    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.tree;
    018
    019
    020/**
    021 * <p>
    022 * A concrete combiner implementation that is able to construct an override
    023 * combination.
    024 * </p>
    025 * <p>
    026 * An <em>override combination</em> means that nodes in the first node
    027 * structure take precedence over nodes in the second, or - in other words -
    028 * nodes of the second structure are only added to the resulting structure if
    029 * they do not occur in the first one. This is especially suitable for dealing
    030 * with the properties of configurations that are defined in an
    031 * {@code override} section of a configuration definition file (hence the
    032 * name).
    033 * </p>
    034 * <p>
    035 * This combiner will iterate over the second node hierarchy and find all nodes
    036 * that are not contained in the first hierarchy; these are added to the result.
    037 * If a node can be found in both structures, it is checked whether a
    038 * combination (in a recursive way) can be constructed for the two, which will
    039 * then be added. Per default, nodes are combined, which occur only once in both
    040 * structures. This test is implemented in the {@code canCombine()}
    041 * method.
    042 * </p>
    043 * <p>
    044 * As is true for the {@link UnionCombiner}, for this combiner
    045 * list nodes are important. The {@code addListNode()} can be called to
    046 * declare certain nodes as list nodes. This has the effect that these nodes
    047 * will never be combined.
    048 * </p>
    049 *
    050 * @since 1.3
    051 */
    052public class OverrideCombiner extends NodeCombiner
    053{
    054    /**
    055     * Constructs an override combination for the passed in node structures.
    056     *
    057     * @param node1 the first node
    058     * @param node2 the second node
    059     * @return the resulting combined node structure
    060     */
    061    @Override
    062    public ImmutableNode combine(final ImmutableNode node1,
    063            final ImmutableNode node2)
    064    {
    065        final ImmutableNode.Builder result = new ImmutableNode.Builder();
    066        result.name(node1.getNodeName());
    067
    068        // Process nodes from the first structure, which override the second
    069        for (final ImmutableNode child : node1.getChildren())
    070        {
    071            final ImmutableNode child2 = canCombine(node1, node2, child);
    072            if (child2 != null)
    073            {
    074                result.addChild(combine(child, child2));
    075            }
    076            else
    077            {
    078                result.addChild(child);
    079            }
    080        }
    081
    082        // Process nodes from the second structure, which are not contained
    083        // in the first structure
    084        for (final ImmutableNode child : node2.getChildren())
    085        {
    086            if (HANDLER.getChildrenCount(node1, child.getNodeName()) < 1)
    087            {
    088                result.addChild(child);
    089            }
    090        }
    091
    092        // Handle attributes and value
    093        addAttributes(result, node1, node2);
    094        result.value((node1.getValue() != null) ? node1.getValue() : node2
    095                .getValue());
    096
    097        return result.create();
    098    }
    099
    100    /**
    101     * Handles the attributes during a combination process. First all attributes
    102     * of the first node are added to the result. Then all attributes of the
    103     * second node, which are not contained in the first node, are also added.
    104     *
    105     * @param result the resulting node
    106     * @param node1 the first node
    107     * @param node2 the second node
    108     */
    109    protected void addAttributes(final ImmutableNode.Builder result,
    110            final ImmutableNode node1, final ImmutableNode node2)
    111    {
    112        result.addAttributes(node1.getAttributes());
    113        for (final String attr : node2.getAttributes().keySet())
    114        {
    115            if (!node1.getAttributes().containsKey(attr))
    116            {
    117                result.addAttribute(attr,
    118                        HANDLER.getAttributeValue(node2, attr));
    119            }
    120        }
    121    }
    122
    123    /**
    124     * Tests if a child node of the second node can be combined with the given
    125     * child node of the first node. If this is the case, the corresponding node
    126     * will be returned, otherwise <b>null</b>. This implementation checks
    127     * whether the child node occurs only once in both hierarchies and is no
    128     * known list node.
    129     *
    130     * @param node1 the first node
    131     * @param node2 the second node
    132     * @param child the child node (of the first node)
    133     * @return a child of the second node, with which a combination is possible
    134     */
    135    protected ImmutableNode canCombine(final ImmutableNode node1,
    136            final ImmutableNode node2, final ImmutableNode child)
    137    {
    138        if (HANDLER.getChildrenCount(node2, child.getNodeName()) == 1
    139                && HANDLER.getChildrenCount(node1, child.getNodeName()) == 1
    140                && !isListNode(child))
    141        {
    142            return HANDLER.getChildren(node2, child.getNodeName()).get(0);
    143        }
    144        return null;
    145    }
    146}