previous   next   contents   index

31 March 2003

1. Document Object Model XPath

Editor:
Ray Whitmer, Netscape/AOL

Table of contents

1.1. Introduction

XPath 1.0 [XPath 1.0] is becoming an important part of a variety of many specifications including XForms, XPointer, XSL, XML Query, and so on. It is also a clear advantage for user applications which use DOM to be able to use XPath expressions to locate nodes automatically and declaratively.

This specification was created to map between the Document Object Model's representation of the W3C Information Set and XPath's model to permit XPath functions to be supplied and results returned within the framework of DOM APIs in a standard, interoperable way, allowing also for liveness of data, which is not addressed by the XPath specification but is present in results coming from the DOM hierarchy.

1.2. Mapping DOM to XPath

This section presents a mapping between the Document Object Model [DOM Level 2 Core] and the XPath 1.0 [XPath 1.0] model for the purposes of implementing the APIs.

1.2.1. Element Nodes

The DOM model uses Element nodes to represent Element Information Items. These nodes of a document are directly used to represent the elements of an XPath result.

1.2.2. Attribute Nodes

The DOM model uses Attr nodes to represent Attribute Information Items of attribute and namespace attribute properties of Element Information Item. These nodes have no parent, but have an ownerElement which can be used as XPath defines an attribute's parent.

XPath 1.0 does not make available the namespace attributes of an element. The DOM implementation of XPath 1.0 using these defined interfaces never directly returns Attr nodes of namespace attributes, but returned Element nodes still contain them.

1.2.3. Namespace Nodes

The XPath model expects namespace nodes for each in-scope namespace to be attached to each element. DOM only maintains the namespace attributes instead of replicating in-scope namespaces on each Element where they are in-scope. The DOM implementation of XPath produces a new node of type XPATH_NAMESPACE_NODE, defined in the XPathNamespace interface, to properly preserve identity and ordering in a way that is compatible with XPath. This node type is only visible using the XPath evaluation methods.

The set of in-scope namespaces of an element is the default xml namespace combined with the contributions of namespace attributes of the current and all ancestor elements. In addition to explicit namespace attributes, any element has an implicit declaration of its own prefix, if any, or if no prefix then of the default namespace, which is enforced during namespace serialization, fixup, and lookup, which must be added to the set of in-scope namespaces when generating namespace nodes for an element. This causes the set of namespace nodes to be consistent with serialization, fixup, and lookup of namespaces in DOM Level 3.

1.2.4. Text Nodes

The XPath model relies on the XML Information Set [XML Information set] ands represents Character Information Items in a single logical text node where DOM may have multiple fragmented Text nodes due to cdata sections, entity references, etc. Instead of returning multiple nodes where XPath sees a single logical text node, only the first non-empty DOM Text or CDATASection node of any logical XPath text will be returned in the node set. Applications using XPath in an environment with fragmented text nodes must manually gather the text of a single logical text node possibly from multiple nodes beginning with the first Text node or CDATASection node returned by the implementation.

Note: In an attempt to better implement the XML Information Set, DOM Level 3 Core [DOM Level 3 Core] adds the attribute wholeText on the Text interface for retrieving the whole text for logically-adjacent Text nodes and the method replaceWholeText for replacing those nodes.

1.2.5. Entity Reference Nodes

The DOM model may represent Unexpanded Entity Reference Information Items or may provide the position and URI of expanded entity hierarchies by using EntityReference nodes. XPath 1.0 does not preserve corresponding information.

Where the node represents an unexpanded entity reference, it is skipped as dictated by the XPath specifications for all infoset items besides those specifically processed.

Where there is a hierarchy underneath the node, these nodes are processed as though they were siblings of the entity reference, as is consistent with the rest of the DOM specification.

EntityReference nodes found within a DOM hierarchy are never returned as a node of the result, but returned nodes may contain or be contained within an EntityReference node. Text may be split partially inside and partially outside of an EntityReference node, but this is solved by handling Text nodes as described in the previous section.

