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Java StAX parser is a Java API which is used to parse XML documents and query the necessary information. This API is event based and hence we need not load entire XML document to query it. As the parser identifies each event, the corresponding action is performed only when the client program implements the event.
In this chapter, we are going to see how to query an XML document to get necessary information.
Query XML Using Java StAX Parser
Following are the steps we need to follow to query an XML document using Java StAX parser −
- Step 1: Creating XMLInputFactory instance
- Step 2: Reading the XML
- Step 3: Parsing the XML
- Step 4: Querying the Elements
Refer this chapter for first three steps
Step 4: Querying the Elements
After following the first three steps, we have XMLEventReader to get events from the XML document. Using various events and by implementing them, we can query the XML document. Let us see how we can do this in detail.
Querying Elements by Text Content
We can get the text content in any element of an XML document by using getData().This method of Characters interface returns the character data of the current event in the form of a String. Using this method, we can query all the elements to find the required text content.
Example
The cars.xml file shown below has many carname elements with different text content. Let us query this file to find out if “Bentley 2” car is present.
<?xml version = "1.0"?>
<cars>
<carname company="Ferarri" >Ferarri 101</carname>
<carname company="Lamborgini">Lamborgini 001</carname>
<carname company="Lamborgini">Lamborgini 002</carname>
<carname company="Lamborgini">Lamborgini 003</carname>
<carname company="Bentley">Bentley 1</carname>
<carname company="Bentley">Bentley 2</carname>
<carname company="Bentley">Bentley 3</carname>
</cars>
The following QueryTextContent.java program reads the cars.xml file using a FileReader object. When the CHARACTERS event type is encountered, the getData() method is used to get the text content. If that data is equal to “Bentley 2″, then we are updating the ”found” boolean variable. In the END_DOCUMENT event, we are printing it on the console.
import java.io.FileReader;
import javax.xml.stream.XMLEventReader;
import javax.xml.stream.XMLInputFactory;
import javax.xml.stream.XMLStreamConstants;
import javax.xml.stream.events.Characters;
import javax.xml.stream.events.XMLEvent;
public class QueryTextContent {
public static void main(String args[]) {
try {
//Creating XMLInputFactory instance
XMLInputFactory factory = XMLInputFactory.newInstance();
//Reading the XML
FileReader fileReader = new FileReader("cars.xml");
//Parsing the XML
XMLEventReader eventReader =
factory.createXMLEventReader(fileReader);
//Querying the XML
boolean found=false;
while(eventReader.hasNext()) {
XMLEvent event = eventReader.nextEvent();
if(event.getEventType()==XMLStreamConstants.CHARACTERS) {
Characters characters = event.asCharacters();
String textContent = characters.getData();
if(textContent.equals("Bentley 2"))
found=true;
}
if(event.getEventType()==XMLStreamConstants.END_DOCUMENT) {
if(found)
System.out.println("Bentley 2 car is found");
else
System.out.println("Bentley 2 car is not found");
}
}
} catch(Exception e) {
e.printStackTrace();
}
}
}
Output
Since, Bentley 2 car is present in cars.xml file, it prints that it is found.
Bentley 2 car is found
Querying Elements by Attributes
The getAttributeByName() method of an Element interface takes the QName object which is the qualified XML name of an attribute and returns the Attribute object. Further, the getValue() method of Attribute interface is used to get the value of the attribute in the form of a String.
Example 1
The cars.xml file that we have used in the previous example is parsed in the following QueryAttributes.java program to count the number of Bentley cars present in the XML file. A count variable is incremented each time the company attribute of the carname element is equal to Bentley.
import java.io.FileReader;
import javax.xml.namespace.QName;
import javax.xml.stream.XMLEventReader;
import javax.xml.stream.XMLInputFactory;
import javax.xml.stream.XMLStreamConstants;
import javax.xml.stream.events.Attribute;
import javax.xml.stream.events.StartElement;
import javax.xml.stream.events.XMLEvent;
public class QueryExample2 {
public static void main(String args[]) {
try {
//Creating XMLInputFactory instance
XMLInputFactory factory = XMLInputFactory.newInstance();
//Reading the XML
FileReader fileReader = new FileReader("cars.xml");
//Parsing the XML
XMLEventReader eventReader =
factory.createXMLEventReader(fileReader);
//Querying the XML document
int count=0;
while(eventReader.hasNext()) {
XMLEvent event = eventReader.nextEvent();
if(event.getEventType()==XMLStreamConstants.START_ELEMENT) {
StartElement element=event.asStartElement();
QName qname=new QName("company");
Attribute attr=element.getAttributeByName(qname);
if(attr!=null && attr.getValue().equals("Bentley"))
count++;
}
}
System.out.println("No.of Bentley cars found : " + count);
} catch(Exception e) {
e.printStackTrace();
}
}
}
The number of Bentley cars in the XML file are displayed.
