Package org.apache.commons.configuration.tree.xpath

Class XPathExpressionEngine

java.lang.Object

org.apache.commons.configuration.tree.xpath.XPathExpressionEngine

All Implemented Interfaces:

ExpressionEngine

public class XPathExpressionEngine
extends java.lang.Object
implements ExpressionEngine

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:

1. 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.
2. 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

Version:

$Id: XPathExpressionEngine.java 1206563 2011-11-26 19:47:26Z oheger $

Author:

Commons Configuration team

Constructor Summary

Constructors

Constructor	Description
XPathExpressionEngine()

Method Summary

Modifier and Type	Method	Description
protected org.apache.commons.jxpath.JXPathContext	createContext(ConfigurationNode root, java.lang.String key)	Creates the JXPathContext used for executing a query.
protected void	initNodeAddData(NodeAddData data, java.lang.String path)	Initializes most properties of a NodeAddData object.
java.lang.String	nodeKey(ConfigurationNode node, java.lang.String parentKey)	Returns a (canonical) key for the given node based on the parent's key.
NodeAddData	prepareAdd(ConfigurationNode root, java.lang.String key)	Prepares an add operation for a configuration property.
java.util.List<ConfigurationNode>	query(ConfigurationNode root, java.lang.String key)	Executes a query.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

XPathExpressionEngine

public XPathExpressionEngine()

Method Detail

query

public java.util.List<ConfigurationNode> query(ConfigurationNode root,
                                               java.lang.String key)

Executes a query. The passed in property key is directly passed to a JXPath context.

Specified by:

query in interface ExpressionEngine

Parameters:

root - the configuration root node

key - the query to be executed

Returns:

a list with the nodes that are selected by the query

nodeKey

public java.lang.String nodeKey(ConfigurationNode node,
                                java.lang.String parentKey)

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 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.

Specified by:

nodeKey in interface ExpressionEngine

Parameters:

node - the node for which a key is to be constructed

parentKey - the key of the parent node

Returns:

the key for the given node

prepareAdd

public NodeAddData prepareAdd(ConfigurationNode root,
                              java.lang.String key)

Prepares an add operation for a configuration property. The expected format of the passed in key is explained in the class comment.

Specified by:

prepareAdd in interface ExpressionEngine

Parameters:

root - the configuration's root node

key - the key describing the target of the add operation and the path of the new node

Returns:

a data object to be evaluated by the calling configuration object

createContext

protected org.apache.commons.jxpath.JXPathContext createContext(ConfigurationNode root,
                                                                java.lang.String key)

Creates the JXPathContext used for executing a query. This method will create a new context and ensure that it is correctly initialized.

Parameters:

root - the configuration root node

key - the key to be queried

Returns:

the new context

initNodeAddData

protected void initNodeAddData(NodeAddData data,
                               java.lang.String path)

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.

Parameters:

data - the data object to initialize

path - the path of the new node