Java and XML – Tutorial

Posted: February 28, 2012 in Java Posts
Tags: , , , , , , , , , , , , , , , ,

1. XML Introduction

1.1. XML Overview

XML stands for Extensible Markup Language and was defined 1998 by the World Wide Web Consortium (W3C).

A XML document consists out of elements, each element has a start tag, content and an end tag. A XML document must have exactly one root element, e.g. one tag which encloses the remaining tags. XML makes a difference between capital and non-capital letters.

A XML file is required to be well-formated.

Well-formated XML must apply to the following conditions:

  • A XML document always starts with a prolog (see below for an explanation of what a prolog is)
  • Every tag has a closing tag.
  • All tags are completely nested.

 

A XML file is valid if it is well-formated and if it is contains a link to a XML schema and is valid according to the schema.

In general the following is considered as advantages in using XML for data processing / representation.

  • XML is Plain text
  • XML allows the data identification without any display information
  • Style can be defined via XSL
  • Easily processed due to it regular and consistent notation
  • XML files are hierarchical

 

1.2. XML Example

The following is a valid, well-formated XML file.

 

				
<?xml version="1.0"?>
<!-- This is a comment -->
<address>
	<name>Lars </name>
	<street> Test </street>
	<telephon number= "0123"/>
</address>

 

1.3. XML Elements

1.3.1. Prolog

A XML document always starts with a prolog which describes XML file. This prolog can be minimal, e.g. <?xml version=”1.0″?> or can contain other information, e.g. the encoding, e.g. <?xml version=”1.0″ encoding=”UTF-8″ standalone=”yes” ?>

1.3.2. Empty Tag

A tag which doesn’t enclose any content is know as an “empty tag”. For example: <flag/>

1.3.3. Commends

Comments in XML are defined as: <! COMMENT>

2. Java XML Overview

Java contains several methods to access XML. The following is a short overview of the available methods.

Java provides the DOM API (Document Object Model). In DOM your access the XML document over an object tree. DOM can be used to read and write XML files.

SAX (Simple API for XML) is an Java API to sequential reading of XML files. SAX can only read XML documents. SAX provides an Event-Driven XML Processing following the Push-Parsing Model. What this model means is that in SAX, Applications will register Listeners in the form of Handlers to the Parser and will get notified through Call-back methods. Here the SAX Parser takes the control over Application thread by Pushing Events to the Application. Both DOM and Sax are older API’s and I recommend not to use them anymore.

Stax (Streaming API for XML) is an API for reading and writing XML Documents. Introduced in Java 6.0 and considered as superior to SAX and DOM.

Java Architecture for XML Binding ( JAXB ) is a Java standard that defines how Java objects are converted to/from XML (specified using a standard set of mappings. JAXB defines a programmer API for reading and writing Java objects to / from XML documents and a service provider which / from from XML documents allows the selection of the JAXB implementation JAXB applies a lot of defaults thus making reading and writing of XML via Java very easy.

The following will explain the Stax interface, for an introduction into JAXB please see JAXB tutorial.

3. Streaming API for XML (StaX)

3.1. Overview

Streaming API for XML, called StaX, is an API for reading and writing XML Documents.

StaX is a Pull-Parsing model. Application can take the control over parsing the XML documents by pulling (taking) the events from the parser.

The core StaX API falls into two categories and they are listed below. They are

  • Cursor API
  • Event Iterator API

 

Applications can any of these two API for parsing XML documents. The following will focus on the event iterator API as I consider it more convenient to use.

3.2. Event Iterator API

The event iterator API has two main interfaces: XMLEventReader for parsing XML and XMLEventWriter for generating XML.

3.3. XMLEventReader – Read XML Example

This example is stored in project “de.vogella.xml.stax.reader”.

Applications loop over the entire document requesting for the Next Event. The Event Iterator API is implemented on top of Cursor API.

In this example we will read the following XML document and create objects from it. file.

