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17 package org.apache.commons.configuration.tree;
18
19 import java.util.Iterator;
20 import java.util.LinkedList;
21 import java.util.List;
22
23 /***
24 * <p>
25 * A specialized implementation of the <code>NodeCombiner</code> interface
26 * that constructs a union from two passed in node hierarchies.
27 * </p>
28 * <p>
29 * The given source hierarchies are traversed and their nodes are added to the
30 * resulting structure. Under some circumstances two nodes can be combined
31 * rather than adding both. This is the case if both nodes are single children
32 * (no lists) of their parents and do not have values. The corresponding check
33 * is implemented in the <code>findCombineNode()</code> method.
34 * </p>
35 * <p>
36 * Sometimes it is not possible for this combiner to detect whether two nodes
37 * can be combined or not. Consider the following two node hierarchies:
38 * </p>
39 * <p>
40 *
41 * <pre>
42 * Hierarchy 1:
43 *
44 * Database
45 * +--Tables
46 * +--Table
47 * +--name [users]
48 * +--fields
49 * +--field
50 * | +--name [uid]
51 * +--field
52 * | +--name [usrname]
53 * ...
54 * </pre>
55 *
56 * </p>
57 * <p>
58 *
59 * <pre>
60 * Hierarchy 2:
61 *
62 * Database
63 * +--Tables
64 * +--Table
65 * +--name [documents]
66 * +--fields
67 * +--field
68 * | +--name [docid]
69 * +--field
70 * | +--name [docname]
71 * ...
72 * </pre>
73 *
74 * </p>
75 * <p>
76 * Both hierarchies contain data about database tables. Each describes a single
77 * table. If these hierarchies are to be combined, the result should probably
78 * look like the following:
79 * <p>
80 *
81 * <pre>
82 * Database
83 * +--Tables
84 * +--Table
85 * | +--name [users]
86 * | +--fields
87 * | +--field
88 * | | +--name [uid]
89 * | ...
90 * +--Table
91 * +--name [documents]
92 * +--fields
93 * +--field
94 * | +--name [docid]
95 * ...
96 * </pre>
97 *
98 * </p>
99 * <p>
100 * i.e. the <code>Tables</code> nodes should be combined, while the
101 * <code>Table</code> nodes should both be added to the resulting tree. From
102 * the combiner's point of view there is no difference between the
103 * <code>Tables</code> and the <code>Table</code> nodes in the source trees,
104 * so the developer has to help out and give a hint that the <code>Table</code>
105 * nodes belong to a list structure. This can be done using the
106 * <code>addListNode()</code> method; this method expects the name of a node,
107 * which should be treated as a list node. So if
108 * <code>addListNode("Table");</code> was called, the combiner knows that it
109 * must not combine the <code>Table</code> nodes, but add it both to the
110 * resulting tree.
111 * </p>
112 *
113 * @author <a
114 * href="http://jakarta.apache.org/commons/configuration/team-list.html">Commons
115 * Configuration team</a>
116 * @version $Id: UnionCombiner.java 439648 2006-09-02 20:42:10Z oheger $
117 * @since 1.3
118 */
119 public class UnionCombiner extends NodeCombiner
120 {
121 /***
122 * Combines the given nodes to a new union node.
123 *
124 * @param node1 the first source node
125 * @param node2 the second source node
126 * @return the union node
127 */
128 public ConfigurationNode combine(ConfigurationNode node1,
129 ConfigurationNode node2)
130 {
131 ViewNode result = createViewNode();
132 result.setName(node1.getName());
133 result.appendAttributes(node1);
134 result.appendAttributes(node2);
135
136
137 List children2 = new LinkedList(node2.getChildren());
138 for (Iterator it = node1.getChildren().iterator(); it.hasNext();)
139 {
140 ConfigurationNode child1 = (ConfigurationNode) it.next();
141 ConfigurationNode child2 = findCombineNode(node1, node2, child1,
142 children2);
143 if (child2 != null)
144 {
145 result.addChild(combine(child1, child2));
146 children2.remove(child2);
147 }
148 else
149 {
150 result.addChild(child1);
151 }
152 }
153
154
155 for (Iterator it = children2.iterator(); it.hasNext();)
156 {
157 result.addChild((ConfigurationNode) it.next());
158 }
159
160 return result;
161 }
162
163 /***
164 * <p>
165 * Tries to find a child node of the second source node, with whitch a child
166 * of the first source node can be combined. During combining of the source
167 * nodes an iteration over the first source node's children is performed.
168 * For each child node it is checked whether a corresponding child node in
169 * the second source node exists. If this is the case, these corresponsing
170 * child nodes are recursively combined and the result is added to the
171 * combined node. This method implements the checks whether such a recursive
172 * combination is possible. The actual implementation tests the following
173 * conditions:
174 * </p>
175 * <p>
176 * <ul>
177 * <li>In both the first and the second source node there is only one child
178 * node with the given name (no list structures).</li>
179 * <li>The given name is not in the list of known list nodes, i.e. it was
180 * not passed to the <code>addListNode()</code> method.</li>
181 * <li>None of these matching child nodes has a value.</li>
182 * </ul>
183 * </p>
184 * <p>
185 * If all of these tests are successfull, the matching child node of the
186 * second source node is returned. Otherwise the result is <b>null</b>.
187 * </p>
188 *
189 * @param node1 the first source node
190 * @param node2 the second source node
191 * @param child the child node of the first source node to be checked
192 * @param children a list with all children of the second source node
193 * @return the matching child node of the second source node or <b>null</b>
194 * if there is none
195 */
196 protected ConfigurationNode findCombineNode(ConfigurationNode node1,
197 ConfigurationNode node2, ConfigurationNode child, List children)
198 {
199 if (child.getValue() == null && !isListNode(child)
200 && node1.getChildrenCount(child.getName()) == 1
201 && node2.getChildrenCount(child.getName()) == 1)
202 {
203 ConfigurationNode child2 = (ConfigurationNode) node2.getChildren(
204 child.getName()).iterator().next();
205 if (child2.getValue() == null)
206 {
207 return child2;
208 }
209 }
210 return null;
211 }
212 }