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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.List;
22  
23  import org.apache.commons.configuration.interpol.ConfigurationInterpolator;
24  import org.apache.commons.configuration.reloading.Reloadable;
25  import org.apache.commons.configuration.tree.ConfigurationNode;
26  
27  /**
28   * <p>
29   * A specialized hierarchical configuration class that wraps a single node of
30   * its parent configuration.
31   * </p>
32   * <p>
33   * Configurations of this type are initialized with a parent configuration and a
34   * configuration node of this configuration. This node becomes the root node of
35   * the subnode configuration. All property accessor methods are evaluated
36   * relative to this root node. A good use case for a
37   * {@code SubnodeConfiguration} is when multiple properties from a
38   * specific sub tree of the whole configuration need to be accessed. Then a
39   * {@code SubnodeConfiguration} can be created with the parent node of
40   * the affected sub tree as root node. This allows for simpler property keys and
41   * is also more efficient.
42   * </p>
43   * <p>
44   * A subnode configuration and its parent configuration operate on the same
45   * hierarchy of configuration nodes. So if modifications are performed at the
46   * subnode configuration, these changes are immediately visible in the parent
47   * configuration. Analogously will updates of the parent configuration affect
48   * the subnode configuration if the sub tree spanned by the subnode
49   * configuration's root node is involved.
50   * </p>
51   * <p>
52   * There are however changes at the parent configuration, which cause the
53   * subnode configuration to become detached. An example for such a change is a
54   * reload operation of a file-based configuration, which replaces all nodes of
55   * the parent configuration. The subnode configuration per default still
56   * references the old nodes. Another example are list structures: a subnode
57   * configuration can be created to point on the <em>i</em>th element of the
58   * list. Now list elements can be added or removed, so that the list elements'
59   * indices change. In such a scenario the subnode configuration would always
60   * point to the same list element, regardless of its current index.
61   * </p>
62   * <p>
63   * To solve these problems and make a subnode configuration aware of
64   * such structural changes of its parent, it is possible to associate a
65   * subnode configuration with a configuration key. This can be done by calling
66   * the {@code setSubnodeKey()} method. If here a key is set, the subnode
67   * configuration will evaluate it on each access, thus ensuring that it is
68   * always in sync with its parent. In this mode the subnode configuration really
69   * behaves like a live-view on its parent. The price for this is a decreased
70   * performance because now an additional evaluation has to be performed on each
71   * property access. So this mode should only be used if necessary; if for
72   * instance a subnode configuration is only used for a temporary convenient
73   * access to a complex configuration, there is no need to make it aware for
74   * structural changes of its parent. If a subnode configuration is created
75   * using the {@link HierarchicalConfiguration#configurationAt(String, boolean)
76   * configurationAt()} method of {@code HierarchicalConfiguration}
77   * (which should be the preferred way), with an additional boolean parameter it
78   * can be specified whether the resulting subnode configuration should be
79   * aware of structural changes or not. Then the configuration key will be
80   * automatically set.
81   * </p>
82   * <p>
83   * <em>Note:</em> At the moment support for creating a subnode configuration
84   * that is aware of structural changes of its parent from another subnode
85   * configuration (a "sub subnode configuration") is limited. This only works if
86   * <ol><li>the subnode configuration that serves as the parent for the new
87   * subnode configuration is itself associated with a configuration key and</li>
88   * <li>the key passed in to create the new subnode configuration is not too
89   * complex (if configuration keys are used that contain indices, a corresponding
90   * key that is valid from the parent configuration's point of view cannot be
91   * constructed).</li></ol>
92   * </p>
93   * <p>
94   * When a subnode configuration is created, it inherits the settings of its
95   * parent configuration, e.g. some flags like the
96   * {@code throwExceptionOnMissing} flag or the settings for handling list
97   * delimiters) or the expression engine. If these settings are changed later in
98   * either the subnode or the parent configuration, the changes are not visible
99   * for each other. So you could create a subnode configuration, change its
100  * expression engine without affecting the parent configuration.
