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    }