Web technologies/2014-2015/Laboratory 5
Parsing XML documents
editAn important issue when dealing with XML is parsing the documents. There are several parser types including:
- DOM parsers:
- allow the navigation of the XML document as it were a tree.
- the main drawback is that the document needs to be completely loaded into memory before actually parsing it.
- DOM documents can be either created by parsing an XML file, or by users which want to create an XML file programmatic.
- SAX parsers:
- event-driven API in which the XML document is read sequentially by using callbacks that are triggered when different element types are meet.
- overcomes the DOM’s memory problem, and is fast and efficient at reading files sequentially.
- its problem comes from the fact that it is quite difficult to read random information from inside an XML file.
- FlexML parsers:
- follow the SAX approach and rely on events during the parsing process.
- it does not constitute a parsing library by itself, but instead it converts the DTD file into a parser specification usable with the classical Flex parser generator.
- Pull parsers:
- use an iterator design pattern in order to sequentially read various XML items such as elements, attributes or data.
- this method allows the programmer to write recursive-descent parsers:
- applications in which the structure of the code that handles the parsing looks like the XML they process.
- examples of parsers from this category include: StAX13, and the .NET System.Xml.XmlReader.
- Non-extractive parsers:
- a new technology in which the object oriented modeling of the XML is replaced with 64-bit Virtual Token Descriptors.
- one of the most expressive parser belonging to this category is VTD-XML.
Note: We will use the Apache Xerxes library for these exercises. You can find more information here.
SAX
editSAX (Simple API for XML) is a serial access XML parser. A SAX parser can be found in the Xerces library found here.
The following fragment of code shows how we could use SAX to parse an XML document:
Using Xerces with Java
editimport org.xml.sax.*;
import org.xml.sax.helpers.XMLReaderFactory;
import java.io.IOException;
public class SAXExample {
public static void main(String[] args) throws IOException {
try {
XMLReader parser = XMLReaderFactory.createXMLReader();
ContentHandler handler = new TextExtractor();
parser.setContentHandler(handler);
parser.parse(args[0]);
System.out.println(args[0] + " is well-formed.");
}
catch (SAXException e) {
System.out.println(args[0] + " is not well-formed.");
}
}
}
import org.xml.sax.*;
import java.io.*;
public class TextExtractor implements ContentHandler {
public TextExtractor() { }
// Handle the #PCDATA i.e. the text nodes
public void characters(char[] text, int start, int length) throws SAXException {
System.out.println("Found text node: ");
System.out.println(new String(text).substring(start, start+length));
}
public void setDocumentLocator(Locator locator) {}
// Handles the start of a document event.
public void startDocument() {
System.out.println("Entering document");
}
// Handles the end of a document event.
public void endDocument() {
System.out.println("Leaving document");
}
// Handles the beginning of the scope of a prefix-URI Namespace mapping.
public void startPrefixMapping(String prefix, String uri) {}
// Handles the ending of the scope of a prefix-URI Namespace mapping.
public void endPrefixMapping(String prefix) {}
// Triggers each time a start element is found.
public void startElement(String namespaceURI, String localName, String qualifiedName, Attributes atts) {
System.out.println("Found element: " + localName);
System.out.println("Attributes:");
for (int i=0; i<atts.getLength(); i++) {
System.out.println("Found attribute: " + atts.getLocalName(i) + " with value: " + atts.getValue(i));
}
}
// Triggers each time an end element is found.
public void endElement(String namespaceURI, String localName, String qualifiedName) {
System.out.println("Leaving element: " + localName);
}
// Handles white characters.
public void ignorableWhitespace(char[] text, int start, int length) throws SAXException {}
// Handles the processing instruction. For example it can be called xml with version=1.0 and a certain encoding.
public void processingInstruction(String target, String data){}
// Handles a skipped entity.
public void skippedEntity(String name) {}
}
Using Python3
editfrom xml.sax import handler, make_parser, SAXParseException
class MyHandler(handler.ContentHandler):
def __init__(self):
pass
def startDocument(self):
print('Entering document')
def endDocument(self):
print('Leaving document')
def startElement(self, name, attrs):
print(f'Found element: {name}')
for attr_name, value in attrs.items():
print(f'Found attribute: {attr_name} with value: {value}')
def endElement(self, name):
print(f'Leaving element: {name}')
def characters(self, content):
print(f'Found text node: {content}')
def ignorableWhitespace(self, content):
pass
def processingInstruction(self, target, data):
pass
try:
parser = make_parser()
parser.setContentHandler(MyHandler())
parser.parse('queue.xml')
print('The document is well formed')
except SAXParseException:
print('The document is not well formed')
Links:
DOM
editDOM (Document Object Model) is a convention for representing XML documents. A DOM parser can be found in the Xerces library found here.