101  * </p>
102  * <p>
103  * From its purpose this class is quite similar to
104  * {@link SubsetConfiguration}. The difference is that a subset
105  * configuration of a hierarchical configuration may combine multiple
106  * configuration nodes from different sub trees of the configuration, while all
107  * nodes in a subnode configuration belong to the same sub tree. If an
108  * application can live with this limitation, it is recommended to use this
109  * class instead of {@code SubsetConfiguration} because creating a subset
110  * configuration is more expensive than creating a subnode configuration.
111  * </p>
112  *
113  * @since 1.3
114  * @author <a
115  * href="http://commons.apache.org/configuration/team-list.html">Commons
116  * Configuration team</a>
117  * @version $Id: SubnodeConfiguration.java 1210178 2011-12-04 18:58:51Z oheger $
118  */
119 public class SubnodeConfiguration extends HierarchicalReloadableConfiguration
120 {
121     /**
122      * The serial version UID.
123      */
124     private static final long serialVersionUID = 3105734147019386480L;
125 
126     /** Stores the parent configuration. */
127     private HierarchicalConfiguration parent;
128 
129     /** Stores the key that was used to construct this configuration.*/
130     private String subnodeKey;
131 
132     /**
133      * Creates a new instance of {@code SubnodeConfiguration} and
134      * initializes it with the parent configuration and the new root node.
135      *
136      * @param parent the parent configuration
137      * @param root the root node of this subnode configuration
138      */
139     public SubnodeConfiguration(HierarchicalConfiguration parent, ConfigurationNode root)
140     {
141         super(parent instanceof Reloadable ? ((Reloadable) parent).getReloadLock() : null);
142         if (parent == null)
143         {
144             throw new IllegalArgumentException(
145                     "Parent configuration must not be null!");
146         }
147         if (root == null)
148         {
149             throw new IllegalArgumentException("Root node must not be null!");
150         }
151 
152         setRootNode(root);
153         this.parent = parent;
154         initFromParent(parent);
155     }
156 
157     /**
158      * Returns the parent configuration of this subnode configuration.
159      *
160      * @return the parent configuration
161      */
162     public HierarchicalConfiguration getParent()
163     {
164         return parent;
165     }
166 
167     /**
168      * Returns the key that was used to construct this configuration. If here a
169      * non-<b>null</b> value is returned, the subnode configuration will
170      * always check its parent for structural changes and reconstruct itself if
171      * necessary.
172      *
173      * @return the key for selecting this configuration's root node
174      * @since 1.5
175      */
176     public String getSubnodeKey()
177     {
178         return subnodeKey;
179     }
180 
181     /**
182      * Sets the key to the root node of this subnode configuration. If here a
183      * key is set, the subnode configuration will behave like a live-view on its
184      * parent for this key. See the class comment for more details.
185      *
186      * @param subnodeKey the key used to construct this configuration
187      * @since 1.5
188      */
189     public void setSubnodeKey(String subnodeKey)
190     {
191         this.subnodeKey = subnodeKey;
192     }
193 
194     /**
195      * Returns the root node for this configuration. If a subnode key is set,
196      * this implementation re-evaluates this key to find out if this subnode
197      * configuration needs to be reconstructed. This ensures that the subnode
198      * configuration is always synchronized with its parent configuration.
199      *
200      * @return the root node of this configuration
201      * @since 1.5
202      * @see #setSubnodeKey(String)
203      */
204     @Override
205     public ConfigurationNode getRootNode()
206     {
207         if (getSubnodeKey() != null)
208         {
209             try
210             {
211                 List<ConfigurationNode> nodes = getParent().fetchNodeList(getSubnodeKey());
212                 if (nodes.size() != 1)
213                 {
214                     // key is invalid, so detach this subnode configuration
215                     setSubnodeKey(null);
216                 }
217                 else
218                 {
219                     ConfigurationNode currentRoot = nodes.get(0);
220                     if (currentRoot != super.getRootNode())
221                     {
222                         // the root node was changed due to a change of the
223                         // parent
224                         fireEvent(EVENT_SUBNODE_CHANGED, null, null, true);
225                         setRootNode(currentRoot);
226                         fireEvent(EVENT_SUBNODE_CHANGED, null, null, false);
227                     }
228                     return currentRoot;
229                 }
230             }
231             catch (Exception ex)
232             {
233                 // Evaluation of the key caused an exception. Probably the
234                 // expression engine has changed on the parent. Detach this
235                 // configuration, there is not much we can do about this.
