View Javadoc

    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 }