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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.configuration2.tree;
18  
19  import java.util.Collection;
20  import java.util.List;
21  import java.util.concurrent.atomic.AtomicBoolean;
22  
23  /**
24   * <p>
25   * A specialized {@code NodeModel} implementation that uses a tracked node managed by an {@link InMemoryNodeModel}
26   * object as root node.
27   * </p>
28   * <p>
29   * Models of this type are useful when working on specific sub trees of a nodes structure. This is the case for instance
30   * for a {@code SubnodeConfiguration}.
31   * </p>
32   * <p>
33   * An instance of this class is constructed with an {@link InMemoryNodeModelSupport} object providing a reference to the
34   * underlying {@code InMemoryNodeModel} and the {@link NodeSelector} pointing to the tracked node acting as this model's
35   * root node. The {@code NodeModel} operations are implemented by delegating to the wrapped {@code InMemoryNodeModel}
36   * object specifying the selector to the tracked node as target root node for the update transaction. Note that the
37   * tracked node can become detached at any time. This situation is handled transparently by the implementation of
38   * {@code InMemoryNodeModel}. The reason for using an {@code InMemoryNodeModelSupport} object rather than an
39   * {@code InMemoryNodeModel} directly is that this additional layer of indirection can be used for performing special
40   * initializations on the model before it is returned to the {@code TrackedNodeModel} object. This is needed by some
41   * dynamic configuration implementations, e.g. by {@code CombinedConfiguration}.
42   * </p>
43   * <p>
44   * If the tracked node acting as root node is exclusively used by this model, it should be released when this model is
45   * no longer needed. This can be done manually by calling the {@link #close()} method. It is also possible to pass a
46   * value of <strong>true</strong> to the {@code untrackOnFinalize} argument of the constructor. This causes
47   * {@code close()} to be called automatically if this object gets claimed by the garbage collector.
48   * </p>
49   * <p>
50   * As {@code InMemoryNodeModel}, this class is thread-safe.
51   * </p>
52   *
53   * @since 2.0
54   */
55  public class TrackedNodeModel implements NodeModel<ImmutableNode> {
56      /** Stores the underlying parent model. */
57      private final InMemoryNodeModelSupport parentModelSupport;
58  
59      /** The selector for the managed tracked node. */
60      private final NodeSelector selector;
61  
62      /**
63       * A flag whether the tracked not should be released when this object is finalized.
64       */
65      private final boolean releaseTrackedNodeOnFinalize;
66  
67      /** A flag whether this model has already been closed. */
68      private final AtomicBoolean closed;
69  
70      /**
71       * Creates a new instance of {@code TrackedNodeModel} and initializes it with the given underlying model and the
72       * selector to the root node. The boolean argument controls whether the associated tracked node should be released when
73       * this object gets finalized. This allows the underlying model to free some resources. If used as model within a
74       * {@code SubnodeConfiguration}, there is typically no way to discard the model explicitly. Therefore, it makes sense to
75       * do this automatically on finalization.
76       *
77       * @param modelSupport the underlying {@code InMemoryNodeModelSupport} (must not be <b>null</b>)
78       * @param sel the selector to the root node of this model (must not be <b>null</b>)
79       * @param untrackOnFinalize a flag whether the tracked node should be released on finalization
80       * @throws IllegalArgumentException if a required parameter is missing
81       */
82      public TrackedNodeModel(final InMemoryNodeModelSupport modelSupport, final NodeSelector sel, final boolean untrackOnFinalize) {
83          if (modelSupport == null) {
84              throw new IllegalArgumentException("Underlying model support must not be null!");
85          }
86          if (sel == null) {
87              throw new IllegalArgumentException("Selector must not be null!");
88          }
89  
90          parentModelSupport = modelSupport;
91          selector = sel;
92          releaseTrackedNodeOnFinalize = untrackOnFinalize;
93          closed = new AtomicBoolean();
94      }
95  
96      /**
97       * Gets the {@code InMemoryNodeModelSupport} object which is used to gain access to the underlying node model.
98       *
99       * @return the associated {@code InMemoryNodeModelSupport} object
100      */
101     public InMemoryNodeModelSupport getParentModelSupport() {
102         return parentModelSupport;
103     }
104 
105     /**
106      * Gets the parent model. Operations on this model are delegated to this parent model specifying the selector to the
107      * tracked node.
