    001/*
    002 * Licensed to the Apache Software Foundation (ASF) under one or more
    003 * contributor license agreements.  See the NOTICE file distributed with
    004 * this work for additional information regarding copyright ownership.
    005 * The ASF licenses this file to You under the Apache License, Version 2.0
    006 * (the "License"); you may not use this file except in compliance with
    007 * the License.  You may obtain a copy of the License at
    008 *
    009 *     http://www.apache.org/licenses/LICENSE-2.0
    010 *
    011 * Unless required by applicable law or agreed to in writing, software
    012 * distributed under the License is distributed on an "AS IS" BASIS,
    013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    014 * See the License for the specific language governing permissions and
    015 * limitations under the License.
    016 */
    017package org.apache.commons.configuration2;
    018
    019import org.apache.commons.configuration2.tree.ImmutableNode;
    020import org.apache.commons.configuration2.tree.InMemoryNodeModel;
    021import org.apache.commons.configuration2.tree.InMemoryNodeModelSupport;
    022import org.apache.commons.configuration2.tree.NodeModel;
    023import org.apache.commons.configuration2.tree.NodeSelector;
    024import org.apache.commons.configuration2.tree.TrackedNodeModel;
    025
    026/**
    027 * <p>
    028 * A specialized hierarchical configuration class with a node model that uses a
    029 * tracked node of another node model as its root node.
    030 * </p>
    031 * <p>
    032 * Configurations of this type are initialized with a special {@link NodeModel}
    033 * operating on a specific tracked node of the parent configuration and the
    034 * corresponding {@link NodeSelector}. All property accessor methods are
    035 * evaluated relative to this root node. A good use case for a
    036 * {@code SubnodeConfiguration} is when multiple properties from a specific sub
    037 * tree of the whole configuration need to be accessed. Then a
    038 * {@code SubnodeConfiguration} can be created with the parent node of the
    039 * affected sub tree as root node. This allows for simpler property keys and is
    040 * also more efficient.
    041 * </p>
    042 * <p>
    043 * By making use of a tracked node as root node, a {@code SubnodeConfiguration}
    044 * and its parent configuration initially operate on the same hierarchy of
    045 * configuration nodes. So if modifications are performed at the subnode
    046 * configuration, these changes are immediately visible in the parent
    047 * configuration. Analogously will updates of the parent configuration affect
    048 * the {@code SubnodeConfiguration} if the sub tree spanned by the
    049 * {@code SubnodeConfiguration}'s root node is involved.
    050 * </p>
    051 * <p>
    052 * Note that by making use of a {@code NodeSelector} the
    053 * {@code SubnodeConfiguration} is not associated with a physical node instance,
    054 * but the selection criteria stored in the selector are evaluated after each
    055 * change of the nodes structure. As an example consider that the selector uses
    056 * a key with an index into a list element, say index 2. Now if an update occurs
    057 * on the underlying nodes structure which removes the first element in this
    058 * list structure, the {@code SubnodeConfiguration} still references the element
    059 * with index 2 which is now another one.
    060 * </p>
    061 * <p>
    062 * There are also possible changes of the underlying nodes structure which
    063 * completely detach the {@code SubnodeConfiguration} from its parent
    064 * configuration. For instance, the key referenced by the
    065 * {@code SubnodeConfiguration} could be removed in the parent configuration. If
    066 * this happens, the {@code SubnodeConfiguration} stays functional; however, it
    067 * now operates on a separate node model than its parent configuration. Changes
    068 * made by one configuration are no longer visible for the other one (as the
    069 * node models have no longer overlapping nodes, there is no way to have a
    070 * synchronization here).
    071 * </p>
    072 * <p>
    073 * When a subnode configuration is created, it inherits the settings of its
    074 * parent configuration, e.g. some flags like the
    075 * {@code throwExceptionOnMissing} flag or the settings for handling list
    076 * delimiters) or the expression engine. If these settings are changed later in
    077 * either the subnode or the parent configuration, the changes are not visible
    078 * for each other. So you could create a subnode configuration, and change its
    079 * expression engine without affecting the parent configuration.
    080 * </p>
    081 * <p>
    082 * Because the {@code SubnodeConfiguration} operates on the same nodes structure
    083 * as its parent it uses the same {@code Synchronizer} instance per default.
    084 * This means that locks held on one {@code SubnodeConfiguration} also impact
    085 * the parent configuration and all of its other {@code SubnodeConfiguration}
    086 * objects. You should not change this without a good reason! Otherwise, there
    087 * is the risk of data corruption when multiple threads access these
    088 * configuration concurrently.
    089 * </p>
    090 * <p>
    091 * From its purpose this class is quite similar to {@link SubsetConfiguration}.
    092 * The difference is that a subset configuration of a hierarchical configuration
    093 * may combine multiple configuration nodes from different sub trees of the
    094 * configuration, while all nodes in a subnode configuration belong to the same
    095 * sub tree. If an application can live with this limitation, it is recommended
    096 * to use this class instead of {@code SubsetConfiguration} because creating a
    097 * subset configuration is more expensive than creating a subnode configuration.
    098 * </p>
    099 * <p>
    100 * It is strongly recommended to create {@code SubnodeConfiguration} instances
    101 * only through the {@code configurationAt()} methods of a hierarchical
    102 * configuration. These methods ensure that all necessary initializations are
    103 * done. Creating instances manually without doing proper initialization may
    104 * break some of the functionality provided by this class.
