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1 1 © 2001-2003 Marty Hall, Larry Brown http://www.corewebprogramming.com Web core programming Document Object Model DOM DOM 2 www.corewebprogramming.com Agenda • Introduction to DOM • Java API for XML Parsing (JAXP) • Installation and setup • Steps for DOM parsing • Example – Representing an XML Document as a JTree • DOM or SAX? 2 DOM 3 www.corewebprogramming.com Document Object Model (DOM) • DOM supports navigating and modifying XML documents – Hierarchical tree representation of document • Tree follows standard API • Creating tree is vendor specific • DOM is a language-neutral specification – Bindings exists for Java, C++, CORBA, JavaScript • DOM Versions – DOM 1.0 (1998) – DOM 2.0 Core Specification (2000) – Official Website for DOM •http://www.w3c.org/DOM/ DOM 4 www.corewebprogramming.com DOM Tree Document Document Type Element Attribute Text Comment Element Element Attribute Text Entity Reference Text Attribute Element Text Text 3 DOM 5 www.corewebprogramming.com DOM Advantages and Disadvantages • Advantages – Robust API for the DOM tree – Relatively simple to modify the data structure and extract data • Disadvantages – Stores the entire document in memory – As DOM was written for any language, method naming conventions don’t follow standard Java programming conventions DOM 6 www.corewebprogramming.com Java API for XML Parsing (JAXP) javax.xml.parsers • JAXP provides a vendor-neutral interface to the underlying DOM or SAX parser DocumentBuilderFactory DocumentBuilder SAXParserFactory SAXParser ParserConfigurationException FactoryConfigurationError 4 DOM 7 www.corewebprogramming.com DOM Installation and Setup (JDK 1.4) • All the necessary classes for DOM and JAXP are included with JDK 1.4 • See javax.xml.* packages • For DOM and JAXP with JDK 1.3 see following viewgraphs DOM 8 www.corewebprogramming.com DOM Installation and Setup (JDK 1.3) 1. Download a DOM-compliant parser • Java-based DOM parsers at http://www.xml.com/pub/rg/Java_Parsers • Recommend Apache Xerces-J parser at http://xml.apache.org/xerces-j/ 2. Download the Java API for XML Processing (JAXP) • JAXP is a small layer on top of DOM which supports specifying parsers through system properties versus hard coded • See http://java.sun.com/xml/ • Note: Apache Xerces-J already incorporates JAXP 5 DOM 9 www.corewebprogramming.com DOM Installation and Setup (continued) 3. Set your CLASSPATH to include the DOM (and JAXP) classes set CLASSPATH=xerces_install_dir\xerces.jar; %CLASSPATH% or setenv CLASSPATH xerces_install_dir/xerces.jar: $CLASSPATH • For servlets, place xerces.jar in the server’s lib directory • Note: Tomcat 4.0 is prebundled with xerces.jar • Xerces-J already incorporates JAXP • For other parsers you may need to add jaxp.jar to your classpath and servlet lib directory DOM 10 www.corewebprogramming.com DOM Installation and Setup (continued) 4. Bookmark the DOM Level 2 and JAXP APIs – DOM Level 2 • http://www.w3.org/TR/DOM-Level-2-Core/ – JAXP • http://java.sun.com/xml/jaxp/dist/1.1/ docs/api/index.html 6 DOM 11 www.corewebprogramming.com Steps for DOM Parsing 1. Tell the system which parser you want to use 2. Create a JAXP document builder 3. Invoke the parser to create a Document representing an XML document 4. Normalize the tree 5. Obtain the root node of the tree 6. Examine and modify properties of the node DOM 12 www.corewebprogramming.com Step 1: Specifying a Parser • Approaches to specify a parser – Set a system property for javax.xml.parsers.DocumentBuilder- Factory – Specify the parser in jre_dir/lib/jaxp.properties – Through the J2EE Services API and the class specified in META-INF/services/ javax.xml.parsers. DocumentBuilder- Factory – Use system-dependant default parser (check documentation) 7 DOM 13 www.corewebprogramming.com Specifying a Parser, Example • The following example: – Permits the user to specify the parser through the command line –D option java –Djavax.xml.parser.DocumentBuilderFactory = com.sun.xml.parser.DocumentBuilderFactoryImpl ... – Uses the Apache Xerces parser otherwise public static void main(String[] args) { String jaxpPropertyName = "javax.xml.parsers.DocumentBuilderFactory"; if (System.getProperty(jaxpPropertyName) == null) { String apacheXercesPropertyValue = "org.apache.xerces.jaxp.DocumentBuilderFactoryImpl"; System.setProperty(jaxpPropertyName, apacheXercesPropertyValue); } ... } DOM 14 www.corewebprogramming.com Step 2: Create a JAXP Document Builder • First create an instance of a builder factory, then use that to create a DocumentBuilder