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XML Module

Overview of the XML architecture in Qt

The XML module provides a well-formed XML parser using the SAX2 (Simple API for XML) interface plus an implementation of the DOM Level 2 (Document Object Model).

SAX is an event-based standard interface for XML parsers. The Qt interface follows the design of the SAX2 Java implementation. Its naming scheme was adapted to fit the Qt naming conventions. Details on SAX2 can be found at http://www.megginson.com/SAX/.

Support for SAX2 filters and the reader factory are under development. The Qt implementation does not include the SAX1 compatibility classes present in the Java interface.

For an introduction to Qt's SAX2 classes see "The Qt SAX2 classes".

DOM Level 2 is a W3C Recommendation for XML interfaces that maps the constituents of an XML document to a tree structure. Details and the specification of DOM Level 2 can be found at http://www.w3.org/DOM/. More information about the DOM classes in Qt is provided in the Qt DOM classes.

Qt provides the following XML related classes:

Class Short description
QDomAttr Represents one attribute of a QDomElement
QDomCDATASection Represents an XML CDATA section
QDomCharacterData Represents a generic string in the DOM
QDomComment Represents an XML comment
QDomDocument The representation of an XML document
QDomDocumentFragment Tree of QDomNodes which is usually not a complete QDomDocument
QDomDocumentType The representation of the DTD in the document tree
QDomElement Represents one element in the DOM tree
QDomEntity Represents an XML entity
QDomEntityReference Represents an XML entity reference
QDomImplementation Information about the features of the DOM implementation
QDomNamedNodeMap Collection of nodes that can be accessed by name
QDomNode The base class for all nodes of the DOM tree
QDomNodeList List of QDomNode objects
QDomNotation Represents an XML notation
QDomProcessingInstruction Represents an XML processing instruction
QDomText Represents textual data in the parsed XML document
QXmlAttributes XML attributes
QXmlContentHandler Interface to report logical content of XML data
QXmlDeclHandler Interface to report declaration content of XML data
QXmlDefaultHandler Default implementation of all XML handler classes
QXmlDTDHandler Interface to report DTD content of XML data
QXmlEntityResolver Interface to resolve extern entities contained in XML data
QXmlErrorHandler Interface to report errors in XML data
QXmlInputSource The input data for the QXmlReader subclasses
QXmlLexicalHandler Interface to report lexical content of XML data
QXmlLocator The XML handler classes with information about the actual parsing position
QXmlNamespaceSupport Helper class for XML readers which want to include namespace support
QXmlParseException Used to report errors with the QXmlErrorHandler interface
QXmlReader Interface for XML readers (i.e. for SAX2 parsers)
QXmlSimpleReader Implementation of a simple XML reader (a SAX2 parser)

The Qt SAX2 classes

Introduction to SAX2

The SAX2 interface is an event-driven mechanism to provide the user with document information. An "event" in this context means something reported by the parser, for example, it has encountered a start tag, or an end tag, etc.

To make it less abstract consider the following example:

 
<quote>A quotation.</quote>

Whilst reading (a SAX2 parser is usually referred to as "reader") the above document three events would be triggered:

  1. A start tag occurs (<quote>).
  2. Character data (i.e. text) is found, "A quotation.".
  3. An end tag is parsed (</quote>).

Each time such an event occurs the parser reports it; you can set up event handlers to respond to these events.

Whilst this is a fast and simple approach to read XML documents, manipulation is difficult because data is not stored, simply handled and discarded serially. The DOM interface reads in and stores the whole document in a tree structure; this takes more memory, but makes it easier to manipulate the document's structure..

The Qt XML module provides an abstract class, QXmlReader, that defines the interface for potential SAX2 readers. Qt includes a reader implementation, QXmlSimpleReader, that is easy to adapt through subclassing.

The reader reports parsing events through special handler classes:

Handler class Description
QXmlContentHandler Reports events related to the content of a document (e.g. the start tag or characters).
QXmlDTDHandler Reports events related to the DTD (e.g. notation declarations).
QXmlErrorHandler Reports errors or warnings that occurred during parsing.
QXmlEntityResolver Reports external entities during parsing and allows users to resolve external entities themselves instead of leaving it to the reader.
QXmlDeclHandler Reports further DTD related events (e.g. attribute declarations).
QXmlLexicalHandler Reports events related to the lexical structure of the document (the beginning of the DTD, comments etc.).

These classes are abstract classes describing the interface. The QXmlDefaultHandler class provides a "do nothing" default implementation for all of them. Therefore users only need to overload the QXmlDefaultHandler functions they are interested in.

To read input XML data a special class QXmlInputSource is used.

Apart from those already mentioned, the following SAX2 support classes provide additional useful functionality:

Class Description
QXmlAttributes Used to pass attributes in a start element event.
QXmlLocator Used to obtain the actual parsing position of an event.
QXmlNamespaceSupport Used to implement namespace support for a reader. Note that namespaces do not change the parsing behavior. They are only reported through the handler.

