xml and java: xml, dom4j and xpath – eran toch methodologies in information system development...
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XML and Java: XML, dom4j and Xpath – Eran TochMethodologies in Information System Development
Tutorial: Tutorial: Introduction to XML and Java:Introduction to XML and Java:
XML, dom4j and XPathXML, dom4j and XPath
Eran Toch
Methodologies in the Development of Information Systems
December 2003
XML and Java: XML, dom4j and Xpath – Eran TochMethodologies in Information System Development
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SourcesSources
• Major Sources:– http://www.cis.upenn.edu/~cis550/slides/xml.ppt
CIS550 Course Notes, U. Penn, source for many slides
– http://www.cs.technion.ac.il/~oshmu/236804 - Seminar in Computer Science 4: XML - Technology, Systems and Theory
– http://dom4j.org
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AgendaAgenda
• Short Introduction to XML– What is XML– Structure and Terminology– JAVA APIs for XML: an Overview
• dom4j– Parsing an XML document– Writing to an XML document
• Xpath– Xpath Queries– Xpath in dom4j
• References
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The Structure of XMLThe Structure of XML
• XML consists of tags and text
• Tags come in pairs <date> ...</date>
• They must be properly nested
<date> <day> ... </day> ... </date> --- good
<date> <day> ... </date>... </day> --- bad
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XML textXML text
• XML has only one “basic” type -- text. It is bounded by tags e.g.
<title> The Big Sleep </title> <year> 1935 </ year> --- 1935 is still
text
• XML text is called PCDATA (for parsed character data). It uses a 16-bit encoding, e.g. \&\#x0152 for the Hebrew letter Mem Later we shall see how new types are specified by XML-data
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XML structureXML structure
• Nesting tags can be used to express various structures. E.g. A tuple (record):
<person><name> Jeff Cohen</name><tel> 04-828-1345 </tel><tel> 054-470-778 </tel><email> [email protected] </email>
</person>
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XML structure (cont.)XML structure (cont.)
• We can represent a list by using the same
tag repeatedly:
<addresses> <person> ... </person> <person> ... </person> <person> ... </person> ...</addresses>
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XML structure (cont.)XML structure (cont.)
• Nested tags can be part of a list too:
<addresses><person>
<name> Yossi Orr</name><tel> 04-828-1345 </tel><email> [email protected] </email>
</person><person>
<name> Irma Levy</name><tel> 03-426-1142 </tel><email>[email protected]</email>
</person></addresses>
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TerminologyTerminology
• The segment of an XML document between an opening and a corresponding closing tag is called an element.
• Meta date about an element can appear in an attribute.
<person type=“Friend”> <name>Ortal Derech</name>
<tel>04-8732122</tel> <tel>054-646888</tel>
<email>[email protected]</email> </person>
element
element, a sub-elementof
attribute
text
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XML is tree-likeXML is tree-like
person
name emailtel tel
Malcolm Atchison
(215) 898 4321
(215) 898 4321
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A Complete XML DocumentA Complete XML Document
<?XMLversion ="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE addresses SYSTEM "http://www.technion.ac.il/~erant/addresses.dtd">
<addresses>
<person>
<name> Jeff Cohen</name>
<tel> 04-828-1345 </tel>
<tel> 054-470-778 </tel>
<email> [email protected] </email>
</person>
</addresses>
Tells whether or not this document references an external entity or an external data type specification
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XML Structure DefinitionsXML Structure Definitions
• DTD– Document Type Definition – defines structure
constraints for XML documents
• XML Schema– Same as DTD, more powerful because it includes
facilities to specify the data type of elements and it is based on XML.
• Namespaces– Namespaces are a way of preventing name clashes
among elements from more than one source within the same XML document.
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More StandardsMore Standards
• Xpath– XML Path Language, a language for locating parts of
an XML document.
• Xquery– A query language for XML documents (like SQL…).
• XSLT– XSL Transformations, a language for transforming
XML documents into other XML documents.
• RDF– Resource Description Framework. A formal
knowledge model from the World Wide Web.
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Why Is XML Important?Why Is XML Important?
• Because it exists, and everybody uses it.
• Plain Text - you can create and edit files with anything.
• Data Identification - XML tells you what kind of data you have, not how to display it.
• Separation from style.
• Hierarchical, and easily processed.
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An Overview of the APIsAn Overview of the APIs
• JAXP: Java API for XML Processing – It provides a common interface for creating and using
the standard SAX, DOM, and XSLT APIs.
