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.Collection;
020import java.util.LinkedList;
021import java.util.List;
022
023import org.apache.commons.lang3.StringUtils;
024
025/**
026 * <p>
027 * A default implementation of the {@code ExpressionEngine} interface
028 * providing the &quot;native&quot; expression language for hierarchical
029 * configurations.
030 * </p>
031 * <p>
032 * This class implements a rather simple expression language for navigating
033 * through a hierarchy of configuration nodes. It supports the following
034 * operations:
035 * </p>
036 * <ul>
037 * <li>Navigating from a node to one of its children using the child node
038 * delimiter, which is by the default a dot (&quot;.&quot;).</li>
039 * <li>Navigating from a node to one of its attributes using the attribute node
040 * delimiter, which by default follows the XPATH like syntax
041 * {@code [@&lt;attributeName&gt;]}.</li>
042 * <li>If there are multiple child or attribute nodes with the same name, a
043 * specific node can be selected using a numerical index. By default indices are
044 * written in parenthesis.</li>
045 * </ul>
046 * <p>
047 * As an example consider the following XML document:
048 * </p>
049 *
050 * <pre>
051 *  &lt;database&gt;
052 *    &lt;tables&gt;
053 *      &lt;table type=&quot;system&quot;&gt;
054 *        &lt;name&gt;users&lt;/name&gt;
055 *        &lt;fields&gt;
056 *          &lt;field&gt;
057 *            &lt;name&gt;lid&lt;/name&gt;
058 *            &lt;type&gt;long&lt;/name&gt;
059 *          &lt;/field&gt;
060 *          &lt;field&gt;
061 *            &lt;name&gt;usrName&lt;/name&gt;
062 *            &lt;type&gt;java.lang.String&lt;/type&gt;
063 *          &lt;/field&gt;
064 *         ...
065 *        &lt;/fields&gt;
066 *      &lt;/table&gt;
067 *      &lt;table&gt;
068 *        &lt;name&gt;documents&lt;/name&gt;
069 *        &lt;fields&gt;
070 *          &lt;field&gt;
071 *            &lt;name&gt;docid&lt;/name&gt;
072 *            &lt;type&gt;long&lt;/type&gt;
073 *          &lt;/field&gt;
074 *          ...
075 *        &lt;/fields&gt;
076 *      &lt;/table&gt;
077 *      ...
078 *    &lt;/tables&gt;
079 *  &lt;/database&gt;
080 * </pre>
081 *
082 * <p>
083 * If this document is parsed and stored in a hierarchical configuration object,
084 * for instance the key {@code tables.table(0).name} can be used to find
085 * out the name of the first table. In opposite {@code tables.table.name}
086 * would return a collection with the names of all available tables. Similarly
087 * the key {@code tables.table(1).fields.field.name} returns a collection
088 * with the names of all fields of the second table. If another index is added
089 * after the {@code field} element, a single field can be accessed:
090 * {@code tables.table(1).fields.field(0).name}. The key
091 * {@code tables.table(0)[@type]} would select the type attribute of the
092 * first table.
093 * </p>
094 * <p>
095 * This example works with the default values for delimiters and index markers.
096 * It is also possible to set custom values for these properties so that you can
097 * adapt a {@code DefaultExpressionEngine} to your personal needs.
098 * </p>
099 * <p>
100 * The concrete symbols used by an instance are determined by a
101 * {@link DefaultExpressionEngineSymbols} object passed to the constructor.
102 * By providing a custom symbols object the syntax for querying properties in
103 * a hierarchical configuration can be altered.
104 * </p>
105 * <p>
106 * Instances of this class are thread-safe and can be shared between multiple
107 * hierarchical configuration objects.
108 * </p>
109 *
110 * @since 1.3
111 */
112public class DefaultExpressionEngine implements ExpressionEngine
113{
114    /**
115     * A default instance of this class that is used as expression engine for
116     * hierarchical configurations per default.
117     */
118    public static final DefaultExpressionEngine INSTANCE =
119            new DefaultExpressionEngine(
120                    DefaultExpressionEngineSymbols.DEFAULT_SYMBOLS);
121
122    /** The symbols used by this instance. */
123    private final DefaultExpressionEngineSymbols symbols;
124
125    /** The matcher for node names. */
126    private final NodeMatcher<String> nameMatcher;
127
128    /**
129     * Creates a new instance of {@code DefaultExpressionEngine} and initializes
130     * its symbols.
