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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;
18  
19  import java.util.ArrayList;
20  import java.util.Collections;
21  import java.util.Iterator;
22  import java.util.List;
23  
24  import org.apache.commons.configuration.tree.ConfigurationNode;
25  
26  /***
27   * <p>
28   * A specialized hierarchical configuration class that wraps a single node of
29   * its parent configuration.
30   * </p>
31   * <p>
32   * Configurations of this type are initialized with a parent configuration and a
33   * configuration node of this configuration. This node becomes the root node of
34   * the subnode configuration. All property accessor methods are evaluated
35   * relative to this root node. A good use case for a
36   * <code>SubnodeConfiguration</code> is when multiple properties from a
37   * specific sub tree of the whole configuration need to be accessed. Then a
38   * <code>SubnodeConfiguration</code> can be created with the parent node of
39   * the affected sub tree as root node. This allows for simpler property keys and
40   * is also more efficient.
41   * </p>
42   * <p>
43   * A subnode configuration and its parent configuration operate on the same
44   * hierarchy of configuration nodes. So if modifications are performed at the
45   * subnode configuration, these changes are immideately visible in the parent
46   * configuration. Analogously will updates of the parent configuration affect
47   * the subnode configuration if the sub tree spanned by the subnode
48   * configuration's root node is involved.
49   * </p>
50   * <p>
51   * There are however changes at the parent configuration, which cause the
52   * subnode configuration to become detached. An example for such a change is a
53   * reload operation of a file-based configuration, which replaces all nodes of
54   * the parent configuration. The subnode configuration per default still
55   * references the old nodes. Another example are list structures: a subnode
56   * configuration can be created to point on the <em>i</em>th element of the
57   * list. Now list elements can be added or removed, so that the list elements'
58   * indices change. In such a scenario the subnode configuration would always
59   * point to the same list element, regardless of its current index.
60   * </p>
61   * <p>
62   * To solve these problems and make a subnode configuration aware of
63   * such structural changes of its parent, it is possible to associate a
64   * subnode configuration with a configuration key. This can be done by calling
65   * the <code>setSubnodeKey()</code> method. If here a key is set, the subnode
66   * configuration will evaluate it on each access, thus ensuring that it is
67   * always in sync with its parent. In this mode the subnode configuration really
68   * behaves like a live-view on its parent. The price for this is a decreased
69   * performance because now an additional evaluation has to be performed on each
70   * property access. So this mode should only be used if necessary; if for
71   * instance a subnode configuration is only used for a temporary convenient
72   * access to a complex configuration, there is no need to make it aware for
73   * structural changes of its parent. If a subnode configuration is created
74   * using the <code>{@link HierarchicalConfiguration#configurationAt(String, boolean)
75   * configurationAt()}</code> method of <code>HierarchicalConfiguration</code>
76   * (which should be the preferred way), with an additional boolean parameter it
77   * can be specified whether the resulting subnode configuration should be
78   * aware of structural changes or not. Then the configuration key will be
79   * automatically set.
80   * </p>
81   * <p>
82   * <em>Note:</em> At the moment support for creating a subnode configuration
83   * that is aware of structural changes of its parent from another subnode
84   * configuration (a "sub subnode configuration") is limited. This only works if
85   * <ol><li>the subnode configuration that serves as the parent for the new
86   * subnode configuration is itself associated with a configuration key and</li>
87   * <li>the key passed in to create the new subnode configuration is not too
88   * complex (if configuration keys are used that contain indices, a corresponding
89   * key that is valid from the parent configuration's point of view cannot be
90   * constructed).</li></ol>
91   * </p>
92   * <p>
93   * When a subnode configuration is created, it inherits the settings of its
94   * parent configuration, e.g. some flags like the
95   * <code>throwExceptionOnMissing</code> flag or the settings for handling list
96   * delimiters) or the expression engine. If these settings are changed later in
97   * either the subnode or the parent configuration, the changes are not visible
98   * for each other. So you could create a subnode configuration, change its
99   * expression engine without affecting the parent configuration.
