org.apache.commons.configuration.tree.xpath.XPathExpressionEngine Maven / Gradle / Ivy
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* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package org.apache.commons.configuration.tree.xpath;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.StringTokenizer;
import org.apache.commons.configuration.tree.ConfigurationNode;
import org.apache.commons.configuration.tree.ExpressionEngine;
import org.apache.commons.configuration.tree.NodeAddData;
import org.apache.commons.jxpath.JXPathContext;
import org.apache.commons.jxpath.ri.JXPathContextReferenceImpl;
import org.apache.commons.lang.StringUtils;
/**
*
* A specialized implementation of the ExpressionEngine interface
* that is able to evaluate XPATH expressions.
*
*
* This class makes use of Commons
* JXPath for handling XPath expressions and mapping them to the nodes of a
* hierarchical configuration. This makes the rich and powerful XPATH syntax
* available for accessing properties from a configuration object.
*
*
* For selecting properties arbitrary XPATH expressions can be used, which
* select single or multiple configuration nodes. The associated
* Configuration instance will directly pass the specified property
* keys into this engine. If a key is not syntactically correct, an exception
* will be thrown.
*
*
* For adding new properties, this expression engine uses a specific syntax: the
* "key" of a new property must consist of two parts that are
* separated by whitespace:
*
* - An XPATH expression selecting a single node, to which the new element(s)
* are to be added. This can be an arbitrary complex expression, but it must
* select exactly one node, otherwise an exception will be thrown.
* - The name of the new element(s) to be added below this parent node. Here
* either a single node name or a complete path of nodes (separated by the
* "/" character or "@" for an attribute) can be specified.
*
* Some examples for valid keys that can be passed into the configuration's
* addProperty() method follow:
*
*
*
*
* "/tables/table[1] type"
*
*
*
*
* This will add a new type node as a child of the first
* table element.
*
*
*
*
* "/tables/table[1] @type"
*
*
*
*
* Similar to the example above, but this time a new attribute named
* type will be added to the first table element.
*
*
*
*
* "/tables table/fields/field/name"
*
*
*
*
* This example shows how a complex path can be added. Parent node is the
* tables element. Here a new branch consisting of the nodes
* table, fields, field, and
* name will be added.
*
*
*
*
* "/tables table/fields/field@type"
*
*
*
*
* This is similar to the last example, but in this case a complex path ending
* with an attribute is defined.
*
*
* Note: This extended syntax for adding properties only works
* with the addProperty() method. setProperty() does
* not support creating new nodes this way.
*
*
* From version 1.7 on, it is possible to use regular keys in calls to
* addProperty() (i.e. keys that do not have to contain a
* whitespace as delimiter). In this case the key is evaluated, and the biggest
* part pointing to an existing node is determined. The remaining part is then
* added as new path. As an example consider the key
*
*
* "tables/table[last()]/fields/field/name"
*
*
* If the key does not point to an existing node, the engine will check the
* paths "tables/table[last()]/fields/field",
* "tables/table[last()]/fields",
* "tables/table[last()]", and so on, until a key is
* found which points to a node. Let's assume that the last key listed above can
* be resolved in this way. Then from this key the following key is derived:
* "tables/table[last()] fields/field/name" by appending
* the remaining part after a whitespace. This key can now be processed using
* the original algorithm. Keys of this form can also be used with the
* setProperty() method. However, it is still recommended to use
* the old format because it makes explicit at which position new nodes should
* be added. For keys without a whitespace delimiter there may be ambiguities.
*
*
* @since 1.3
* @author Commons
* Configuration team
* @version $Id: XPathExpressionEngine.java 1152357 2011-07-29 19:56:01Z oheger $
*/
public class XPathExpressionEngine implements ExpressionEngine
{
/** Constant for the path delimiter. */
static final String PATH_DELIMITER = "/";
/** Constant for the attribute delimiter. */
static final String ATTR_DELIMITER = "@";
/** Constant for the delimiters for splitting node paths. */
private static final String NODE_PATH_DELIMITERS = PATH_DELIMITER
+ ATTR_DELIMITER;
/**
* Constant for a space which is used as delimiter in keys for adding
* properties.
*/
private static final String SPACE = " ";
/**
* Executes a query. The passed in property key is directly passed to a
* JXPath context.
*
* @param root the configuration root node
* @param key the query to be executed
* @return a list with the nodes that are selected by the query
*/
public List query(ConfigurationNode root, String key)
{
if (StringUtils.isEmpty(key))
{
List result = new ArrayList(1);
result.add(root);
return result;
}
else
{
JXPathContext context = createContext(root, key);
List result = context.selectNodes(key);
return (result != null) ? result : Collections.EMPTY_LIST;
}
}
/**
* Returns a (canonical) key for the given node based on the parent's key.
* This implementation will create an XPATH expression that selects the
* given node (under the assumption that the passed in parent key is valid).
* As the nodeKey() implementation of
* {@link org.apache.commons.configuration.tree.DefaultExpressionEngine DefaultExpressionEngine}
* this method will not return indices for nodes. So all child nodes of a
* given parent with the same name will have the same key.
