Parsing XML into programming languages

1. Parsing XML into programming languages JAXP, DOM, SAX, JDOM/DOM4J, Xerces, Xalan, JAXB

2. Parsing XML • Goal: read XML files into data structures in programming languages • Possible strategies • Parse by hand with some reusable libraries • Parse into generic tree structure • Parse as sequence of events • Automagically parse to language-specific objects

3. Parsing by-hand • Advantages • Complete control • Good if simple needs – build off of regex package • Disadvantages • Must write the initial code yourself, even if it becomes generalized • Pretty tedious and error prone. • Gets very hard when using schema or DTD to validate

4. Parsing into generic tree structure • Advantages • Industry-wide, language neutral standard exists called DOM (Document Object Model) • Learning DOM for one language makes it easy to learn for any other • As of JAXP 1.2, support for Schema • Have to write much less code to get XML to something you want to manipulate in your program • Disadvantages • Non-intuitive API, doesn’t take full advantage of Java • Still quite a bit of work

5. What is JAXP? • JAXP: Java API for XML Processing • In the Java language, the definition of these standard API’s (together with XSLT API) comprise a set of interfaces known as JAXP • Java also provides standard implementations together with vendor pluggability layer • Some of these come standard with J2SDK, others are only available with Web Services Developers Pack • We will study these shortly

6. Another alternative • JDOM: Native Java published API for representing XML as tree • Like DOM but much more Java-specific, object oriented • However, not supported by other languages • Also, no support for schema • Dom4j another alternative

7. JAXB • JAXB: Java API for XML Bindings • Defines an API for automagically representing XML schema as collections of Java classes. • Most convenient for application programming • Will cover next class

8. DOM

9. About DOM • Stands for Document Object Model • A World Wide Web Consortium (w3c) standard • Standard constantly adding new features – Level 3 Core just released in the past six months • Well cover most of the basics. There’s always more, and it’s always changing.

10. DOM abstraction layer in Java -- architecture Emphasis is on allowing vendors to supply their own DOM Implementation without requiring change to source code Returns specific parser implementation org.w3d.dom.Document

11. Sample Code A factory instance is the parser implementation. Can be changed with runtime System property. Jdk has default. Xerces much better. DocumentBuilderFactor factory = DocumentBuilderFactory.newInstance(); /* set some factory options here */ DocumentBuilder builder = factory.newDocumentBuilder(); Document doc = builder.parse(xmlFile); From the factory one obtains an instance of the parser xmlFile can be an java.io.File, an inputstream, etc. javax.xml.parsers.DocumentBuilderFactory javax.xml.parsers.DocumentBuilder org.w3c.dom.Document For reference. Notice that the Document class comes from the w3c-specified bindings.

12. Validation • Note that by default the parser will not validate against a schema or DTD • As of JAXP1.2, java provides a default parser than can handle most schema features • See next slide for details on how to setup

13. Important: Schema validation String JAXP_SCHEMA_LANGUAGE =      "http://java.sun.com/xml/jaxp/properties/schemaLanguage"; String W3C_XML_SCHEMA =      "http://www.w3.org/2001/XMLSchema"; Next, you need to configure DocumentBuilderFactory to generate a namespace-aware, validating parser that uses XML Schema: … DocumentBuilderFactory factory =     DocumentBuilderFactory.newInstance() factory.setNamespaceAware(true);    factory.setValidating(true); try {    factory.setAttribute(JAXP_SCHEMA_LANGUAGE, W3C_XML_SCHEMA); } catch (IllegalArgumentException x) {    // Happens if the parser does not support JAXP 1.2   ... }

14. Associating document with schema • An xml file can be associated with a schema in two ways • Directly in xml file in regular way • Programmatically from java • Latter is done as: • factory.setAttribute(JAXP_SCHEMA_SOURCE,    new File(schemaSource));

15. A few notes • Factory allows ease of switching parser implementations • Java provides simple DOM implementation, but much better to use vendor-supplied when doing serious work • Xerces, part of apache project, is installed on cluster as Eclipse plugin. We’ll use next week. • Note that some properties are not supported by all parser implementations.

