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1   /*
2    * Licensed to the Apache Software Foundation (ASF) under one or more
3    * contributor license agreements.  See the NOTICE file distributed with
4    * this work for additional information regarding copyright ownership.
5    * The ASF licenses this file to You under the Apache License, Version 2.0
6    * (the "License"); you may not use this file except in compliance with
7    * the License.  You may obtain a copy of the License at
8    *
9    *     http://www.apache.org/licenses/LICENSE-2.0
10   *
11   * Unless required by applicable law or agreed to in writing, software
12   * distributed under the License is distributed on an "AS IS" BASIS,
13   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14   * See the License for the specific language governing permissions and
15   * limitations under the License.
16   */
17  package org.apache.commons.configuration.tree;
18  
19  
20  /**
21   * <p>
22   * A concrete combiner implementation that is able to construct an override
23   * combination.
24   * </p>
25   * <p>
26   * An <em>override combination</em> means that nodes in the first node
27   * structure take precedence over nodes in the second, or - in other words -
28   * nodes of the second structure are only added to the resulting structure if
29   * they do not occur in the first one. This is especially suitable for dealing
30   * with the properties of configurations that are defined in an
31   * {@code override} section of a configuration definition file (hence the
32   * name).
33   * </p>
34   * <p>
35   * This combiner will iterate over the second node hierarchy and find all nodes
36   * that are not contained in the first hierarchy; these are added to the result.
37   * If a node can be found in both structures, it is checked whether a
38   * combination (in a recursive way) can be constructed for the two, which will
39   * then be added. Per default, nodes are combined, which occur only once in both
40   * structures. This test is implemented in the {@code canCombine()}
41   * method.
42   * </p>
43   * <p>
44   * As is true for the {@link UnionCombiner}, for this combiner
45   * list nodes are important. The {@code addListNode()} can be called to
46   * declare certain nodes as list nodes. This has the effect that these nodes
47   * will never be combined.
48   * </p>
49   *
50   * @author <a
51   * href="http://commons.apache.org/configuration/team-list.html">Commons
52   * Configuration team</a>
53   * @version $Id: OverrideCombiner.java 1301991 2012-03-17 20:18:02Z sebb $
54   * @since 1.3
55   */
56  public class OverrideCombiner extends NodeCombiner
57  {
58      /**
59       * Constructs an override combination for the passed in node structures.
60       *
61       * @param node1 the first node
62       * @param node2 the second node
63       * @return the resulting combined node structure
64       */
65      @Override
66      public ConfigurationNode combine(ConfigurationNode node1,
67              ConfigurationNode node2)
68      {
69          ViewNode result = createViewNode();
70          result.setName(node1.getName());
71  
72          // Process nodes from the first structure, which override the second
73          for (ConfigurationNode child : node1.getChildren())
74          {
75              ConfigurationNode child2 = canCombine(node1, node2, child);
76              if (child2 != null)
77              {
78                  result.addChild(combine(child, child2));
79              }
80              else
81              {
82                  result.addChild(child);
83              }
84          }
85  
86          // Process nodes from the second structure, which are not contained
87          // in the first structure
88          for (ConfigurationNode child : node2.getChildren())
89          {
90              if (node1.getChildrenCount(child.getName()) < 1)
91              {
92                  result.addChild(child);
93              }
94          }
95  
96          // Handle attributes and value
97          addAttributes(result, node1, node2);
98          result.setValue((node1.getValue() != null) ? node1.getValue() : node2
99                  .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 }