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}