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XML Processing with DOM and SAX

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XML Processing with DOM and SAX. Dr Kevin McManus http://staffweb.cms.gre.ac.uk/~mk05/web/XML/3/. Document Object Model. We have already seen the Document Object Model (DOM) in the context of JavaScript and HTML DOM is key to "advanced" XML programming
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XML Processing with DOM and SAX Dr Kevin McManus http://staffweb.cms.gre.ac.uk/~mk05/web/XML/3/ University of Greenwich Document Object Model
  • We have already seen the Document Object Model (DOM) in the context of JavaScript and HTML
  • DOM is key to "advanced" XML programming
  • It allows you to manipulate XML documents in ways that are not possible using XSL
  • W3C DOM is language and platform neutral
  • Perl PHP Java JavaScript DOM VB ASP C++ University of Greenwich Dish Garlic Mushrooms Starter Dish Salad Dish Lentil Bake Menu Main Dish Quorn Surprise Dish Ice-cream Dessert <?xml version="1.0" ?> <Menu meal="lunch"> <Starter> <Dish>Garlic Mushrooms</Dish> <Dish diet="vegan">Salad</Dish> </Starter> <Main> <Dish diet="vegan">Lentil Bake</Dish> <Dish diet="vegetarian">Quorn Surprise</Dish> </Main> <Dessert> <Dish>Ice-cream</Dish> </Dessert> </Menu> XML syntax rules mean that well formed XML documents can be represented as a tree of objects The DOM presents a similar (although slightly more complex) view of an XML document What is the DOM?
  • A set of objects types that are used to represent the object tree view of an XML document
  • The objects described in the DOM allow the programmer to read, search, modify, add to and delete from an XML document
  • The DOM provides a standard definition of functionality for document navigation and manipulation
  • A DOM API can be implemented in any programming language
  • such implementations are called bindings
  • The W3C DOM Level 2 includes standards for the core object model, views, events, style, traversal and range
  • University of Greenwich DOM Object Types
  • DOM Level 1 includes 18 object types, e.g.
  • Node - a general type of object - everything is a node as well as belonging to one of the more specialised types
  • Document - to represent the whole document
  • Element - to represent each element (i.e. tag)
  • Attr - to represent attributes
  • Text - a node that represent the textual content of an element
  • NodeList - a list of nodes, e.g. all the elements that are "child" elements of another node.
  • Each object type has a defined set of properties and methods
  • University of Greenwich Text Garlic Mushrooms Element Dish Attr diet Element Starter Element Dish Text Salad Attr meal Attr diet Element Dish Element Menu Document Element Main Text Lentil Bake Element Dish Attr diet Element Dessert Element Dish Text Quorn Surprise Text Ice-cream <?xml version="1.0" ?> <Menu meal="lunch"> <Starter> <Dish>Garlic Mushrooms</Dish> <Dish diet="vegan">Salad</Dish> </Starter> <Main> <Dish diet="vegan">Lentil Bake</Dish> <Dish diet ="vegetarian">Quorn Surprise</Dish> </Main> <Dessert> <Dish>Ice-cream</Dish> </Dessert> </Menu> Menu example redrawn as a DOM tree Note the addition of a top-level document object Quick Quiz Draw a DOM object tree to represent the following XML document - indicate the DOM object types as shown in the previous slide <?xml version="1.0" ?> <temp_records> <record> <town>algiers</town> <temp>23</temp> </record> <record> <town>alicante</town> <temp>24</temp> </record> <record> <town>amsterdam</town> <temp>4</temp> </record> </temp_records> University of Greenwich So what does this give you as a programmer?
  • Each object type described in the DOM has a standard set of methods and properties available.
  • DOM language bindings (e.g. Java) provide a way of calling methods and accessing properties.