    105 * </p>
    106 *
    107 * @since 1.3
    108 */
    109public class SubnodeConfiguration extends BaseHierarchicalConfiguration
    110{
    111    /** Stores the parent configuration. */
    112    private final BaseHierarchicalConfiguration parent;
    113
    114    /** The node selector selecting the root node of this configuration. */
    115    private final NodeSelector rootSelector;
    116
    117    /**
    118     * Creates a new instance of {@code SubnodeConfiguration} and initializes it
    119     * with all relevant properties.
    120     *
    121     * @param parent the parent configuration
    122     * @param model the {@code TrackedNodeModel} to be used for this configuration
    123     * @throws IllegalArgumentException if a required argument is missing
    124     */
    125    public SubnodeConfiguration(final BaseHierarchicalConfiguration parent,
    126                                final TrackedNodeModel model)
    127    {
    128        super(model);
    129        if (parent == null)
    130        {
    131            throw new IllegalArgumentException(
    132                    "Parent configuration must not be null!");
    133        }
    134        if (model == null)
    135        {
    136            throw new IllegalArgumentException("Node model must not be null!");
    137        }
    138
    139        this.parent = parent;
    140        rootSelector = model.getSelector();
    141    }
    142
    143    /**
    144     * Returns the parent configuration of this subnode configuration.
    145     *
    146     * @return the parent configuration
    147     */
    148    public BaseHierarchicalConfiguration getParent()
    149    {
    150        return parent;
    151    }
    152
    153    /**
    154     * Returns the selector to the root node of this configuration.
    155     *
    156     * @return the {@code NodeSelector} to the root node
    157     */
    158    public NodeSelector getRootSelector()
    159    {
    160        return rootSelector;
    161    }
    162
    163    /**
    164     * Closes this sub configuration. This method closes the underlying
    165     * {@link TrackedNodeModel}, thus causing the tracked node acting as root
    166     * node to be released. Per default, this happens automatically when the
    167     * model is claimed by the garbage collector. By calling this method
    168     * explicitly, it can be indicated that this configuration is no longer used
    169     * and that resources used by it can be freed immediately.
    170     */
    171    public void close()
    172    {
    173        getTrackedModel().close();
    174    }
    175
    176    /**
    177     * {@inheritDoc} This implementation returns a newly created node model
    178     * with the correct root node set. Note that this model is not used for
    179     * property access, but only made available to clients that need to
    180     * operate on the node structure of this {@code SubnodeConfiguration}.
    181     * Be aware that the implementation of this method is not very efficient.
    182     */
    183    @Override
    184    public InMemoryNodeModel getNodeModel()
    185    {
    186        final ImmutableNode root =
    187                getParent().getNodeModel().getTrackedNode(getRootSelector());
    188        return new InMemoryNodeModel(root);
    189    }
    190
    191    /**
    192     * Returns the node model of the root configuration.
    193     * {@code SubnodeConfiguration} instances created from a hierarchical
    194     * configuration operate on the same node model, using different nodes as
    195     * their local root nodes. With this method the top-level node model can be
    196     * obtained. It works even in constellations where a
    197     * {@code SubnodeConfiguration} has been created from another
    198     * {@code SubnodeConfiguration}.
    199     *
    200     * @return the root node model
    201     * @since 2.2
    202     */
    203    public InMemoryNodeModel getRootNodeModel()
    204    {
    205        if (getParent() instanceof SubnodeConfiguration)
    206        {
    207            return ((SubnodeConfiguration) getParent()).getRootNodeModel();
    208        }
    209        return getParent().getNodeModel();
    210    }
    211
    212    /**
    213     * {@inheritDoc} This implementation returns a copy of the current node
    214     * model with the same settings. However, it has to be ensured that the
    215     * track count for the node selector is increased.
    216     *
    217     * @return the node model for the clone
    218     */
    219    @Override
    220    protected NodeModel<ImmutableNode> cloneNodeModel()
    221    {
    222        final InMemoryNodeModel parentModel =
    223                (InMemoryNodeModel) getParent().getModel();
    224        parentModel.trackNode(getRootSelector(), getParent());
    225        return new TrackedNodeModel(getParent(), getRootSelector(), true);
    226    }
    227
    228    /**
    229     * {@inheritDoc} This implementation returns a sub selector of the selector
    230     * of this configuration.
    231     */
    232    @Override
    233    protected NodeSelector getSubConfigurationNodeSelector(final String key)
    234    {
    235        return getRootSelector().subSelector(key);
    236    }
    237
    238    /**
    239     * {@inheritDoc} This implementation returns the parent model of the
    240     * {@link TrackedNodeModel} used by this configuration.
    241     */
    242    @Override
    243    protected InMemoryNodeModel getSubConfigurationParentModel()
    244    {
    245        return getTrackedModel().getParentModel();
    246    }
    247
    248    /**
    249     * {@inheritDoc} This implementation makes sure that the correct node model
    250     * (the one of the parent) is used for the new sub configuration.
    251     */
    252    @Override
    253    protected SubnodeConfiguration createSubConfigurationForTrackedNode(
    254            final NodeSelector selector, final InMemoryNodeModelSupport parentModelSupport)
    255    {
    256        return super.createSubConfigurationForTrackedNode(selector, getParent());
    257    }
    258
    259    /**
    260     * Convenience method that returns the tracked model used by this sub
    261     * configuration.
    262     *
    263     * @return the {@code TrackedNodeModel}
    264     */
    265    private TrackedNodeModel getTrackedModel()
    266    {
    267        return (TrackedNodeModel) getModel();
    268    }
    269}