131     *
132     * @param syms the object with the symbols (must not be <b>null</b>)
133     * @throws IllegalArgumentException if the symbols are <b>null</b>
134     */
135    public DefaultExpressionEngine(final DefaultExpressionEngineSymbols syms)
136    {
137        this(syms, null);
138    }
139
140    /**
141     * Creates a new instance of {@code DefaultExpressionEngine} and initializes
142     * its symbols and the matcher for comparing node names. The passed in
143     * matcher is always used when the names of nodes have to be matched against
144     * parts of configuration keys.
145     *
146     * @param syms the object with the symbols (must not be <b>null</b>)
147     * @param nodeNameMatcher the matcher for node names; can be <b>null</b>,
148     *        then a default matcher is used
149     * @throws IllegalArgumentException if the symbols are <b>null</b>
150     */
151    public DefaultExpressionEngine(final DefaultExpressionEngineSymbols syms,
152            final NodeMatcher<String> nodeNameMatcher)
153    {
154        if (syms == null)
155        {
156            throw new IllegalArgumentException("Symbols must not be null!");
157        }
158
159        symbols = syms;
160        nameMatcher =
161                nodeNameMatcher != null ? nodeNameMatcher
162                        : NodeNameMatchers.EQUALS;
163    }
164
165    /**
166     * Returns the {@code DefaultExpressionEngineSymbols} object associated with
167     * this instance.
168     *
169     * @return the {@code DefaultExpressionEngineSymbols} used by this engine
170     * @since 2.0
171     */
172    public DefaultExpressionEngineSymbols getSymbols()
173    {
174        return symbols;
175    }
176
177    /**
178     * {@inheritDoc} This method supports the syntax as described in the class
179     * comment.
180     */
181    @Override
182    public <T> List<QueryResult<T>> query(final T root, final String key,
183            final NodeHandler<T> handler)
184    {
185        final List<QueryResult<T>> results = new LinkedList<>();
186        findNodesForKey(new DefaultConfigurationKey(this, key).iterator(),
187                root, results, handler);
188        return results;
189    }
190
191    /**
192     * {@inheritDoc} This implementation takes the
193     * given parent key, adds a property delimiter, and then adds the node's
194     * name.
195     * The name of the root node is a blank string. Note that no indices are
196     * returned.
197     */
198    @Override
199    public <T> String nodeKey(final T node, final String parentKey, final NodeHandler<T> handler)
200    {
201        if (parentKey == null)
202        {
203            // this is the root node
204            return StringUtils.EMPTY;
205        }
206        final DefaultConfigurationKey key = new DefaultConfigurationKey(this,
207                parentKey);
208            key.append(handler.nodeName(node), true);
209        return key.toString();
210    }
211
212    @Override
213    public String attributeKey(final String parentKey, final String attributeName)
214    {
215        final DefaultConfigurationKey key =
216                new DefaultConfigurationKey(this, parentKey);
217        key.appendAttribute(attributeName);
218        return key.toString();
219    }
220
221    /**
222     * {@inheritDoc} This implementation works similar to {@code nodeKey()};
223     * however, each key returned by this method has an index (except for the
224     * root node). The parent key is prepended to the name of the current node
225     * in any case and without further checks. If it is <b>null</b>, only the
226     * name of the current node with its index is returned.
227     */
228    @Override
229    public <T> String canonicalKey(final T node, final String parentKey,
230            final NodeHandler<T> handler)
231    {
232        final String nodeName = handler.nodeName(node);
233        final T parent = handler.getParent(node);
234        final DefaultConfigurationKey key =
235                new DefaultConfigurationKey(this, parentKey);
236        key.append(StringUtils.defaultString(nodeName));
237
238        if (parent != null)
239        {
240            // this is not the root key
241            key.appendIndex(determineIndex(node, parent, nodeName, handler));
242        }
243        return key.toString();
244    }
245
246    /**
247     * <p>
248     * Prepares Adding the property with the specified key.
249     * </p>
250     * <p>
251     * To be able to deal with the structure supported by hierarchical
252     * configuration implementations the passed in key is of importance,
253     * especially the indices it might contain. The following example should
254     * clarify this: Suppose the current node structure looks like the
255     * following:
256     * </p>
257     * <pre>
258     *  tables
259     *     +-- table
260     *             +-- name = user
261     *             +-- fields
262     *                     +-- field
263     *                             +-- name = uid
264     *                     +-- field
265     *                             +-- name = firstName
266     *                     ...
