Parsing, Creating, and Transforming XML Documents - Beginning Java 7 - page 990

Selecting XML Document Nodes with XPath
XPath isanonXMLdeclarativequerylanguage(definedbytheW3C)forselectingan
XMLdocument'sinfosetitemsasoneormorenodes.Forexample,youcanuseXPath
to locate Listing 10-1 ' s third ingredient element and return this element node.
XPath is often used to simplify access to a DOM tree's nodes, and is also used in
thecontextofXSLT(discussedinthenextsection),typicallytoselectthoseinputdoc-
ument elements (via XPath expressions) that are to be copied to an output document.
Java 7 supports XPath 1.0 via JAXP's XPath API, which is assigned package
javax.xml.xpath .
This section first acquaints you with the XPath 1.0 language. It then demonstrates
howXPathsimplifiestheselectionofaDOMtree'snodes.Lastly,thesectionintroduces
you to three advanced XPath topics.
XPath Language Primer
XPath views an XML document as a tree ofnodes that starts from a root node. XPath
recognizessevenkindsofnodes:element,attribute,text,namespace,processinginstruc-
tion,comment,anddocument.XPathdoesnotrecognizeCDATAsections,entityrefer-
ences, or document type declarations.
Note A tree's root node (a DOM Document instance) is not the same as a doc-
ument's root element. The root node contains the entire document, including the root
element,anycommentsorprocessinginstructionsthatappearbeforetherootelement's
start tag, and any comments or processing instructions that appear after the root ele-
ment's end tag.
XPathprovideslocationpathexpressionsforselectingnodes.A location path expres-
sion locatesnodesviaasequenceof steps startingfromthe context node (therootnode
orsomeotherdocumentnodethatisthecurrentnode).Thereturnedsetofnodesmight
be empty, or it might contain one or more nodes.
Thesimplestlocationpathexpressionselectsthedocument'srootnodeandconsists
of a single forward slash character ( / ). The next simplest location path expression is
thenameofanelement,whichselectsallchildelementsofthecontextnodethathave
that name. For example, ingredient refers to all ingredient child elements of
thecontextnodein Listing10-1 'sXMLdocument.ThisXPathexpressionreturnsaset
of three ingredient nodes when ingredients is the context node. However, if
recipe or instructions happenedtobethecontext node, ingredient would
notreturnanynodes( ingredient isachildof ingredients only).Whenanex-