  • e.g. Nodes (the base type for all entities in the document) includes the following properties:
  • nodeName is the name of the tag for nodes of type Element
  • nodeValue is the text value for a node of type Text
  • University of Greenwich firstChild Element Dish Text Garlic Mushrooms Element Starter lastChild nextSibling Element Dish Text Salad parentNode Node Relationships The childNodes property of Element Starter would contain a NodeList of the two child nodes. Quick Quiz What is the firstChild of Element Starter's firstChild? What is the lastChild of Element Starter's firstChild? University of Greenwich Server Side Processing With PHP uses an XML file like the earlier quiz but with more records listTempsSimple.php <html><head><title>World Temperatures</title><head> <body><h1>World Temperatures</h1> <table width="50%"> <?php $doc = new DOMDocument(); $xmlString = ''; foreach ( file('tempsDTD.xml') as $node ) { $xmlString .= trim($node); } $doc->loadXML($xmlString); $records = $doc->documentElement->childNodes; for ($i=0; $i<$records->length; $i++) { $townName = $records->item($i)->firstChild->textContent; $tempValue = $records->item($i)->lastChild->textContent; print "<tr><td>$townName</td><td>$tempValue</td></tr>\n"; } ?> </table> </body></html> read the XML file into a single string with no whitespace parse the XML string into a DOM structure in memory get the top node list temperature_records loop over the child nodes get the town names and temperature values format the results as a table University of Greenwich Quick Quiz In the above example which lines use properties that are part of the DOM? University of Greenwich Server Side Processing With PHP Requires that PHP is configured with DOM PHP5 is bundled with a standard compliant DOM PHP4 supported an ideosyncratic DOM University of Greenwich Reading and Parsing XML with PHP
  • Reading and parsing the XML file requires that all whitespace is removed or you end up with twice as many nodes as you expected. $xmlString = ''; foreach ( file('temperatures.xml') as $node ) { $xmlString .= trim($node); } $doc->loadXML($xmlString);
  • There is a purpose built XML file reading and parsing function. $doc->load('temperatures.xml')
  • but this requires removing whitespace from the XML file first
  • see listFlatTemps.php
  • University of Greenwich listTempsPaging.php University of Greenwich <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en-gb"> <head> <title>World Temperatures</title> </head> <body> <h1>World Temperatures</h1> <?php $doc = new DOMDocument(); $xmlString = ''; foreach ( file('tempsDTD.xml') as $node ) { $xmlString .= trim($node); } $doc->loadXML($xmlString); # Validate the XML against it's DTD $valid = ( $doc->validate() ) ? 'valid' : 'not valid'; echo "<p>This document is $valid</p>\n"; $records = $doc->documentElement->childNodes; # Calculate the first and last records $pageLength = 7; $first = ( isset($_GET['next']) ) ? $_GET['next'] : 0; $last = ( $records->length - $first < $pageLength ) ? $records->length : $first + $pageLength; ?> listTempsPaging.php validate the XML against it’s DTD if it's the first call then "next" will not be set so start with the first record calculate the last record on the page University of Greenwich listTempsPaging.php <table width="50%"> <?php # Print the records for this page for ( $i=$first; $i<$last; $i++ ) { $townName = $records->item($i)->firstChild->textContent; $tempValue = $records->item($i)->lastChild->textContent; echo "<tr><td>$townName</td><td>$tempValue</td></tr>\n"; } ?> </table> <?php # Format an anchor tag to link to the next page echo '<p>'; if ( $last < $records->length - 1 ) { if ( $first > 0 ) { echo '<a href="listTempsPaging.php?next=' . ($last-2*$pageLength) . '">previous page &lt;&lt;</a> &nbsp; &nbsp;'; } echo '<a href="listTempsPaging.php?next=' . $last . '">next page &gt;&gt;</a></p>'; } else { echo '<a href="listTempsPaging.php?next=' . ($last-2*$pageLength) . '">previous page &lt;&lt;</a></p>'; } ?> </body></html> output records from from first to last-1 University of Greenwich listTempsPaging.php
  • Can you do more with the DOM than this?