1.2.6. Comment Nodes

The DOM model uses Comment nodes to represent Comment Information Items. These nodes of a document are directly used to represent the comments of an XPath result.

1.2.7. Processing Instruction Nodes

The DOM model uses ProcessingInstruction nodes to represent Processing Instruction Information Items. These nodes of a document are directly used to represent the processing instructions of an XPath result.

1.2.8. Document order

The document order of nodes in the DOM Core has been defined to be compatible with the XPath document order. The XPath DOM extends the document order of the DOM Core to include the XPathNamespace nodes. Element nodes occur before their children. The attribute nodes and namespace nodes of an element occur before the children of the element. The namespace nodes are defined to occur before the attribute nodes. The relative order of namespace nodes is implementation-dependent. The relative order of attribute nodes is implementation-dependent. The compareTreePosition method on the Node interface defined in the DOM Core must compare the XPathNamespace nodes using this extended document order if the XPath DOM module is supported.

Note: It is possible that in future versions of XPath, the order of namespace nodes or other aspects of document order may change incompatibly.

1.3. Conformance

This section explains conformance to DOM Level 3 XPath Module.

A DOM implementation must not return true to hasFeature("xpath", "3.0") unless the implementation conforms to that module. As documented in [DOM Level 3 Core], if a null or empty string is passed in for the second parameter, then conformance is still required to some version of the DOM XPath Module or false must be returned.

A conformant implementation is DOM Level 3 XPath must support all the interfaces as specified in that specification. In addition to implementing the interfaces in the DOM XPath Module, a conforming implementation must correctly implement each part of the XPath 1.0 specification when evaluating expressions including Location Paths, Expressions, the Core Function Library, and the mapping between DOM and the XPath 1.0 data model described in the DOM Level 3 XPath Module. The XPath id()function must return the corresponding element, if any, returned by the DOM method Document.getElementById.

After meeting the requirements for conformance, a conforming implementation may implement additional functions and variables. Applications which evaluate expressions using these extensions will not necessarily be portable to other implementations of the DOM Level 3 XPath Module.

1.4. Interfaces

An implementation is DOM Level 3 XPath conformant if it supports the Core module defined in [DOM Level 2 Core] and the module defined in this specification. An implementation conforms to a DOM module if it supports all the interfaces for that module and the associated semantics.

A DOM application may use the hasFeature(feature, version) method of the DOMImplementation interface with parameter values "XPath" and "3.0" (respectively) to determine whether or not the XPath module is supported by the implementation. In order to fully support this module, an implementation must also support the "Core" feature defined in the DOM Level 2 Core specification [DOM Level 2 Core].

A DOM implementation must not return true to the hasFeature(feature, version) method of the DOMImplementation interface for that feature unless the implementation conforms to that module. The version number for the feature used in this document is "3.0".

Exception XPathException

A new exception has been created for exceptions specific to these XPath interfaces.


IDL Definition
exception XPathException {
  unsigned short   code;
};
// XPathExceptionCode
const unsigned short      INVALID_EXPRESSION_ERR         = 51;
const unsigned short      TYPE_ERR                       = 52;

Definition group XPathExceptionCode
Defined Constants
INVALID_EXPRESSION_ERR
If the expression has a syntax error or otherwise is not a legal expression according to the rules of the specific XPathEvaluator or contains specialized extension functions or variables not supported by this implementation.
TYPE_ERR
If the expression cannot be converted to return the specified type.
Interface XPathEvaluator

The evaluation of XPath expressions is provided by XPathEvaluator. In a DOM implementation which supports the XPath 3.0 feature, as described above, the XPathEvaluator interface will be implemented on the same object which implements the Document interface permitting it to be obtained by the usual binding-specific method such as casting or by using the DOM Level 3 getInterface method. In this case the implementation obtained from the Document supports the XPath DOM module and is compatible with the XPath 1.0 specification.

Evaluation of expressions with specialized extension functions or variables may not work in all implementations and is, therefore, not portable. XPathEvaluator implementations may be available from other sources that could provide specific support for specialized extension functions or variables as would be defined by other specifications.