108      *
109      * @return the parent model
110      */
111     public InMemoryNodeModel getParentModel() {
112         return getParentModelSupport().getNodeModel();
113     }
114 
115     /**
116      * Gets the {@code NodeSelector} pointing to the tracked node managed by this model.
117      *
118      * @return the tracked node selector
119      */
120     public NodeSelector getSelector() {
121         return selector;
122     }
123 
124     /**
125      * Returns the flag whether the managed tracked node is to be released when this object gets finalized. This method
126      * returns the value of the corresponding flag passed to the constructor. If result is true, the underlying model is
127      * asked to untrack the managed node when this object is claimed by the GC.
128      *
129      * @return a flag whether the managed tracked node should be released when this object dies
130      * @see InMemoryNodeModel#untrackNode(NodeSelector)
131      */
132     public boolean isReleaseTrackedNodeOnFinalize() {
133         return releaseTrackedNodeOnFinalize;
134     }
135 
136     @Override
137     public void setRootNode(final ImmutableNode newRoot) {
138         getParentModel().replaceTrackedNode(getSelector(), newRoot);
139     }
140 
141     @Override
142     public NodeHandler<ImmutableNode> getNodeHandler() {
143         return getParentModel().getTrackedNodeHandler(getSelector());
144     }
145 
146     @Override
147     public void addProperty(final String key, final Iterable<?> values, final NodeKeyResolver<ImmutableNode> resolver) {
148         getParentModel().addProperty(key, getSelector(), values, resolver);
149     }
150 
151     @Override
152     public void addNodes(final String key, final Collection<? extends ImmutableNode> nodes, final NodeKeyResolver<ImmutableNode> resolver) {
153         getParentModel().addNodes(key, getSelector(), nodes, resolver);
154     }
155 
156     @Override
157     public void setProperty(final String key, final Object value, final NodeKeyResolver<ImmutableNode> resolver) {
158         getParentModel().setProperty(key, getSelector(), value, resolver);
159     }
160 
161     @Override
162     public List<QueryResult<ImmutableNode>> clearTree(final String key, final NodeKeyResolver<ImmutableNode> resolver) {
163         return getParentModel().clearTree(key, getSelector(), resolver);
164     }
165 
166     @Override
167     public void clearProperty(final String key, final NodeKeyResolver<ImmutableNode> resolver) {
168         getParentModel().clearProperty(key, getSelector(), resolver);
169     }
170 
171     /**
172      * {@inheritDoc} This implementation clears the sub tree spanned by the associate tracked node. This has the side effect
173      * that this in any case becomes detached.
174      *
175      * @param resolver the {@code NodeKeyResolver}.
176      */
177     @Override
178     public void clear(final NodeKeyResolver<ImmutableNode> resolver) {
179         getParentModel().clearTree(null, getSelector(), resolver);
180     }
181 
182     /**
183      * {@inheritDoc} This implementation returns the tracked node instance acting as root node of this model.
184      */
185     @Override
186     public ImmutableNode getInMemoryRepresentation() {
187         return getNodeHandler().getRootNode();
188     }
189 
190     /**
191      * Closes this model. This causes the tracked node this model is based upon to be released (i.e.
192      * {@link InMemoryNodeModel#untrackNode(NodeSelector)} is called). This method should be called when this model is no
193      * longer needed. This implementation is idempotent; it is safe to call {@code close()} multiple times - only the first
194      * invocation has an effect. After this method has been called this model can no longer be used because there is no
195      * guarantee that the node can still be accessed from the parent model.
196      */
197     public void close() {
198         if (closed.compareAndSet(false, true)) {
199             getParentModel().untrackNode(getSelector());
200         }
201     }
202 
203     /**
204      * {@inheritDoc} This implementation calls {@code close()} if the {@code untrackOnFinalize} flag was set when this
205      * instance was constructed. While this is not 100 percent reliable, it is better than keeping the tracked node hanging
206      * around. Note that it is not a problem if {@code close()} already had been invoked manually because this method is
207      * idempotent.
208      *
209      * @see #close()
210      */
211     @Override
212     protected void finalize() throws Throwable {
213         if (isReleaseTrackedNodeOnFinalize()) {
214             close();
215         }
216         super.finalize();
217     }
218 }