Using XML Schemas
Questions
Usage
Using Entities and CDATA Sections
XPath 2.0 support for XML Schema 1.1 validation
User defined error messages, for XML Schema 1.1 assertion failures
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, and also with the JAXP
validation API using the XSD 1.0 Schema factory. No special classes are
required to parse documents that use a schema.
For XML Schema 1.1 validation, the preferred way is 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);
Similar to XML Schema 1.0 validation using SAXParser and DOMParser classes that was
available earlier with Xerces, the same has been enhanced to support XML Schema 1.1
validation as well. To be able to do this, we need to construct the XSD 1.1 Schema factory
with the following java statement, SchemaFactory.newInstance("http://www.w3.org/XML/XMLSchema/v1.1")
and do the subsequent validation.
You can also refer to the JAXP sample, SourceValidator, where you
can validate XML documents against 1.1 schemas by specifying an option "-xsd11"
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:
xsi:noNamespaceSchemaLocation='document.xsd'>
...
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.
xmlns:xsi='http://www.w3.org/2001/XMLSchema-instance'
xsi:schemaLocation='http://my.com document.xsd'>
...
Review the sample file, 'data/personal.xsd' for an example of an XML
Schema grammar.
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.
How an XPath 2.0 engine is used for XML Schema 1.1 assertions and CTAs?
XML Schema 1.1 assertions and CTAs require an XPath processor during evaluation. For XSD 1.1 assertions,
full
XPath 2.0
support is required. For XSD 1.1 CTAs the XSD schema engines can
provide full XPath 2.0 support, or they can implement a
smaller XPath subset
as defined by the XSD 1.1 language. For CTAs Xerces uses the XSD 1.1 CTA XPath subset language by default, but can be made to use the
full XPath 2.0 support by setting the value of Xerces feature
to 'true'.
The native CTA XPath 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 CTAs), Xerces-J uses an
XPath 2.0 engine
Xerces-J does bundle along an XPath 2.0 engine jar for these features (that requires JDK 1.4 or later).
We would like to acknowledge, work of following people for the XPath 2.0 support that comes with Xerces's
XML Schema 1.1 processor: Andrea Bittau (responsible for the original design and implementation, of XPath 2.0
processor that's been used by Xerces. Andrea donated his XPath 2.0 processor to the Eclipse Foundation, that
Xerces uses), Dave Carver (Dave helped to set up software infrastructure at Eclipse Foundation's Web Tools Platform project,
for testing the XPath 2.0 processor as per W3C XPath 2.0 test suite. He also helped to improve compliance of
XPath 2.0 processor, to the W3C XPath 2.0 test suite by providing numerous bug fixes and implementation),
Jesper Steen Moeller (Jesper as an Eclipse's Web Tools Platform committer, helped to improve the implementation
of the XPath 2.0 processor).
How to specify a user defined error message, when an XML Schema 1.1 assertion returns a 'false' result?
When evaluation of an XSD 1.1 assertion fails the Xerces XML Schema validator would produce a default error message,
which would say that which element or attribute was not validated successfully by an assertion and the schema type involved
during validation. It is however possible to, specify an user-defined error message for assertion failures. This is done by
specifying an attribute
"message"
in the XML namespace
'http://xerces.apache.org'
on an
xs:assert
or
xs:assertion
element in the schema. In this case, the value of attribute
"message"
is a text of user-defined assertion error message which is generated by the Xerces XML Schema validator when an assertion fails.
While using
xs:assertion
facet within simple types, if a user defined error message is used, a user may
dynamically construct the assertion error message by getting the value of XPath 2.0 context variable
$value
into the error message. Following is an example of such an assertion error message:
xerces:message="The number {$value} is not divisible by 2"
. With such a description of assertion error message,
a value from XML instance document is assigned to a variable reference
{$value}
which is a defined keyword.
During a validation failure, an error message for this example would be produced as follows,
"The number 3 is not divisible by 2"
(the variable reference
{$value}
is assigned a value 3).
User-defined error messages for assertion failures are not a standard feature of the XML Schema 1.1 specification, and
are a Xerces extension to help XML Schema document authors to specify a problem domain specific error messages.
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
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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