IDL Definition
interface XPathEvaluator {
  XPathExpression    createExpression(in DOMString expression, 
                                      in XPathNSResolver resolver)
                                        raises(XPathException, 
                                               DOMException);
  XPathNSResolver    createNSResolver(in Node nodeResolver);
  DOMObject          evaluate(in DOMString expression, 
                              in Node contextNode, 
                              in XPathNSResolver resolver, 
                              in unsigned short type, 
                              in DOMObject result)
                                        raises(XPathException, 
                                               DOMException);
};

Methods
createExpression
Creates a parsed XPath expression with resolved namespaces. This is useful when an expression will be reused in an application since it makes it possible to compile the expression string into a more efficient internal form and preresolve all namespace prefixes which occur within the expression.
Parameters
expression of type DOMString
The XPath expression string to be parsed.
resolver of type XPathNSResolver
The resolver permits translation of all prefixes, including the xml namespace prefix, within the XPath expression into appropriate namespace URIs. If this is specified as null, any namespace prefix within the expression will result in DOMException being thrown with the code NAMESPACE_ERR.
Return Value

XPathExpression

The compiled form of the XPath expression.
Exceptions

XPathException

INVALID_EXPRESSION_ERR: Raised if the expression is not legal according to the rules of the XPathEvaluator.

DOMException

NAMESPACE_ERR: Raised if the expression contains namespace prefixes which cannot be resolved by the specified XPathNSResolver.
createNSResolver
Adapts any DOM node to resolve namespaces so that an XPath expression can be easily evaluated relative to the context of the node where it appeared within the document. This adapter works like the DOM Level 3 method lookupNamespaceURI on nodes in resolving the namespaceURI from a given prefix using the current information available in the node's hierarchy at the time lookupNamespaceURI is called. also correctly resolving the implicit xml prefix.
Parameters
nodeResolver of type Node
The node to be used as a context for namespace resolution.
Return Value

XPathNSResolver

XPathNSResolver which resolves namespaces with respect to the definitions in scope for a specified node.
No Exceptions
evaluate
Evaluates an XPath expression string and returns a result of the specified type if possible.
Parameters
expression of type DOMString
The XPath expression string to be parsed and evaluated.
contextNode of type Node
The context is context node for the evaluation of this XPath expression. If the XPathEvaluator was obtained by casting the Document then this must be owned by the same document and must be a Document, Element, Attribute, Text, CDATASection, Comment, ProcessingInstruction, or XPathNamespace node. If the context node is a Text or a CDATASection, then the context is interpreted as the whole logical text node as seen by XPath, unless the node is empty in which case it may not serve as the XPath context.
resolver of type XPathNSResolver
The resolver permits translation of all prefixes, including the xml namespace prefix, within the XPath expression into appropriate namespace URIs. If this is specified as null, any namespace prefix within the expression will result in DOMException being thrown with the code NAMESPACE_ERR.
type of type unsigned short
If a specific type is specified, then the result will be returned as the corresponding type.
For XPath 1.0 results, this must be one of the codes of the XPathResult interface.
result of type DOMObject
The result specifies a specific result object which may be reused and returned by this method. If this is specified as nullor the implementation does not reuse the specified result, a new result object will be constructed and returned.
For XPath 1.0 results, this object will be of type XPathResult.
Return Value

DOMObject

The result of the evaluation of the XPath expression.
For XPath 1.0 results, this object will be of type XPathResult.
Exceptions

XPathException

INVALID_EXPRESSION_ERR: Raised if the expression is not legal according to the rules of the XPathEvaluatori

TYPE_ERR: Raised if the result cannot be converted to return the specified type.

DOMException

NAMESPACE_ERR: Raised if the expression contains namespace prefixes which cannot be resolved by the specified XPathNSResolver.

WRONG_DOCUMENT_ERR: The Node is from a document that is not supported by this XPathEvaluator.

