Schema Processing using JAXP

Applications can invoke schema processing using the APIs provided in JAXP 1.3. This makes Saxon interchangeable with other schema processors implementing this interface. There is full information on these APIs in the Java documentation. The two main mechanisms are the Validator class, and the ValidatorHandler class. Sample applications using these interfaces are provided in the samples/java directory of the saxon-resources download (see SchemaValidatorExample.java and SchemaValidatorHandlerExample.java). Saxon also supplies the class com.saxonica.ee.jaxp.ValidatingReader, which implements the SAX2 XMLReader interface, allowing it to be used as a schema-validating XML parser.

The main steps are:

1. Create a SchemaFactory, by calling SchemaFactory.newInstance() with the argument "http://www.w3.org/2001/XMLSchema", and with the Java system properties set up to ensure that Saxon is loaded as the chosen schema processor. Saxon will normally be loaded as the default schema processor if Saxon-EE is present on the classpath, but to make absolutely sure, set the system property javax.xml.validation.SchemaFactory:http://www.w3.org/2001/XMLSchema to the value com.saxonica.ee.jaxp.SchemaFactoryImpl. Note that if you set this property using a property file, colons in the property name must be escaped as "\:".

2. Process a schema document by calling one of the several newSchema() methods on the returned SchemaFactory.

3. Create either a Validator or a ValidatorHandler from this returned Schema.

4. Use the Validator or ValidatorHandler to process one or more source documents.

Saxon also provides the class SchemaFactory11 which automatically enables support for XSD 1.1. When the JAXP search mechanism is used, this schema factory will be selected if the schema language required is set to http://www.w3.org/XML/XMLSchema/v1.1. Saxon also recognizes the generic language identifier http://www.w3.org/XML/XMLSchema and the XSD 1.0 identifier http://www.w3.org/XML/XMLSchema/vX.Y as requests for an XSD 1.0 processor.

Note that additional schemas referenced from the xsi:schemaLocation attributes within the source documents will be loaded as necessary. A target namespace is ignored if there is already a loaded schema for that namespace; Saxon makes no attempt to load multiple schemas for the same namespace and check them for consistency.

Although the API is defined in such a way that a Validator or ValidatorHandler is created for a particular Schema, in the Saxon implementation the schema components that are available to the validator are not only the components within that schema, but all the components that form part of any schema registered with the Configuration.

Another way to control validation from a Java application is to run a JAXP identity transformation, having first set the option to perform schema validation. The following code (from the sample application QuickValidator.java) illustrates this:

If you set an ErrorListener on the TransformerFactory, then you can control the way that error messages are output.

If you want to validate against a schema without hard-coding the URI of the schema into the source document, you can do this by pre-loading the schema into the TransformerFactory. This extended example (again from the sample application QuickValidator.java) illustrates this:

You can preload as many schemas as you like using the addSchema() method. Such schemas are parsed, validated, and compiled once, and can be used as often as you like for validating multiple source documents. You cannot unload a schema once it has been loaded. If you want to remove or replace a schema, start afresh with a new TransformerFactory.

Behind the scenes, the TransformerFactory uses a Configuration object to hold all the configuration information. The basic Saxon product (Saxon-HE and Saxon-PE) uses the class net.sf.saxon.TransformerFactoryImpl for the TransformerFactory, and net.sf.saxon.Configuration for the underlying configuration information. The schema-aware product (Saxon-EE) subclasses these with com.saxonica.config.EnterpriseTransformerFactory and com.saxonica.config.EnterpriseConfiguration respectively. You can get hold of the Configuration object by casting the TransformerFactory to a Saxon TransformerFactorImpl and calling the getConfiguration() method. This gives you more precise control, for example it allows you to retrieve the Schema object containing the schema components for a given target namespace, and to inspect the compiled schema to establish its properties. See the JavaDoc documentation for further details.

Saxon currently implements its own API for access to the schema components. This API should be regarded as temporary. In the longer term, it is possible that Saxon will offer an API for schema access that has been proposed in a member submission to W3C.

The programming approach outlined above, of using an identity transformer, is suitable for a wide class of applications. For example, it enables you to insert a validation step into a SAX-based pipeline. However, for finer control, there are lower-level interfaces available in Saxon that you can also use. See for example the JavaDoc for the EnterpriseConfiguration class, which includes methods such as getElementValidator(). This constructs a Receiver which acts as a validating XML event filter. This can be inserted into a pipeline of Receivers. Saxon also provides classes to bridge between SAX events and Receiver events: ReceivingContentHandler and ContentHandlerProxy respectively.