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 use 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 the Cursor API.
In this example we will read the following XML document and create objects from it.
<?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 an 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".
Let's 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 automatically format the XML file 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 an XMLOutputFactory
XMLOutputFactory outputFactory = XMLOutputFactory.newInstance();
// create XMLEventWriter
XMLEventWriter eventWriter = outputFactory
.createXMLEventWriter(new FileOutputStream(configFile));
// create an 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).
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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 for reading an 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 an XPathFactory
XPathFactory xFactory = XPathFactory.newInstance();
// create an 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 than 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();
}
}