Features

The behaviour of an XML reader depends on its support for certain optional features. For example, a reader may have the feature "report attributes used for namespace declarations and prefixes along with the local name of a tag". Like every other feature this has a unique name represented by a URI: it is called http://xml.org/sax/features/namespace-prefixes.

The Qt SAX2 implementation can report whether the reader has particular functionality using the QXmlReader::hasFeature() function. Available features can be tested with QXmlReader::feature(), and switched on or off using QXmlReader::setFeature().

Consider the example

 
<document xmlns:book = 'http://trolltech.com/fnord/book/'
          xmlns      = 'http://trolltech.com/fnord/' >
A reader that does not support the http://xml.org/sax/features/namespace-prefixes feature would report the element name document but not its attributes xmlns:book and xmlns with their values. A reader with the feature http://xml.org/sax/features/namespace-prefixes reports the namespace attributes if the feature is switched on.

Other features include http://xml.org/sax/features/namespace (namespace processing, implies http://xml.org/sax/features/namespace-prefixes) and http://xml.org/sax/features/validation (the ability to report validation errors).

Whilst SAX2 leaves it to the user to define and implement whatever features are required, support for http://xml.org/sax/features/namespace (and thus http://xml.org/sax/features/namespace-prefixes) is mandantory. The QXmlSimpleReader implementation of QXmlReader, supports them, and can do namespace processing.

QXmlSimpleReader is not validating, so it does not support http://xml.org/sax/features/validation.

Namespace support via features

As we have seen in the previous section we can configure the behavior of the reader when it comes to namespace processing. This is done by setting and unsetting the http://xml.org/sax/features/namespaces and http://xml.org/sax/features/namespace-prefixes features.

They influence the reporting behavior in the following way:

  1. Namespace prefixes and local parts of elements and attributes can be reported.
  2. The qualified names of elements and attributes are reported.
  3. QXmlContentHandler::startPrefixMapping() and QXmlContentHandler::endPrefixMapping() are called by the reader.
  4. Attributes that declare namespaces (i.e. the attribute xmlns and attributes starting with xmlns:) are reported.

Consider the following element:

<author xmlns:fnord = 'http://trolltech.com/fnord/'
             title="Ms" 
             fnord:title="Goddess" 
             name="Eris Kallisti"/>
With http://xml.org/sax/features/namespace-prefixes set to TRUE the reader will report four attributes; but with the namespace-prefixes feature set to FALSE only three, with the xmlns:fnord attribute defining a namespace being "invisible" to the reader.

The http://xml.org/sax/features/namespaces feature is responsible for reporting local names, namespace prefixes and URIs. With http://xml.org/sax/features/namespaces set to TRUE the parser will report title as the local name of the fnord:title attribute, fnord being the namespace prefix and http://trolltech.com/fnord/ as the namespace URI. When http://xml.org/sax/features/namespaces is FALSE none of them are reported.

In the current implementation the Qt XML classes follow the definition that the prefix xmlns itself isn't associated with any namespace at all (see http://www.w3.org/TR/1999/REC-xml-names-19990114/#ns-using). Therefore even with http://xml.org/sax/features/namespaces and http://xml.org/sax/features/namespace-prefixes both set to TRUE the reader won't return either a local name, a namespace prefix or a namespace URI for xmlns:fnord.

This might be changed in the future following the W3C suggestion http://www.w3.org/2000/xmlns/ to associate xmlns with the namespace http://www.w3.org/2000/xmlns.

As the SAX2 standard suggests, QXmlSimpleReader defaults to having http://xml.org/sax/features/namespaces set to TRUE and http://xml.org/sax/features/namespace-prefixes set to FALSE. When changing this behavior using QXmlSimpleReader::setFeature() note that the combination of both features set to FALSE is illegal.

For a practical demonstration of how the two features affect the output of the reader run the tagreader with features example.

Summary

QXmlSimpleReader implements the following behavior:

(namespaces, namespace-prefixes) Namespace prefix and local part Qualified names Prefix mapping xmlns attributes
(TRUE, FALSE) Yes Yes* Yes No
(TRUE, TRUE) Yes Yes Yes Yes
(FALSE, TRUE) No* Yes No* Yes
(FALSE, FALSE) Illegal

* The behavior of these entries is not specified by SAX.

Properties

Properties are a more general concept. They have a unique name, represented as an URI, but their value is void*. Thus nearly anything can be used as a property value. This concept involves some danger, though: there is no means of ensuring type-safety; the user must take care that they pass the right type. Properties are useful if a reader supports special handler classes.

The URIs used for features and properties often look like URLs, e.g. http://xml.org/sax/features/namespace. This does not mean that the data required is at this address. It is simply a way of defining unique names.

Anyone can define and use new SAX2 properties for their readers. Property support is not mandatory.