*
* @param node the node for which a key is to be constructed
* @param parentKey the key of the parent node
* @return the key for the given node
*/
public String nodeKey(ConfigurationNode node, String parentKey)
{
if (parentKey == null)
{
// name of the root node
return StringUtils.EMPTY;
}
else if (node.getName() == null)
{
// paranoia check for undefined node names
return parentKey;
}
else
{
StringBuffer buf = new StringBuffer(parentKey.length()
+ node.getName().length() + PATH_DELIMITER.length());
if (parentKey.length() > 0)
{
buf.append(parentKey);
buf.append(PATH_DELIMITER);
}
if (node.isAttribute())
{
buf.append(ATTR_DELIMITER);
}
buf.append(node.getName());
return buf.toString();
}
}
/**
* Prepares an add operation for a configuration property. The expected
* format of the passed in key is explained in the class comment.
*
* @param root the configuration's root node
* @param key the key describing the target of the add operation and the
* path of the new node
* @return a data object to be evaluated by the calling configuration object
*/
public NodeAddData prepareAdd(ConfigurationNode root, String key)
{
if (key == null)
{
throw new IllegalArgumentException(
"prepareAdd: key must not be null!");
}
String addKey = key;
int index = findKeySeparator(addKey);
if (index < 0)
{
addKey = generateKeyForAdd(root, addKey);
index = findKeySeparator(addKey);
}
List nodes = query(root, addKey.substring(0, index).trim());
if (nodes.size() != 1)
{
throw new IllegalArgumentException(
"prepareAdd: key must select exactly one target node!");
}
NodeAddData data = new NodeAddData();
data.setParent((ConfigurationNode) nodes.get(0));
initNodeAddData(data, addKey.substring(index).trim());
return data;
}
/**
* Creates the JXPathContext used for executing a query. This
* method will create a new context and ensure that it is correctly
* initialized.
*
* @param root the configuration root node
* @param key the key to be queried
* @return the new context
*/
protected JXPathContext createContext(ConfigurationNode root, String key)
{
JXPathContext context = JXPathContext.newContext(root);
context.setLenient(true);
return context;
}
/**
* Initializes most properties of a NodeAddData object. This
* method is called by prepareAdd() after the parent node has
* been found. Its task is to interpret the passed in path of the new node.
*
* @param data the data object to initialize
* @param path the path of the new node
*/
protected void initNodeAddData(NodeAddData data, String path)
{
String lastComponent = null;
boolean attr = false;
boolean first = true;
StringTokenizer tok = new StringTokenizer(path, NODE_PATH_DELIMITERS,
true);
while (tok.hasMoreTokens())
{
String token = tok.nextToken();
if (PATH_DELIMITER.equals(token))
{
if (attr)
{
invalidPath(path, " contains an attribute"
+ " delimiter at an unallowed position.");
}
if (lastComponent == null)
{
invalidPath(path,
" contains a '/' at an unallowed position.");
}
data.addPathNode(lastComponent);
lastComponent = null;
}
else if (ATTR_DELIMITER.equals(token))
{
if (attr)
{
invalidPath(path,
" contains multiple attribute delimiters.");
}
if (lastComponent == null && !first)
{
invalidPath(path,
" contains an attribute delimiter at an unallowed position.");
}
if (lastComponent != null)
{
data.addPathNode(lastComponent);
}
attr = true;
lastComponent = null;
}
else
{
lastComponent = token;
}
first = false;
}
if (lastComponent == null)
{
invalidPath(path, "contains no components.");
}
data.setNewNodeName(lastComponent);
data.setAttribute(attr);
}
/**
* Tries to generate a key for adding a property. This method is called if a
* key was used for adding properties which does not contain a space
* character. It splits the key at its single components and searches for
* the last existing component. Then a key compatible for adding properties
* is generated.
*
* @param root the root node of the configuration
* @param key the key in question
* @return the key to be used for adding the property
*/
private String generateKeyForAdd(ConfigurationNode root, String key)
{
int pos = key.lastIndexOf(PATH_DELIMITER, key.length());
while (pos >= 0)
{
String keyExisting = key.substring(0, pos);
if (!query(root, keyExisting).isEmpty())
{
StringBuffer buf = new StringBuffer(key.length() + 1);
buf.append(keyExisting).append(SPACE);
buf.append(key.substring(pos + 1));
return buf.toString();
}
pos = key.lastIndexOf(PATH_DELIMITER, pos - 1);
}
return SPACE + key;
}
/**
* Helper method for throwing an exception about an invalid path.
*
* @param path the invalid path
* @param msg the exception message
*/
private void invalidPath(String path, String msg)
{
throw new IllegalArgumentException("Invalid node path: \"" + path
+ "\" " + msg);
}
/**
* Determines the position of the separator in a key for adding new
* properties. If no delimiter is found, result is -1.
*
* @param key the key
* @return the position of the delimiter
*/
private static int findKeySeparator(String key)
{
int index = key.length() - 1;
while (index >= 0 && !Character.isWhitespace(key.charAt(index)))
{
index--;
}
return index;
}
// static initializer: registers the configuration node pointer factory
static
{
JXPathContextReferenceImpl
.addNodePointerFactory(new ConfigurationNodePointerFactory());
}
}