XML and SAX (A quick overview) ● What is XML? ● What are SAX and DOM? ● Using SAX

What is XML? ● Textfiles built from text content marked up with text tags ● The tags provide meaning ● Although it is similar to HTML, all tags in XML must be well formed ● Start tags must be balanced by end tags... ● Similarly, tags are case sensitive in XML

XML Documents ● XML Documents contain one root node Zippy The Pinehad Politician Clown ● The tags within an XML document form a tree structure. ● All tags must be contained within the root tags.

Tags ● Tags can have attributes ● The tags within an XML document form a tree structure. ● All tags must be contained within the root tags.

DTDs and Schemas ● An XML document can be checked for “validity”. ● What makes a “valid” XML document can be defined within: ● DTD: Document Type Definitions ● Defines which tags must be present and where they can be seen ● Convoluted syntax ● Schema ● Similar to DTDs except they are defined in XML ● DTDs aren't used very much anymore due to the overly complex nature of their syntax ● DTDs and Schemas are NOT necessary to using XML. If you choose to use a validating parser (and validation is enabled), you must have a DTD or Schema.

What are SAX and DOM? ● DOM (Document Object Model) describes a language neutral object model which can represent an XML document. ● The types defined in DOM have been defined using an interface definition language (IDL) as defined by the OMG (Object Management Group) ● DOM parsers parse an XML file into the DOM types. The user/programmer traverses these structures to pull out relevant portions of the document ● SAX (Simple API for XML) is an API for parsing XML documents ● It is an event-based push model of parsing. ● As the document is parsed, events are generated. ● The programmer must create handlers to deal with these events

Using SAX ● To get started with SAX, the developer must obtain a reference to an XMLReader object. This can be obtained in the following ways: XMLReader aReader = XMLReaderFactory.createXMLReader(); XMLReader aReader = XMLReaderFactory.createXMLReader( “org.apache.xerces.parsers.SAXParser”); ● In the first call, the programmer is asking the system for an instance of the default parser. In the second, the programmer is asking for an instance of a specific parser. ● The second call requires that a file xerces.jar is in the CLASSPATH.

Defining Content Handlers ● Once a Reader has been instantiated, it must be provided with a ContentHandler ● The ContentHandler interface defines the events which are generated as a result of parsing the XML document. ● The interface is as follows: setDocumentLocator(Locator locator); startDocument() throws SAXException; endDocument() throws SAXException; startPrefixMapping(String prefix, String uri) throws SAXException; endPrefixMapping(String prefix) throws SAXException; startElement(String namespaceURI, String localName, String qualifiedName, Attributes attrs) throws SAXException; endElement(String namespaceURI, String localName, String qualifiedName) throws SAXException; characters(char[] ch, int start, int length) throws SAXException; ignorableWhitespace(char[] ch, int start, int length) throws SAXException; processingInstruction(String target, String data) throws SAXException; skippedEntity(String name) throws SAXException;

The DefaultHandler Class ● To implement a ContentHandler, you must provide an implementation for all of the methods defined in the ContentHandler interface. ● However, not all methods are necessary for all contexts ● The DefaultHandler class contains a null implementation of all the methods defined in the ContentHandler interface. ● You can subclass DefaultHandler and override only those methods you require. ● Generally, most people provide an implementation for: ● startElement – (Called when an start tag is parsed) ● endElement – (Called when an end tag is parsed) ● characters – (called when text between tags is parsed)

Design Elements to Think about ● When you are doing your 1 st assignment, I recommend that you use SAX ● Think about the design of SAX while you are using it, we will be revisting SAX at a later time ● We will be evaluating its design to see if we can make improvements.