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XML

XML. eXtensible Markup Language. HTML and XML. XML stands for e X tensible M arkup L anguage. HTML is used to mark up text so it can be displayed to users. XML is used to mark up data so it can be processed by computers.

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XML

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  1. XML eXtensible Markup Language

  2. HTML and XML XML stands for eXtensible Markup Language HTML is used to mark up text so it can be displayed to users XML is used to mark up data so it can be processed by computers HTML describes both structure (e.g. <p>, <h2>, <em>) and appearance (e.g. <br>, <font>, <i>) XML describes only content, or “meaning” In XML, we make up our own tags HTML uses a fixed, unchangeable set of tags

  3. HTML and XML, II • HTML and XML look similar, because they are both SGML languages (SGML = Standard Generalized Markup Language) • Both HTML and XML use elements enclosed in tags (e.g. <body>This is an element</body>) • Both use tag attributes (e.g.,<font face="Verdana" size="1" color="red">) • Both use entities(&lt;,&gt;,&amp;,&quot;,&apos;) • More precisely, • HTML is defined in SGML • XML is a (very small) subset of SGML

  4. HTML and XML, III • HTML is for humans • HTML describes web pages • You don’t want to see error messages about web pages you visit • Browsers ignore and/or correct as many HTML errors as they can, so HTML is often sloppy • XML is for computers • XML describes data • The rules are strict and errors are not allowed • In this way, XML is like a programming language • Current versions of most browsers can display XML • However, browser support of XML tends to be inconsistent

  5. XML-related technologies • DTD (Document Type Definition) and XML Schemas are used to define legal XML tags and their attributes for particular purposes • CSS (Cascading Style Sheets) describe how to display HTML or XML in a browser • XSLT (eXtensible Stylesheet Language Transformations) and XPath are used to translate from one form of XML to another • DOM (Document Object Model), SAX (Simple API for XML, and JAXP (Java API for XML Processing) are all APIs for XML parsing.

  6. Example XML document <?xml version="1.0"?> <weatherReport> <date>7/1/2013</date> <city>Ghaziabad</city> <state>UP</state> <country>India</country> <highscale="F">103</high> <low scale="F">70</low> <morning>cloudy</morning> <afternoon>Sunny &amp; hot</afternoon> <evening>Clear and Cooler</evening> </weatherReport> From: XML: A Primer, by Simon St. Laurent

  7. Overall structure • An XML document may start with one or more processing instructions (PIs) or directives: • <?xml version="1.0"?><?xml-stylesheet type="text/xsl" href="ss.xsl"?> • Following the directives, there must be exactly one tag, called the root element, containing all the rest of the XML: • <weatherReport> ...</weatherReport>

  8. XML building blocks • Aside from the directives, an XML document is built from: • elements: high in <high scale="F">103</high> • tags, in pairs:<high scale="F">103</high> • attributes: <high scale="F">103</high> • entities: <afternoon>Sunny&amp; hot</afternoon> • character data, which may be: • parsed (processed as XML)--this is the default -PCDATA • unparsed (all characters stand for themselves) - CDATA

  9. Elements and attributes • Attributes and elements are somewhat interchangeable • Example using just elements: • <name> <first>David</first> <last>Matuszek</last></name> • Example using attributes: • <name first="David" last="Matuszek"></name> • You will find that elements are easier to use in your programs--this is a good reason to prefer them • Attributes often contain metadata, such as unique IDs • Generally speaking, browsers display only elements (values enclosed by tags), not tags and attributes

  10. Well-formed XML • Every element must have both a start tag and an end tag, e.g. <name> ... </name> • But empty elements can be abbreviated: <break />. • XML tags are case sensitive • XML tags may not begin with the letters xml, in any combination of cases • Elements must be properly nested, e.g. not<b><i>bold and italic</b></i> • Every XML document must have one and only one root element • The values of attributes must be enclosed in single or double quotes, e.g. <time unit="days"> • Character data cannot contain < or &

  11. Entities • Five special characters must be written as entities: • &amp; for & (almost always necessary) • &lt; for < (almost always necessary) • &gt; for > (not usually necessary) • &quot; for " (necessary inside double quotes) • &apos; for ' (necessary inside single quotes) • These entities can be used even in places where they are not absolutely required • These are the only predefined entities in XML

