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XPath is a language for finding information in an XML file. You can say that XPath is (sort of) SQL for XML files. XPath is used to navigate through elements and attributes in an XML document. You can also use XPath to traverse through an XML file in Java.
XPath comes with powerful expressions that can be used to parse an xml document and retrieve relevant information.
For demo, let us consider an xml file that holds information of employees.
<?xml version="1.0"?>
<Employees>
<Employee emplid="1111" type="admin">
<firstname>John</firstname>
<lastname>Watson</lastname>
<age>30</age>
<email>johnwatson@sh.com</email>
</Employee>
<Employee emplid="2222" type="admin">
<firstname>Sherlock</firstname>
<lastname>Homes</lastname>
<age>32</age>
<email>sherlock@sh.com</email>
</Employee>
<Employee emplid="3333" type="user">
<firstname>Jim</firstname>
<lastname>Moriarty</lastname>
<age>52</age>
<email>jim@sh.com</email>
</Employee>
<Employee emplid="4444" type="user">
<firstname>Mycroft</firstname>
<lastname>Holmes</lastname>
<age>41</age>
<email>mycroft@sh.com</email>
</Employee>
</Employees>
I have saved this file at path C:\employees.xml. We will use this xml file in our demo and will try to fetch useful information using XPath. Before we start lets check few facts from above xml file.
emplid type which defines whether an employee is admin or user.firstname, lastname, age and email Let’s get started…
In order to understand XPath, first we need to understand basics of DOM parsing in Java. Java provides powerful implementation of domparser in form of below API.
First, we need to create a document builder using DocumentBuilderFactory class. Just follow the code. It’s pretty much self explainatory.
import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.parsers.ParserConfigurationException;
//...
DocumentBuilderFactory builderFactory =
DocumentBuilderFactory.newInstance();
DocumentBuilder builder = null;
try {
builder = builderFactory.newDocumentBuilder();
} catch (ParserConfigurationException e) {
e.printStackTrace();
}
Once we have a document builder object. We uses it to parse XML file and create a document object.
import org.w3c.dom.Document;
import java.io.IOException;
import org.xml.sax.SAXException;
//...
try {
Document document = builder.parse(
new FileInputStream("c:\\employees.xml"));
} catch (SAXException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
In above code, we are parsing an XML file from filesystem. Sometimes you might want to parse XML specified as String value instead of reading it from file. Below code comes handy to parse XML specified as String.
String xml = ...;
Document xmlDocument = builder.parse(new ByteArrayInputStream(xml.getBytes()));
Once we have document object. We are ready to use XPath. Just create an xpath object using XPathFactory.
import javax.xml.xpath.XPath;
import javax.xml.xpath.XPathFactory;
//...
XPath xPath = XPathFactory.newInstance().newXPath();
Use xpath object to complie an XPath expression and evaluate it on document. In below code we read email address of employee having employee id = 3333. Also we have specified APIs to read an XML node and a nodelist.
String expression = "/Employees/Employee[@emplid='3333']/email";
//read a string value
String email = xPath.compile(expression).evaluate(xmlDocument);
//read an xml node using xpath
Node node = (Node) xPath.compile(expression).evaluate(xmlDocument, XPathConstants.NODE);
//read a nodelist using xpath
NodeList nodeList = (NodeList) xPath.compile(expression).evaluate(xmlDocument, XPathConstants.NODESET);
As mentioned above, XPath uses a path expression to select nodes or list of node from an xml document. Heres a list of useful paths and expression that can be used to select any node/nodelist from an xml document.
| Expression | Description |
|---|---|
nodename | Selects all nodes with the name “nodename” |
/ | Selects from the root node |
// | Selects nodes in the document from the current node that match the selection no matter where they are |
. | Selects the current node |
.. | Selects the parent of the current node |
@ | Selects attributes |
employee | Selects all nodes with the name “employee” |
employees/employee | Selects all employee elements that are children of employees |
//employee | Selects all book elements no matter where they are in the document |
Below list of expressions are called Predicates. The Predicates are defined in square brackets [ ... ]. They are used to find a specific node or a node that contains a specific value.
| Path Expression | Result |
|---|---|
/employees/employee[1] | Selects the first employee element that is the child of the employees element. |
/employees/employee[last()] | Selects the last employee element that is the child of the employees element |
/employees/employee[last()-1] | Selects the last but one employee element that is the child of the employees element |
//employee[@type='admin'] | Selects all the employee elements that have an attribute named type with a value of ‘admin’ |
There are other useful expressions that you can use to query the data.
