This package provides an object-model neutral API for the evaluation of XPath expressions and access to the evaluation environment.

The following XML standards apply:

- XML Path Language (XPath) Version 1.0

## XPath Overview

The XPath language provides a simple, concise syntax for selecting nodes from an XML document. XPath also provides rules for converting a node in an XML document object model (DOM) tree to a boolean, double, or string value. XPath is a W3C-defined language and an official W3C recommendation; the W3C hosts the XML Path Language (XPath) Version 1.0 specification.

XPath started in life in 1999 as a supplement to the XSLT and XPointer languages, but has more recently become popular as a stand-alone language, as a single XPath expression can be used to replace many lines of DOM API code.

### XPath Expressions

An XPath expression is composed of a location path and one or more optional predicates. Expressions may also include XPath variables.

The following is an example of a simple XPath expression:

    /foo/bar

This example would select the `<bar>` element in an XML document such as the following:

    <foo>
    <bar/>
    </foo>

The expression `/foo/bar` is an example of a location path. While XPath location paths resemble Unix-style file system paths, an important distinction is that XPath expressions return all nodes that match the expression. Thus, all three `<bar>` elements in the following document would be selected by the `/foo/bar` expression:

    <foo>
    <bar/>
    <bar/>
    <bar/>
    </foo>

A special location path operator, `//`, selects nodes at any depth in an XML document. The following example selects all `<bar>` elements regardless of their location in a document:

    //bar

A wildcard operator, *, causes all element nodes to be selected. The following example selects all children elements of a `<foo>` element:

    /foo/*

In addition to element nodes, XPath location paths may also address attribute nodes, text nodes, comment nodes, and processing instruction nodes. The following table gives examples of location paths for each of these node types:

Location Path | Description
`/foo/bar/@id` | Selects the attribute `id` of the `<bar>` element
`/foo/bar/text()` | Selects the text nodes of the `<bar>` element. No distinction is made between escaped and non-escaped character data.
`/foo/bar/comment()` | Selects all comment nodes contained in the `<bar>` element.
`/foo/bar/processing-instruction()` | Selects all processing-instruction nodes contained in the `<bar>` element.

Predicates allow for refining the nodes selected by an XPath location path. Predicates are of the form `[expression]`. The following example selects all `<foo>` elements that contain an `include` attribute with the value of `true`:

    //foo[@include='true']

Predicates may be appended to each other to further refine an expression, such as:

    //foo[@include='true'][@mode='bar']

### Using the XPath API

The following example demonstrates using the XPath API to select one or more nodes from an XML document:

    XPath xpath = XPathFactory.newInstance().newXPath();
    String expression = "/widgets/widget";
    InputSource inputSource = new InputSource("widgets.xml");
    NodeList nodes = (NodeList) xpath.evaluate(expression, inputSource, XPathConstants.NODESET);

### XPath Expressions and Types

While XPath expressions select nodes in the XML document, the XPath API allows the selected nodes to be coalesced into one of the following other data types:

- `Boolean`
- `Number`
- `String`

The desired return type is specified by a {@link javax.xml.namespace.QName} parameter in method call used to evaluate the expression, which is either a call to `XPathExpression.evalute(...)` or to one of the `XPath.evaluate(...)` convenience methods. The allowed QName values are specified as constants in the {@link javax.xml.xpath.XPathConstants} class; they are:

- {@link javax.xml.xpath.XPathConstants#NODESET}
- {@link javax.xml.xpath.XPathConstants#NODE}
- {@link javax.xml.xpath.XPathConstants#STRING}
- {@link javax.xml.xpath.XPathConstants#BOOLEAN}
- {@link javax.xml.xpath.XPathConstants#NUMBER}

When a `Boolean` return type is requested, `Boolean.TRUE` is returned if one or more nodes were selected; otherwise, `Boolean.FALSE` is returned.

The `String` return type is a convenience for retrieving the character data from a text node, attribute node, comment node, or processing-instruction node. When used on an element node, the value of the child text nodes is returned.

The `Number` return type attempts to coalesce the text of a node to a `double` data type.

### XPath Context

XPath location paths may be relative to a particular node in the document, known as the `context`. Consider the following XML document:

    <widgets>
    <widget>
    <manufacturer/>
    <dimensions/>
    </widget>
    </widgets>

The `<widget>` element can be selected with the following XPath API code:

    // parse the XML as a W3C Document
    DocumentBuilder builder = DocumentBuilderFactory.newInstance().newDocumentBuilder();
    Document document = builder.parse(new File("/widgets.xml"));

    XPath xpath = XPathFactory.newInstance().newXPath();
    String expression = "/widgets/widget";
    Node widgetNode = (Node) xpath.evaluate(expression, document, XPathConstants.NODE);

With a reference to the `<widget>` element, a relative XPath expression can now written to select the `<manufacturer>` child element:

    XPath xpath = XPathFactory.newInstance().newXPath();
    String expression = "manufacturer";
    Node manufacturerNode = (Node) xpath.evaluate(expression, widgetNode, XPathConstants.NODE);

- Author Ben Galbraith
- Author Norman Walsh
- Author Jeff Suttor
- See XML Path Language (XPath) Version 1.0
- Since 1.5