  12. XML declaration • The XML declaration looks like this:<?xml version="1.0" encoding="UTF-8" standalone="yes"?> • The XML declaration is not required by browsers, but is required by most XML processors (so include it!) • If present, the XML declaration must be first--not even whitespace should precede it • Note that the brackets are <? and ?> • version="1.0"is required (this is the only version so far) • encoding can be "UTF-8" (ASCII) or "UTF-16" (Unicode), or something else, or it can be omitted • standalone tells whether there is a separate DTD

  13. Processing instructions • PIs (Processing Instructions) may occur anywhere in the XML document (but usually first) • A PI is a command to the program processing the XML document to handle it in a certain way • XML documents are typically processed by more than one program • Programs that do not recognize a given PI should just ignore it • General format of a PI: <?target instructions?> • Example: <?xml-stylesheet type="text/css" href="mySheet.css"?>

  14. Comments • <!-- This is a comment in both HTML and XML --> • Comments can be put anywhere in an XML document • Comments are useful for: • Explaining the structure of an XML document • Commenting out parts of the XML during development and testing • Comments are not elements and do not have an end tag • The blanks after <!-- and before --> are optional • The character sequence -- cannot occur in the comment • The closing bracket must be --> • Comments are not displayed by browsers, but can be seen by anyone who looks at the source code

  15. CDATA • By default, all text inside an XML document is parsed • You can force text to be treated as unparsed character data by enclosing it in <![CDATA[ ... ]]> • Any characters, even & and <, can occur inside a CDATA • Whitespace inside a CDATA is (usually) preserved • The only real restriction is that the character sequence]]>cannot occur inside a CDATA • CDATA is useful when your text has a lot of illegal characters (for example, if your XML document contains some HTML text)

  16. Names in XML • Names (as used for tags and attributes) must begin with a letter or underscore, and can consist of: • Letters • Digits & Special symbols • . (dot) • - (hyphen) • _(underscore) • : (colon) should be used only for namespaces • Combining characters and extenders (not used in English)

  17. Namespaces • Recall that DTDs are used to define the tags that can be used in an XML document • An XML document may reference more than one DTD • Namespaces are a way to specify which DTD defines a given tag • XML, like Java, uses qualified names • This helps to avoid collisions between names • Java: myObject.myVariable • XML: myDTD:myTag • Note that XML uses a colon (:) rather than a dot (.) • <book xmlns:dave="http://www.abc.org/ns">

  18. Review of XML rules • Start with <?xml version="1"?> • XML is case sensitive • You must have exactly one root element that encloses all the rest of the XML • Every element must have a closing tag • Elements must be properly nested • Attribute values must be enclosed in double or single quotation marks • There are only five predeclared entities (&amp; &lt; &gt; &quot; &apos;)

  19. Another well-structured example • <novel> <foreword> <paragraph>This is the great American novel. </paragraph></foreword> <chapter number="1"> <paragraph>It was a dark and stormy night. </paragraph> <paragraph>Suddenly, a shot rang out! </paragraph> </chapter></novel>

  20. novel foreword chapternumber="1" paragraph paragraph paragraph This is the greatAmerican novel. It was a darkand stormy night. Suddenly, a shotrang out! XML as a tree • An XML document represents a hierarchy; a hierarchy is a tree

  21. Valid XML • You can make up your own XML tags and attributes, but... • ...any program that uses the XML must know what to expect! • A DTD (Document Type Definition) defines what tags are legal and where they can occur in the XML • XML is well-structured if it follows the rules given earlier • In addition, XML is valid if it declares a DTD and conforms to that DTD • A DTD can be included in the XML, but is typically a separate document • Errors in XML documents will stop XML programs • Some alternatives to DTDs are XML Schemas.

