Class XPathExpressionEngine

java.lang.Object
org.apache.commons.configuration2.tree.xpath.XPathExpressionEngine
All Implemented Interfaces:
ExpressionEngine

public class XPathExpressionEngine extends 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
  • Field Details

    • PATH_DELIMITER

      static final String PATH_DELIMITER
      Constant for the path delimiter.
      See Also:
    • ATTR_DELIMITER

      static final String ATTR_DELIMITER
      Constant for the attribute delimiter.
      See Also:
    • NODE_PATH_DELIMITERS

      private static final String NODE_PATH_DELIMITERS
      Constant for the delimiters for splitting node paths.
      See Also:
    • SPACE

      private static final String SPACE
      Constant for a space which is used as delimiter in keys for adding properties.
      See Also:
    • BUF_SIZE

      private static final int BUF_SIZE
      Constant for a default size of a key buffer.
      See Also:
    • START_INDEX

      private static final char START_INDEX
      Constant for the start of an index expression.
      See Also:
    • END_INDEX

      private static final char END_INDEX
      Constant for the end of an index expression.
      See Also:
    • contextFactory

      private final XPathContextFactory contextFactory
      The internally used context factory.
  • Constructor Details

    • XPathExpressionEngine

      public XPathExpressionEngine()
      Creates a new instance of XPathExpressionEngine with default settings.
    • XPathExpressionEngine

      XPathExpressionEngine(XPathContextFactory factory)
      Creates a new instance of XPathExpressionEngine and sets the context factory. This constructor is mainly used for testing purposes.
      Parameters:
      factory - the XPathContextFactory
  • Method Details

    • query

      public <T> List<QueryResult<T>> query(T root, String key, NodeHandler<T> handler)
      Finds the nodes and/or attributes that are matched by the specified key. This is the main method for interpreting property keys. An implementation must traverse the given root node and its children to find all results that are matched by the given key. If the key is not correct in the syntax provided by that implementation, it is free to throw a (runtime) exception indicating this error condition. The passed in NodeHandler can be used to gather the required information from the node object. This implementation interprets the passed in key as an XPATH expression.
      Specified by:
      query in interface ExpressionEngine
      Type Parameters:
      T - the type of the node to be processed
      Parameters:
      root - the root node of a hierarchy of nodes
      key - the key to be evaluated
      handler - the NodeHandler for accessing the node
      Returns:
      a list with the results that are matched by the key (should never be null)
    • nodeKey

      public <T> String nodeKey(T node, String parentKey, NodeHandler<T> handler)
      Returns the key for the specified node in the expression language supported by an implementation. This method is called whenever a property key for a node has to be constructed, e.g. by the getKeys() method. This implementation creates 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 does not return indices for nodes. So all child nodes of a given parent with the same name have the same key.
      Specified by:
      nodeKey in interface ExpressionEngine
      Type Parameters:
      T - the type of the node to be processed
      Parameters:
      node - the node, for which the key must be constructed
      parentKey - the key of this node's parent (can be null for the root node)
      handler - the NodeHandler for accessing the node
      Returns:
      this node's key
    • attributeKey

      public String attributeKey(String parentKey, String attributeName)
      Description copied from interface: ExpressionEngine
      Returns the key of an attribute. The passed in parentKey must reference the parent node of the attribute. A concrete implementation must concatenate this parent key with the attribute name to a valid key for this attribute.
      Specified by:
      attributeKey in interface ExpressionEngine
      Parameters:
      parentKey - the key to the node owning this attribute
      attributeName - the name of the attribute in question
      Returns:
      the resulting key referencing this attribute
    • canonicalKey

      public <T> String canonicalKey(T node, String parentKey, NodeHandler<T> handler)
      Determines a "canonical" key for the specified node in the expression language supported by this implementation. This means that always a unique key if generated pointing to this specific node. For most concrete implementations, this means that an index is added to the node name to ensure that there are no ambiguities with child nodes having the same names. This implementation works similar to nodeKey(), but always adds an index expression to the resulting key.
      Specified by:
      canonicalKey in interface ExpressionEngine
      Type Parameters:
      T - the type of the node to be processed
      Parameters:
      node - the node, for which the key must be constructed
      parentKey - the key of this node's parent (can be null for the root node)
      handler - the NodeHandler for accessing the node
      Returns:
      the canonical key of this node
    • prepareAdd

      public <T> NodeAddData<T> prepareAdd(T root, String key, NodeHandler<T> handler)
      Returns information needed for an add operation. This method gets called when new properties are to be added to a configuration. An implementation has to interpret the specified key, find the parent node for the new elements, and provide all information about new nodes to be added. The expected format of the passed in key is explained in the class comment.
      Specified by:
      prepareAdd in interface ExpressionEngine
      Type Parameters:
      T - the type of the node to be processed
      Parameters:
      root - the root node
      key - the key for the new property
      handler - the NodeHandler for accessing the node
      Returns:
      an object with all information needed for the add operation
    • createContext

      private <T> org.apache.commons.jxpath.JXPathContext createContext(T root, NodeHandler<T> handler)
      Creates the JXPathContext to be used for executing a query. This method delegates to the context factory.
      Parameters:
      root - the configuration root node
      handler - the node handler
      Returns:
      the new context
    • createNodeAddData

      <T> NodeAddData<T> createNodeAddData(String path, QueryResult<T> parentNodeResult)
      Creates a NodeAddData object as a result of a prepareAdd() operation. This method interprets the passed in path of the new node.
      Type Parameters:
      T - the type of the nodes involved
      Parameters:
      path - the path of the new node
      parentNodeResult - the parent node
    • getContextFactory

      XPathContextFactory getContextFactory()
      Gets the XPathContextFactory used by this instance.
      Returns:
      the XPathContextFactory
    • generateKeyForAdd

      private <T> String generateKeyForAdd(T root, String key, NodeHandler<T> handler)
      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 key for adding properties is generated.
      Parameters:
      root - the root node of the configuration
      key - the key in question
      handler - the node handler
      Returns:
      the key to be used for adding the property
    • determineIndex

      private static <T> int determineIndex(T parent, T child, NodeHandler<T> handler)
      Determines the index of the given child node in the node list of its parent.
      Type Parameters:
      T - the type of the nodes involved
      Parameters:
      parent - the parent node
      child - the child node
      handler - the node handler
      Returns:
      the index of this child node
    • invalidPath

      private static void invalidPath(String path, String msg)
      Helper method for throwing an exception about an invalid path.
      Parameters:
      path - the invalid path
      msg - the exception message
    • findKeySeparator

      private static int findKeySeparator(String key)
      Determines the position of the separator in a key for adding new properties. If no delimiter is found, result is -1.
      Parameters:
      key - the key
      Returns:
      the position of the delimiter
    • convertResults

      private static <T> List<QueryResult<T>> convertResults(List<?> results)
      Converts the objects returned as query result from the JXPathContext to query result objects.
      Type Parameters:
      T - the type of results to be produced
      Parameters:
      results - the list with results from the context
      Returns:
      the result list
    • createResult

      private static <T> QueryResult<T> createResult(Object resObj)
      Creates a QueryResult object from the given result object of a query. Because of the node pointers involved result objects can only be of two types:
      • nodes of type T
      • attribute results already wrapped in QueryResult objects
      This method performs a corresponding cast. Warnings can be suppressed because of the implementation of the query functionality.
      Type Parameters:
      T - the type of the result to be produced
      Parameters:
      resObj - the query result object
      Returns:
      the QueryResult