 

				
<?xml version="1.0" encoding="UTF-8"?>
<config>
	<item date="January 2009">
		<mode>1</mode>
		<unit>900</unit>
		<current>1</current>
		<interactive>1</interactive>
	</item>
	<item date="February 2009">
		<mode>2</mode>
		<unit>400</unit>
		<current>2</current>
		<interactive>5</interactive>
	</item>
	<item date="December 2009">
		<mode>9</mode>
		<unit>5</unit>
		<current>100</current>
		<interactive>3</interactive>
	</item>
</config>

 

Define therefore the following class to store the individual entries of the XML file.

 

				
package de.vogella.xml.stax.model;

public class Item {
	private String date; 
	private String mode;
	private String unit;
	private String current;
	private String interactive;

	public String getDate() {
		return date;
	}

	public void setDate(String date) {
		this.date = date;
	}
	public String getMode() {
		return mode;
	}
	public void setMode(String mode) {
		this.mode = mode;
	}
	public String getUnit() {
		return unit;
	}
	public void setUnit(String unit) {
		this.unit = unit;
	}
	public String getCurrent() {
		return current;
	}
	public void setCurrent(String current) {
		this.current = current;
	}
	public String getInteractive() {
		return interactive;
	}
	public void setInteractive(String interactive) {
		this.interactive = interactive;
	}

	@Override
	public String toString() {
		return "Item [current=" + current + ", date=" + date + ", interactive="
				+ interactive + ", mode=" + mode + ", unit=" + unit + "]";
	}
}

 

The following reads the XML file and creates a List of object Items from the entries in the XML file.

 

				
package de.vogella.xml.stax.read;

import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.InputStream;
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

import javax.xml.stream.XMLEventReader;
import javax.xml.stream.XMLInputFactory;
import javax.xml.stream.XMLStreamException;
import javax.xml.stream.events.Attribute;
import javax.xml.stream.events.EndElement;
import javax.xml.stream.events.StartElement;
import javax.xml.stream.events.XMLEvent;

import de.vogella.xml.stax.model.Item;

public class StaXParser {
	static final String DATE = "date";
	static final String ITEM = "item";
	static final String MODE = "mode";
	static final String UNIT = "unit";
	static final String CURRENT = "current";
	static final String INTERACTIVE = "interactive";

	@SuppressWarnings({ "unchecked", "null" })
	public List<Item> readConfig(String configFile) {
		List<Item> items = new ArrayList<Item>();
		try {
			// First create a new XMLInputFactory
			XMLInputFactory inputFactory = XMLInputFactory.newInstance();
			// Setup a new eventReader
			InputStream in = new FileInputStream(configFile);
			XMLEventReader eventReader = inputFactory.createXMLEventReader(in);
			// Read the XML document
			Item item = null;

			while (eventReader.hasNext()) {
				XMLEvent event = eventReader.nextEvent();

				if (event.isStartElement()) {
					StartElement startElement = event.asStartElement();
					// If we have a item element we create a new item
					if (startElement.getName().getLocalPart() == (ITEM)) {
						item = new Item();
						// We read the attributes from this tag and add the date
						// attribute to our object
						Iterator<Attribute> attributes = startElement
								.getAttributes();
						while (attributes.hasNext()) {
							Attribute attribute = attributes.next();
							if (attribute.getName().toString().equals(DATE)) {
								item.setDate(attribute.getValue());
							}

						}
					}

					if (event.isStartElement()) {
						if (event.asStartElement().getName().getLocalPart()
								.equals(MODE)) {
							event = eventReader.nextEvent();
							item.setMode(event.asCharacters().getData());
							continue;
						}
					}
					if (event.asStartElement().getName().getLocalPart()
							.equals(UNIT)) {
						event = eventReader.nextEvent();
						item.setUnit(event.asCharacters().getData());
						continue;
					}

					if (event.asStartElement().getName().getLocalPart()
							.equals(CURRENT)) {
						event = eventReader.nextEvent();
						item.setCurrent(event.asCharacters().getData());
						continue;
					}

					if (event.asStartElement().getName().getLocalPart()
							.equals(INTERACTIVE)) {
						event = eventReader.nextEvent();
						item.setInteractive(event.asCharacters().getData());
						continue;
					}
				}
				// If we reach the end of an item element we add it to the list
				if (event.isEndElement()) {
					EndElement endElement = event.asEndElement();
					if (endElement.getName().getLocalPart() == (ITEM)) {
						items.add(item);
					}
				}