16. Document object • Once a Document object is obtained, rich API to manipulate. • First call is usually Element root = doc.getDocumentElement(); This gets the root element of the Document as an instance of the Element class • Note that Element subclasses Node and has methods getType(), getName(), and getValue(), and getChildNodes()

17. Types of Nodes • Note that there are many types of Nodes (ie subclasses of Node: Attr, CDATASection, Comment, Document, DocumentFragment, DocumentType, Element, Entity, EntityReference, Notation, ProcessingInstruction, Text Each of these has a special and non-obvious associated type, value, and name. Standards are language-neutral and are specified on chart on following slide Important: keep this chart nearby when using DOM

19. Transforming XML

20. The JAXP Transformation Packages • JAXP Transformation APIs: • javax.xml.transform • This package defines the factory class you use to get a Transformer object. You then configure the transformer with input (Source) and output (Result) objects, and invoke its transform() method to make the transformation happen. The source and result objects are created using classes from one of the other three packages. • javax.xml.transform.dom • Defines the DOMSource and DOMResult classes that let you use a DOM as an input to or output from a transformation. • javax.xml.transform.sax • Defines the SAXSource and SAXResult classes that let you use a SAX event generator as input to a transformation, or deliver SAX events as output to a SAX event processor. • javax.xml.transform.stream • Defines the StreamSource and StreamResult classes that let you use an I/O stream as an input to or output from a transformation.

21. Transformer Architecture

22. Writing DOM to XML public class WriteDOM{ public static void main(String[] argv) throws Exception{ File f = new File(argv[0]); DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance(); DocumentBuilder builder = factory.newDocumentBuilder(); Document document = builder.parse(f); TransformerFactory tFactory = TransformerFactory.newInstance(); Transformer transformer = tFactory.newTransformer(); DOMSource source = new DOMSource(document); StreamResult result = new StreamResult(System.out); transformer.transform(source, result); } }

23. Creating a DOM from scratch • Sometimes you may want to create a DOM tree directly in memory. This is done with: DocumentBuilderFactory factory =  DocumentBuilderFactory.newInstance();          DocumentBuilder builder =         factory.newDocumentBuilder();         document = builder.newDocument();

24. Manipulating Nodes • Once the root node is obtained, typical tree methods exist to manipulate other elements: boolean node.hasChildNodes() NodeList node.getChildNodes() Node node.getNextSibling() Node node.getParentNode() String node.getValue(); String node.getName(); String node.getText(); void setNodeValue(String nodeValue); Node insertBefore(Node new, Node ref);

25. SAX Simple API for XML Processing

26. About SAX • SAX in Java is hosted on source forge • SAX is not a w3c standard • Originated purely in Java • Other languages have chosen to implement in their own ways based on this prototype

27. SAX vs. … • Please don’t compare unrelated things: • SAX is an alternative to DOM, but realize that DOM is often built on top of SAX • SAX and DOM do not compete with JAXP • They do both compete with JAXB implementations

28. How a SAX parser works • SAX parser scans an xml stream on the fly and responds to certain parsing events as it encounters them. • This is very different than digesting an entire XML document into memory. • Much faster, requires less memory. • However, need to reparse if you need to revisit data.

29. Obtaining a SAX parser • Important classes javax.xml.parsers.SAXParserFactory; javax.xml.parsers.SAXParser; javax.xml.parsers.ParserConfigurationException; //get the parser SAXParserFactory factory = SAXParserFactory.newInstance(); SAXParser saxParser = factory.newSAXParser(); //parse the document saxParser.parse( new File(argv[0]), handler);

30. DefaultHandler • Note that an event handler has to be passed to the SAX parser. • This must implement the interface org.xml.sax.ContentHandler; • Easier to extend the adapter org.xml.sax.helpers.DefaultHandler

31. Overriding Handler methods • Most important methods to override • void startDocument() • Called once when document parsing begins • void endDocument() • Called once when parsing ends • void startElement(...) • Called each time an element begin tag is encountered • void endElement(...) • Called each time an element end tag is encountered • void characters(...) • Called randomly between startElement and endElement calls to accumulated character data

32. startElement • public void startElement( String namespaceURI, //if namespace assoc String sName, //nonqualified name String qName, //qualified name Attributes attrs) //list of attributes • Attribute info is obtained by querying Attributes objects.