• JAXB: Java Architecture for XML Binding – defines a mechanism for writing out Java objects as
XML.
• JDOM– Represents an XML file as a tree of objects
(sophisticated version of DOM)
• dom4j– Lightweight version of JDOM.
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AgendaAgenda
• Introduction to XML– What is XML– Structure and Terminology– JAVA APIs for XML: an Overview
• dom4j– Parsing an XML document– Writing to an XML document
• Xpath– Xpath Queries– Xpath in dom4j
• References
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dom4jdom4j
• An Open Source XML framework for Java.
• Allows you to read, write, navigate, create and modify XML documents.
• Integrates with DOM and SAX.
• Full XPath support.
• XSLT Support.
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Download and UseDownload and Use
• Go to: http://dom4j.org.
• Go to http://dom4j.org/download.html, and download the latest release (current = 1.4).
• Unzip.
• Don’t forget the classpath. When working in an IDE, don’t forget to add the log4j.jar library.
• Javadoc: http://dom4j.org/apidocs/index.html.
• Quick start guide: http://dom4j.org/guide.html.
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Opening an XML DocumentOpening an XML Document
import org.dom4j.*;public class Foo {
public Document parse(String id) throws DocumentException{
SAXReader reader = new SAXReader();Document document = reader.read(id);return document;
}}
We can read: file, URL, InputStream, String
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Example XML FileExample XML File
<?xml version="1.0" encoding="UTF-8" ?><salesdata xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="C:\Documents and Settings\eran\My Documents\Academic\Courses\XML\xpath_ass_schema.xsd">
<year><theyear>1997</theyear><region><name>central</name><sales unit="millions">34</sales></region><region><name>east</name><sales unit="millions">34</sales></region><region><name>west</name><sales unit="millions">32</sales></region>
</year><year>
<theyear>1998</theyear><region><name>east</name><sales unit="millions">35</sales></region>region><name>west</name><sales unit="millions">42</sales> </region>
</year></salesdata>
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Accessing XML ElementsAccessing XML Elements
public void dump(Document document) throws DocumentException{
Element root = document.getRootElement();for (Iterator i = root.elementIterator(); i.hasNext(); ) {
Element element = (Element)i.next();System.out.println(element.getQualifiedName());System.out.println(element.getTextTrim());System.out.println(element.elementText("theyear"));
}}
Accessing root element
Retrieving child elements
Retrieving element text
Retrieving element name
Retrieving the text of the child element “theyear”
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Accessing XML Elements – cont’dAccessing XML Elements – cont’d
• What will be the output of dump()?
year
1997year
1998
Why?
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Accessing XML Elements RecursivelyAccessing XML Elements Recursively
public void go(Element element, int depth){for (int d=0; d<depth; d++){
System.out.print(" ");}System.out.print(element.getQualifiedName());System.out.println(" "+ element.getTextTrim());for (Iterator i = element.elementIterator(); i.hasNext(); ) {
Element son = (Element)i.next();go(son, depth+1);
}}
What will be the output?
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Accessing Recursively – cont’dAccessing Recursively – cont’dsalesdata year theyear 1997 region name central sales 34 region name east sales 34 region name west sales 32 year theyear 1998 region name east sales 35 region name west sales 42
The whole XML tree, element names + values
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Creating an XML documentCreating an XML document
public Document createDocument() {Document document = DocumentHelper.createDocument();Element root = document.addElement("phonebook");
Element address1 = root.addElement("address").addAttribute("name", "Yuval").addAttribute("category", "family").addText("Ehud 3, Jerusalem");
Element address2 = root.addElement("address").addAttribute("name", "Ortal").addAttribute("category", "friends").addText("Kibbutz Givaat Haim");return document;
}
Creating root element
Adding elementsWhat will we get when running go()?
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Creating an XML document – cont’dCreating an XML document – cont’d
phonebook address Ehud 3, Jerusalem address Kibbutz Givaat Haim
XML tree structure of the new document
FileWriter out = new FileWriter("C:\\addresses.xml");document.write(out);String XML = document.asXML()
Writing the XML document to a fileRetrieving the
XML itself as string
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Client ProgramClient Program
public static void main(String[] args) {Foo foo = new Foo();try{
Document doc = foo.parse("C:\\Documents and Settings\\eran\\My Documents\\Academic\\Courses\\XML\\sales.xml");foo.dump(doc);foo.go(doc.getRootElement(), 0);foo.xpath(doc);Document newDoc = foo.createDocument();foo.go(newDoc.getRootElement(), 0);FileWriter out = new FileWriter( "C:\\addresses.xml" );newDoc.write(out);
}catch (Exception E){
System.out.println(E);}
}
Opening the file
Dumping and printed recursively
Creating a new document
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AgendaAgenda
• Introduction to XML– What is XML– Structure and Terminology– JAVA APIs for XML: an Overview
• dom4j– Parsing an XML document– Writing to an XML document
• Xpath– Xpath Queries– Xpath in dom4j
• References
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Xpath - IntroductionXpath - Introduction
• XML Path Language. XPath is a language for addressing parts of an XML document.
