    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
    025 * configuration nodes.
    026 * </p>
    027 * <p>
    028 * The methods provided by this class accept a {@link ConfigurationNodeVisitor}
    029 * and visit all nodes in a hierarchy starting from a given root node. Because a
    030 * {@link NodeHandler} has to be passed in, too, arbitrary types of nodes can be
    031 * processed. The {@code walk()} methods differ in the order in which nodes are
    032 * visited. Details can be found in the method documentation.
    033 * </p>
    034 * <p>
    035 * An instance of this class does not define any state; therefore, it can be
    036 * shared and used concurrently. The {@code INSTANCE} member field can be used
    037 * for accessing a default instance. If desired (e.g. for testing purposes), new
    038 * instances can be created.
    039 * </p>
    040 *
    041 * @since 2.0
    042 */
    043public class NodeTreeWalker
    044{
    045    /** The default instance of this class. */
    046    public static final NodeTreeWalker INSTANCE = new NodeTreeWalker();
    047
    048    /**
    049     * Visits all nodes in the hierarchy represented by the given root node in
    050     * <em>depth first search</em> manner. This means that first
    051     * {@link ConfigurationNodeVisitor#visitBeforeChildren(Object, NodeHandler)}
    052     * is called on a node, then recursively all of its children are processed,
    053     * and eventually
    054     * {@link ConfigurationNodeVisitor#visitAfterChildren(Object, NodeHandler)}
    055     * gets invoked.
    056     *
    057     * @param root the root node of the hierarchy to be processed (may be
    058     *        <b>null</b>, then this call has no effect)
    059     * @param visitor the {@code ConfigurationNodeVisitor} (must not be
    060     *        <b>null</b>)
    061     * @param handler the {@code NodeHandler} (must not be <b>null</b>)
    062     * @param <T> the type of the nodes involved
    063     * @throws IllegalArgumentException if a required parameter is <b>null</b>
    064     */
    065    public <T> void walkDFS(final T root, final ConfigurationNodeVisitor<T> visitor,
    066            final NodeHandler<T> handler)
    067    {
    068        if (checkParameters(root, visitor, handler))
    069        {
    070            dfs(root, visitor, handler);
    071        }
    072    }
    073
    074    /**
    075     * Visits all nodes in the hierarchy represented by the given root node in
    076     * <em>breadth first search</em> manner. This means that the nodes are
    077     * visited in an order corresponding to the distance from the root node:
    078     * first the root node is visited, then all direct children of the root
    079     * node, then all direct children of the first child of the root node, etc.
    080     * In this mode of traversal, there is no direct connection between the
    081     * encounter of a node and its children. <strong>Therefore, on the visitor
    082     * object only the {@code visitBeforeChildren()} method gets
    083     * called!</strong>.
    084     *
    085     * @param root the root node of the hierarchy to be processed (may be
    086     *        <b>null</b>, then this call has no effect)
    087     * @param visitor the {@code ConfigurationNodeVisitor} (must not be
    088     *        <b>null</b>)
    089     * @param handler the {@code NodeHandler} (must not be <b>null</b>)
    090     * @param <T> the type of the nodes involved
    091     * @throws IllegalArgumentException if a required parameter is <b>null</b>
    092     */
    093    public <T> void walkBFS(final T root, final ConfigurationNodeVisitor<T> visitor,
    094            final NodeHandler<T> handler)
    095    {
    096        if (checkParameters(root, visitor, handler))
    097        {
    098            bfs(root, visitor, handler);
    099        }
    100    }
    101
    102    /**
    103     * Recursive helper method for performing a DFS traversal.
    104     *
    105     * @param node the current node
    106     * @param visitor the visitor
    107     * @param handler the handler
    108     * @param <T> the type of the nodes involved
    109     */
    110    private static <T> void dfs(final T node, final ConfigurationNodeVisitor<T> visitor,
    111            final NodeHandler<T> handler)
    112    {
    113        if (!visitor.terminate())
    114        {
    115            visitor.visitBeforeChildren(node, handler);
    116            for (final T c : handler.getChildren(node))
    117            {
    118                dfs(c, visitor, handler);
    119            }
    120            if (!visitor.terminate())
    121            {
    122                visitor.visitAfterChildren(node, handler);
    123            }
    124        }
    125    }
    126
    127    /**
    128     * Helper method for performing a BFS traversal. Implementation node: This
    129     * method organizes the nodes to be visited in structures on the heap.
    130     * Therefore, it can deal with larger structures than would be the case in a
    131     * recursive approach (where the stack size limits the size of the
    132     * structures which can be traversed).
    133     *
    134     * @param root the root node to be navigated
    135     * @param visitor the visitor
    136     * @param handler the handler
    137     * @param <T> the type of the nodes involved
    138     */
    139    private static <T> void bfs(final T root, final ConfigurationNodeVisitor<T> visitor,
    140            final NodeHandler<T> handler)
    141    {
    142        final List<T> pendingNodes = new LinkedList<>();
    143        pendingNodes.add(root);
    144        boolean cancel = false;
    145
    146        while (!pendingNodes.isEmpty() && !cancel)
    147        {
    148            final T node = pendingNodes.remove(0);
    149            visitor.visitBeforeChildren(node, handler);
    150            cancel = visitor.terminate();
    151            for (final T c : handler.getChildren(node))
    152            {
    153                pendingNodes.add(c);
    154            }
    155        }
    156    }
    157
    158    /**
    159     * Helper method for checking the parameters for the walk() methods. If
    160     * mandatory parameters are missing, an exception is thrown. The return
    161     * value indicates whether an operation can be performed.
    162     *
    163     * @param root the root node
    164     * @param visitor the visitor
    165     * @param handler the handler
    166     * @param <T> the type of the nodes involved
    167     * @return <b>true</b> if a walk operation can be performed, <b>false</b>
    168     *         otherwise
    169     * @throws IllegalArgumentException if a required parameter is missing
    170     */
    171    private static <T> boolean checkParameters(final T root,
    172            final ConfigurationNodeVisitor<T> visitor, final NodeHandler<T> handler)
    173    {
    174        if (visitor == null)
    175        {
    176            throw new IllegalArgumentException("Visitor must not be null!");
    177        }
    178        if (handler == null)
    179        {
    180            throw new IllegalArgumentException("NodeHandler must not be null!");
    181        }
    182        return root != null;
    183    }
    184}