implemented in the <code>canCombine()</code>
    042     * method.
    043     * </p>
    044     * <p>
    045     * As is true for the <code>{@link UnionCombiner}</code>, for this combiner
    046     * list nodes are important. The <code>addListNode()</code> can be called to
    047     * declare certain nodes as list nodes. This has the effect that these nodes
    048     * will never be combined.
    049     * </p>
    050     *
    051     * @author <a
    052     * href="http://commons.apache.org/configuration/team-list.html">Commons
    053     * Configuration team</a>
    054     * @version $Id: OverrideCombiner.java 561230 2007-07-31 04:17:09Z rahul $
    055     * @since 1.3
    056     */
    057    public class OverrideCombiner extends NodeCombiner
    058    {
    059        /**
    060         * Constructs an override combination for the passed in node structures.
    061         *
    062         * @param node1 the first node
    063         * @param node2 the second node
    064         * @return the resulting combined node structure
    065         */
    066        public ConfigurationNode combine(ConfigurationNode node1,
    067                ConfigurationNode node2)
    068        {
    069            ViewNode result = createViewNode();
    070            result.setName(node1.getName());
    071
    072            // Process nodes from the first structure, which override the second
    073            for (Iterator it = node1.getChildren().iterator(); it.hasNext();)
    074            {
    075                ConfigurationNode child = (ConfigurationNode) it.next();
    076                ConfigurationNode child2 = canCombine(node1, node2, child);
    077                if (child2 != null)
    078                {
    079                    result.addChild(combine(child, child2));
    080                }
    081                else
    082                {
    083                    result.addChild(child);
    084                }
    085            }
    086
    087            // Process nodes from the second structure, which are not contained
    088            // in the first structure
    089            for (Iterator it = node2.getChildren().iterator(); it.hasNext();)
    090            {
    091                ConfigurationNode child = (ConfigurationNode) it.next();
    092                if (node1.getChildrenCount(child.getName()) < 1)
    093                {
    094                    result.addChild(child);
    095                }
    096            }
    097
    098            // Handle attributes and value
    099            addAttributes(result, node1, node2);
    100            result.setValue((node1.getValue() != null) ? node1.getValue() : node2
    101                    .getValue());
    102
    103            return result;
    104        }
    105
    106        /**
    107         * Handles the attributes during a combination process. First all attributes
    108         * of the first node will be added to the result. Then all attributes of the
    109         * second node, which are not contained in the first node, will also be
    110         * added.
    111         *
    112         * @param result the resulting node
    113         * @param node1 the first node
    114         * @param node2 the second node
    115         */
    116        protected void addAttributes(ViewNode result, ConfigurationNode node1,
    117                ConfigurationNode node2)
    118        {
    119            result.appendAttributes(node1);
    120            for (Iterator it = node2.getAttributes().iterator(); it.hasNext();)
    121            {
    122                ConfigurationNode attr = (ConfigurationNode) it.next();
    123                if (node1.getAttributeCount(attr.getName()) == 0)
    124                {
    125                    result.addAttribute(attr);
    126                }
    127            }
    128        }
    129
    130        /**
    131         * Tests if a child node of the second node can be combined with the given
    132         * child node of the first node. If this is the case, the corresponding node
    133         * will be returned, otherwise <b>null</b>. This implementation checks
    134         * whether the child node occurs only once in both hierarchies and is no
    135         * known list node.
    136         *
    137         * @param node1 the first node
    138         * @param node2 the second node
    139         * @param child the child node (of the first node)
    140         * @return a child of the second node, with which a combination is possible
    141         */
    142        protected ConfigurationNode canCombine(ConfigurationNode node1,
    143                ConfigurationNode node2, ConfigurationNode child)
    144        {
    145            if (node2.getChildrenCount(child.getName()) == 1
    146                    && node1.getChildrenCount(child.getName()) == 1
    147                    && !isListNode(child))
    148            {
    149                return (ConfigurationNode) node2.getChildren(child.getName())
    150                        .get(0);
    151            }
    152            else
    153            {
    154                return null;
    155            }
    156        }
    157    }