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Introduction
 

This package contains an implementation of the W3C XML Schema Language, a recommendation of the Worldwide Web Consortium available in three parts: XML Schema: Primer and XML Schema: Structures and XML Schema: Datatypes. We consider this implementation to be complete except for the limitations cited below. The implementation has been extensively tested and known problem areas are set out below. In this document we also discuss our interpretation of the specifications where it seems to us particularly likely that others might arrive at a different interpretation. Please read this document before using this package.


Limitations
 
  • Length, minLength, and maxLength facets are limited to the value 2147483647. Items larger than this limit will not be validated correctly.
  • Year and seconds values in date/time datatypes are limited to the value 2147483647. Items larger than this limit will not be validated correctly.

Problem Areas/Known Bugs
 
  • Due to changes in the specification for Decimals in JDK1.3, not all decimals that should be invalid according to the Schema specs may be found by the parser;
  • For larger values of maxOccurs, a StackOverflowError may occur. The workaround for this problem, if your application permits it, is to change the maxOccurs value to unbounded.

Interpretation of Areas that are Unclear or Implementation-Dependent
 
  • QName: the specification does not define the unit of length. We chose to implement the length facet as being in units of XML characters in the value space ( # of chars in namespace URI + ":" + local part );
  • We have interpreted the specs as requiring <keyref> Identity Constraints to refer to <key> or <unique> identity constraints within the scope of the elements to which the <keyref> is attached. This interpretation is at variance with the Schema Primer, which contains an example with a <keyref> declared on an element used inside the element of its corresponding <key>;

Other Notes
 

The parsers contained in this package are able to read and validate XML documents with the grammar specified in either DTD or XML Schema format. The schema is specified by the xsi:schemaLocation or xsi:noNamespaceSchemaLocation attribute on the root element of the document. The xsi prefix must be bound to the Schema document instance namespace, as specified by the Recommendation. See the sample provided in the Usage section.


Usage
 

In this release, schema validation has been integrated with the regular SAXParser and DOMParser classes. No special classes are required to parse documents that use a schema.

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.


PSVI and Schema Component API
 

Xerces provides experimental PSVI support. The PSVI augmentations can be accessed via interfaces in the org.apache.xerces.xni.psvi package.

From within an XMLDocumentHandler, one can retrieve PSVI information when certain methods are called. For example,

import org.apache.xerces.xni.psvi.*;

  ...

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();
    ...
}

Certain PSVI item is only available on certain XMLDocumentHandler methods. For more information, please refer to the API documentation of the PSVI interfaces.

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's used for assessment. These components and the schema itself are represented by interfaces in the org.apache.xerces.impl.xs.psvi 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.xni.psvi.*;
import org.apache.xerces.impl.xs.psvi.*;

  ...

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


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