    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.ArrayList;
    020import java.util.Collections;
    021import java.util.Iterator;
    022import java.util.LinkedList;
    023import java.util.List;
    024
    025import org.apache.commons.lang3.builder.ToStringBuilder;
    026
    027/**
    028 * <p>
    029 * A class for selecting a specific node based on a key or a set of keys.
    030 * </p>
    031 * <p>
    032 * An instance of this class is initialized with the key of a node. It is also
    033 * possible to concatenate multiple keys - e.g. if a sub key is to be
    034 * constructed from another sub key. {@code NodeSelector} provides the
    035 * {@code select()} method which evaluates the wrapped keys on a specified root
    036 * node and returns the resulting unique target node. The class expects that the
    037 * key(s) stored in an instance select exactly one target node. If this is not
    038 * the case, result is <b>null</b> indicating that the selection criteria are
    039 * not sufficient.
    040 * </p>
    041 * <p>
    042 * Implementation node: Instances of this class are immutable. They can be
    043 * shared between arbitrary components.
    044 * </p>
    045 *
    046 * @since 2.0
    047 */
    048public class NodeSelector
    049{
    050    /** Stores the wrapped keys. */
    051    private final List<String> nodeKeys;
    052
    053    /**
    054     * Creates a new instance of {@code NodeSelector} and initializes it with
    055     * the key to the target node.
    056     *
    057     * @param key the key
    058     */
    059    public NodeSelector(final String key)
    060    {
    061        this(Collections.singletonList(key));
    062    }
    063
    064    /**
    065     * Creates a new instance of {@code NodeSelector} and initializes it with
    066     * the list of keys to be used as selection criteria.
    067     *
    068     * @param keys the keys for selecting nodes
    069     */
    070    private NodeSelector(final List<String> keys)
    071    {
    072        nodeKeys = keys;
    073    }
    074
    075    /**
    076     * Applies this {@code NodeSelector} on the specified root node. This method
    077     * applies the selection criteria stored in this object and tries to
    078     * determine a single target node. If this is successful, the target node is
    079     * returned. Otherwise, result is <b>null</b>.
    080     *
    081     * @param root the root node on which to apply this selector
    082     * @param resolver the {@code NodeKeyResolver}
    083     * @param handler the {@code NodeHandler}
    084     * @return the selected target node or <b>null</b>
    085     */
    086    public ImmutableNode select(final ImmutableNode root,
    087            final NodeKeyResolver<ImmutableNode> resolver,
    088            final NodeHandler<ImmutableNode> handler)
    089    {
    090        List<ImmutableNode> nodes = new LinkedList<>();
    091        final Iterator<String> itKeys = nodeKeys.iterator();
    092        getFilteredResults(root, resolver, handler, itKeys.next(), nodes);
    093
    094        while (itKeys.hasNext())
    095        {
    096            final String currentKey = itKeys.next();
    097            final List<ImmutableNode> currentResults =
    098                    new LinkedList<>();
    099            for (final ImmutableNode currentRoot : nodes)
    100            {
    101                getFilteredResults(currentRoot, resolver, handler, currentKey,
    102                        currentResults);
    103            }
    104            nodes = currentResults;
    105        }
    106
    107        return nodes.size() == 1 ? nodes.get(0) : null;
    108    }
    109
    110    /**
    111     * Creates a sub {@code NodeSelector} object which uses the key(s) of this
    112     * selector plus the specified key as selection criteria. This is useful
    113     * when another selection is to be performed on the results of a first
    114     * selector.
    115     *
    116     * @param subKey the additional key for the sub selector
    117     * @return the sub {@code NodeSelector} instance
    118     */
    119    public NodeSelector subSelector(final String subKey)
    120    {
    121        final List<String> keys = new ArrayList<>(nodeKeys.size() + 1);
    122        keys.addAll(nodeKeys);
    123        keys.add(subKey);
    124        return new NodeSelector(keys);
    125    }
    126
    127    /**
    128     * Compares this object with another one. Two instances of
    129     * {@code NodeSelector} are considered equal if they have the same keys as
    130     * selection criteria.
    131     *
    132     * @param obj the object to be compared
    133     * @return a flag whether these objects are equal
    134     */
    135    @Override
    136    public boolean equals(final Object obj)
    137    {
    138        if (this == obj)
    139        {
    140            return true;
    141        }
    142        if (!(obj instanceof NodeSelector))
    143        {
    144            return false;
    145        }
    146
    147        final NodeSelector c = (NodeSelector) obj;
    148        return nodeKeys.equals(c.nodeKeys);
    149    }
    150
    151    /**
    152     * Returns a hash code for this object.
    153     *
    154     * @return a hash code
    155     */
    156    @Override
    157    public int hashCode()
    158    {
    159        return nodeKeys.hashCode();
    160    }
    161
    162    /**
    163     * Returns a string representation for this object. This string contains the
    164     * keys to be used as selection criteria.
    165     *
    166     * @return a string for this object
    167     */
    168    @Override
    169    public String toString()
    170    {
    171        return new ToStringBuilder(this).append("keys", nodeKeys).toString();
    172    }
    173
    174    /**
    175     * Executes a query for a given key and filters the results for nodes only.
    176     *
    177     * @param root the root node for the query
    178     * @param resolver the {@code NodeKeyResolver}
    179     * @param handler the {@code NodeHandler}
    180     * @param key the key
    181     * @param nodes here the results are stored
    182     */
    183    private void getFilteredResults(final ImmutableNode root,
    184            final NodeKeyResolver<ImmutableNode> resolver,
    185            final NodeHandler<ImmutableNode> handler, final String key,
    186            final List<ImmutableNode> nodes)
    187    {
    188        final List<QueryResult<ImmutableNode>> results =
    189                resolver.resolveKey(root, key, handler);
    190        for (final QueryResult<ImmutableNode> result : results)
    191        {
    192            if (!result.isAttributeResult())
    193            {
    194                nodes.add(result.getNode());
    195            }
    196        }
    197    }
    198}