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