Selecting the XPath implementation

The JAXP API is designed on the basis that when your application invokes XPathFactory.newInstance(), an XPath engine is selected by examining the values of system properties and searching the classpath. If you rely on this mechanism, then your application may end up running with an XPath engine on which it has never been tested. Since different XPath engines can differ in many significant respects (most notably, the version of XPath that they support), this can easily lead to application failures. Saxon therefore no longer identifies itself (in the JAR file manifest) as a JAXP XPath supplier. If you want to load Saxon as your XPath engine, you need to select it explicitly; it's not enough to just put it on the classpath.

An application using the JAXP 1.3 XPath API starts by instantiating a factory class. This is done by calling:

XPathFactory xpathFactory = XPathFactory.newInstance(objectModel); XPath xpath = xpathFactory.newXPath();

Here objectModel is a URI that identifies the object model you are using. Saxon recognizes the following values for the object model:

Symbolic name	Meaning
XPathConstants.DOM_OBJECT_MODEL	The DOM object model
NamespaceConstant.OBJECT_MODEL_SAXON	Saxon's native object model. This means anything that implements the `NodeInfo` interface, including the standard tree, the tiny tree, and third-party implementations of `NodeInfo`.
NamespaceConstant.OBJECT_MODEL_JDOM	The JDOM object model
NamespaceConstant.OBJECT_MODEL_XOM	The XOM object model
NamespaceConstant.OBJECT_MODEL_DOM4J	The DOM4J object model

If you want to use Saxon as your XPath implementation, you must instantiate the class net.sf.saxon.xpath.XPathFactoryImpl directly. If you want to take advantage of features in Saxon-PE (Professional Edition), use com.saxonica.config.ProfessionalXPathFactory, or for Saxon-EE (Enterprise Edition) use com.saxonica.config.EnterpriseXPathFactory.

It is important to note that a compiled XPath expression can only be used with a source document that was built using the same Saxon Configuration. When you create an XPathFactory, a Saxon Configuration is created automatically. You can extract this configuration and use it to build source documents. Alternatively, there is a constructor that allows you to create an XPathFactory that uses a preexisting Configuration.

Saxon's implementation of java.xml.xpath.XPath is the class net.sf.saxon.xpath.XPathEvaluator. This class provides a few simple configuration interfaces to set the source document, the static context, and the context node, plus a number of methods for evaluating XPath expressions.

The XPath object allows you to set the static context for evaluating XPath expressions (you can pre-declare namespaces, variables, and functions), and to compile XPath expressions in this context. A compiled XPath expression (an object of class XPathExpression) can then be evaluated, with a supplied node (represented by a class in the selected object model) supplied as the context node. For further details, see the Javadoc specifications and the supplied example applications (as described in JAXP XPath API).