No.of Bentley cars found : 3
Example 2
In this example, we are going to retrieve the information of a particular student based on their roll number. Here is the student.xml file we need to query −
<?xml version = "1.0"?>
<class>
<student rollno = "393">
<firstname>dinkar</firstname>
<lastname>kad</lastname>
<nickname>dinkar</nickname>
<marks>85</marks>
</student>
<student rollno = "493">
<firstname>Vaneet</firstname>
<lastname>Gupta</lastname>
<nickname>vinni</nickname>
<marks>95</marks>
</student>
<student rollno = "593">
<firstname>jasvir</firstname>
<lastname>singn</lastname>
<nickname>jazz</nickname>
<marks>90</marks>
</student>
</class>
In the following QueryStudent.java program, we have checked for the events START_ELEMENT, CHARACTERS and END_ELEMENT and performed actions accordingly. when the roll number attribute is equal to 393, we are printing the entire information of the student.
import java.io.FileReader;
import java.util.Iterator;
import javax.xml.stream.XMLEventReader;
import javax.xml.stream.XMLInputFactory;
import javax.xml.stream.XMLStreamConstants;
import javax.xml.stream.events.Attribute;
import javax.xml.stream.events.Characters;
import javax.xml.stream.events.EndElement;
import javax.xml.stream.events.StartElement;
import javax.xml.stream.events.XMLEvent;
public class QueryStudent {
public static void main(String[] args) {
boolean bFirstName = false;
boolean bLastName = false;
boolean bNickName = false;
boolean bMarks = false;
boolean isRequestRollNo = false;
try {
//Creating XMLInputFactory instance
XMLInputFactory factory = XMLInputFactory.newInstance();
//Reading the XML
FileReader fileReader = new FileReader("student.xml");
//Parsing the XML
XMLEventReader eventReader =
factory.createXMLEventReader(fileReader);
//Querying the XML
String requestedRollNo = "393";
while(eventReader.hasNext()) {
XMLEvent event = eventReader.nextEvent();
switch(event.getEventType()) {
case XMLStreamConstants.START_ELEMENT:
StartElement startElement = event.asStartElement();
String qName = startElement.getName().getLocalPart();
if (qName.equalsIgnoreCase("student")) {
Iterator<Attribute> attributes = startElement.getAttributes();
String rollNo = attributes.next().getValue();
if(rollNo.equalsIgnoreCase(requestedRollNo)) {
System.out.println("Start Element : student");
System.out.println("Roll No : " + rollNo);
isRequestRollNo = true;
}
} else if (qName.equalsIgnoreCase("firstname")) {
bFirstName = true;
} else if (qName.equalsIgnoreCase("lastname")) {
bLastName = true;
} else if (qName.equalsIgnoreCase("nickname")) {
bNickName = true;
}
else if (qName.equalsIgnoreCase("marks")) {
bMarks = true;
}
break;
case XMLStreamConstants.CHARACTERS:
Characters characters = event.asCharacters();
if(bFirstName && isRequestRollNo) {
System.out.println("First Name: " + characters.getData());
bFirstName = false;
}
if(bLastName && isRequestRollNo) {
System.out.println("Last Name: " + characters.getData());
bLastName = false;
}
if(bNickName && isRequestRollNo) {
System.out.println("Nick Name: " + characters.getData());
bNickName = false;
}
if(bMarks && isRequestRollNo) {
System.out.println("Marks: " + characters.getData());
bMarks = false;
}
break;
case XMLStreamConstants.END_ELEMENT:
EndElement endElement = event.asEndElement();
if(endElement.getName().getLocalPart().equalsIgnoreCase(
"student") && isRequestRollNo) {
System.out.println("End Element : student");
System.out.println();
isRequestRollNo = false;
}
break;
}
}
} catch (Exception e) {
e.printStackTrace();
}
}
}
All the information of the student with roll number 393 is displayed.
Start Element : student Roll No : 393 First Name: dinkar Last Name: kad Nick Name: dinkar Marks: 85 End Element : student
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