This tutorial offers a thorough exploration of XML processing in Java, including parsing methods compliant with W3C standards: DOM and SAX. Learn how to convert XML into a tree of objects with DOM, allowing random access, and delve into SAX for efficient event-driven parsing. The guide covers practical examples, such as the structure of XML documents and the use of DTD for validation. With easy-to-follow explanations and code snippets, you’ll gain essential skills for manipulating and accessing XML data within Java applications.
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Comprehensive Guide to XML Processing in Java: DOM and SAX Parsers Explained
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Processing XML with Java A comprehensive tutorial about XML processing with Java XML tutorial of W3Schools
Parsers • What is a parser? Formal grammar Analyzed Data Input Parser The structure(s) of the input, according to the atomic elements and their relationships (as described in the grammar)
XML-Parsing Standards • We will consider two parsing methods that implement W3C standards for accessing XML • DOM • convert XML into a tree of objects • “random access” protocol • SAX • “serial access” protocol • event-driven parsing
world.dtd <!ELEMENTcountries(country*)> <!ELEMENTcountry(name,population?,city*)> <!ATTLISTcountrycontinentCDATA#REQUIRED> <!ELEMENTname (#PCDATA)> <!ELEMENTcity (name)> <!ATTLISTcitycapital(yes|no) "no"> <!ELEMENTpopulation (#PCDATA)> <!ATTLISTpopulationyearCDATA#IMPLIED> <!ENTITYeu"Europe"> <!ENTITYas"Asia"> <!ENTITY af"Africa"> <!ENTITY am"America"> <!ENTITY au"Australia"> parsed Not parsed default value As opposed to required Open world.xml in your browser Check world2.xml for #PCDATA exmaple
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
Application API XML File DOM Parser DOM Tree Using a DOM Tree in memory
Creating a DOM Tree • A DOM tree is generated by a DocumentBuilder • The builder is generated by a factory, in order to be implementation independent • The factory is chosen according to the system configuration DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance(); DocumentBuilder builder = factory.newDocumentBuilder(); Document doc = builder.parse("world.xml");
Configuring the Factory • The methods of the document-builder factory enable you to configure the properties of the document building • For example • factory.setValidating(true) • factory.setIgnoringComments(false) Read more about DocumentBuilderFactory Class, DocumentBuilder Class
The Node Interface • The nodes of the DOM tree include • a special root (denoted document) • The Document interface retrieved by builder.parse(…) actually extends the Node Interface • element nodes • text nodes and CDATA sections • attributes • comments • and more ... • Every node in the DOM tree implements the Node interface This is very unintuitive (some would even say this is a bloated design)
Figure as appears in : “The XML Companion” - Neil Bradley NodeList NamedNodeMap A light-weight fragment of the document. Can hold several sub-trees Interfaces in a DOM Tree DocumentFragment Document Text CDATASection CharacterData Comment Attr Element Node DocumentType Notation Entity EntityReference Run FragmentVsElement with 1st argument fragment/element ProcessingInstruction DocumentType
Document Type Element Attribute Attribute Element Element Text Text Element Entity Reference Text Text Comment Text Interfaces in the DOM Tree Document
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()
Not a very OO approach… reminds one of simulating OO using unions in C…. Node Properties Only Element Nodes have attributes… some would say this should’ve been a property of the Element derived class and not of Node. • Every node has (a dubious design and…) • a type • a name • a value • attributes • The roles of these properties differ according to the node types • Nodes of different types implement different interfaces (that extend Node) Very few nodes have both a significant name and a significant value
import org.w3c.dom.*; import javax.xml.parsers.*; publicclass EchoWithDom { publicstaticvoid main(String[] args) throwsException { DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance(); factory.setIgnoringElementContentWhitespace(true); DocumentBuilder builder = factory.newDocumentBuilder(); Document doc = builder.parse(“world.xml"); new EchoWithDom().echo(doc); } e.g white spaces used for indentation in non mixed data elements
privatevoid echo(Node n) { print(n); if (n.getNodeType() == Node.ELEMENT_NODE) { NamedNodeMap atts = n.getAttributes(); ++depth; for (int i = 0; i < atts.getLength(); i++) echo(atts.item(i)); --depth; } depth++; for (Node child = n.getFirstChild(); child != null; child = child.getNextSibling()) echo(child); depth--; } Attribute nodes are not included…
privateint depth = 0; privateString[] NODE_TYPES = { "", "ELEMENT", "ATTRIBUTE", "TEXT", "CDATA", "ENTITY_REF", "ENTITY", "PROCESSING_INST", "COMMENT", "DOCUMENT", "DOCUMENT_TYPE", "DOCUMENT_FRAG", "NOTATION" }; privatevoid print(Node n) { for (int i = 0; i < depth; i++) System.out.print(" "); System.out.print(NODE_TYPES[n.getNodeType()] + ":"); System.out.print("Name: "+ n.getNodeName()); System.out.print(" Value: "+ n.getNodeValue()+"\n"); }} run EchoWithDom, pay attention to the default values
Normalizing the DOM Tree • Normalizing a DOM Tree has two effects: • Combine adjacent textual nodes • Eliminate empty textual nodes • To normalize, apply the normalize() method to the document element Created by node manipulation…
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.
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. • This is called event-driven programming. Most GUI programs are written using this paradigm. • The handlers are programmer’s implementation of standard Java API (i.e., interfaces and classes)
<?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 …
Used to create a SAX Parser Handles document events: start tag, end tag, etc. Handles Parser Errors Handles DTD Handles Entities
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