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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.tree;
    18
    19  import java.util.Iterator;
    20
    21  /***
    22   * <p>
    23   * A concrete combiner implementation that is able to construct an override
    24   * combination.
    25   * </p>
    26   * <p>
    27   * An <em>override combination</em> means that nodes in the first node
    28   * structure take precedence over nodes in the second, or - in other words -
    29   * nodes of the second structure are only added to the resulting structure if
    30   * they do not occure in the first one. This is especially suitable for dealing
    31   * with the properties of configurations that are defined in an
    32   * <code>override</code> section of a configuration definition file (hence the
    33   * name).
    34   * </p>
    35   * <p>
    36   * This combiner will iterate over the second node hierarchy and find all nodes
    37   * that are not contained in the first hierarchy; these are added to the result.
    38   * If a node can be found in both structures, it is checked whether a
    39   * combination (in a recursive way) can be constructed for the two, which will
    40   * then be added. Per default, nodes are combined, which occur only once in both
    41   * structures. This test is implemented in the <code>canCombine()</code>
    42   * method.
    43   * </p>
    44   * <p>
    45   * As is true for the <code>{@link UnionCombiner}</code>, for this combiner
    46   * list nodes are important. The <code>addListNode()</code> can be called to
    47   * declare certain nodes as list nodes. This has the effect that these nodes
    48   * will never be combined.
    49   * </p>
    50   *
    51   * @author <a
    52   * href="http://commons.apache.org/configuration/team-list.html">Commons
    53   * Configuration team</a>
    54   * @version $Id: OverrideCombiner.java 561230 2007-07-31 04:17:09Z rahul $
    55   * @since 1.3
    56   */
    57  public class OverrideCombiner extends NodeCombiner
    58  {
    59      /***
    60       * Constructs an override combination for the passed in node structures.
    61       *
    62       * @param node1 the first node
    63       * @param node2 the second node
    64       * @return the resulting combined node structure
    65       */
    66      public ConfigurationNode combine(ConfigurationNode node1,
    67              ConfigurationNode node2)
    68      {
    69          ViewNode result = createViewNode();
    70          result.setName(node1.getName());
    71
    72          // Process nodes from the first structure, which override the second
    73          for (Iterator it = node1.getChildren().iterator(); it.hasNext();)
    74          {
    75              ConfigurationNode child = (ConfigurationNode) it.next();
    76              ConfigurationNode child2 = canCombine(node1, node2, child);
    77              if (child2 != null)
    78              {
    79                  result.addChild(combine(child, child2));
    80              }
    81              else
    82              {
    83                  result.addChild(child);
    84              }
    85          }
    86
    87          // Process nodes from the second structure, which are not contained
    88          // in the first structure
    89          for (Iterator it = node2.getChildren().iterator(); it.hasNext();)
    90          {
    91              ConfigurationNode child = (ConfigurationNode) it.next();
    92              if (node1.getChildrenCount(child.getName()) < 1)
    93              {
    94                  result.addChild(child);
    95              }
    96          }
    97
    98          // Handle attributes and value
    99          addAttributes(result, node1, node2);
    100         result.setValue((node1.getValue() != null) ? node1.getValue() : node2
    101                 .getValue());
    102
    103         return result;
    104     }
    105
    106     /***
    107      * Handles the attributes during a combination process. First all attributes
    108      * of the first node will be added to the result. Then all attributes of the
    109      * second node, which are not contained in the first node, will also be
    110      * added.
    111      *
    112      * @param result the resulting node
    113      * @param node1 the first node
    114      * @param node2 the second node
    115      */
    116     protected void addAttributes(ViewNode result, ConfigurationNode node1,
    117             ConfigurationNode node2)
    118     {
    119         result.appendAttributes(node1);
    120         for (Iterator it = node2.getAttributes().iterator(); it.hasNext();)
    121         {
    122             ConfigurationNode attr = (ConfigurationNode) it.next();
    123             if (node1.getAttributeCount(attr.getName()) == 0)
    124             {
    125                 result.addAttribute(attr);
    126             }
    127         }
    128     }
    129
    130     /***
    131      * Tests if a child node of the second node can be combined with the given
    132      * child node of the first node. If this is the case, the corresponding node
    133      * will be returned, otherwise <b>null</b>. This implementation checks
    134      * whether the child node occurs only once in both hierarchies and is no
    135      * known list node.
    136      *
    137      * @param node1 the first node
    138      * @param node2 the second node
    139      * @param child the child node (of the first node)
    140      * @return a child of the second node, with which a combination is possible
    141      */
    142     protected ConfigurationNode canCombine(ConfigurationNode node1,
    143             ConfigurationNode node2, ConfigurationNode child)
    144     {
    145         if (node2.getChildrenCount(child.getName()) == 1
    146                 && node1.getChildrenCount(child.getName()) == 1
    147                 && !isListNode(child))
    148         {
    149             return (ConfigurationNode) node2.getChildren(child.getName())
    150                     .get(0);
    151         }
    152         else
    153         {
    154             return null;
    155         }
    156     }
    157 }