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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  import java.util.LinkedList;
    21  import java.util.List;
    22
    23  /***
    24   * <p>
    25   * A specialized implementation of the <code>NodeCombiner</code> interface
    26   * that constructs a union from two passed in node hierarchies.
    27   * </p>
    28   * <p>
    29   * The given source hierarchies are traversed and their nodes are added to the
    30   * resulting structure. Under some circumstances two nodes can be combined
    31   * rather than adding both. This is the case if both nodes are single children
    32   * (no lists) of their parents and do not have values. The corresponding check
    33   * is implemented in the <code>findCombineNode()</code> method.
    34   * </p>
    35   * <p>
    36   * Sometimes it is not possible for this combiner to detect whether two nodes
    37   * can be combined or not. Consider the following two node hierarchies:
    38   * </p>
    39   * <p>
    40   *
    41   * <pre>
    42   * Hierarchy 1:
    43   *
    44   * Database
    45   *   +--Tables
    46   *        +--Table
    47   *             +--name [users]
    48   *             +--fields
    49   *                   +--field
    50   *                   |    +--name [uid]
    51   *                   +--field
    52   *                   |    +--name [usrname]
    53   *                     ...
    54   * </pre>
    55   *
    56   * </p>
    57   * <p>
    58   *
    59   * <pre>
    60   * Hierarchy 2:
    61   *
    62   * Database
    63   *   +--Tables
    64   *        +--Table
    65   *             +--name [documents]
    66   *             +--fields
    67   *                   +--field
    68   *                   |    +--name [docid]
    69   *                   +--field
    70   *                   |    +--name [docname]
    71   *                     ...
    72   * </pre>
    73   *
    74   * </p>
    75   * <p>
    76   * Both hierarchies contain data about database tables. Each describes a single
    77   * table. If these hierarchies are to be combined, the result should probably
    78   * look like the following:
    79   * <p>
    80   *
    81   * <pre>
    82   * Database
    83   *   +--Tables
    84   *        +--Table
    85   *        |    +--name [users]
    86   *        |    +--fields
    87   *        |          +--field
    88   *        |          |    +--name [uid]
    89   *        |            ...
    90   *        +--Table
    91   *             +--name [documents]
    92   *             +--fields
    93   *                   +--field
    94   *                   |    +--name [docid]
    95   *                     ...
    96   * </pre>
    97   *
    98   * </p>
    99   * <p>
    100  * i.e. the <code>Tables</code> nodes should be combined, while the
    101  * <code>Table</code> nodes should both be added to the resulting tree. From
    102  * the combiner's point of view there is no difference between the
    103  * <code>Tables</code> and the <code>Table</code> nodes in the source trees,
    104  * so the developer has to help out and give a hint that the <code>Table</code>
    105  * nodes belong to a list structure. This can be done using the
    106  * <code>addListNode()</code> method; this method expects the name of a node,
    107  * which should be treated as a list node. So if
    108  * <code>addListNode("Table");</code> was called, the combiner knows that it
    109  * must not combine the <code>Table</code> nodes, but add it both to the
    110  * resulting tree.
    111  * </p>
    112  *
    113  * @author <a
    114  * href="http://commons.apache.org/configuration/team-list.html">Commons
    115  * Configuration team</a>
    116  * @version $Id: UnionCombiner.java 561230 2007-07-31 04:17:09Z rahul $
    117  * @since 1.3
    118  */
    119 public class UnionCombiner extends NodeCombiner
    120 {
    121     /***
    122      * Combines the given nodes to a new union node.
    123      *
    124      * @param node1 the first source node
    125      * @param node2 the second source node
    126      * @return the union node
    127      */
    128     public ConfigurationNode combine(ConfigurationNode node1,
    129             ConfigurationNode node2)
    130     {
    131         ViewNode result = createViewNode();
    132         result.setName(node1.getName());
    133         result.appendAttributes(node1);
    134         result.appendAttributes(node2);
    135
    136         // Check if nodes can be combined
    137         List children2 = new LinkedList(node2.getChildren());
    138         for (Iterator it = node1.getChildren().iterator(); it.hasNext();)
    139         {
    140             ConfigurationNode child1 = (ConfigurationNode) it.next();
    141             ConfigurationNode child2 = findCombineNode(node1, node2, child1,
    142                     children2);
    143             if (child2 != null)
    144             {
    145                 result.addChild(combine(child1, child2));
    146                 children2.remove(child2);
    147             }
    148             else
    149             {
    150                 result.addChild(child1);
    151             }
    152         }
    153
    154         // Add remaining children of node 2
    155         for (Iterator it = children2.iterator(); it.hasNext();)
    156         {
    157             result.addChild((ConfigurationNode) it.next());
    158         }
    159
    160         return result;
    161     }
    162
    163     /***
    164      * <p>
    165      * Tries to find a child node of the second source node, with whitch a child
    166      * of the first source node can be combined. During combining of the source
    167      * nodes an iteration over the first source node's children is performed.
    168      * For each child node it is checked whether a corresponding child node in
    169      * the second source node exists. If this is the case, these corresponsing
    170      * child nodes are recursively combined and the result is added to the
    171      * combined node. This method implements the checks whether such a recursive
    172      * combination is possible. The actual implementation tests the following
    173      * conditions:
    174      * </p>
    175      * <p>
    176      * <ul>
    177      * <li>In both the first and the second source node there is only one child
    178      * node with the given name (no list structures).</li>
    179      * <li>The given name is not in the list of known list nodes, i.e. it was
    180      * not passed to the <code>addListNode()</code> method.</li>
    181      * <li>None of these matching child nodes has a value.</li>
    182      * </ul>
    183      * </p>
    184      * <p>
    185      * If all of these tests are successfull, the matching child node of the
    186      * second source node is returned. Otherwise the result is <b>null</b>.
    187      * </p>
    188      *
    189      * @param node1 the first source node
    190      * @param node2 the second source node
    191      * @param child the child node of the first source node to be checked
    192      * @param children a list with all children of the second source node
    193      * @return the matching child node of the second source node or <b>null</b>
    194      * if there is none
    195      */
    196     protected ConfigurationNode findCombineNode(ConfigurationNode node1,
    197             ConfigurationNode node2, ConfigurationNode child, List children)
    198     {
    199         if (child.getValue() == null && !isListNode(child)
    200                 && node1.getChildrenCount(child.getName()) == 1
    201                 && node2.getChildrenCount(child.getName()) == 1)
    202         {
    203             ConfigurationNode child2 = (ConfigurationNode) node2.getChildren(
    204                     child.getName()).iterator().next();
    205             if (child2.getValue() == null)
    206             {
    207                 return child2;
    208             }
    209         }
    210         return null;
    211     }
    212 }