NOT_SUPPORTED_ERR: The Node is not a type permitted as an XPath context node or the request type is not permitted by this XPathEvaluator.
Interface XPathExpression

The XPathExpression interface represents a parsed and resolved XPath expression.


IDL Definition
interface XPathExpression {
  DOMObject          evaluate(in Node contextNode, 
                              in unsigned short type, 
                              in DOMObject result)
                                        raises(XPathException, 
                                               DOMException);
};

Methods
evaluate
Evaluates this XPath expression and returns a result.
Parameters
contextNode of type Node
The context is context node for the evaluation of this XPath expression.
If the XPathEvaluator was obtained by casting the Document then this must be owned by the same document and must be a Document, Element, Attribute, Text, CDATASection, Comment, ProcessingInstruction, or XPathNamespace node.
If the context node is a Text or a CDATASection, then the context is interpreted as the whole logical text node as seen by XPath, unless the node is empty in which case it may not serve as the XPath context.
type of type unsigned short
If a specific type is specified, then the result will be coerced to return the specified type relying on XPath conversions and fail if the desired coercion is not possible. This must be one of the type codes of XPathResult.
result of type DOMObject
The result specifies a specific result object which may be reused and returned by this method. If this is specified as nullor the implementation does not reuse the specified result, a new result object will be constructed and returned.
For XPath 1.0 results, this object will be of type XPathResult.
Return Value

DOMObject

The result of the evaluation of the XPath expression.
For XPath 1.0 results, this object will be of type XPathResult.
Exceptions

XPathException

TYPE_ERR: Raised if the result cannot be converted to return the specified type.

DOMException

WRONG_DOCUMENT_ERR: The Node is from a document that is not supported by the XPathEvaluator that created this XPathExpression.

NOT_SUPPORTED_ERR: The Node is not a type permitted as an XPath context node or the request type is not permitted by this XPathExpression.
Interface XPathNSResolver

The XPathNSResolver interface permit prefix strings in the expression to be properly bound to namespaceURI strings. XPathEvaluator can construct an implementation of XPathNSResolver from a node, or the interface may be implemented by any application.


IDL Definition
interface XPathNSResolver {
  DOMString          lookupNamespaceURI(in DOMString prefix);
};

Methods
lookupNamespaceURI
Look up the namespace URI associated to the given namespace prefix. The XPath evaluator must never call this with a null or empty argument, because the result of doing this is undefined.
Parameters
prefix of type DOMString
The prefix to look for.
Return Value

DOMString

Returns the associated namespace URI or null if none is found.
No Exceptions
Interface XPathResult

The XPathResult interface represents the result of the evaluation of an XPath 1.0 expression within the context of a particular node. Since evaluation of an XPath expression can result in various result types, this object makes it possible to discover and manipulate the type and value of the result.


IDL Definition
interface XPathResult {

  // XPathResultType
  const unsigned short      ANY_TYPE                       = 0;
  const unsigned short      NUMBER_TYPE                    = 1;
  const unsigned short      STRING_TYPE                    = 2;
  const unsigned short      BOOLEAN_TYPE                   = 3;
  const unsigned short      UNORDERED_NODE_ITERATOR_TYPE   = 4;
  const unsigned short      ORDERED_NODE_ITERATOR_TYPE     = 5;
  const unsigned short      UNORDERED_NODE_SNAPSHOT_TYPE   = 6;
  const unsigned short      ORDERED_NODE_SNAPSHOT_TYPE     = 7;
  const unsigned short      ANY_UNORDERED_NODE_TYPE        = 8;
  const unsigned short      FIRST_ORDERED_NODE_TYPE        = 9;

  readonly attribute unsigned short  resultType;
  readonly attribute double          numberValue;
                                        // raises(XPathException) on retrieval

  readonly attribute DOMString       stringValue;
                                        // raises(XPathException) on retrieval

  readonly attribute boolean         booleanValue;
                                        // raises(XPathException) on retrieval

  readonly attribute Node            singleNodeValue;
                                        // raises(XPathException) on retrieval

  readonly attribute boolean         invalidIteratorState;
  readonly attribute unsigned long   snapshotLength;
                                        // raises(XPathException) on retrieval

  Node               iterateNext()
                                        raises(XPathException, 
                                               DOMException);
  Node               snapshotItem(in unsigned long index)
                                        raises(XPathException);
};

Definition group XPathResultType

An integer indicating what type of result this is.