  22. Viewing XML • XML is designed to be processed by computer programs, not to be displayed to humans • Nevertheless, almost all current browsers can display XML documents • For best results, update your browsers to the newest available versions • Remember: HTML is designed to be viewed,XML is designed to be used

  23. Parsers What is a parser? • A program that analyses the grammatical structure of an input, with respect to a given formal grammar • The parser determines how a sentence can be constructed from the grammar of the language by describing the atomic elements of the input and the relationship among them

  24. XML-Parsing Standards We will consider two parsing methods that implement W3C standards for accessing XML • SAX (Simple API for XML) • event-driven parsing • “serial access” protocol • Read only API • DOM (Document Object Model) • convert XML into a tree of objects • “random access” protocol • Can update XML document (insert/delete nodes)

  25. 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) • Similar to an I/O-Stream, goes in one direction

  26. Sample Data Orders Data in XML: several orders, each with several items each item has a part number and a quantity <orders> <order> <onum>1020</onum> <takenBy>1000</takenBy> <customer>1111</customer> <recDate>10-DEC 94</recDate> <items> <item> <pnum>10506</pnum> <quantity>1</quantity> </item> <item> <pnum>10507</pnum> <quantity>1</quantity> </item> <item> <pnum>10508</pnum> <quantity>2</quantity> </item> <item> <pnum>10509</pnum> <quantity>3</quantity> </item> </items> </order> ... </orders>

  27. Sample Data Orders Data in XML: several orders, each with several items each item has a part number and a quantity Parsing Event <orders> <order> <onum>1020</onum> <takenBy>1000</takenBy> <customer>1111</customer> <recDate>10-DEC 94</recDate> <items> <item> <pnum>10506</pnum> <quantity>1</quantity> </item> startDocument <item> <pnum>10507</pnum> <quantity>1</quantity> </item> <item> <pnum>10508</pnum> <quantity>2</quantity> </item> <item> <pnum>10509</pnum> <quantity>3</quantity> </item> </items> </order> ... </orders> endDocument

  28. Sample Data Orders Data in XML: several orders, each with several items each item has a part number and a quantity <orders> <order> <onum>1020</onum> <takenBy>1000</takenBy> <customer>1111</customer> <recDate>10-DEC-94</recDate> <items> <item> <pnum>10506</pnum> <quantity>1</quantity> </item> <item> <pnum>10507</pnum> <quantity>1</quantity> </item> <item> <pnum>10508</pnum> <quantity>2</quantity> </item> <item> <pnum>10509</pnum> <quantity>3</quantity> </item> </items> </order> ... </orders> endElement startElement

  29. Sample Data Orders Data in XML: several orders, each with several items each item has a part number and a quantity <orders> <order> <onum>1020</onum> <takenBy>1000</takenBy> <customer>1111</customer> <recDate>10-DEC-94</recDate> <items> <item> <pnum>10506</pnum> <quantity>1</quantity> </item> <item> <pnum>10507</pnum> <quantity>1</quantity> </item> <item> <pnum>10508</pnum> <quantity>2</quantity> </item> <item> <pnum>10509</pnum> <quantity>3</quantity> </item> </items> </order> ... </orders> characters

  30. <?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 …

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

  32. SAX API Two important classes in the SAX API: SAXParserandHandlerBase. A new SAXParser object is created by: public SAXParser() Register a SAX handler with the parser object to receive notifications of various parser events by: public void setDocumentHandler(DocumentHandler h) A similar method is available to register an error handler: public void setErrorHandler(ErrorHandler h)

  33. SAX API - Continued • The HandlerBase class is a default base class for all handlers. • It implements the default behavior for the various handlers. • Application writers extend this class by rewriting the following event handler methods: public void startDocument() throws SAXException public void endDocument() throws SAXException public void startElement() throws SAXException public void endElement() throws SAXException public void characters() throws SAXException public void warning() throws SAXException public void error() throws SAXException

  34. Creating a SAX Parser import org.xml.sax.*; import oracle.xml.parser.v2.SAXParser; public class SampleApp extends HandlerBase { // Global variables declared here static public void main(String [] args){ Parser parser = new SAXParser(); SampleApp app = new SampleApp(); parser.setDocumentHandler(app); parser.setErrorHandler(app); try { parser.parse(createURL(args[0]).toString()); } catch (SAXParseException e) { System.out.println(e.getMessage()); } } }

  35. DOM Parser • DOM = Document Object Model • Parser creates a tree object out of the document • User accesses data by traversing the tree • The tree and its traversal conform to a W3C standard • The API allows for constructing, accessing and manipulating the structure and content of XML documents

