1. 1 SAX and more… CS 236607, Spring 2008/9

2. 2 SAX Parser SAX = Simple API for XML XML is read sequentially When a parsing event happens, the parser invokes the corresponding method of the corresponding handler The handlers are programmer’s implementation of standard Java API (i.e., interfaces and classes)

3. Israel 6,199,008 Jerusalem Ashdod France 60,424,213

4. Israel 6,199,008 Jerusalem Ashdod France 60,424,213 Start Document

5. Israel 6,199,008 Jerusalem Ashdod France 60,424,213 Start Element

6. Israel 6,199,008 Jerusalem Ashdod France 60,424,213 Start Element

7. Israel 6,199,008 Jerusalem Ashdod France 60,424,213 Comment

8. Israel 6,199,008 Jerusalem Ashdod France 60,424,213 Start Element

9. Israel 6,199,008 Jerusalem Ashdod France 60,424,213 Characters

10. Israel 6,199,008 Jerusalem Ashdod France 60,424,213 End Element

11. Israel 6,199,008 Jerusalem Ashdod France 60,424,213 End Element

12. Israel 6,199,008 Jerusalem Ashdod France 60,424,213 End Document

13. 13 SAX Parsers SAX Parser When you see the start of the document do … When you see the start of an element do … When you see the end of an element do ….

14. Used to create a SAX Parser Handles document events: start tag, end tag, etc. Handles Parser Errors Handles DTD Handles Entities

15. 15 Creating a Parser The SAX interface is an accepted standard There are many implementations of many vendors  Standard API does not include an actual implementation, but Sun provides one with JDK We would like to be able to change the implementation used, without changing any code in the program  How this is done?

16. 16 System Properties Properties are configuration values managed as key/value pairs. In each pair, the key and value are both String values. Properties The System class maintains a Properties object that describes the configuration of the current working environment.  System properties include information about the current user, the current version of the Java runtime, and the character used to separate components of a file path name. Java standard System Properties. Java standard System Properties

17. 17 Factory Design Pattern Have a “factory” class that creates the actual parsers  org.xml.sax.helpers.XMLReaderFactory The factory checks configurations, mainly the value of a system property, that specify the implementation In order to change the implementation, simply change the system property Read more about XMLReaderFactory ClassXMLReaderFactory Class

18. 18 Creating a SAX Parser import org.xml.sax.*; import org.xml.sax.helpers.*; public class EchoWithSax { public static void main(String[] args) throws Exception { System.setProperty("org.xml.sax.driver", "org.apache.xerces.parsers.SAXParser"); XMLReader reader = XMLReaderFactory.createXMLReader(); reader.parse("world.xml"); } Read more about XMLReader Interface, Xerces SAXParser classXMLReader InterfaceXerces SAXParser class Implements XMLReader (not a must) The factory will look for this…

19. 19 Implementing the Content Handler A SAX parser invokes methods such as startDocument, startElement and endElement of its content handler as it runs In order to react to parsing events we must:  implement the ContentHandler interface  set the parser’s content handler with an instance of our ContentHandler implementation

20. 20 ContentHandler Methods startDocument - parsing begins endDocument - parsing ends startElement - an opening tag is encountered endElement - a closing tag is encountered characters - text (CDATA) is encountered ignorableWhitespace - white spaces that should be ignored (according to the DTD) and more... Read more about ContentHandler InterfaceContentHandler Interface

21. 21 The Default Handler The class DefaultHandler implements all handler interfaces (usually, in an empty manner)  i.e., ContentHandler, EntityResolver, DTDHandler, ErrorHandler An easy way to implement the ContentHandler interface is to extend DefaultHandler Read more about DefaultHandler ClassDefaultHandler Class

22. A Content Handler Example import org.xml.sax.helpers.DefaultHandler; import org.xml.sax.*; public class EchoHandler extends DefaultHandler { int depth = 0; public void print(String line) { for(int i=0; i<depth; ++i) System.out.print(" "); System.out.println(line); }

23. 23 A Content Handler Example public void startDocument() throws SAXException { print("BEGIN"); } public void endDocument() throws SAXException { print("END"); } public void startElement(String ns, String localName, String qName, Attributes attrs) throws SAXException { print("Element " + qName + "{"); ++depth; for (int i = 0; i < attrs.getLength(); ++i) print(attrs.getLocalName(i) + "=" + attrs.getValue(i)); } We will discuss this interface later…

24. 24 A Content Handler Example public void endElement(String ns, String lName, String qName) throws SAXException { --depth; print("}"); } public void characters(char buf[], int offset, int len) throws SAXException { String s = new String(buf, offset, len).trim(); ++depth; print(s); --depth; } }

25. 25 Fixing The Parser public class EchoWithSax { public static void main(String[] args) throws Exception { XMLReader reader = XMLReaderFactory.createXMLReader(); reader.setContentHandler(new EchoHandler()); reader.parse("world.xml"); } What would happen without this line?

26. 26 Empty Elements What do you think happens when the parser parses an empty element?

