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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 "native" 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 (".").</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>[@<attributeName>]</code>.</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 * <database> 052 * <tables> 053 * <table type="system"> 054 * <name>users</name> 055 * <fields> 056 * <field> 057 * <name>lid</name> 058 * <type>long</name> 059 * </field> 060 * <field> 061 * <name>usrName</name> 062 * <type>java.lang.String</type> 063 * </field> 064 * ... 065 * </fields> 066 * </table> 067 * <table> 068 * <name>documents</name> 069 * <fields> 070 * <field> 071 * <name>docid</name> 072 * <type>long</type> 073 * </field> 074 * ... 075 * </fields> 076 * </table> 077 * ... 078 * </tables> 079 * </database> 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 * @author <a 112 * href="http://commons.apache.org/configuration/team-list.html">Commons 113 * Configuration team</a> 114 */ 115public class DefaultExpressionEngine implements ExpressionEngine 116{ 117 /** 118 * A default instance of this class that is used as expression engine for 119 * hierarchical configurations per default. 120 */ 121 public static final DefaultExpressionEngine INSTANCE = 122 new DefaultExpressionEngine( 123 DefaultExpressionEngineSymbols.DEFAULT_SYMBOLS); 124 125 /** The symbols used by this instance. */ 126 private final DefaultExpressionEngineSymbols symbols; 127 128 /** The matcher for node names. */ 129 private final NodeMatcher<String> nameMatcher; 130 131 /** 132 * Creates a new instance of {@code DefaultExpressionEngine} and initializes 133 * its symbols. 134 * 135 * @param syms the object with the symbols (must not be <b>null</b>) 136 * @throws IllegalArgumentException if the symbols are <b>null</b> 137 */ 138 public DefaultExpressionEngine(final DefaultExpressionEngineSymbols syms) 139 { 140 this(syms, null); 141 } 142 143 /** 144 * Creates a new instance of {@code DefaultExpressionEngine} and initializes 145 * its symbols and the matcher for comparing node names. The passed in 146 * matcher is always used when the names of nodes have to be matched against 147 * parts of configuration keys. 148 * 149 * @param syms the object with the symbols (must not be <b>null</b>) 150 * @param nodeNameMatcher the matcher for node names; can be <b>null</b>, 151 * then a default matcher is used 152 * @throws IllegalArgumentException if the symbols are <b>null</b> 153 */ 154 public DefaultExpressionEngine(final DefaultExpressionEngineSymbols syms, 155 final NodeMatcher<String> nodeNameMatcher) 156 { 157 if (syms == null) 158 { 159 throw new IllegalArgumentException("Symbols must not be null!"); 160 } 161 162 symbols = syms; 163 nameMatcher = 164 (nodeNameMatcher != null) ? nodeNameMatcher 165 : NodeNameMatchers.EQUALS; 166 } 167 168 /** 169 * Returns the {@code DefaultExpressionEngineSymbols} object associated with 170 * this instance. 171 * 172 * @return the {@code DefaultExpressionEngineSymbols} used by this engine 173 * @since 2.0 174 */ 175 public DefaultExpressionEngineSymbols getSymbols() 176 { 177 return symbols; 178 } 179 180 /** 181 * {@inheritDoc} This method supports the syntax as described in the class 182 * comment. 183 */ 184 @Override 185 public <T> List<QueryResult<T>> query(final T root, final String key, 186 final NodeHandler<T> handler) 187 { 188 final List<QueryResult<T>> results = new LinkedList<>(); 189 findNodesForKey(new DefaultConfigurationKey(this, key).iterator(), 190 root, results, handler); 191 return results; 192 } 193 194 /** 195 * {@inheritDoc} This implementation takes the 196 * given parent key, adds a property delimiter, and then adds the node's 197 * name. 198 * The name of the root node is a blank string. Note that no indices are 199 * returned. 200 */ 201 @Override 202 public <T> String nodeKey(final T node, final String parentKey, final NodeHandler<T> handler) 203 { 204 if (parentKey == null) 205 { 206 // this is the root node 207 return StringUtils.EMPTY; 208 } 209 final DefaultConfigurationKey key = new DefaultConfigurationKey(this, 210 parentKey); 211 key.append(handler.nodeName(node), true); 212 return key.toString(); 213 } 214 215 @Override 216 public String attributeKey(final String parentKey, final String attributeName) 217 { 218 final DefaultConfigurationKey key = 219 new DefaultConfigurationKey(this, parentKey); 220 key.appendAttribute(attributeName); 221 return key.toString(); 222 } 223 224 /** 225 * {@inheritDoc} This implementation works similar to {@code nodeKey()}; 226 * however, each key returned by this method has an index (except for the 227 * root node). The parent key is prepended to the name of the current node 228 * in any case and without further checks. If it is <b>null</b>, only the 229 * name of the current node with its index is returned. 