| Questions |

- Usage
- XPath 2.0 support for XML Schema 1.1 validation
- Using Entities and CDATA Sections
- XML Schema API
- Changes to PSVI
- Accessing PSVI via XNI
- Accessing PSVI via DOM
- Accessing PSVI via SAX
- Accessing PSVI via the JAXP 1.4 Validation API
- Parsing and analyzing an XML schema
- Using XSLoader to get an XSModel

| Answers |

| How do I validate against XML schema? |

XML Schema 1.0 validation has been integrated with the regular SAXParser and DOMParser classes. No special classes are required to parse documents that use a schema.

For XML Schema 1.1 validation, you'll need to use the JAXP validation API, using the XSD 1.1 Schema factory. Here's an example:

    import javax.xml.transform.Source;
    import javax.xml.transform.stream.StreamSource;
    import javax.xml.validation.Schema;
    import javax.xml.validation.SchemaFactory;
    import javax.xml.validation.Validator;

    ...

    StreamSource[] schemaDocuments = /* created by your application */;
    Source instanceDocument = /* created by your application */;

    SchemaFactory sf = SchemaFactory.newInstance(
        "http://www.w3.org/XML/XMLSchema/v1.1");
    Schema s = sf.newSchema(schemaDocuments);
    Validator v = s.newValidator();
    v.validate(instanceDocument);

You can also refer to the JAXP sample, SourceValidator, where you can validate XML documents against 1.1 schemas by specifying the "-xsd11" flag when running the sample.

Each document that uses XML Schema grammars must specify the location of the grammars it uses by using an xsi:schemaLocation attribute if they use namespaces, and an xsi:noNamespaceSchemaLocation attribute otherwise. These are usually placed on the root / top-level element in the document, though they may occur on any element; for more details see XML Schema Part 1 section 4.3.2. Here is an example with no target namespace:

    <document
        xmlns:xsi='http://www.w3.org/2001/XMLSchema-instance'
        xsi:noNamespaceSchemaLocation='document.xsd'>
        ...
    </document>

Here is an example with a target namespace. Note that it is an error to specify a different namespace than the target namespace defined in the Schema.

    <document
        xmlns='http://my.com'
        xmlns:xsi='http://www.w3.org/2001/XMLSchema-instance'
        xsi:schemaLocation='http://my.com document.xsd'>
        ...
    </document>

Review the sample file, 'data/personal.xsd' for an example of an XML Schema grammar.

| How Xerces-J uses an XPath 2.0 engine for XML Schema 1.1 assertions and type alternatives? |

XML Schema 1.1 'assertions' and 'type alternatives' require an XPath 2.0 processor for evaluation. For XSD 1.1 assertions, full XPath 2.0 support is required. For XSD 1.1 type alternatives, XML schema engines can provide full XPath 2.0 support or they can implement a smaller XPath 2.0 subset, as defined by the XSD 1.1 language. Within the type alternatives implementation, Xerces-J first attempts to compile the XPath expression with a native Xerces "XPath subset" parser. If parsing with the native Xerces "XPath subset" parser fails, Xerces-J transparently switches over to another processor which supports all of XPath 2.0. The native processor in Xerces-J was written for efficiency, so you will likely get better performance if your XPath expressions fall within the minimum subset defined by the XML Schema 1.1 specification. For full XPath 2.0 evaluation (for XSD 1.1 'assertions', and optionally for 'type alternatives'), Xerces-J uses the Eclipse/PsychoPath XPath 2.0 engine.

Xerces-J bundles a PsychoPath XPath 2.0 jar (which requires JDK 1.4 or later).

Users should be aware that more recent releases of the PsychoPath XPath engine may have better conformance to the W3C XPath 2.0 language than the PsychoPath XPath 2.0 jar that's bundled with Apache Xerces-J. Builds of the PsychoPath XPath 2.0 jar are available for download from following location, Eclipse Web Tools Platform (WTP) Project - The latest 'Released' or a 'Stable milestone' WTP version can be downloaded from here (from which the PsychoPath XPath 2.0 jar can be extracted). The latest PsychoPath XPath 2.0 engine builds at Eclipse require a minimum JDK level of 1.5.

| How does the XML Schema processor treat entities and CDATA sections? |

According to the XML Infoset the infoset items contributing to the [character information item] are: characters in the document, whether literally, as a character reference, or within a CDATA section, or within Entity Reference. The XML Schema specification "requires as a precondition for assessment an information set as defined in [XML-Infoset]" (Appendix D) and thus Xerces might attempt to normalize data in an entity reference or CDATA section. To preserve character data within entity references and CDATA sections, turn off http://apache.org/xml/features/validation/schema/normalized-value feature.

| Does Xerces provide access to the post schema validation infoset (PSVI)? |

Xerces implements the XML Schema API specification that defines an API for accessing and querying the post schema validation infoset (PSVI) as defined in Contributions to the post-schema-validation infoset (Appendix C.2). This API also defines interfaces for loading XML schema documents.