236                 setSubnodeKey(null);
237             }
238         }
239 
240         return super.getRootNode(); // use stored root node
241     }
242 
243     /**
244      * Returns a hierarchical configuration object for the given sub node.
245      * This implementation will ensure that the returned
246      * {@code SubnodeConfiguration} object will have the same parent than
247      * this object.
248      *
249      * @param node the sub node, for which the configuration is to be created
250      * @return a hierarchical configuration for this sub node
251      */
252     @Override
253     protected SubnodeConfiguration createSubnodeConfiguration(ConfigurationNode node)
254     {
255         SubnodeConfiguration result = new SubnodeConfiguration(getParent(), node);
256         getParent().registerSubnodeConfiguration(result);
257         return result;
258     }
259 
260     /**
261      * Returns a hierarchical configuration object for the given sub node that
262      * is aware of structural changes of its parent. Works like the method with
263      * the same name, but also sets the subnode key for the new subnode
264      * configuration, so it can check whether the parent has been changed. This
265      * only works if this subnode configuration has itself a valid subnode key.
266      * So if a subnode configuration that should be aware of structural changes
267      * is created from an already existing subnode configuration, this subnode
268      * configuration must also be aware of such changes.
269      *
270      * @param node the sub node, for which the configuration is to be created
271      * @param subnodeKey the construction key
272      * @return a hierarchical configuration for this sub node
273      * @since 1.5
274      */
275     @Override
276     protected SubnodeConfiguration createSubnodeConfiguration(
277             ConfigurationNode node, String subnodeKey)
278     {
279         SubnodeConfiguration result = createSubnodeConfiguration(node);
280 
281         if (getSubnodeKey() != null)
282         {
283             // construct the correct subnode key
284             // determine path to root node
285             List<ConfigurationNode> lstPathToRoot = new ArrayList<ConfigurationNode>();
286             ConfigurationNode top = super.getRootNode();
287             ConfigurationNode nd = node;
288             while (nd != top)
289             {
290                 lstPathToRoot.add(nd);
291                 nd = nd.getParentNode();
292             }
293 
294             // construct the keys for the nodes on this path
295             Collections.reverse(lstPathToRoot);
296             String key = getSubnodeKey();
297             for (ConfigurationNode pathNode : lstPathToRoot)
298             {
299                 key = getParent().getExpressionEngine().nodeKey(pathNode, key);
300             }
301             result.setSubnodeKey(key);
302         }
303 
304         return result;
305     }
306 
307     /**
308      * Creates a new node. This task is delegated to the parent.
309      *
310      * @param name the node's name
311      * @return the new node
312      */
313     @Override
314     protected Node createNode(String name)
315     {
316         return getParent().createNode(name);
317     }
318 
319     /**
320      * Initializes this subnode configuration from the given parent
321      * configuration. This method is called by the constructor. It will copy
322      * many settings from the parent.
323      *
324      * @param parentConfig the parent configuration
325      */
326     protected void initFromParent(HierarchicalConfiguration parentConfig)
327     {
328         setExpressionEngine(parentConfig.getExpressionEngine());
329         setListDelimiter(parentConfig.getListDelimiter());
330         setDelimiterParsingDisabled(parentConfig.isDelimiterParsingDisabled());
331         setThrowExceptionOnMissing(parentConfig.isThrowExceptionOnMissing());
332     }
333 
334     /**
335      * Creates a ConfigurationInterpolator with a chain to the parent's
336      * interpolator.
337      *
338      * @return the new interpolator
339      */
340     @Override
341     protected ConfigurationInterpolator createInterpolator()
342     {
343         ConfigurationInterpolator interpolator = super.createInterpolator();
344         interpolator.setParentInterpolator(getParent().getInterpolator());
345         return interpolator;
346     }
347 }