object DocumentBuilderFactory builderFactory = DocumentBuilderFactory.newInstance(); DocumentBuilder builder = builderFactory.newDocumentBuilder(); – A builder is basically a wrapper around a specific XML parser – To set up namespace awareness and validation, use builderFactory.setNamespaceAware(true) builderFactory.setValidating(true) 8 DOM 15 www.corewebprogramming.com Step3: Invoke the Parser to Create a Document • Call the parse method of the DocumentBuilder, supplying an XML document (input stream) Document document = builder.parse(someInputStream); – The Document class represents the parsed result in a tree structure – The XML document can be represented as a: • URI, represented as a string • InputStream • org.xml.sax.InputSource DOM 16 www.corewebprogramming.com Step 4: Normalize the Tree • Normalization has two affects: – Combines textual nodes that span multiple lines – Eliminates empty textual nodes document.getDocumentElement().normalize(); 9 DOM 17 www.corewebprogramming.com Step 5: Obtain the Root Node of the Tree • Traversing and modifying the tree begins at the root node Element rootElement = document.getDocumentElement(); – An Element is a subclass of the more general Node class and represents an XML element – A Node represents all the various components of an XML document • Document, Element, Attribute, Entity, Text, CDATA, Processing Instruction, Comment, etc. DOM 18 www.corewebprogramming.com Step 6: Examine and Modify Properties of the Node • Examine the various node properties – getNodeName • Returns the name of the element – getNodeType • Returns the node type • Compare to Node constants –DOCUMENT_NODE, ELEMENT_NODE, etc. – getAttributes • Returns a NamedNodeMap (collection of nodes, each representing an attribute) – Obtain particular attribute node through getNamedItem – getChildNodes • Returns a NodeList collection of all the children 10 DOM 19 www.corewebprogramming.com Step 6: Examine and Modify Properties of the Node (cont) • Modify the document – setNodeValue • Assigns the text value of the node – appendChild • Adds a new node to the list of children – removeChild • Removes the child node from the list of children – replaceChild • Replace a child with a new node DOM 20 www.corewebprogramming.com DOM Example: Representing an XML Document as a JTree • Approach – Each XML document element is represented as a tree node (in the JTree) – Each tree node is either the element name or the element name followed by a list of attributes 11 DOM 21 www.corewebprogramming.com DOM Example: Representing an XML Document as a JTree • Approach (cont.) – The following steps are performed: 1. Parse and normalize the XML document and then obtain the root node 2. Turn the root note into a JTree node • The element name (getNodeName) is used for the tree node label • If attributes are present (node.getAttributes), then include them in the label enclosed in parentheses 3. Look up child elements (getChildNodes) and turn them into JTree nodes, linking to their parent tree node 4. Recursively apply step 3 to all child nodes DOM 22 www.corewebprogramming.com DOM Example: XMLTree import java.awt.*; import javax.swing.*; import javax.swing.tree.*; import java.io.*; import org.w3c.dom.*; import javax.xml.parsers.*; /** Given a filename or a name and an input stream, * this class generates a JTree representing the * XML structure contained in the file or stream. * Parses with DOM then copies the tree structure * (minus text and comment nodes). */ public class XMLTree extends JTree { public XMLTree(String filename) throws IOException { this(filename, new FileInputStream(new File(filename))); } public XMLTree(String filename, InputStream in) { super(makeRootNode(in)); } 12 DOM 23 www.corewebprogramming.com DOM Example: XMLTree (continued) private static DefaultMutableTreeNode makeRootNode(InputStream in) { try { // Use the system property // javax.xml.parsers.DocumentBuilderFactory (set either // from Java code or by using the -D option to "java"). DocumentBuilderFactory builderFactory = DocumentBuilderFactory.newInstance(); DocumentBuilder builder = builderFactory.newDocumentBuilder(); Document document = builder.parse(in); document.getDocumentElement().normalize(); Element rootElement = document.getDocumentElement(); DefaultMutableTreeNode rootTreeNode = buildTree(rootElement); return(rootTreeNode); } catch(Exception e) { String errorMessage = "Error making root node: " + e; System.err.println(errorMessage); e.printStackTrace(); return(new DefaultMutableTreeNode(errorMessage)); } } DOM 24 www.corewebprogramming.com DOM Example: XMLTree (continued) ... private static DefaultMutableTreeNode buildTree(Element rootElement) { // Make a JTree node for the root, then make JTree // nodes for each child and add them to the root node. // The addChildren method is recursive. DefaultMutableTreeNode rootTreeNode = new DefaultMutableTreeNode(treeNodeLabel(rootElement)); addChildren(rootTreeNode, rootElement); return(rootTreeNode); } ... 