27. 27 Attributes Interface The Attributes interface provides an access to all attributes of an element  getLength(), getQName(i), getValue(i), getType(i), getValue(qname), etc. The following are possible types for attributes: CDATA, ID, IDREF, IDREFS, NMTOKEN, NMTOKENS, ENTITY, ENTITIES, NOTATION Read more about Attributes InterfaceAttributes Interface #attributes

28. 28 ErrorHandler Interface We implement ErrorHandler to receive error events (similar to implementing ContentHandler ) DefaultHandler implements ErrorHandler in an empty fashion, so we can extend it (as before) An ErrorHandler is registered with  reader.setErrorHandler(handler); Three methods:  void error(SAXParseException ex);  void fatalError(SAXParserExcpetion ex);  void warning(SAXParserException ex);

29. 29 Parsing Errors Fatal errors disable the parser from continuing parsing  For example, the document is not well formed, an unknown XML version is declared, etc. Errors (that is recoverable ones) occur for example when the parser is validating and validity constrains are violated Warnings occur when abnormal (yet legal) conditions are encountered  For example, an entity is declared twice in the DTD

30. 30 Usually appear with external non-xml resources and describe their type EntityResolver and DTDHandler The interface EntityResolver enables the programmer to specify a new source for translation of external entities The interface DTDHandler enables the programmer to react to notations (unparsed entities) declarations inside the DTD ( Notation Example )notations Notation Example e.g. external DTD Read more about EntityResolver Interface, DTDHandler InterfaceEntityResolver Interface DTDHandler Interface

31. 31 Features and Properties SAX parsers can be configured by setting their features and properties Syntax:  reader.setFeature("feature-url", boolean)  reader.setProperty("property-url", Object) Standard feature URLs have the form: http://xml.org/sax/features/feature-name Standard property URLs have the form http://xml.org/sax/properties/prop-name

32. 32 Feature/Property Examples Features:  namespaces - are namespaces supported?  validation - does the parser validate (against the declared DTD) ?  http://apache.org/xml/features/nonvalidating/load-external-dtd Ignore the DTD? (spec. to Xerces implementation ) Properties:  lexical-handler - see the next slide... Read more about FeaturesFeatures Read more about PropertiesProperties

33. 33 Lexical Events Lexical events have to do with the way that a document was written and not with its content Examples:  A comment is a lexical event ( )  The use of an entity is a lexical event ( > ) These can be dealt with by implementing the LexicalHandler interface, and setting the lexical- handler property to an instance of the handler

34. 34 LexicalHandler Methods comment(char[] ch, int start, int length) startCDATA() endCDATA() startEntity(java.lang.String name) endEntity(java.lang.String name) and more... Read more about LexicalHandler InterfaceLexicalHandler Interface

35. 35 Different Approaches

36. 36 The DOM Approach Tree-based API : map an XML document into an internal tree structure, then allow an application to navigate that tree. The application is active. Provides random-access. Remember that it is possible to construct a parse tree using an event-based API, and it is possible to use an event-based API to traverse an in-memory tree. (Actually DOM is a level above SAX)

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38. The DOM Tree

39. 39 The SAX Approach Event based API. The Application is passive. Provides Serial access parser. Given the following XML document: This XML document, when passed through a SAX parser, will generate the following sequence of events (Pushing)…

40. XML Processing Instruction, named xml, with attributes version equal to "1.0" and encoding equal to "UTF-8" XML Element start, named RootElement, with an attribute param equal to "value" XML Element start, named FirstElement XML Text node, with data equal to "Some Text" (note: text processing, with regard to spaces, can be changed) XML Element end, named FirstElement XML Element start, named SecondElement, with an attribute param2 equal to "something" XML Text node, with data equal to "Pre-Text" XML Element start, named Inline XML Text node, with data equal to "Inlined text" XML Element end, named Inline XML Text node, with data equal to "Post-text." XML Element end, named SecondElement XML Element end, named RootElement

41. 41 Pull vs. Push SAX is known as a push framework  the parser has the initivative  the programmer must react to events An alternative is a pull framework  the programmer has the initiative  the parser must react to requests

42. 42 StAX - Streaming API for XML An API to read and write XML documents in the Java programming language (not like DOM & SAX that were written for Parsing). The StAX approach: the programmatic entry point is a cursor that represents a point within the document. The application moves the cursor forward - 'pulling' the information from the parser as it needs. Thus, the application is active. Not part of the standard Java API.

43. 43 For example, here's a simple bit of code that iterates through an XML document and prints out the names of the different elements it encounters: Here's the beginning of the output when I ran this across a simple well-formed HTML file: html head title meta meta link meta...

44. 44 JDOM An implementation of generic XML trees in Java. JDOM provides a way to represent XML document for easy and efficient reading, manipulation, and writing. one can construct a tree of elements, then generate a XML file from it, like:

45. 45 SAX vs. DOM A more detailed comparison

46. 46 Parser Efficiency The DOM object built by DOM parsers is usually complicated and requires more memory storage than the XML file itself  A lot of time is spent on construction before use  For some very large documents, this may be impractical SAX parsers store only local information that is encountered during the serial traversal Hence, programming with SAX parsers is, in general, more efficient

47. 47 Programming using SAX is Difficult In some cases, programming with SAX is difficult:  How can we find, using a SAX parser, elements e1 with ancestor e2 ?  How can we find, using a SAX parser, elements e1 that have a descendant element e2 ?  How can we find the element e1 referenced by the IDREF attribute of e2 ?

48. 48 Node Navigation SAX parsers do not provide access to elements other than the one currently visited in the serial (DFS) traversal of the document In particular,  They do not read backwards  They do not enable access to elements by ID or name DOM parsers enable any traversal method Hence, using DOM parsers is usually more comfortable

49. 49 More DOM Advantages You can save time and effort if you send and receive DOM objects instead of XML files  But, DOM object are generally larger than the source DOM parsers provide a natural integration of XML reading and manipulating  e.g., “cut and paste” of XML fragments

50. 50 Which should we use? DOM vs. SAX If your document is very large and you only need a few elements – use SAX If you need to manipulate (i.e., change) the XML – use DOM If you need to access the XML many times – use DOM (assuming the file is not too large)

51. 51 Resources used for this presentation The Hebrew University of Jerusalem – CS Faculty. Wikipedia An Introduction to XML and Web Technologies – Course’s Literature. http://www.saxproject.org/event.html http://www.xml.com/