230 */ 231 @Override 232 public <T> String canonicalKey(final T node, final String parentKey, 233 final NodeHandler<T> handler) 234 { 235 final String nodeName = handler.nodeName(node); 236 final T parent = handler.getParent(node); 237 final DefaultConfigurationKey key = 238 new DefaultConfigurationKey(this, parentKey); 239 key.append(StringUtils.defaultString(nodeName)); 240 241 if (parent != null) 242 { 243 // this is not the root key 244 key.appendIndex(determineIndex(node, parent, nodeName, handler)); 245 } 246 return key.toString(); 247 } 248 249 /** 250 * <p> 251 * Prepares Adding the property with the specified key. 252 * </p> 253 * <p> 254 * To be able to deal with the structure supported by hierarchical 255 * configuration implementations the passed in key is of importance, 256 * especially the indices it might contain. The following example should 257 * clarify this: Suppose the current node structure looks like the 258 * following: 259 * </p> 260 * <pre> 261 * tables 262 * +-- table 263 * +-- name = user 264 * +-- fields 265 * +-- field 266 * +-- name = uid 267 * +-- field 268 * +-- name = firstName 269 * ... 270 * +-- table 271 * +-- name = documents 272 * +-- fields 273 * ... 274 * </pre> 275 * <p> 276 * In this example a database structure is defined, e.g. all fields of the 277 * first table could be accessed using the key 278 * {@code tables.table(0).fields.field.name}. If now properties are 279 * to be added, it must be exactly specified at which position in the 280 * hierarchy the new property is to be inserted. So to add a new field name 281 * to a table it is not enough to say just 282 * </p> 283 * <pre> 284 * config.addProperty("tables.table.fields.field.name", "newField"); 285 * </pre> 286 * <p> 287 * The statement given above contains some ambiguity. For instance it is not 288 * clear, to which table the new field should be added. If this method finds 289 * such an ambiguity, it is resolved by following the last valid path. Here 290 * this would be the last table. The same is true for the {@code field}; 291 * because there are multiple fields and no explicit index is provided, a 292 * new {@code name} property would be added to the last field - which 293 * is probably not what was desired. 294 * </p> 295 * <p> 296 * To make things clear explicit indices should be provided whenever 297 * possible. In the example above the exact table could be specified by 298 * providing an index for the {@code table} element as in 299 * {@code tables.table(1).fields}. By specifying an index it can 300 * also be expressed that at a given position in the configuration tree a 301 * new branch should be added. In the example above we did not want to add 302 * an additional {@code name} element to the last field of the table, 303 * but we want a complete new {@code field} element. This can be 304 * achieved by specifying an invalid index (like -1) after the element where 305 * a new branch should be created. Given this our example would run: 306 * </p> 307 * <pre> 308 * config.addProperty("tables.table(1).fields.field(-1).name", "newField"); 309 * </pre> 310 * <p> 311 * With this notation it is possible to add new branches everywhere. We 312 * could for instance create a new {@code table} element by 313 * specifying 314 * </p> 315 * <pre> 316 * config.addProperty("tables.table(-1).fields.field.name", "newField2"); 317 * </pre> 318 * <p> 319 * (Note that because after the {@code table} element a new branch is 320 * created indices in following elements are not relevant; the branch is new 321 * so there cannot be any ambiguities.) 322 * </p> 323 * 324 * @param <T> the type of the nodes to be dealt with 325 * @param root the root node of the nodes hierarchy 326 * @param key the key of the new property 327 * @param handler the node handler 328 * @return a data object with information needed for the add operation 329 */ 330 @Override 331 public <T> NodeAddData<T> prepareAdd(final T root, final String key, final NodeHandler<T> handler) 332 { 333 final DefaultConfigurationKey.KeyIterator it = new DefaultConfigurationKey( 334 this, key).iterator(); 335 if (!it.hasNext()) 336 { 337 throw new IllegalArgumentException( 338 "Key for add operation must be defined!"); 339 } 340 341 final T parent = findLastPathNode(it, root, handler); 342 final List<String> pathNodes = new LinkedList<>(); 343 344 while (it.hasNext()) 345 { 346 if (!it.isPropertyKey()) 347 { 348 throw new IllegalArgumentException( 349 "Invalid key for add operation: " + key 350 + " (Attribute key in the middle.)"); 351 } 352 pathNodes.add(it.currentKey()); 353 it.next(); 354 } 355 356 return new NodeAddData<>(parent, it.currentKey(), !it.isPropertyKey(), 357 pathNodes); 358 } 359 360 /** 361 * Recursive helper method for evaluating a key. This method processes all 362 * facets of a configuration key, traverses the tree of properties and 363 * fetches the results of all matching properties. 