100  * </p>
101  * <p>
102  * From its purpose this class is quite similar to
103  * <code>{@link SubsetConfiguration}</code>. The difference is that a subset
104  * configuration of a hierarchical configuration may combine multiple
105  * configuration nodes from different sub trees of the configuration, while all
106  * nodes in a subnode configuration belong to the same sub tree. If an
107  * application can live with this limitation, it is recommended to use this
108  * class instead of <code>SubsetConfiguration</code> because creating a subset
109  * configuration is more expensive than creating a subnode configuration.
110  * </p>
111  *
112  * @since 1.3
113  * @author Oliver Heger
114  * @version $Id: SubnodeConfiguration.java 531254 2007-04-22 18:54:57Z oheger $
115  */
116 public class SubnodeConfiguration extends HierarchicalConfiguration
117 {
118     /***
119      * The serial version UID.
120      */
121     private static final long serialVersionUID = 3105734147019386480L;
122 
123     /*** Stores the parent configuration. */
124     private HierarchicalConfiguration parent;
125 
126     /*** Stores the key that was used to construct this configuration.*/
127     private String subnodeKey;
128 
129     /***
130      * Creates a new instance of <code>SubnodeConfiguration</code> and
131      * initializes it with the parent configuration and the new root node.
132      *
133      * @param parent the parent configuration
134      * @param root the root node of this subnode configuration
135      */
136     public SubnodeConfiguration(HierarchicalConfiguration parent, ConfigurationNode root)
137     {
138         if (parent == null)
139         {
140             throw new IllegalArgumentException(
141                     "Parent configuration must not be null!");
142         }
143         if (root == null)
144         {
145             throw new IllegalArgumentException("Root node must not be null!");
146         }
147 
148         setRootNode(root);
149         this.parent = parent;
150         initFromParent(parent);
151     }
152 
153     /***
154      * Returns the parent configuration of this subnode configuration.
155      *
156      * @return the parent configuration
157      */
158     public HierarchicalConfiguration getParent()
159     {
160         return parent;
161     }
162 
163     /***
164      * Returns the key that was used to construct this configuration. If here a
165      * non-<b>null</b> value is returned, the subnode configuration will
166      * always check its parent for structural changes and reconstruct itself if
167      * necessary.
168      *
169      * @return the key for selecting this configuration's root node
170      * @since 1.5
171      */
172     public String getSubnodeKey()
173     {
174         return subnodeKey;
175     }
176 
177     /***
178      * Sets the key to the root node of this subnode configuration. If here a
179      * key is set, the subnode configuration will behave like a live-view on its
180      * parent for this key. See the class comment for more details.
181      *
182      * @param subnodeKey the key used to construct this configuration
183      * @since 1.5
184      */
185     public void setSubnodeKey(String subnodeKey)
186     {
187         this.subnodeKey = subnodeKey;
188     }
189 
190     /***
191      * Returns the root node for this configuration. If a subnode key is set,
192      * this implementation re-evaluates this key to find out if this subnode
193      * configuration needs to be reconstructed. This ensures that the subnode
194      * configuration is always synchronized with its parent configuration.
195      *
196      * @return the root node of this configuration
197      * @since 1.5
198      * @see #setSubnodeKey(String)
199      */
200     public ConfigurationNode getRootNode()
201     {
202         if (getSubnodeKey() != null)
203         {
204             try
205             {
206                 List nodes = getParent().fetchNodeList(getSubnodeKey());
207                 if (nodes.size() != 1)
208                 {
209                     // key is invalid, so detach this subnode configuration
210                     setSubnodeKey(null);
211                 }
212                 else
213                 {
214                     ConfigurationNode currentRoot = (ConfigurationNode) nodes
215                             .get(0);
216                     if (currentRoot != super.getRootNode())
217                     {
218                         // the root node was changed due to a change of the
219                         // parent
220                         setRootNode(currentRoot);
221                     }
222                     return currentRoot;
223                 }
224             }
225             catch (Exception ex)
226             {
227                 // Evaluation of the key caused an exception. Probably the
228                 // expression engine has changed on the parent. Detach this
229                 // configuration, there is not much we can do about this.
