XML & Databases. Tutorial. 2. Parsing XML. Universität Konstanz. Database & Information Systems Group Prof. Marc H. Scholl

Transcription

1 XML & Databases Tutorial Christian Grün, Database & Information Systems Group University of, Winter 2007/08

2 DOM Document Object Model Idea mapping the whole XML document to main memory The XML Processor parses the XML document constructs the document tree in main memory Your Application explores arbitrary document nodes bla! creates, changes and removes elements <B> <A> <XML> <B> + blabla <A> <XML><A><B>bla!</B><B>blabla</B></A></XML> Seite 2

3 SAX Serial Access Parser Idea serial document parsing The XML Parser sequentially parses the document input sends events whenever XML snippets are found Your Application registers methods which are called by the parser processes the relevant tokens Start Parsing start <XML> start <A> start <B> text bla! end </B> start <B> text blabla end </B> end </A> end <XML> <XML><A><B>bla!</B><B>blabla</B></A></XML> Seite 3

4 DOM vs SAX vs DOM Which one is better? depends on your actual application needs SAX + fast access to document nodes, real-time parsing unidirectional (no backward parsing, limited tests on well-formedness) no document manipulation DOM + complete document model memory overhead (normally 6 8 times bigger than source document) no processing possible before document is completely read Seite 4

5 Java & DOM JAVA Architecture since JDK 1.4: DOM & SAX are included in Java distribution architecture still allows different implementations: W3C s DOM specifications are defined as interfaces in org.w3c.dom helper classes are defined in javax.xml.parsers (JAXP Java API for XML Proc.) Application JAXP java.xml.parsers DOM Spec s org.w3c.dom DOM Implementations Seite 5

6 Java & DOM Example Parsing & Dumping an XML File import javax.xml.parsers.*; import org.w3c.dom.*; class XMLTester { XMLTester() { try { DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance(); DocumentBuilder docbuilder = factory.newdocumentbuilder(); Document doc = docbuilder.parse("input.xml"); System.out.println(doc); catch(exception e) { e.printstacktrace(); Seite 6

7 Java & DOM Data Structure Nodes Node is the primary DOM data type for all node types: Document, Element, Text, Attribute Nodes,... the node type can be determined with (surprise ) getnodetype() a Node instance offers numerous methods to explore the tree: getchildren(), getattributes(), node types are explained in more detail in the Java docs NodeList NodeList objects are simple lists. They contain the following methods: getlength() returns the number of stored nodes the method item(int i) returns the specified node Seite 7

8 Java & DOM Example Reversing Document Nodes public void reverse(document doc) { Stack<Node> stack = new Stack<Node>(); Element root = doc.getdocumentelement(); NodeList elements = root.getchildnodes(); while(elements.getlength() > 0) { elementstack.push(elements.item(0)); root.removechild(elements.item(0)); while(stack.size() > 0) { root.appendchild(stack.pop()); Seite 8

9 Java & SAX JAVA Architecture does not follow W3C specifications extra package org.xml.sax same helper classes as before: package javax.xml.parsers Instantiation similar to DOM: import javax.xml.parsers.*; import org.xml.sax.*;... SAXParserFactory factory = SAXParserFactory.newInstance(); SAXParser parser = factory.newsaxparser(); XMLReader reader = parser.getxmlreader(); reader.parse("input.xml");... Seite 9

10 Java & SAX Events Callback Events the code of the previous slide works correctly but we get no results reason: SAX sends callback events, but we didn't get informed callbacks are realized as Interfaces: ContentHandler, ErrorHandler,... To register implement callbacks in your class: class... implements XYZHandler register them in the XMLReader: reader.setxyzhandler(this); To get callbacks override the available callback functions: startelement(), endelement(),... Seite 10

11 Java & SAX Events Events Handlers four different event handlers exist: ContentHandler, ErrorHandler, DTDHandler, EntityResolver disadvantage of interfaces: all methods have to be implemented the DefaultHandler class provides an empty implementation of all SAX handlers. Error Handler functions for severity levels, defined in the XML specification: warning() dependent on the parser implementation error() errors are found, but parsing is continued fatalerror() parsing is interrupted Seite 11

12 Java & SAX Content Handler Start of a Tag startelement(string URI, String ln, String qname, Attributes ats) qname is the simple tag name ( quick name ) ats contains the list of attributes End of a Tag endelement(string URI, String ln, String qname) Element Content characters(char[] chars, int start, int end) char array with offset to first and last character performance issue function may be called several times for a single content content must be manually merged Seite 12

13 import javax.xml.parsers.*; import org.xml.sax.*; import org.xml.sax.helpers.*; class XMLContentParser extends DefaultHandler { public static void main(string[] args) throws Exception { new XMLContentParser(); XMLContentParser() throws Exception { SAXParserFactory factory = SAXParserFactory.newInstance(); SAXParser parser = factory.newsaxparser(); XMLReader reader = parser.getxmlreader(); reader.setcontenthandler(this); reader.parse("input.xml"); public void startelement(string uri, String ln, String qname, Attributes atts) { System.out.println(qName); Seite 13

14 Java & StAX JAVA Architecture StAX (Streaming API for XML) is included since JDK 1.6 Pull-API: document elements are actively requested from the parser: XMLInputFactory factory = XMLInputFactory.newInstance(); XMLStreamReader parser = factory.createxmlstreamreader(new FileReader("input.xml")); while(parser.hasnext()) { int code = parser.next(); if(code == XMLStreamConstants.START_ELEMENT) { System.out.println("Tag: " + parser.getlocalname());... Seite 14