    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        for (final ImmutableNode child : node1) {
    057            final ImmutableNode child2 = canCombine(node1, node2, child);
    058            if (child2 != null) {
    059                result.addChild(combine(child, child2));
    060            } else {
    061                result.addChild(child);
    062            }
    063        }
    064
    065        // Process nodes from the second structure, which are not contained
    066        // in the first structure
    067        for (final ImmutableNode child : node2) {
    068            if (HANDLER.getChildrenCount(node1, child.getNodeName()) < 1) {
    069                result.addChild(child);
    070            }
    071        }
    072
    073        // Handle attributes and value
    074        addAttributes(result, node1, node2);
    075        result.value(node1.getValue() != null ? node1.getValue() : node2.getValue());
    076
    077        return result.create();
    078    }
    079
    080    /**
    081     * Handles the attributes during a combination process. First all attributes of the first node are added to the result.
    082     * Then all attributes of the second node, which are not contained in the first node, are also added.
    083     *
    084     * @param result the resulting node
    085     * @param node1 the first node
    086     * @param node2 the second node
    087     */
    088    protected void addAttributes(final ImmutableNode.Builder result, final ImmutableNode node1, final ImmutableNode node2) {
    089        result.addAttributes(node1.getAttributes());
    090        for (final String attr : node2.getAttributes().keySet()) {
    091            if (!node1.getAttributes().containsKey(attr)) {
    092                result.addAttribute(attr, HANDLER.getAttributeValue(node2, attr));
    093            }
    094        }
    095    }
    096
    097    /**
    098     * 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
    099     * case, the corresponding node will be returned, otherwise <b>null</b>. This implementation checks whether the child
    100     * node occurs only once in both hierarchies and is no known list node.
    101     *
    102     * @param node1 the first node
    103     * @param node2 the second node
    104     * @param child the child node (of the first node)
    105     * @return a child of the second node, with which a combination is possible
    106     */
    107    protected ImmutableNode canCombine(final ImmutableNode node1, final ImmutableNode node2, final ImmutableNode child) {
    108        if (HANDLER.getChildrenCount(node2, child.getNodeName()) == 1 && HANDLER.getChildrenCount(node1, child.getNodeName()) == 1 && !isListNode(child)) {
    109            return HANDLER.getChildren(node2, child.getNodeName()).get(0);
    110        }
    111        return null;
    112    }
    113}