    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 * <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     * Constructs an override combination for the passed in node structures.
    045     *
    046     * @param node1 the first node
    047     * @param node2 the second node
    048     * @return the resulting combined node structure
    049     */
    050    @Override
    051    public ImmutableNode combine(final ImmutableNode node1, final ImmutableNode node2) {
    052        final ImmutableNode.Builder result = new ImmutableNode.Builder();
    053        result.name(node1.getNodeName());
    054
    055        // Process nodes from the first structure, which override the second
    056        node1.forEach(child -> {
    057            final ImmutableNode child2 = canCombine(node1, node2, child);
    058            result.addChild(child2 != null ? combine(child, child2) : child);
    059        });
    060
    061        // Process nodes from the second structure, which are not contained
    062        // in the first structure
    063        node2.stream().filter(child -> HANDLER.getChildrenCount(node1, child.getNodeName()) < 1).forEach(result::addChild);
    064
    065        // Handle attributes and value
    066        addAttributes(result, node1, node2);
    067        result.value(node1.getValue() != null ? node1.getValue() : node2.getValue());
    068
    069        return result.create();
    070    }
    071
    072    /**
    073     * Handles the attributes during a combination process. First all attributes of the first node are added to the result.
    074     * Then all attributes of the second node, which are not contained in the first node, are also added.
    075     *
    076     * @param result the resulting node
    077     * @param node1 the first node
    078     * @param node2 the second node
    079     */
    080    protected void addAttributes(final ImmutableNode.Builder result, final ImmutableNode node1, final ImmutableNode node2) {
    081        result.addAttributes(node1.getAttributes());
    082        node2.getAttributes().keySet().forEach(attr -> {
    083            if (!node1.getAttributes().containsKey(attr)) {
    084                result.addAttribute(attr, HANDLER.getAttributeValue(node2, attr));
    085            }
    086        });
    087    }
    088
    089    /**
    090     * 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
    091     * case, the corresponding node will be returned, otherwise <b>null</b>. This implementation checks whether the child
    092     * node occurs only once in both hierarchies and is no known list node.
    093     *
    094     * @param node1 the first node
    095     * @param node2 the second node
    096     * @param child the child node (of the first node)
    097     * @return a child of the second node, with which a combination is possible
    098     */
    099    protected ImmutableNode canCombine(final ImmutableNode node1, final ImmutableNode node2, final ImmutableNode child) {
    100        if (HANDLER.getChildrenCount(node2, child.getNodeName()) == 1 && HANDLER.getChildrenCount(node1, child.getNodeName()) == 1 && !isListNode(child)) {
    101            return HANDLER.getChildren(node2, child.getNodeName()).get(0);
    102        }
    103        return null;
    104    }
    105}