  36. <?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>

  37. The DOM Tree

  38. Application API XML File DOM Parser DOM Tree Using a DOM Tree

  39. The Node Interface • The nodes of the DOM tree include • a special root (denoted document) • element nodes • text nodes and CDATA sections • attributes • comments • and more ... • Every node in the DOM tree implements the Node interface

  40. Node Navigation • Every node has a specific location in tree • Node interface specifies methods for tree navigation • Node getFirstChild(); • Node getLastChild(); • Node getNextSibling(); • Node getPreviousSibling(); • Node getParentNode(); • NodeList getChildNodes(); • NamedNodeMap getAttributes()

  41. Node Navigation (cont) getPreviousSibling() getFirstChild() getChildNodes() getParentNode() getLastChild() getNextSibling()

  42. DOM Parsing Example Consider the following XML data file describing geographical information about states in U.S. <?xml version="1.0" standalone="no"?> <!DOCTYPE geography SYSTEM "geo.dtd"> <geography> <state id="georgia"> <scode>GA</scode> <sname>Georgia</sname> <capital idref="atlanta"/> <citiesin idref="atlanta"/> <citiesin idref="columbus"/> <citiesin idref="savannah"/> <citiesin idref="macon"/> <nickname>Peach State</nickname> <population>6478216</population> </state> ... <city id="atlanta"> <ccode>ATL</ccode> <cname>Atlanta</cname> <stateof idref="georgia"/> </city> ... </geography>

  43. Geography XML Data DTD <!ELEMENT geography (state|city)*> <!ELEMENT state (scode, sname, capital, citiesin*, nickname, population)> <!ATTLIST state id ID #REQUIRED> <!ELEMENT scode (#PCDATA)> <!ELEMENT sname (#PCDATA)> <!ELEMENT capital EMPTY> <!ATTLIST capital idref IDREF #REQUIRED> <!ELEMENT citiesin EMPTY> <!ATTLIST citiesin idref IDREF #REQUIRED> <!ELEMENT nickname (#PCDATA)> <!ELEMENT population (#PCDATA)> <!ELEMENT city (ccode, cname, stateof)> <!ATTLIST city id ID #IMPLIED> <!ELEMENT ccode (#PCDATA)> <!ELEMENT cname (#PCDATA)> <!ELEMENT stateof EMPTY> <!ATTLIST stateof idref IDREF #REQUIRED>

  44. Geography Data: SQL Schema (Oracle Objects) create type city_type as object ( ccode varchar2(15), cname varchar2(50) ); create type cities_in_table as table of city_type; create table state ( scode varchar2(15), sname varchar2(50), nickname varchar2(100), population number(30), capital city_type, cities_in cities_in_table, primary key (scode)) nested table cities_in store as cities_tab;

  45. Create DOM Parser Object import org.w3c.dom.*; import org.w3c.dom.Node; import oracle.xml.parser.v2.*; public class CreateGeoData { static public void main(String[] argv) throws SQLException { // Get an instance of the parser DOMParser parser = new DOMParser(); // Set various parser options: validation on, // warnings shown, error stream set to stderr. parser.setErrorStream(System.err); parser.setValidationMode(true); parser.showWarnings(true); // Parse the document. parser.parse(url); // Obtain the document. XMLDocument doc = parser.getDocument();

  46. Traverse DOM Tree NodeList sl = doc.getElementsByTagName("state"); NodeList cl = doc.getElementsByTagName("city"); XMLNode e = (XMLNode) sl.item(j); scode = e.valueOf("scode"); sname = e.valueOf("sname"); nickname = e.valueOf("nickname"); population = Long.parseLong(e.valueOf("population")); XMLNode child = (XMLNode) e.getFirstChild(); while (child != null) { if (child.getNodeName().equals("capital")) break; child = (XMLNode) child.getNextSibling(); } NamedNodeMap nnm = child.getAttributes(); XMLNode n = (XMLNode) nnm.item(0);

  47. Node Manipulation • Children of a node in a DOM tree can be manipulated - added, edited, deleted, moved, copied, etc. • To constructs new nodes, use the methods of Document • createElement,createAttribute, createTextNode, createCDATASection etc. • To manipulate a node, use the methods of Node • appendChild, insertBefore, removeChild,replaceChild, setNodeValue, cloneNode(boolean deep)etc.

  48. SAX vs DOM Parsing: 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

  49. 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?

  50. 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

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