The validation package contains classes used to validate document instances against a schema.

This validation is all performed "on-the-fly", by a collection of classes that implement Saxon's (SAX-like) Receiver interface, which is notified of start/end element events, attributes, and text nodes, as they occur. A pipeline of such classes is used both while processing input documents and while writing output documents, and in both cases, validation classes can be added to the pipeline.

Local validation of elements is controlled using the ValidationStack class, which manages a stack of validators for each element that is currently open. The most interesting of these is the ComplexContentValidator, which makes use of a finite state machine created by compiling a ComplexType in the schema. The finite state machine is represented by the AutomatonState object corresponding to the initial (entry) state of the machine.

Global validation of uniqueness and referential constraints is controlled using the ConstraintChecker class. The maintains a set of Watch classes that are interested in elements matching particular (restricted) XPath expressions. The restricted XPath expression used in XML Schema (which is similar to, but not identical to, an XPath pattern) is represented by a Selection object. Implementations of the Watch class are used to handle the different kinds of constraint that a schema can specify.

None of the classes in this package will normally be used directly by user applications.