  • Yes there are masses of properties and methods defined for reading and writing DOM objects
  • appendChild(), removeChild()
  • insertBefore(), insertAfter()
  • createElement()
  • Refer to the W3C for the full specification
  • although this is a less than easily readable document
  • Plenty of other resources
  • http://www.w3.org/TR/1998/REC-DOM-Level-1-19981001/ http://www.mozilla.org/docs/dom/domref/dom_shortTOC.html University of Greenwich XML processing at the client-side
  • The previous two examples show processing of an XML document on the server-side
  • XHTML was delivered to the client-side
  • no browser compatibility problems
  • IE5+, NN6+ and Mozilla have considerable support for processing XML documents at the client-side
  • they allow XSLT and CSS style sheets to be applied to the document
  • DOM on the client allows manipulation of a downloaded XML document using JavaScript
  • JavaScript also uses DOM to manipulate the XHTML document
  • The following examples work on both IE and Netscape/Mozilla
  • not Opera
  • some interesting code to cope with browser variations
  • University of Greenwich Client Side Processing With JavaScript the XML is loaded into the browser with the HTML body onLoad event but is not visible the button onClick event calls JavaScript code that loops through the DOM to display the records University of Greenwich listTempsStrict.html <?xml version="1.0" encoding="UTF-8"?> <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1 //EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml11.dtd"> <html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en-gb"> <head> <script type="text/javascript"><!-- // thanks to quirksmode.org for this idea if ( document.implementation.createDocument ) { var doc = document.implementation.createDocument("", "", null); } else if ( window.ActiveXObject ) { var doc = new ActiveXObject("Microsoft.XMLDOM"); } else { alert('Your browser can\'t handle this script'); } Netscape and Mozilla Internet Explorer Instead of sniffing the browser by interrogating the navigator object this script tests the browser functionality in order to correctly instantiate an XML DOM object University of Greenwich listTempsStrict.html function doIt() { var output = "" var records = doc.documentElement.childNodes; for ( var i = 0; i < records.length; i++ ) { townName = records[i].firstChild.firstChild.nodeValue; tempValue = records[i].lastChild.firstChild.nodeValue; output += "<br />Temperature in " + townName + " is " + tempValue; } document.getElementById("output").innerHTML = output; } --></script> <title></title> </head> <body onload="doc.load('flatTemps.xml')"> <form action="dummy"><p> <input type="button" onclick="doIt()" value="Display temperatures" /> <br /><span id="output"></span> </p></form> </body> </html> onLoad event handler loads the XML file into the XML DOM object the output goes in here University of Greenwich listTempsStrict.html
  • This example is much the same as the PHP version
  • JavaScript dot notation allows concatenation of node addressing townName = records[i].firstChild.firstChild.nodeValue;
  • Mozilla browsers offer the methoddocument.implementation.createDocument()
  • accepts parameters for namespace and root node of the document should you need them
  • Internet Explorer uses an ActiveX object
  • IE also supports XML data islands
  • a good idea but not part of XHTML 1.0 strict and not supported by other browsers <xmlid="XMLisland"src="booklist.xml">
  • What does this example do that you couldn't do using an XSLT style sheet?
  • University of Greenwich searchTempsStrict.html University of Greenwich searchTempsStrict.html <snip> function doIt() { var records = doc.documentElement.childNodes; found = false; for ( i= 0; i < records.length; i++ ) { townName = records[i].firstChild.firstChild.nodeValue; if (townName == document.forms[0].town.value) { tempValue = records[i].lastChild.firstChild.nodeValue; message = "Temperature in " + townName + " is " + tempValue; found = true; } } if (found == false) message = "Sorry not found"; document.getElementById("output").innerHTML = message; } --></script> </head> <body onload="doc.load('flatTemps.xml')“> <form action="dummy"><p> Enter town: <input type="text" name="town" size="10"/> <input type="button" onclick="doIt()" value="Display temperatures"/> <br /><span id="output"></span> </p></form> </body></html> loop over all records if the record matches the form input data return the record University of Greenwich searchTempsStrict.html
  • This is very similar to the previous example
  • An XML DOM object is created by JavaScript in the HTML header
  • An XML file is read into the XML DOM object when the page has been rendered
  • body tag onLoad event handler
  • this document is not displayed
  • DOM is searched to match data from the input type text
  • better to implement this sort of application at the client rather than the server
  • why would this not be good with PHP?
