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SAX and more…

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

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SAX and more…

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

  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. <?xmlversion="1.0"?> <!DOCTYPEcountriesSYSTEM"world.dtd"> <countries> <country continent="&as;"> <!--israel--> <name>Israel</name> <population year="2001">6,199,008</population> <city capital="yes"><name>Jerusalem</name></city> <city><name>Ashdod</name></city> </country> <country continent="&eu;"> <name>France</name> <population year="2004">60,424,213</population> </country> </countries>

  4. <?xmlversion="1.0"?> <!DOCTYPEcountriesSYSTEM"world.dtd"> <countries> <country continent="&as;"> <!--israel--> <name>Israel</name> <population year="2001">6,199,008</population> <city capital="yes"><name>Jerusalem</name></city> <city><name>Ashdod</name></city> </country> <country continent="&eu;"> <name>France</name> <population year="2004">60,424,213</population> </country> </countries> Start Document

  5. <?xmlversion="1.0"?> <!DOCTYPEcountriesSYSTEM"world.dtd"> <countries> <country continent="&as;"> <!--israel--> <name>Israel</name> <population year="2001">6,199,008</population> <city capital="yes"><name>Jerusalem</name></city> <city><name>Ashdod</name></city> </country> <country continent="&eu;"> <name>France</name> <population year="2004">60,424,213</population> </country> </countries> Start Element

  6. <?xmlversion="1.0"?> <!DOCTYPEcountriesSYSTEM"world.dtd"> <countries> <country continent="&as;"> <!--israel--> <name>Israel</name> <population year="2001">6,199,008</population> <city capital="yes"><name>Jerusalem</name></city> <city><name>Ashdod</name></city> </country> <country continent="&eu;"> <name>France</name> <population year="2004">60,424,213</population> </country> </countries> Start Element

  7. <?xmlversion="1.0"?> <!DOCTYPEcountriesSYSTEM"world.dtd"> <countries> <country continent="&as;"> <!--israel--> <name>Israel</name> <population year="2001">6,199,008</population> <city capital="yes"><name>Jerusalem</name></city> <city><name>Ashdod</name></city> </country> <country continent="&eu;"> <name>France</name> <population year="2004">60,424,213</population> </country> </countries> Comment

  8. <?xmlversion="1.0"?> <!DOCTYPEcountriesSYSTEM"world.dtd"> <countries> <country continent="&as;"> <!--israel--> <name>Israel</name> <population year="2001">6,199,008</population> <city capital="yes"><name>Jerusalem</name></city> <city><name>Ashdod</name></city> </country> <country continent="&eu;"> <name>France</name> <population year="2004">60,424,213</population> </country> </countries> Start Element

  9. <?xmlversion="1.0"?> <!DOCTYPEcountriesSYSTEM"world.dtd"> <countries> <country continent="&as;"> <!--israel--> <name>Israel</name> <population year="2001">6,199,008</population> <city capital="yes"><name>Jerusalem</name></city> <city><name>Ashdod</name></city> </country> <country continent="&eu;"> <name>France</name> <population year="2004">60,424,213</population> </country> </countries> Characters

  10. <?xmlversion="1.0"?> <!DOCTYPEcountriesSYSTEM"world.dtd"> <countries> <country continent="&as;"> <!--israel--> <name>Israel</name> <population year="2001">6,199,008</population> <city capital="yes"><name>Jerusalem</name></city> <city><name>Ashdod</name></city> </country> <country continent="&eu;"> <name>France</name> <population year="2004">60,424,213</population> </country> </countries> End Element

  11. <?xmlversion="1.0"?> <!DOCTYPEcountriesSYSTEM"world.dtd"> <countries> <country continent="&as;"> <!--israel--> <name>Israel</name> <population year="2001">6,199,008</population> <city capital="yes"><name>Jerusalem</name></city> <city><name>Ashdod</name></city> </country> <country continent="&eu;"> <name>France</name> <population year="2004">60,424,213</population> </country> </countries> End Element

  12. <?xmlversion="1.0"?> <!DOCTYPEcountriesSYSTEM"world.dtd"> <countries> <country continent="&as;"> <!--israel--> <name>Israel</name> <population year="2001">6,199,008</population> <city capital="yes"><name>Jerusalem</name></city> <city><name>Ashdod</name></city> </country> <country continent="&eu;"> <name>France</name> <population year="2004">60,424,213</population> </country> </countries> End Document

  13. <?xml version="1.0"?> . . . SAX Parsers When you see the start of the document do … SAX Parser 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. 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. System Properties • Propertiesare configuration values managed as key/value pairs. In each pair, the key and value are both String values. • 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.

  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 Class

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

  19. Implementing the Content Handler • A SAX parser invokes methods such as startDocument, startElement and endElement of its content handleras 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. 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 Interface

  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 ContentHandlerinterface is to extend DefaultHandler Read more about DefaultHandler Class

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

  23. A Content Handler Example publicvoid startDocument() throws SAXException { print("BEGIN"); } publicvoid endDocument() throws SAXException { print("END"); } publicvoid 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. A Content Handler Example publicvoid endElement(String ns, String lName, String qName) throws SAXException { --depth; print("}"); } publicvoid characters(char buf[], int offset, int len) throws SAXException { String s = newString(buf, offset, len).trim(); ++depth; print(s); --depth; } }

  25. Fixing The Parser publicclass EchoWithSax { publicstaticvoid main(String[] args) throwsException { XMLReader reader = XMLReaderFactory.createXMLReader(); reader.setContentHandler(new EchoHandler()); reader.parse("world.xml"); } } What would happen without this line?

  26. Empty Elements • What do you think happens when the parser parses an empty element? <rating stars="five"/>

  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 #attributes Read more about Attributes Interface

  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) • AnErrorHandler is registered with • reader.setErrorHandler(handler); • Three methods: • void error(SAXParseException ex); • void fatalError(SAXParserExcpetion ex); • void warning(SAXParserException ex);

  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. 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 ) e.g. external DTD Usually appear with external non-xml resources and describe their type Read more about EntityResolver Interface, DTDHandler Interface

  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. 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 Features Read more about Properties

  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 (<!-- comment -->) • The use of an entity is a lexical event (&gt;) • These can be dealt with by implementing the LexicalHandler interface, and setting the lexical-handler property to an instance of the handler

  34. LexicalHandler Methods • comment(char[]ch, intstart, intlength) • startCDATA() • endCDATA() • startEntity(java.lang.Stringname) • endEntity(java.lang.Stringname) • and more... Read more about LexicalHandler Interface

  35. Different Approaches

  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)

  37. <?xmlversion="1.0"?> <!DOCTYPEcountriesSYSTEM"world.dtd"> <countries> <country continent="&as;"> <name>Israel</name> <population year="2001">6,199,008</population> <city capital="yes"><name>Jerusalem</name></city> <city><name>Ashdod</name></city> </country> <country continent="&eu;"> <name>France</name> <population year="2004">60,424,213</population> </country> </countries>

  38. The DOM Tree

  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. 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. 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. 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: htmlheadtitlemetametalinkmeta...

  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. SAX vs. DOM A more detailed comparison

  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. 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. 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. 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. 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)

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