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 019import java.util.LinkedList; 020import java.util.List; 021 022/** 023 * <p> 024 * A class providing different algorithms for traversing a hierarchy of configuration nodes. 025 * </p> 026 * <p> 027 * The methods provided by this class accept a {@link ConfigurationNodeVisitor} and visit all nodes in a hierarchy 028 * starting from a given root node. Because a {@link NodeHandler} has to be passed in, too, arbitrary types of nodes can 029 * be processed. The {@code walk()} methods differ in the order in which nodes are visited. Details can be found in the 030 * method documentation. 031 * </p> 032 * <p> 033 * An instance of this class does not define any state; therefore, it can be shared and used concurrently. The 034 * {@code INSTANCE} member field can be used for accessing a default instance. If desired (e.g. for testing purposes), 035 * new instances can be created. 036 * </p> 037 * 038 * @since 2.0 039 */ 040public class NodeTreeWalker { 041 /** The default instance of this class. */ 042 public static final NodeTreeWalker INSTANCE = new NodeTreeWalker(); 043 044 /** 045 * Visits all nodes in the hierarchy represented by the given root node in <em>depth first search</em> manner. This 046 * means that first {@link ConfigurationNodeVisitor#visitBeforeChildren(Object, NodeHandler)} is called on a node, then 047 * recursively all of its children are processed, and eventually 048 * {@link ConfigurationNodeVisitor#visitAfterChildren(Object, NodeHandler)} gets invoked. 049 * 050 * @param root the root node of the hierarchy to be processed (may be <b>null</b>, then this call has no effect) 051 * @param visitor the {@code ConfigurationNodeVisitor} (must not be <b>null</b>) 052 * @param handler the {@code NodeHandler} (must not be <b>null</b>) 053 * @param <T> the type of the nodes involved 054 * @throws IllegalArgumentException if a required parameter is <b>null</b> 055 */ 056 public <T> void walkDFS(final T root, final ConfigurationNodeVisitor<T> visitor, final NodeHandler<T> handler) { 057 if (checkParameters(root, visitor, handler)) { 058 dfs(root, visitor, handler); 059 } 060 } 061 062 /** 063 * Visits all nodes in the hierarchy represented by the given root node in <em>breadth first search</em> manner. This 064 * means that the nodes are visited in an order corresponding to the distance from the root node: first the root node is 065 * visited, then all direct children of the root node, then all direct children of the first child of the root node, 066 * etc. In this mode of traversal, there is no direct connection between the encounter of a node and its children. 067 * <strong>Therefore, on the visitor object only the {@code visitBeforeChildren()} method gets called!</strong>. 068 * 069 * @param root the root node of the hierarchy to be processed (may be <b>null</b>, then this call has no effect) 070 * @param visitor the {@code ConfigurationNodeVisitor} (must not be <b>null</b>) 071 * @param handler the {@code NodeHandler} (must not be <b>null</b>) 072 * @param <T> the type of the nodes involved 073 * @throws IllegalArgumentException if a required parameter is <b>null</b> 074 */ 075 public <T> void walkBFS(final T root, final ConfigurationNodeVisitor<T> visitor, final NodeHandler<T> handler) { 076 if (checkParameters(root, visitor, handler)) { 077 bfs(root, visitor, handler); 078 } 079 } 080 081 /** 082 * Recursive helper method for performing a DFS traversal. 083 * 084 * @param node the current node 085 * @param visitor the visitor 086 * @param handler the handler 087 * @param <T> the type of the nodes involved 088 */ 089 private static <T> void dfs(final T node, final ConfigurationNodeVisitor<T> visitor, final NodeHandler<T> handler) { 090 if (!visitor.terminate()) { 091 visitor.visitBeforeChildren(node, handler); 092 handler.getChildren(node).forEach(c -> dfs(c, visitor, handler)); 093 if (!visitor.terminate()) { 094 visitor.visitAfterChildren(node, handler); 095 } 096 } 097 } 098 099 /** 100 * Helper method for performing a BFS traversal. Implementation node: This method organizes the nodes to be visited in 101 * structures on the heap. Therefore, it can deal with larger structures than would be the case in a recursive approach 102 * (where the stack size limits the size of the structures which can be traversed). 103 * 104 * @param root the root node to be navigated 105 * @param visitor the visitor 106 * @param handler the handler 107 * @param <T> the type of the nodes involved 108 */ 109 private static <T> void bfs(final T root, final ConfigurationNodeVisitor<T> visitor, final NodeHandler<T> handler) { 110 final List<T> pendingNodes = new LinkedList<>(); 111 pendingNodes.add(root); 112 boolean cancel = false; 113 114 while (!pendingNodes.isEmpty() && !cancel) { 115 final T node = pendingNodes.remove(0); 116 visitor.visitBeforeChildren(node, handler); 117 cancel = visitor.terminate(); 118 pendingNodes.addAll(handler.getChildren(node)); 119 } 120 } 121 122 /** 123 * Helper method for checking the parameters for the walk() methods. If mandatory parameters are missing, an exception 124 * is thrown. The return value indicates whether an operation can be performed. 125 * 126 * @param root the root node 127 * @param visitor the visitor 128 * @param handler the handler 129 * @param <T> the type of the nodes involved 130 * @return <b>true</b> if a walk operation can be performed, <b>false</b> otherwise 131 * @throws IllegalArgumentException if a required parameter is missing 132 */ 133 private static <T> boolean checkParameters(final T root, final ConfigurationNodeVisitor<T> visitor, final NodeHandler<T> handler) { 134 if (visitor == null) { 135 throw new IllegalArgumentException("Visitor must not be null!"); 136 } 137 if (handler == null) { 138 throw new IllegalArgumentException("NodeHandler must not be null!"); 139 } 140 return root != null; 141 } 142}