DOM handles XML files as being made of the following types of nodes:
- Document node
- Element nodes
- Attribute nodes
- Leaf nodes:
- Text nodes
- Comment nodes
- Processing instruction nodes
- CDATA nodes
- Entity reference nodes
- Document type nodes
- Non-tree nodes;
Using Xerces and Java:
editThe following fragment of code shows how we could use DOM to traverse an XML tree:
import javax.xml.parsers.*; // JAXP
import org.w3c.dom.Node;
import org.w3c.dom.NodeList;
import org.xml.sax.SAXException;
import java.io.IOException;
public class DOMExample {
public static void main(String[] args) {
DOMExample iterator = new DOMExample();
try {
// Use JAXP to find a parser
DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
// Turn on namespace support
factory.setNamespaceAware(true);
DocumentBuilder parser = factory.newDocumentBuilder();
// Read the entire document into memory
Node document = parser.parse(args[0]);
// Process it starting at the root
iterator.followNode(document);
}
catch (SAXException e) {
System.out.println(args[0] + " is not well-formed.");
System.out.println(e.getMessage());
}
catch (IOException e) {
System.out.println(e);
}
catch (ParserConfigurationException e) {
System.out.println("Could not locate a JAXP parser");
}
}
public void followNode(Node node) throws IOException {
// Print information on node.
System.out.println("Node name:" + node.getNodeName());
System.out.println("Node type:" + node.getNodeType());
System.out.println("Node local name:" + node.getLocalName());
System.out.println("Node value:" + node.getNodeValue());
// Process the children.
NodeList children = node.getChildNodes();
for (int i = 0; i < children.getLength(); i++) {
Node child = children.item(i);
// Recursion on child.
followNode(child);
}
}
}
DOM also allows users to create a new XML document or change the structure of an already existing one:
import java.io.IOException;
import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import org.w3c.dom.Document;
import org.w3c.dom.Node;
import org.w3c.dom.Element;
public class DOMCreatorExample {
public static void main(String[] av) throws IOException {
DOMCreatorExample dc = new DOMCreatorExample();
Document doc = dc.makeXML();
}
public Document makeXML() {
try {
DocumentBuilderFactory fact = DocumentBuilderFactory.newInstance();
DocumentBuilder parser = fact.newDocumentBuilder();
Document doc = parser.newDocument();
Node root = doc.createElement("books");
doc.appendChild(root);
Node book = doc.createElement("book");
((Element) book).setAttribute("title", "Processing XML with Java");
((Element) book).setAttribute("author", "Elliotte Rusty Harold");
book.appendChild(doc.createTextNode("A complete tutorial about writing Java programs that read and write XML documents."));
root.appendChild(book);
return doc;
} catch (Exception ex) {
ex.printStackTrace();
return null;
}
}
}
Using Python3:
editimport xml.dom.minidom as mdom
try:
dom = mdom.parse('queue.xml')
except Exception:
print('Document is not well formed')
exit(1)
def follow_nodes(element):
print(f'Node name: {element.nodeName}')
print(f'Node type: {type(element)}')
try:
print(f'Node local name: {element.nodeLocalName}')
except Exception:
pass
print(f'Node Value: {element.nodeValue}')
for elem in element.childNodes:
follow_nodes(elem)
follow_nodes(dom.firstChild)
To programmatically create an XML document using the xml.dom module:
import xml.dom.minidom as mdom
def create_xml():
doc = mdom.Document()
root = doc.createElement('books')
doc.appendChild(root)
book = doc.createElement('book')
book.setAttribute('title', 'Processing XML with Java')
book.setAttribute('author', 'Elliotte Rusty Harold')
book.appendChild(doc.createTextNode("A complete tutorial about writing Java programs that read and write XML documents."));
root.appendChild(book)
return root
with open('out.xml', 'w') as f:
create_xml().writexml(f)
Links:
Exercises
edit- Parse the XML created in your assignment from Laboratory 3 using both SAX and DOM. Print out the parsing time of each method (hint: use System.currentTimeMillis() to get the start and end time).
- Create the XML from your assignment in Laboratory 3 using DOM. Print the result to an XML file.
- In order to save resulted xml to file you can use:
import javax.xml.transform.stream.StreamResult;
.....
TransformerFactory transformerFactory = TransformerFactory.newInstance();
Transformer transformer = transformerFactory.newTransformer();
DOMSource source = new DOMSource(doc);
StreamResult result = new StreamResult(new File("<location/of/file>"));
transformer.transform(source, result);
.......
Or you can use the writexml method from the minimal DOM implementation, minidom if you are using Python.
Gabriel Iuhasz, 2019-10-30, iuhasz.gabriel@e-uvt.ro