267     *     +-- table
268     *             +-- name = documents
269     *             +-- fields
270     *                    ...
271     * </pre>
272     * <p>
273     * In this example a database structure is defined, e.g. all fields of the
274     * first table could be accessed using the key
275     * {@code tables.table(0).fields.field.name}. If now properties are
276     * to be added, it must be exactly specified at which position in the
277     * hierarchy the new property is to be inserted. So to add a new field name
278     * to a table it is not enough to say just
279     * </p>
280     * <pre>
281     * config.addProperty(&quot;tables.table.fields.field.name&quot;, &quot;newField&quot;);
282     * </pre>
283     * <p>
284     * The statement given above contains some ambiguity. For instance it is not
285     * clear, to which table the new field should be added. If this method finds
286     * such an ambiguity, it is resolved by following the last valid path. Here
287     * this would be the last table. The same is true for the {@code field};
288     * because there are multiple fields and no explicit index is provided, a
289     * new {@code name} property would be added to the last field - which
290     * is probably not what was desired.
291     * </p>
292     * <p>
293     * To make things clear explicit indices should be provided whenever
294     * possible. In the example above the exact table could be specified by
295     * providing an index for the {@code table} element as in
296     * {@code tables.table(1).fields}. By specifying an index it can
297     * also be expressed that at a given position in the configuration tree a
298     * new branch should be added. In the example above we did not want to add
299     * an additional {@code name} element to the last field of the table,
300     * but we want a complete new {@code field} element. This can be
301     * achieved by specifying an invalid index (like -1) after the element where
302     * a new branch should be created. Given this our example would run:
303     * </p>
304     * <pre>
305     * config.addProperty(&quot;tables.table(1).fields.field(-1).name&quot;, &quot;newField&quot;);
306     * </pre>
307     * <p>
308     * With this notation it is possible to add new branches everywhere. We
309     * could for instance create a new {@code table} element by
310     * specifying
311     * </p>
312     * <pre>
313     * config.addProperty(&quot;tables.table(-1).fields.field.name&quot;, &quot;newField2&quot;);
314     * </pre>
315     * <p>
316     * (Note that because after the {@code table} element a new branch is
317     * created indices in following elements are not relevant; the branch is new
318     * so there cannot be any ambiguities.)
319     * </p>
320     *
321     * @param <T> the type of the nodes to be dealt with
322     * @param root the root node of the nodes hierarchy
323     * @param key the key of the new property
324     * @param handler the node handler
325     * @return a data object with information needed for the add operation
326     */
327    @Override
328    public <T> NodeAddData<T> prepareAdd(final T root, final String key, final NodeHandler<T> handler)
329    {
330        final DefaultConfigurationKey.KeyIterator it = new DefaultConfigurationKey(
331                this, key).iterator();
332        if (!it.hasNext())
333        {
334            throw new IllegalArgumentException(
335                    "Key for add operation must be defined!");
336        }
337
338        final T parent = findLastPathNode(it, root, handler);
339        final List<String> pathNodes = new LinkedList<>();
340
341        while (it.hasNext())
342        {
343            if (!it.isPropertyKey())
344            {
345                throw new IllegalArgumentException(
346                        "Invalid key for add operation: " + key
347                                + " (Attribute key in the middle.)");
348            }
349            pathNodes.add(it.currentKey());
350            it.next();
351        }
352
353        return new NodeAddData<>(parent, it.currentKey(), !it.isPropertyKey(),
354                pathNodes);
355    }
356
357    /**
358     * Recursive helper method for evaluating a key. This method processes all
359     * facets of a configuration key, traverses the tree of properties and
360     * fetches the results of all matching properties.