364 * 365 * @param <T> the type of nodes to be dealt with 366 * @param keyPart the configuration key iterator 367 * @param node the current node 368 * @param results here the found results are stored 369 * @param handler the node handler 370 */ 371 protected <T> void findNodesForKey( 372 final DefaultConfigurationKey.KeyIterator keyPart, final T node, 373 final Collection<QueryResult<T>> results, final NodeHandler<T> handler) 374 { 375 if (!keyPart.hasNext()) 376 { 377 results.add(QueryResult.createNodeResult(node)); 378 } 379 380 else 381 { 382 final String key = keyPart.nextKey(false); 383 if (keyPart.isPropertyKey()) 384 { 385 processSubNodes(keyPart, findChildNodesByName(handler, node, key), 386 results, handler); 387 } 388 if (keyPart.isAttribute() && !keyPart.hasNext()) 389 { 390 if (handler.getAttributeValue(node, key) != null) 391 { 392 results.add(QueryResult.createAttributeResult(node, key)); 393 } 394 } 395 } 396 } 397 398 /** 399 * Finds the last existing node for an add operation. This method traverses 400 * the node tree along the specified key. The last existing node on this 401 * path is returned. 402 * 403 * @param <T> the type of the nodes to be dealt with 404 * @param keyIt the key iterator 405 * @param node the current node 406 * @param handler the node handler 407 * @return the last existing node on the given path 408 */ 409 protected <T> T findLastPathNode(final DefaultConfigurationKey.KeyIterator keyIt, 410 final T node, final NodeHandler<T> handler) 411 { 412 final String keyPart = keyIt.nextKey(false); 413 414 if (keyIt.hasNext()) 415 { 416 if (!keyIt.isPropertyKey()) 417 { 418 // Attribute keys can only appear as last elements of the path 419 throw new IllegalArgumentException( 420 "Invalid path for add operation: " 421 + "Attribute key in the middle!"); 422 } 423 final int idx = 424 keyIt.hasIndex() ? keyIt.getIndex() : handler 425 .getMatchingChildrenCount(node, nameMatcher, 426 keyPart) - 1; 427 if (idx < 0 428 || idx >= handler.getMatchingChildrenCount(node, 429 nameMatcher, keyPart)) 430 { 431 return node; 432 } 433 return findLastPathNode(keyIt, 434 findChildNodesByName(handler, node, keyPart).get(idx), 435 handler); 436 } 437 return node; 438 } 439 440 /** 441 * Called by {@code findNodesForKey()} to process the sub nodes of 442 * the current node depending on the type of the current key part (children, 443 * attributes, or both). 444 * 445 * @param <T> the type of the nodes to be dealt with 446 * @param keyPart the key part 447 * @param subNodes a list with the sub nodes to process 448 * @param nodes the target collection 449 * @param handler the node handler 450 */ 451 private <T> void processSubNodes(final DefaultConfigurationKey.KeyIterator keyPart, 452 final List<T> subNodes, final Collection<QueryResult<T>> nodes, final NodeHandler<T> handler) 453 { 454 if (keyPart.hasIndex()) 455 { 456 if (keyPart.getIndex() >= 0 && keyPart.getIndex() < subNodes.size()) 457 { 458 findNodesForKey((DefaultConfigurationKey.KeyIterator) keyPart 459 .clone(), subNodes.get(keyPart.getIndex()), nodes, handler); 460 } 461 } 462 else 463 { 464 for (final T node : subNodes) 465 { 466 findNodesForKey((DefaultConfigurationKey.KeyIterator) keyPart 467 .clone(), node, nodes, handler); 468 } 469 } 470 } 471 472 /** 473 * Determines the index of the given node based on its parent node. 474 * 475 * @param node the current node 476 * @param parent the parent node 477 * @param nodeName the name of the current node 478 * @param handler the node handler 479 * @param <T> the type of the nodes to be dealt with 480 * @return the index of this node 481 */ 482 private <T> int determineIndex(final T node, final T parent, final String nodeName, 483 final NodeHandler<T> handler) 484 { 485 return findChildNodesByName(handler, parent, nodeName).indexOf(node); 486 } 487 488 /** 489 * Returns a list with all child nodes of the given parent node which match 490 * the specified node name. The match is done using the current node name 491 * matcher. 492 * 493 * @param handler the {@code NodeHandler} 494 * @param parent the parent node 495 * @param nodeName the name of the current node 496 * @param <T> the type of the nodes to be dealt with 497 * @return a list with all matching child nodes 498 */ 499 private <T> List<T> findChildNodesByName(final NodeHandler<T> handler, final T parent, 500 final String nodeName) 501 { 502 return handler.getMatchingChildren(parent, nameMatcher, nodeName); 503 } 504}