13 DOM 25 www.corewebprogramming.com DOM Example: XMLTree (continued) private static void addChildren (DefaultMutableTreeNode parentTreeNode, Node parentXMLElement) { // Recursive method that finds all the child elements and adds // them to the parent node. Nodes corresponding to the graphical // JTree will have the word "tree" in the variable name. NodeList childElements = parentXMLElement.getChildNodes(); for(int i=0; i<childElements.getLength(); i++) { Node childElement = childElements.item(i); if (!(childElement instanceof Text || childElement instanceof Comment)) { DefaultMutableTreeNode childTreeNode = new DefaultMutableTreeNode (treeNodeLabel(childElement)); parentTreeNode.add(childTreeNode); addChildren(childTreeNode, childElement); } } } DOM 26 www.corewebprogramming.com DOM Example: XMLTree (continued) ... private static String treeNodeLabel(Node childElement) { NamedNodeMap elementAttributes = childElement.getAttributes(); String treeNodeLabel = childElement.getNodeName(); if (elementAttributes != null && elementAttributes.getLength() > 0) { treeNodeLabel = treeNodeLabel + " ("; int numAttributes = elementAttributes.getLength(); for(int i=0; i<numAttributes; i++) { Node attribute = elementAttributes.item(i); if (i > 0) { treeNodeLabel = treeNodeLabel + ", "; } treeNodeLabel = treeNodeLabel + attribute.getNodeName() + "=" + attribute.getNodeValue(); } treeNodeLabel = treeNodeLabel + ")"; } return(treeNodeLabel); } } 14 DOM 27 www.corewebprogramming.com DOM Example: XMLFrame import java.awt.*; import javax.swing.*; import java.io.*; public class XMLFrame extends JFrame { public static void main(String[] args) { String jaxpPropertyName = "javax.xml.parsers.DocumentBuilderFactory"; // Pass the parser factory in on the command line with // -D to override the use of the Apache parser. if (System.getProperty(jaxpPropertyName) == null) { String apacheXercesPropertyValue = "org.apache.xerces.jaxp.DocumentBuilderFactoryImpl"; System.setProperty(jaxpPropertyName, apacheXercesPropertyValue); } ... DOM 28 www.corewebprogramming.com DOM Example: XMLFrame (continued) String[] extensions = { "xml", "tld" }; WindowUtilities.setNativeLookAndFeel(); String filename = ExtensionFileFilter.getFileName(".", "XML Files", extensions); new XMLFrame(filename); } public XMLFrame(String filename) { try { WindowUtilities.setNativeLookAndFeel(); JTree tree = new XMLTree(filename); JFrame frame = new JFrame(filename); frame.addWindowListener(new ExitListener()); Container content = frame.getContentPane(); content.add(new JScrollPane(tree)); frame.pack(); frame.setVisible(true); } catch(IOException ioe) { System.out.println("Error creating tree: " + ioe); } } } 15 DOM 29 www.corewebprogramming.com DOM Example: perennials.xml <?xml version="1.0"?> <!DOCTYPE perennials SYSTEM "dtds/perennials.dtd"> <perennials> <daylily status="in-stock"> <cultivar>Luxury Lace</cultivar> <award> <name>Stout Medal</name> <year>1965</year> </award> <award> <name note="small-flowered">Annie T. Giles</name> <year>1965</year> </award> <award> <name>Lenington All-American</name> <year>1970</year> </award> <bloom code="M">Midseason</bloom> <cost discount="3" currency="US">11.75</cost> </daylily> ... <perennials> DOM 30 www.corewebprogramming.com DOM Example: Results 16 DOM 31 www.corewebprogramming.com DOM Example: Results (continued) DOM 32 www.corewebprogramming.com DOM or SAX? • DOM – Suitable for small documents – Easily modify document – Memory intensive; load the complete XML document • SAX – Suitable for large documents; saves significant amounts of memory – Only traverse document once, start to end – Event driven – Limited standard functions 17 DOM 33 www.corewebprogramming.com Summary • DOM is a tree representation of an XML document in memory – DOM provides a robust API to easily modify and extract data from an XML document • JAXP provides a vendor-neutral interface to the underlying DOM or SAX parser • Every component of the XML document is represent as a Node • Use normalization to combine text elements spanning multiple lines 34 © 2001-2003 Marty Hall, Larry Brown http://www.corewebprogramming.com Web core programming Questions?
