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
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 1206482 2011-11-26 16:34:01Z oheger $
054 * @since 1.3
055 */
056 public 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 (ConfigurationNode) node2.getChildren(child.getName())
147 .get(0);
148 }
149 else
150 {
151 return null;
152 }
153 }
154 }