			}
		} catch (FileNotFoundException e) {
			e.printStackTrace();
		} catch (XMLStreamException e) {
			e.printStackTrace();
		}
		return items;
	}

}

 

You can test the parser via the following test program. Please note that the file config.xml must exist in the Java project folder.

 

				
package de.vogella.xml.stax.read;

import java.util.List;

import de.vogella.xml.stax.model.Item;

public class TestRead {
	public static void main(String args[]) {
		StaXParser read = new StaXParser();
		List<Item> readConfig = read.readConfig("config.xml");
		for (Item item : readConfig) {
			System.out.println(item);
		}
	}
}

 

3.4. Write XML File- Example

This example is stored in project “de.vogella.xml.stax.writer”.

Lets assume you would like to write the following simple XML file.

 

				
<?xml version="1.0" encoding="UTF-8"?>
<config>
	<mode>1</mode>
	<unit>900</unit>
	<current>1</current>
	<interactive>1</interactive>
</config>

 

StaX does not provide functionality to format the XML file automatically. So you have to add end-of-lines and tab information to your XML file.

 

				
package de.vogella.xml.stax.writer;

import java.io.FileOutputStream;

import javax.xml.stream.XMLEventFactory;
import javax.xml.stream.XMLEventWriter;
import javax.xml.stream.XMLOutputFactory;
import javax.xml.stream.XMLStreamException;
import javax.xml.stream.events.Characters;
import javax.xml.stream.events.EndElement;
import javax.xml.stream.events.StartDocument;
import javax.xml.stream.events.StartElement;
import javax.xml.stream.events.XMLEvent;

public class StaxWriter {
	private String configFile;

	public void setFile(String configFile) {
		this.configFile = configFile;
	}

	public void saveConfig() throws Exception {
		// Create a XMLOutputFactory
		XMLOutputFactory outputFactory = XMLOutputFactory.newInstance();
		// Create XMLEventWriter
		XMLEventWriter eventWriter = outputFactory
				.createXMLEventWriter(new FileOutputStream(configFile));
		// Create a EventFactory
		XMLEventFactory eventFactory = XMLEventFactory.newInstance();
		XMLEvent end = eventFactory.createDTD("\n");
		// Create and write Start Tag
		StartDocument startDocument = eventFactory.createStartDocument();
		eventWriter.add(startDocument);

		// Create config open tag
		StartElement configStartElement = eventFactory.createStartElement("",
				"", "config");
		eventWriter.add(configStartElement);
		eventWriter.add(end);
		// Write the different nodes
		createNode(eventWriter, "mode", "1");
		createNode(eventWriter, "unit", "901");
		createNode(eventWriter, "current", "0");
		createNode(eventWriter, "interactive", "0");

		eventWriter.add(eventFactory.createEndElement("", "", "config"));
		eventWriter.add(end);
		eventWriter.add(eventFactory.createEndDocument());
		eventWriter.close();
	}

	private void createNode(XMLEventWriter eventWriter, String name,
			String value) throws XMLStreamException {

		XMLEventFactory eventFactory = XMLEventFactory.newInstance();
		XMLEvent end = eventFactory.createDTD("\n");
		XMLEvent tab = eventFactory.createDTD("\t");
		// Create Start node
		StartElement sElement = eventFactory.createStartElement("", "", name);
		eventWriter.add(tab);
		eventWriter.add(sElement);
		// Create Content
		Characters characters = eventFactory.createCharacters(value);
		eventWriter.add(characters);
		// Create End node
		EndElement eElement = eventFactory.createEndElement("", "", name);
		eventWriter.add(eElement);
		eventWriter.add(end);

	}

}

 

And a little test.

