    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}