To set or query properties the following functions are provided: QXmlReader::setProperty(), QXmlReader::property() and QXmlReader::hasProperty().

Further reading

More information about XML (e.g. namespaces) can be found in the introduction to the Qt XML module.

The Qt DOM classes

Introduction to DOM

DOM provides an interface to access and change the content and structure of an XML file. It makes a hierarchical view of the document (a tree view). Thus -- in contrast to the SAX2 interface -- an object model of the document is resident in memory after parsing which makes manipulation easy.

All DOM nodes in the document tree are subclasses of QDomNode. The document itself is represented as a QDomDocument object.

Here are the available node classes and their potential child classes:

With QDomNodeList and QDomNamedNodeMap two collection classes are provided: QDomNodeList is a list of nodes, and QDomNamedNodeMap is used to handle unordered sets of nodes (often used for attributes).

The QDomImplementation class allows the user to query features of the DOM implementation.

Further reading

To get started please refer to the QDomDocument documentation.

An introduction to namespaces

Parts of the Qt XML module documentation assume that you are familiar with XML namespaces. Here we present a brief introduction; skip to Qt XML documentation conventions if you already know this material.

Namespaces are a concept introduced into XML to allow a more modular design. With their help data processing software can easily resolve naming conflicts in XML documents.

Consider the following example:

<document>
<book>
  <title>Practical XML</title>
  <author title="Ms" name="Eris Kallisti"/>
  <chapter>
    <title>A Namespace Called fnord</title>
  </chapter>
</book>
</document>

Here we find three different uses of the name title. If you wish to process this document you will encounter problems because each of the titles should be displayed in a different manner -- even though they have the same name.

The solution would be to have some means of identifying the first occurrence of title as the title of a book, i.e. to use the title element of a book namespace to distinguish it from, for example, the chapter title, e.g.:

<book:title>Practical XML</book:title>

book in this case is a prefix denoting the namespace.

Before we can apply a namespace to element or attribute names we must declare it.

Namespaces are URIs like http://trolltech.com/fnord/book/. This does not mean that data must be available at this address; the URI is simply used to provide a unique name.

We declare namespaces in the same way as attributes; strictly speaking they are attributes. To make for example http://trolltech.com/fnord/ the document's default XML namespace xmlns we write

xmlns="http://trolltech.com/fnord/"

To distinguish the http://trolltech.com/fnord/book/ namespace from the default, we must supply it with a prefix:

xmlns:book="http://trolltech.com/fnord/book/"

A namespace that is declared like this can be applied to element and attribute names by prepending the appropriate prefix and a ":" delimiter. We have already seen this with the book:title element.

Element names without a prefix belong to the default namespace. This rule does not apply to attributes: an attribute without a prefix does not belong to any of the declared XML namespaces at all. Attributes always belong to the "traditional" namespace of the element in which they appear. A "traditional" namespace is not an XML namespace, it simply means that all attribute names belonging to one element must be different. Later we will see how to assign an XML namespace to an attribute.

Due to the fact that attributes without prefixes are not in any XML namespace there is no collision between the attribute title (that belongs to the author element) and for example the title element within a chapter.

Let's clarify this with an example:

<document xmlns:book = 'http://trolltech.com/fnord/book/'
          xmlns      = 'http://trolltech.com/fnord/' >
<book>
  <book:title>Practical XML</book:title>
  <book:author xmlns:fnord = 'http://trolltech.com/fnord/'
               title="Ms"
               fnord:title="Goddess"
               name="Eris Kallisti"/>
  <chapter>
    <title>A Namespace Called fnord</title>
  </chapter>
</book>
</document>

Within the document element we have two namespaces declared. The default namespace http://trolltech.com/fnord/ applies to the book element, the chapter element, the appropriate title element and of course to document itself.

The book:author and book:title elements belong to the namespace with the URI http://trolltech.com/fnord/book/.

The two book:author attributes title and name have no XML namespace assigned. They are only members of the "traditional" namespace of the element book:author, meaning that for example two title attributes in book:author are forbidden.

In the above example we circumvent the last rule by adding a title attribute from the http://trolltech.com/fnord/ namespace to book:author: the fnord:title comes from the namespace with the prefix fnord that is declared in the book:author element.

Clearly the fnord namespace has the same namespace URI as the default namespace. So why didn't we simply use the default namespace we'd already declared? The answer is quite complex:

With the Qt XML classes elements and attributes can be accessed in two ways: either by refering to their qualified names consisting of the namespace prefix and the "real" name (or local name) or by the combination of local name and namespace URI.

More information on XML namespaces can be found at http://www.w3.org/TR/REC-xml-names/.

Conventions used in Qt XML documentation

The following terms are used to distinguish the parts of names within the context of namespaces:

Elements without a ":" (like chapter in the example) do not have a namespace prefix. In this case the local part and the qualified name are identical (i.e. chapter).


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Qt 3.2.1