For more information please refer to the interfaces.

| The Xerces 2.6.2 release fixes a documentation bug in the XML Schema API. In particular in the XSModel interface the order of the parameters in getTypeDefinition, getNotationDeclaration, getModelGroupDefinition, getElementDeclaration, getAttributeDeclaration, getAttributeGroup methods have been changed from (String namespace, String name) to (String name, String namespace).

| What happened to the PSVI interfaces in org.apache.xerces.xni.psvi? |

| The PSVI interfaces which used to be part of the `org.apache.xerces.xni.psvi` and `org.apache.xerces.impl.xs.psvi` packages were modified and have been moved into the XML Schema API.

| How do I access PSVI via XNI? |

From within an `XMLDocumentHandler`, one can retrieve PSVI information while in the scope of the document handler start/end element calls:

    import org.apache.xerces.xs.*;

    ...

    public void startElement(QName element, XMLAttributes attributes,
        Augmentations augs) {
        ElementPSVI elemPSVI = (ElementPSVI)augs.getItem("ELEMENT_PSVI");
        // get PSVI items of this element out of elemPSVI
        short attemp = elemPSVI.getValidationAttempted();
        short validity = elemPSVI.getValidity();
        ...
    }

| For more information, please refer to the API documentation for the XML Schema API.

The above code shows how to retrieve PSVI information after elements/attributes are assessed. The other kind of information PSVI offers is the property [schema information]. This property exposes all schema components in the schema that are used for assessment. These components and the schema itself are represented by interfaces in the `org.apache.xerces.xs` package.

[schema information] property is only available on the `endElement` method for the validation root. When this method is called, information about various components can be retrieved by

    import org.apache.xerces.xs.*;

    ...

    public void endElement(QName element, Augmentations augs) {
        ElementPSVI elemPSVI = (ElementPSVI)augs.getItem("ELEMENT_PSVI");
        XSModel xsModel = elemPSVI.getSchemaInformation();
        // get a list of [namespace schema information information item]s,
        // one for each namespace.
        XSNamespaceItemList nsItems = xsModel.getNamespaceItems();
        ...

        // get an element declaration of the specified name and namespace
        XSElementDeclaration elemDecl = xsModel.getElementDeclaration
            (namespace, name);
        ...
    }

| How do I access PSVI via DOM? |

Use the http://apache.org/xml/properties/dom/document-class-name property to set `org.apache.xerces.dom.PSVIDocumentImpl` as the implementation of the `org.w3c.dom.Document` interface. In the resulting DOM tree, you may cast `org.w3c.dom.Element` to the `org.apache.xerces.xs.ElementPSVI` and `org.w3c.dom.Attr` to the `org.apache.xerces.xs.AttributePSVI`.

    import org.apache.xerces.xs.ElementPSVI;
    import org.apache.xerces.xs.XSModel;
    import org.apache.xerces.xs.XSNamedMap;

    ...

    Document document = parser.getDocument();
    Element root = document.getDocumentElement();

    // retrieve PSVI for the root element
    ElementPSVI rootPSVI = (ElementPSVI)root;

    // retrieve the schema used in validation of this document
    XSModel schema = rootPSVI.getSchemaInformation();
    XSNamedMap elementDeclarations =
        schema.getComponents(XSConstants.ELEMENT_DECLARATION);

    // get schema normalized value
    String normalizedValue = rootPSVI.getSchemaNormalizedValue();

| How do I access PSVI via SAX? |

The Xerces SAX parser also implements the `org.apache.xerces.xs.PSVIProvider` interface. Within the scope of the methods handling the start (`org.xml.sax.ContentHandler.startElement`) and end (`org.xml.sax.ContentHandler.endElement`) of an element, applications may use the `PSVIProvider` to retrieve the PSVI related to the element and its attributes.

    import org.apache.xerces.xs.PSVIProvider;
    import javax.xml.parsers.SAXParser;
    import javax.xml.parsers.SAXParserFactory;

    ...

    SAXParserFactory factory = SAXParserFactory.newInstance();
    SAXParser parser = factory.newSAXParser();
    XMLReader reader = parser.getXMLReader();
    PSVIProvider psviProvider = (PSVIProvider)reader;

| How do I access PSVI via the JAXP 1.4 Validation API? |

Like the Xerces SAX parser the implementations of `javax.xml.validation.Validator` and `javax.xml.validation.ValidatorHandler` also implement the `org.apache.xerces.xs.PSVIProvider` interface. Within the scope of the methods handling the start (`org.xml.sax.ContentHandler.startElement`) and end (`org.xml.sax.ContentHandler.endElement`) of an element, applications may use the `PSVIProvider` to retrieve the PSVI related to the element and its attributes.

| How do I parse and analyze an XML schema? |

| Please, refer to the Examining Grammars FAQ.

| Can I parse and query XML Schema components in memory? |

Yes, the XML Schema API specification defines an interface called `XSLoader` which provides methods for loading XML Schema documents into an `XSModel`. The `XSImplementation` interface provides a method to create an `XSLoader` using the DOM Level 3 Bootstraping mechanism. An application can get a reference to an `XSImplementation` using the `getDOMImplementation` method on the `DOMImplementationRegistry` object with the feature string "XS-Loader". To create an `XSLoader` you need to do something like this:

    import org.w3c.dom.DOMImplementationRegistry;
    import org.apache.xerces.xs.XSImplementation;
    import org.apache.xerces.xs.XSLoader;

    ...

    // Get DOM Implementation using DOM Registry
    System.setProperty(DOMImplementationRegistry.PROPERTY,
        "org.apache.xerces.dom.DOMXSImplementationSourceImpl");
    DOMImplementationRegistry registry = DOMImplementationRegistry.newInstance();

    XSImplementation impl =
        (XSImplementation) registry.getDOMImplementation("XS-Loader");

    XSLoader schemaLoader = impl.createXSLoader(null);

    ...

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