  • Not at all clear how you would do this with XSLT
  • University of Greenwich AJAX
  • Asynchronous JavaScript and XML (AJAX)
  • not a technology in itself
  • a "new" approach combining a number of existing technologies
  • XHTML
  • CSS
  • JavaScript
  • DOM
  • XML
  • XSLT
  • XMLHttpRequest object
  • Web applications that make incremental updates
  • without reloading the entire browser page
  • faster and more responsive to user actions
  • University of Greenwich ajax.html University of Greenwich ajax.html if ( document.implementation.createDocument ) { var xmlDoc = document.implementation.createDocument("", "", null); xmlDoc.onload = doIt; } else if ( window.ActiveXObject ) { var xmlDoc = new ActiveXObject("Microsoft.XMLDOM"); xmlDoc.onreadystatechange = function () { if (xmlDoc.readyState == 4) doIt(); } } else { alert('Your browser can\'t handle this script'); } function doIt() { var output = ‘<table><tr><th> &nbsp; Location &nbsp; </th><th>Temperature</th></tr>'; var records = xmlDoc.documentElement.childNodes; for ( var i = 0; i < records.length; i++ ) { townName = records[i].firstChild.firstChild.nodeValue; tempValue = records[i].lastChild.firstChild.nodeValue; output += '<tr><td class="right">'+townName+'</td><td class="centre">'+tempValue+'</td></tr>'; } output += '</table>'; document.getElementById("data").innerHTML = output; } --></script> </head> <body> <h1>JavaScript AJAX Example</h1> <p> <input type="button" onclick="xmlDoc.load('flatTemps1.xml')" value="flatTemps1.xml" /> <input type="button" onclick="xmlDoc.load('flatTemps2.xml')" value="flatTemps2.xml" /> <input type="button" onclick="xmlDoc.load('flatTemps3.xml')" value="flatTemps3.xml" /> <input type="button" onclick="xmlDoc.load('flatTemps4.xml')" value="flatTemps4.xml" /> <input type="button" onclick="xmlDoc.load('flatTemps5.xml')" value="flatTemps5.xml" /> </p> <div id="data"><table><tr><th> &nbsp; Location &nbsp; </th><th>Temperature</th></tr></table></div> </body></html> callback functions for the XML read completed event event loads an XML file ajax.html
  • Page requests an XML document in response to a user event
  • button onClick
  • The callback function doIt() is called when the XML document read completes
  • DoIt() parses the XML using DOM
  • updates the page content without re-loading the entire page
  • AJAX applications usually use the XMLHttpRequest object
  • this one doesn’t
  • for more information:
  • read the article by Jesse James Garrett
  • look at SAJAX – the Simple AJAX toolkit
  • University of Greenwich XMLHttpRequest object
  • Originally developed by Microsoft
  • Now widely supported
  • Provides useful functionality
  • Not necessarily asynchronous
  • use a callback mechanism like the previous example
  • Not necessarily XML
  • txt, JSON, etc.
  • University of Greenwich SAX – the Simple API for XML
  • An alternative to DOM
  • SAX is a stream based processor
  • rather than reading the XML into memory the XML is processed as a stream
  • each XML tag encountered acts as an event that triggers a handler
  • Current version has for some time been SAX 2.0.1
  • not a W3C standard – saxproject.org
  • Originally Java only now available in several language bindings
  • PHP, Perl, Python, C++
  • The “XML for <SCRIPT>” project provides a JavaScript DOM Level 2, Xpath and SAX processor for cross platform client side XML processing
  • More efficient than DOM for many applications
  • SAX lacks the intuitive approach of DOM
  • less flexible
  • requires more code to achieve similar results
  • requires less memory to achieve similar results
  • University of Greenwich SAX “DOM and SAX have different philosophies on how to parse xml. The SAX engine is essentially event-driven. When it comes across a tag, it calls an appropriate function to handle it. This makes SAX very fast and efficient. However, it feels like you're trapped inside an eternal loop when writing code. You find yourself using many global variables and conditional statements. On the other hand, the DOM method is somewhat memory intensive. It loads an entire XML document into memory as a hierarchy. The upside is that all of the data is available to the programmer. This approach is more intuitive, easier to use, and affords better readability.” Matt Dunford University of Greenwich saxMenu.php saxMenu.php <head><title>SAX Menu</title></head> <body> <h1>SAX Menu</h1> <form action="saxMenu.php" method="post"> <p> <input type="checkbox" name="chkVegan" />Vegan<br /> <input type="checkbox" name="chkVege" />Vegetarian<br /> <input type="checkbox" name="chkFish" />Fish<br /> <input type="checkbox" name="chkMeat" />Meat<br /> <input type="submit" name="getMenu" value="Get Menu" /> </p></form> <?php if ( isset( $_POST['getMenu']) ) { $diet = ( isset($_POST['chkVegan']) ? 'vegan ' : '' ); $diet .= ( isset($
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