 

				
package de.vogella.xml.stax.writer;

public class TestWrite {

	public static void main(String[] args) {
		StaxWriter configFile = new StaxWriter();
		configFile.setFile("config2.xml");
		try {
			configFile.saveConfig();
		} catch (Exception e) {
			e.printStackTrace();
		}
	}
}

 

For another (more complex example of using Stax please see Reading and creating RSS feeds via Java (with Stax)

4. XPath

4.1. Overview

XPath (XML Path Language) is a language for selecting / searching nodes from an XML document. Java 5 introduced the javax.xml.xpath package which provides a XPath library.

The following explains how to use XPath to query an XML document via Java.

4.2. Using XPath

The following explains how to use XPath. Create a new Java project called “UsingXPath”.

Create the following xml file.

 

				
<?xml version="1.0" encoding="UTF-8"?>
<people>
	<person>
		<firstname>Lars</firstname>
		<lastname>Vogel</lastname>
		<city>Heidelberg</city>
	</person>
	<person>
		<firstname>Jim</firstname>
		<lastname>Knopf</lastname>
		<city>Heidelberg</city>
	</person>
	<person>
		<firstname>Lars</firstname>
		<lastname>Strangelastname</lastname>
		<city>London</city>
	</person>
	<person>
		<firstname>Landerman</firstname>
		<lastname>Petrelli</lastname>
		<city>Somewhere</city>
	</person>
	<person>
		<firstname>Lars</firstname>
		<lastname>Tim</lastname>
		<city>SomewhereElse</city>
	</person>
</people>

 

Create a new package “myxml” and a new Java class “QueryXML”.

 

				
package myxml;

import java.io.IOException;

import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.parsers.ParserConfigurationException;
import javax.xml.xpath.XPath;
import javax.xml.xpath.XPathConstants;
import javax.xml.xpath.XPathExpression;
import javax.xml.xpath.XPathExpressionException;
import javax.xml.xpath.XPathFactory;

import org.w3c.dom.Document;
import org.w3c.dom.NodeList;
import org.xml.sax.SAXException;

public class QueryXML {
	public void query() throws ParserConfigurationException, SAXException,
			IOException, XPathExpressionException {
		// Standard of reading a XML file
		DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
		factory.setNamespaceAware(true);
		DocumentBuilder builder;
		Document doc = null;
		XPathExpression expr = null;
		builder = factory.newDocumentBuilder();
		doc = builder.parse("person.xml");

		// Create a XPathFactory
		XPathFactory xFactory = XPathFactory.newInstance();

		// Create a XPath object
		XPath xpath = xFactory.newXPath();

		// Compile the XPath expression
		expr = xpath.compile("//person[firstname='Lars']/lastname/text()");
		// Run the query and get a nodeset
		Object result = expr.evaluate(doc, XPathConstants.NODESET);

		// Cast the result to a DOM NodeList
		NodeList nodes = (NodeList) result;
		for (int i=0; i<nodes.getLength();i++){
			System.out.println(nodes.item(i).getNodeValue());
		}

		// New XPath expression to get the number of people with name lars
		expr = xpath.compile("count(//person[firstname='Lars'])");
		// Run the query and get the number of nodes
		Double number = (Double) expr.evaluate(doc, XPathConstants.NUMBER);
		System.out.println("Number of objects " +number);

		// Do we have more then 2 people with name lars?
		expr = xpath.compile("count(//person[firstname='Lars']) >2");
		// Run the query and get the number of nodes
		Boolean check = (Boolean) expr.evaluate(doc, XPathConstants.BOOLEAN);
		System.out.println(check);

	}

	public static void main(String[] args) throws XPathExpressionException, ParserConfigurationException, SAXException, IOException {
		QueryXML process = new QueryXML();
		process.query();
	}
}

 

5. Thank you

 

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