230                 setSubnodeKey(null);
231             }
232         }
233 
234         return super.getRootNode(); // use stored root node
235     }
236 
237     /***
238      * Returns a hierarchical configuration object for the given sub node.
239      * This implementation will ensure that the returned
240      * <code>SubnodeConfiguration</code> object will have the same parent than
241      * this object.
242      *
243      * @param node the sub node, for which the configuration is to be created
244      * @return a hierarchical configuration for this sub node
245      */
246     protected SubnodeConfiguration createSubnodeConfiguration(ConfigurationNode node)
247     {
248         SubnodeConfiguration result = new SubnodeConfiguration(getParent(), node);
249         getParent().registerSubnodeConfiguration(result);
250         return result;
251     }
252 
253     /***
254      * Returns a hierarchical configuration object for the given sub node that
255      * is aware of structural changes of its parent. Works like the method with
256      * the same name, but also sets the subnode key for the new subnode
257      * configuration, so it can check whether the parent has been changed. This
258      * only works if this subnode configuration has itself a valid subnode key.
259      * So if a subnode configuration that should be aware of structural changes
260      * is created from an already existing subnode configuration, this subnode
261      * configuration must also be aware of such changes.
262      *
263      * @param node the sub node, for which the configuration is to be created
264      * @param subnodeKey the construction key
265      * @return a hierarchical configuration for this sub node
266      * @since 1.5
267      */
268     protected SubnodeConfiguration createSubnodeConfiguration(
269             ConfigurationNode node, String subnodeKey)
270     {
271         SubnodeConfiguration result = createSubnodeConfiguration(node);
272 
273         if (getSubnodeKey() != null)
274         {
275             // construct the correct subnode key
276             // determine path to root node
277             List lstPathToRoot = new ArrayList();
278             ConfigurationNode top = super.getRootNode();
279             ConfigurationNode nd = node;
280             while (nd != top)
281             {
282                 lstPathToRoot.add(nd);
283                 nd = nd.getParentNode();
284             }
285 
286             // construct the keys for the nodes on this path
287             Collections.reverse(lstPathToRoot);
288             String key = getSubnodeKey();
289             for (Iterator it = lstPathToRoot.iterator(); it.hasNext();)
290             {
291                 key = getParent().getExpressionEngine().nodeKey(
292                         (ConfigurationNode) it.next(), key);
293             }
294             result.setSubnodeKey(key);
295         }
296 
297         return result;
298     }
299 
300     /***
301      * Creates a new node. This task is delegated to the parent.
302      *
303      * @param name the node's name
304      * @return the new node
305      */
306     protected Node createNode(String name)
307     {
308         return getParent().createNode(name);
309     }
310 
311     /***
312      * Initializes this subnode configuration from the given parent
313      * configuration. This method is called by the constructor. It will copy
314      * many settings from the parent.
315      *
316      * @param parentConfig the parent configuration
317      */
318     protected void initFromParent(HierarchicalConfiguration parentConfig)
319     {
320         setExpressionEngine(parentConfig.getExpressionEngine());
321         setListDelimiter(parentConfig.getListDelimiter());
322         setDelimiterParsingDisabled(parentConfig.isDelimiterParsingDisabled());
323         setThrowExceptionOnMissing(parentConfig.isThrowExceptionOnMissing());
324     }
325 
326     /***
327      * Performs interpolation. This implementation will ask the parent
328      * configuration to perform the interpolation so that variables can be
329      * evaluated in the global context.
330      *
331      * @param value the value to be interpolated
332      */
333     protected Object interpolate(Object value)
334     {
335         return getParent().interpolate(value);
336     }
337 }