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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  import java.util.Iterator;
20  
21  /***
22   * <p>
23   * A concrete combiner implementation that is able to construct an override
24   * combination.
25   * </p>
26   * <p>
27   * An <em>override combination</em> means that nodes in the first node
28   * structure take precedence over nodes in the second, or - in other words -
29   * nodes of the second structure are only added to the resulting structure if
30   * they do not occure in the first one. This is especially suitable for dealing
31   * with the properties of configurations that are defined in an
32   * <code>override</code> section of a configuration definition file (hence the
33   * name).
34   * </p>
35   * <p>
36   * This combiner will iterate over the second node hierarchy and find all nodes
37   * that are not contained in the first hierarchy; these are added to the result.
38   * If a node can be found in both structures, it is checked whether a
39   * combination (in a recursive way) can be constructed for the two, which will
40   * then be added. Per default, nodes are combined, which occur only once in both
41   * structures. This test is implemented in the <code>canCombine()</code>
42   * method.
43   * </p>
44   * <p>
45   * As is true for the <code>{@link UnionCombiner}</code>, for this combiner
46   * list nodes are important. The <code>addListNode()</code> can be called to
47   * declare certain nodes as list nodes. This has the effect that these nodes
48   * will never be combined.
49   * </p>
50   *
51   * @author <a
52   * href="http://jakarta.apache.org/commons/configuration/team-list.html">Commons
53   * Configuration team</a>
54   * @version $Id: OverrideCombiner.java 439648 2006-09-02 20:42:10Z oheger $
55   * @since 1.3
56   */
57  public class OverrideCombiner extends NodeCombiner
58  {
59      /***
60       * Constructs an override combination for the passed in node structures.
61       *
62       * @param node1 the first node
63       * @param node2 the second node
64       * @return the resulting combined node structure
65       */
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 (Iterator it = node1.getChildren().iterator(); it.hasNext();)
74          {
75              ConfigurationNode child = (ConfigurationNode) it.next();
76              ConfigurationNode child2 = canCombine(node1, node2, child);
77              if (child2 != null)
78              {
79                  result.addChild(combine(child, child2));
80              }
81              else
82              {
83                  result.addChild(child);
84              }
85          }
86  
87          // Process nodes from the second structure, which are not contained
88          // in the first structure
89          for (Iterator it = node2.getChildren().iterator(); it.hasNext();)
90          {
91              ConfigurationNode child = (ConfigurationNode) it.next();
92              if (node1.getChildrenCount(child.getName()) < 1)
93              {
94                  result.addChild(child);
95              }
96          }
97  
98          // Handle attributes and value
99          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 }