361     *
362     * @param <T> the type of nodes to be dealt with
363     * @param keyPart the configuration key iterator
364     * @param node the current node
365     * @param results here the found results are stored
366     * @param handler the node handler
367     */
368    protected <T> void findNodesForKey(
369            final DefaultConfigurationKey.KeyIterator keyPart, final T node,
370            final Collection<QueryResult<T>> results, final NodeHandler<T> handler)
371    {
372        if (!keyPart.hasNext())
373        {
374            results.add(QueryResult.createNodeResult(node));
375        }
376
377        else
378        {
379            final String key = keyPart.nextKey(false);
380            if (keyPart.isPropertyKey())
381            {
382                processSubNodes(keyPart, findChildNodesByName(handler, node, key),
383                        results, handler);
384            }
385            if (keyPart.isAttribute() && !keyPart.hasNext())
386            {
387                if (handler.getAttributeValue(node, key) != null)
388                {
389                    results.add(QueryResult.createAttributeResult(node, key));
390                }
391            }
392        }
393    }
394
395    /**
396     * Finds the last existing node for an add operation. This method traverses
397     * the node tree along the specified key. The last existing node on this
398     * path is returned.
399     *
400     * @param <T> the type of the nodes to be dealt with
401     * @param keyIt the key iterator
402     * @param node the current node
403     * @param handler the node handler
404     * @return the last existing node on the given path
405     */
406    protected <T> T findLastPathNode(final DefaultConfigurationKey.KeyIterator keyIt,
407            final T node, final NodeHandler<T> handler)
408    {
409        final String keyPart = keyIt.nextKey(false);
410
411        if (keyIt.hasNext())
412        {
413            if (!keyIt.isPropertyKey())
414            {
415                // Attribute keys can only appear as last elements of the path
416                throw new IllegalArgumentException(
417                        "Invalid path for add operation: "
418                                + "Attribute key in the middle!");
419            }
420            final int idx =
421                    keyIt.hasIndex() ? keyIt.getIndex() : handler
422                            .getMatchingChildrenCount(node, nameMatcher,
423                                    keyPart) - 1;
424            if (idx < 0
425                    || idx >= handler.getMatchingChildrenCount(node,
426                            nameMatcher, keyPart))
427            {
428                return node;
429            }
430            return findLastPathNode(keyIt,
431                    findChildNodesByName(handler, node, keyPart).get(idx),
432                    handler);
433        }
434        return node;
435    }
436
437    /**
438     * Called by {@code findNodesForKey()} to process the sub nodes of
439     * the current node depending on the type of the current key part (children,
440     * attributes, or both).
441     *
442     * @param <T> the type of the nodes to be dealt with
443     * @param keyPart the key part
444     * @param subNodes a list with the sub nodes to process
445     * @param nodes the target collection
446     * @param handler the node handler
447     */
448    private <T> void processSubNodes(final DefaultConfigurationKey.KeyIterator keyPart,
449            final List<T> subNodes, final Collection<QueryResult<T>> nodes, final NodeHandler<T> handler)
450    {
451        if (keyPart.hasIndex())
452        {
453            if (keyPart.getIndex() >= 0 && keyPart.getIndex() < subNodes.size())
454            {
455                findNodesForKey((DefaultConfigurationKey.KeyIterator) keyPart
456                        .clone(), subNodes.get(keyPart.getIndex()), nodes, handler);
457            }
458        }
459        else
460        {
461            for (final T node : subNodes)
462            {
463                findNodesForKey((DefaultConfigurationKey.KeyIterator) keyPart
464                        .clone(), node, nodes, handler);
465            }
466        }
467    }
468
469    /**
470     * Determines the index of the given node based on its parent node.
471     *
472     * @param node the current node
473     * @param parent the parent node
474     * @param nodeName the name of the current node
475     * @param handler the node handler
476     * @param <T> the type of the nodes to be dealt with
477     * @return the index of this node
478     */
479    private <T> int determineIndex(final T node, final T parent, final String nodeName,
480                                          final NodeHandler<T> handler)
481    {
482        return findChildNodesByName(handler, parent, nodeName).indexOf(node);
483    }
484
485    /**
486     * Returns a list with all child nodes of the given parent node which match
487     * the specified node name. The match is done using the current node name
488     * matcher.
489     *
490     * @param handler the {@code NodeHandler}
491     * @param parent the parent node
492     * @param nodeName the name of the current node
493     * @param <T> the type of the nodes to be dealt with
494     * @return a list with all matching child nodes
495     */
496    private <T> List<T> findChildNodesByName(final NodeHandler<T> handler, final T parent,
497            final String nodeName)
498    {
499        return handler.getMatchingChildren(parent, nameMatcher, nodeName);
500    }
501}