33. Characters • public void characters( char buf[], //buffer of chars accumulated int offset, //begin element of chars int len) //number of chars • Note, characters may be called more than once between begin tag / end tag • Also, mixed-content elements require careful handling

34. Entity references • Recall that entity references are special character sequences for referring to characters that have special meaning in XML syntax • ‘<‘ is &lt • ‘>’ is &gt • In SAX these are automatically converted and passed to the characters stream unless they are part of a CDATA section

35. Choosing a Parser • Choosing your Parser Implementation • If no other factory class is specified, the default SAXParserFactory class is used. To use a different manufacturer's parser, you can change the value of the environment variable that points to it. You can do that from the command line, like this: • java -Djavax.xml.parsers.SAXParserFactory=yourFactoryHere ... • The factory name you specify must be a fully qualified class name (all package prefixes included). For more information, see the documentation in the newInstance() method of the SAXParserFactory class.

36. Validating SAX Parsers String JAXP_SCHEMA_LANGUAGE =      "http://java.sun.com/xml/jaxp/properties/schemaLanguage"; String W3C_XML_SCHEMA =      "http://www.w3.org/2001/XMLSchema"; Next, you need to configure DocumentBuilderFactory to generate a namespace-aware, validating parser that uses XML Schema: … SaxParserFactory factory =     SaxParserFactory.newInstance() factory.setNamespaceAware(true);    factory.setValidating(true); try {    factory.setAttribute(JAXP_SCHEMA_LANGUAGE, W3C_XML_SCHEMA); } catch (IllegalArgumentException x) {    // Happens if the parser does not support JAXP 1.2   ... }

37. Transforming arbitrary data structures using SAX and Transformer

38. Goal • Now that we know SAX and a little about Transformations, there are some cool things we can do. • One immediate thing is to create xml files from plain text files using the help of a faux SAX parser • Turns out to be more robust than doing by hand

39. Transformers • Recall that transformers easily let us go between any source and result by arbitrary wirings of • StreamSource / StreamResult • SAXSource / SAXResult • DOMSource / DOMResult • We used this to write a DOM tree to an XML file • Now we will use a SAXSource together with a StreamResult to convert our text file

40. Strategy • We construct our own SAXParser – ie a class that implements the XMLReader interface • This class must have a parse method (among others) • We use parse to read our input file and fire the appropriate SAX events.

41. What? • What are we really doing here? • We’re having the SAXParser pretend as though it has encountered certain SAX XML events when it reads the text file. • Exactly where we pretend these things occur is where the appropriate XML will get written by the transformer

42. Main snippet public static void main (String argv []){ StudentReader parser = new StudentReader(); TransformerFactory tFactory = TransformerFactory.newInstance(); Transformer transformer = tFactory.newTransformer(); FileReader fr = new FileReader(“students.txt”); BufferedReader br = new BufferedReader(fr); InputSource inputSource = new InputSource(fr); SAXSource source = new SAXSource(parser, inputSource); StreamResult result = new StreamResult(System.out); transformer.transform(source, result); } Create SAX “parser” create transformer Use text File as Transformer source Use text as result

43. XMLReader implementation • To have a valid SAXSource we need a class that implements • XMLReader interface • public void parse(InputSource input) • public void setContentHandler(ContentHandler handler) • public ContentHandler getContentHandler() • . • . • . • Shown are the important methods for a simple app

44. End