If a specific type is specified, then the result will be returned as the corresponding type, using XPath type conversions where required and possible.

Defined Constants
ANY_TYPE
This code does not represent a specific type. An evaluation of an XPath expression will never produce this type. If this type is requested, then the evaluation returns whatever type naturally results from evaluation of the expression.
If the natural result is a node set when ANY_TYPE was requested, then UNORDERED_NODE_ITERATOR_TYPE is always the resulting type. Any other representation of a node set must be explicitly requested.
ANY_UNORDERED_NODE_TYPE
The result is a node set as defined by [XPath 1.0] and will be accessed as a single node, which may be nullif the node set is empty. Document modification does not invalidate the node, but may mean that the result node no longer corresponds to the current document. This is a convenience that permits optimization since the implementation can stop once any node in the resulting set has been found.
If there is more than one node in the actual result, the single node returned might not be the first in document order.
BOOLEAN_TYPE
The result is a boolean as defined by [XPath 1.0]. Document modification does not invalidate the boolean, but may mean that reevaluation would not yield the same boolean.
FIRST_ORDERED_NODE_TYPE
The result is a node set as defined by [XPath 1.0] and will be accessed as a single node, which may be null if the node set is empty. Document modification does not invalidate the node, but may mean that the result node no longer corresponds to the current document. This is a convenience that permits optimization since the implementation can stop once the first node in document order of the resulting set has been found.
If there are more than one node in the actual result, the single node returned will be the first in document order.
NUMBER_TYPE
The result is a number as defined by [XPath 1.0]. Document modification does not invalidate the number, but may mean that reevaluation would not yield the same number.
ORDERED_NODE_ITERATOR_TYPE
The result is a node set as defined by [XPath 1.0] that will be accessed iteratively, which will produce document-ordered nodes. Document modification invalidates the iteration.
ORDERED_NODE_SNAPSHOT_TYPE
The result is a node set as defined by [XPath 1.0] that will be accessed as a snapshot list of nodes that will be in original document order. Document modification does not invalidate the snapshot but may mean that reevaluation would not yield the same snapshot and nodes in the snapshot may have been altered, moved, or removed from the document.
STRING_TYPE
The result is a string as defined by [XPath 1.0]. Document modification does not invalidate the string, but may mean that the string no longer corresponds to the current document.
UNORDERED_NODE_ITERATOR_TYPE
The result is a node set as defined by [XPath 1.0] that will be accessed iteratively, which may not produce nodes in a particular order. Document modification invalidates the iteration.
This is the default type returned if the result is a node set and ANY_TYPE is requested.
UNORDERED_NODE_SNAPSHOT_TYPE
The result is a node set as defined by [XPath 1.0] that will be accessed as a snapshot list of nodes that may not be in a particular order. Document modification does not invalidate the snapshot but may mean that reevaluation would not yield the same snapshot and nodes in the snapshot may have been altered, moved, or removed from the document.
Attributes
booleanValue of type boolean, readonly
The value of this boolean result.
Exceptions on retrieval

XPathException

TYPE_ERR: raised if resultType is not BOOLEAN_TYPE.
invalidIteratorState of type boolean, readonly
Signifies that the iterator has become invalid. True if resultType is UNORDERED_NODE_ITERATOR_TYPE or ORDERED_NODE_ITERATOR_TYPE and the document has been modified since this result was returned.
numberValue of type double, readonly
The value of this number result. If the native double type of the DOM binding does not directly support the exact IEEE 754 result of the XPath expression, then it is up to the definition of the binding to specify how the XPath number is converted to the native binding number.
Exceptions on retrieval