• Enables node locating and retrieving, very much like directory accessing in file systems.
• Limited (but not bad) filtering and querying abilities.
• Retrieved the actual PCDATA or node sets
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Xpath – Simple Path SelectionXpath – Simple Path Selection
Xpath Expression: /salesdata/year/theyear
<theyear>1997</theyear>
<theyear>1998</theyear>
/salesdata/year[2]/theyear
<theyear>1998</theyear>
“/” signifies child-of
Filtering the level – getting only the second year element
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Xpath – ConditionsXpath – Conditions
/salesdata/year/region[sales > 34]
<region>
<name>east</name>
<sales unit="millions">35</sales>
</region>
<region>
<name>west</name>
<sales unit="millions">42</sales>
</region>
Going down to region, and filtering according to the sales element
/salesdata/year/region[sales > 34]/name
?
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Xpath – Traveling Up the TreeXpath – Traveling Up the Tree
/salesdata/year/region[sales > 34]/parent::year/theyear
<theyear>1998</theyear>
Going up the XML tree (and then down again)
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Xpath – Traveling Down FastXpath – Traveling Down Fast
/descendant::sales
<sales unit="millions">34</sales>
<sales unit="millions">34</sales>
<sales unit="millions">32</sales>
<sales unit="millions">35</sales>
<sales unit="millions">42</sales>
//sales
Going all the way down, until the sales element
Same same
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Xpath – Advanced QueriesXpath – Advanced Queries
• The years (text nodes) for which sales data exists:
//region[name=\"west\" and sales > 32]/sales[@unit='millions']/ancestor::year/theyear
<theyear>1998</theyear>
Logical operators
Accessing attributes
ancestor is same as parent but goes all the way up to year
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Xpath – Advanced Queries (cont’d)Xpath – Advanced Queries (cont’d)
• The years (text nodes) in which the west region sales were higher than the east region sales; sales may be expressed in thousands or in millions:
year[region[name="west"]/sales[@unit='millions'*1000 or @unit='thousands'] > region[name="east"]/sales[@unit='millions‘*1000 or @unit='thousands']]/theyear/text()
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Xpath in dom4jXpath in dom4j
• Xpath queries can be used in dom4j:
public void xpath(Document document) {XPath xpathSelector =
DocumentHelper.createXPath("/salesdata/year/theyear");List results = xpathSelector.selectNodes(document);for (Iterator iter = results.iterator(); iter.hasNext(); ) {
Element element = (Element) iter.next(); System.out.println(element.asXML());
}}
Xpath expression is fed to the xpathSelector
The nodes are selected from the document, according to the xpath query
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AgendaAgenda
• Introduction to XML– What is XML– Structure and Terminology– JAVA APIs for XML: an Overview
• dom4j– Parsing an XML document– Writing to an XML document
• Xpath– Xpath Queries– Xpath in dom4j
• References
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References - XMLReferences - XML
• XML tutorial:– http://www.w3schools.com/xml/default.asp
• XML Specification from w3c:– http://www.w3.org/XML/
• The Java/XML Tutorial:– http://java.sun.com/xml/tutorial_intro.html
• DTD Tutorial:– http://www.xmlfiles.com/dtd/
• XML Schema Tutorial:– http://www.w3schools.com/schema/default.asp
• XML Schema Resource Page:– http://www.w3.org/XML/Schema
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dom4jdom4j
• Web site:– http://dom4j.org/
• Javadocs:– http://dom4j.org/apidocs/index.html
• Quick Start:– http://dom4j.org/guide.html
• Cookbook (main functionality):– http://dom4j.org/cookbook.html
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XpathXpath
• Xpath specification:– http://www.w3.org/TR/xpath
• Xpath tutorial:– http://www.w3schools.com/xpath/default.asp
• Xpath tutorial (extended):– http://www.zvon.org/xxl/XPathTutorial/General/
examples.html
• Xpath reference:– http://www.vbxml.com/xsl/XPathRef.asp