Learning Java, 4th Edition

XPath

XPath is an expression language for addressing parts of an XML document. You can think of XPath expressions as sort of like regular expressions for XML. They let you pull out parts of an XML document based on patterns. In the case of XPath, the patterns are more concerned with structural information than with character content and the values returned may be either simple text or “live” DOM nodes. With XPath, we can query an XML document for all of the elements with a certain name or in a certain parent-child relationship. We can also apply fairly sophisticated tests or predicates to the nodes, which allows us to construct complex queries such as this one: give me all of the Animals with a Weight greater than the number 400 and a Temperament of irritable whose animalClass attribute is mammal.

The full XPath specification has many features and includes both a compact and more verbose syntax. We won’t try to cover it all here, but the basics are easy and it’s important to know them because XPath expressions are at the core of XSL transformations and other APIs that refer to parts of XML documents. The full specification does not make great bedtime reading, but can be found at http://www.w3.org/TR/xpath.

Nodes

An XPath expression addresses a Node in an XML document tree. The node may be an element (possibly with children) like <animal>...</animal> or it may be a lower-level document node representing an attribute (e.g., animalClass="mammal"), a CDATA block, or even a comment. All ...