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 */
017package org.apache.commons.configuration.tree;
018
019
020/**
021 * <p>
022 * A concrete combiner implementation that is able to construct an override
023 * combination.
024 * </p>
025 * <p>
026 * An <em>override combination</em> means that nodes in the first node
027 * structure take precedence over nodes in the second, or - in other words -
028 * nodes of the second structure are only added to the resulting structure if
029 * they do not occur in the first one. This is especially suitable for dealing
030 * with the properties of configurations that are defined in an
031 * {@code override} section of a configuration definition file (hence the
032 * name).
033 * </p>
034 * <p>
035 * This combiner will iterate over the second node hierarchy and find all nodes
036 * that are not contained in the first hierarchy; these are added to the result.
037 * If a node can be found in both structures, it is checked whether a
038 * combination (in a recursive way) can be constructed for the two, which will
039 * then be added. Per default, nodes are combined, which occur only once in both
040 * structures. This test is implemented in the {@code canCombine()}
041 * method.
042 * </p>
043 * <p>
044 * As is true for the {@link UnionCombiner}, for this combiner
045 * list nodes are important. The {@code addListNode()} can be called to
046 * declare certain nodes as list nodes. This has the effect that these nodes
047 * will never be combined.
048 * </p>
049 *
050 * @author <a
051 * href="http://commons.apache.org/configuration/team-list.html">Commons
052 * Configuration team</a>
053 * @version $Id: OverrideCombiner.java 1301991 2012-03-17 20:18:02Z sebb $
054 * @since 1.3
055 */
056public class OverrideCombiner extends NodeCombiner
057{
058    /**
059     * Constructs an override combination for the passed in node structures.
060     *
061     * @param node1 the first node
062     * @param node2 the second node
063     * @return the resulting combined node structure
064     */
065    @Override
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 (ConfigurationNode child : node1.getChildren())
074        {
075            ConfigurationNode child2 = canCombine(node1, node2, child);
076            if (child2 != null)
077            {
078                result.addChild(combine(child, child2));
079            }
080            else
081            {
082                result.addChild(child);
083            }
084        }
085
086        // Process nodes from the second structure, which are not contained
087        // in the first structure
088        for (ConfigurationNode child : node2.getChildren())
089        {
090            if (node1.getChildrenCount(child.getName()) < 1)
091            {
092                result.addChild(child);
093            }
094        }
095
096        // Handle attributes and value
097        addAttributes(result, node1, node2);
098        result.setValue((node1.getValue() != null) ? node1.getValue() : node2
099                .getValue());
100
101        return result;
102    }
103
104    /**
105     * Handles the attributes during a combination process. First all attributes
106     * of the first node will be added to the result. Then all attributes of the
107     * second node, which are not contained in the first node, will also be
108     * added.
109     *
110     * @param result the resulting node
111     * @param node1 the first node
112     * @param node2 the second node
113     */
114    protected void addAttributes(ViewNode result, ConfigurationNode node1,
115            ConfigurationNode node2)
116    {
117        result.appendAttributes(node1);
118        for (ConfigurationNode attr : node2.getAttributes())
119        {
120            if (node1.getAttributeCount(attr.getName()) == 0)
121            {
122                result.addAttribute(attr);
123            }
124        }
125    }
126
127    /**
128     * Tests if a child node of the second node can be combined with the given
129     * child node of the first node. If this is the case, the corresponding node
130     * will be returned, otherwise <b>null</b>. This implementation checks
131     * whether the child node occurs only once in both hierarchies and is no
132     * known list node.
133     *
134     * @param node1 the first node
135     * @param node2 the second node
136     * @param child the child node (of the first node)
137     * @return a child of the second node, with which a combination is possible
138     */
139    protected ConfigurationNode canCombine(ConfigurationNode node1,
140            ConfigurationNode node2, ConfigurationNode child)
141    {
142        if (node2.getChildrenCount(child.getName()) == 1
143                && node1.getChildrenCount(child.getName()) == 1
144                && !isListNode(child))
145        {
146            return node2.getChildren(child.getName()).get(0);
147        }
148        else
149        {
150            return null;
151        }
152    }
153}