XPathException

TYPE_ERR: raised if resultType is not NUMBER_TYPE.
resultType of type unsigned short, readonly
A code representing the type of this result, as defined by the type constants.
singleNodeValue of type Node, readonly
The value of this single node result, which may be null.
Exceptions on retrieval

XPathException

TYPE_ERR: raised if resultType is not ANY_UNORDERED_NODE_TYPE or FIRST_ORDERED_NODE_TYPE.
snapshotLength of type unsigned long, readonly
The number of nodes in the result snapshot. Valid values for snapshotItem indices are 0 to snapshotLength-1 inclusive.
Exceptions on retrieval

XPathException

TYPE_ERR: raised if resultType is not UNORDERED_NODE_SNAPSHOT_TYPE or ORDERED_NODE_SNAPSHOT_TYPE.
stringValue of type DOMString, readonly
The value of this string result.
Exceptions on retrieval

XPathException

TYPE_ERR: raised if resultType is not STRING_TYPE.
Methods
iterateNext
Iterates and returns the next node from the node set or nullif there are no more nodes.
Return Value

Node

Returns the next node.
Exceptions

XPathException

TYPE_ERR: raised if resultType is not UNORDERED_NODE_ITERATOR_TYPE or ORDERED_NODE_ITERATOR_TYPE.

DOMException

INVALID_STATE_ERR: The document has been mutated since the result was returned.
No Parameters
snapshotItem
Returns the indexth item in the snapshot collection. If index is greater than or equal to the number of nodes in the list, this method returns null. Unlike the iterator result, the snapshot does not become invalid, but may not correspond to the current document if it is mutated.
Parameters
index of type unsigned long
Index into the snapshot collection.
Return Value

Node

The node at the indexth position in the NodeList, or null if that is not a valid index.
Exceptions

XPathException

TYPE_ERR: raised if resultType is not UNORDERED_NODE_SNAPSHOT_TYPE or ORDERED_NODE_SNAPSHOT_TYPE.
Interface XPathNamespace

The XPathNamespace interface is returned by XPathResult interfaces to represent the XPath namespace node type that DOM lacks. There is no public constructor for this node type. Attempts to place it into a hierarchy or a NamedNodeMap result in a DOMException with the code HIERARCHY_REQUEST_ERR. This node is read only, so methods or setting of attributes that would mutate the node result in a DOMException with the code NO_MODIFICATION_ALLOWED_ERR.

The core specification describes attributes of the Node interface that are different for different node types but does not describe XPATH_NAMESPACE_NODE, so here is a description of those attributes for this node type. All attributes of Node not described in this section have a null or false value.

ownerDocument matches the ownerDocument of the ownerElement even if the element is later adopted.

nodeName is always the string "#namespace".

prefix is the prefix of the namespace represented by the node.

localName is the same as prefix.

nodeType is equal to XPATH_NAMESPACE_NODE.

namespaceURI is the namespace URI of the namespace represented by the node.

nodeValue is the same as namespaceURI.

adoptNode, cloneNode, and importNode fail on this node type by raising a DOMException with the code NOT_SUPPORTED_ERR.

Note: In future versions of the XPath specification, the definition of a namespace node may be changed incomatibly, in which case incompatible changes to field values may be required to implement versions beyond XPath 1.0.


IDL Definition
interface XPathNamespace : Node {

  // XPathNodeType
  const unsigned short      XPATH_NAMESPACE_NODE           = 13;

  readonly attribute Element         ownerElement;
};

Definition group XPathNodeType

An integer indicating which type of node this is.

Note: There is currently only one type of node which is specific to XPath. The numbers in this list must not collide with the values assigned to core node types.

Defined Constants
XPATH_NAMESPACE_NODE
The node is a Namespace.
Attributes
ownerElement of type Element, readonly
The Element on which the namespace was in scope when it was requested. This does not change on a returned namespace node even if the document changes such that the namespace goes out of scope on that element and this node is no longer found there by XPath.
previous   next   contents   index