Read this w3school page for more details: http://www.w3schools.com/xpath/xpath_syntax.asp
Below are few examples of using different expressions of xpath to fetch some information from xml document.
Below expression will read firstname of all the employees.
String expression = "/Employees/Employee/firstname";
System.out.println(expression);
NodeList nodeList = (NodeList) xPath.compile(expression).evaluate(xmlDocument, XPathConstants.NODESET);
for (int i = 0; i < nodeList.getLength(); i++) {
System.out.println(nodeList.item(i).getFirstChild().getNodeValue());
}
Output:
John
Sherlock
Jim
Mycroft
Below expression will read employee information for employee with emplid = 2222. Check how we used API to retrieve node information and then traveresed this node to print xml tag and its value.
String expression = "/Employees/Employee[@emplid='2222']";
System.out.println(expression);
Node node = (Node) xPath.compile(expression).evaluate(xmlDocument, XPathConstants.NODE);
if(null != node) {
nodeList = node.getChildNodes();
for (int i = 0;null!=nodeList && i < nodeList.getLength(); i++) {
Node nod = nodeList.item(i);
if(nod.getNodeType() == Node.ELEMENT_NODE)
System.out.println(nodeList.item(i).getNodeName() + " : " + nod.getFirstChild().getNodeValue());
}
}
Output:
firstname : Sherlock
lastname : Homes
age : 32
email : sherlock@sh.com
This is again a predicate example to read firstname of all employee who are admin (defined by type=admin).
String expression = "/Employees/Employee[@type='admin']/firstname";
System.out.println(expression);
NodeList nodeList = (NodeList) xPath.compile(expression).evaluate(xmlDocument, XPathConstants.NODESET);
for (int i = 0; i < nodeList.getLength(); i++) {
System.out.println(nodeList.item(i).getFirstChild().getNodeValue());
}
Output:
John
Sherlock
See how we used predicate to filter employees who has age > 40.
String expression = "/Employees/Employee[age>40]/firstname";
NodeList nodeList = (NodeList) xPath.compile(expression).evaluate(xmlDocument, XPathConstants.NODESET);
for (int i = 0; i < nodeList.getLength(); i++) {
System.out.println(nodeList.item(i).getFirstChild().getNodeValue());
}
Output:
Jim
Mycroft
Within predicates, you can use position() to identify the position of xml element. Here we are filtering first two employees using position().
String expression = "/Employees/Employee[position() <= 2]/firstname";
NodeList nodeList = (NodeList) xPath.compile(expression).evaluate(xmlDocument, XPathConstants.NODESET);
for (int i = 0; i < nodeList.getLength(); i++) {
System.out.println(nodeList.item(i).getFirstChild().getNodeValue());
}
Output:
John
Sherlock
In order to execute this source, just create a basic Java project in your IDE or just save below code in Main.java and execute. It will need employees.xml file as input. Copy the employee xml defined in start of this tutorial at c:\\employees.xml.
package net.viralpatel.java;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
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.XPathExpressionException;
import javax.xml.xpath.XPathFactory;
import org.w3c.dom.Document;
import org.w3c.dom.Node;
import org.w3c.dom.NodeList;
import org.xml.sax.SAXException;
public class Main {
public static void main(String[] args) {
try {
FileInputStream file = new FileInputStream(new File("c:/employees.xml"));
DocumentBuilderFactory builderFactory = DocumentBuilderFactory.newInstance();
DocumentBuilder builder = builderFactory.newDocumentBuilder();
Document xmlDocument = builder.parse(file);
XPath xPath = XPathFactory.newInstance().newXPath();
System.out.println("*************************");
String expression = "/Employees/Employee[@emplid='3333']/email";
System.out.println(expression);
String email = xPath.compile(expression).evaluate(xmlDocument);
System.out.println(email);
System.out.println("*************************");
expression = "/Employees/Employee/firstname";
System.out.println(expression);
NodeList nodeList = (NodeList) xPath.compile(expression).evaluate(xmlDocument, XPathConstants.NODESET);
for (int i = 0; i < nodeList.getLength(); i++) {
System.out.println(nodeList.item(i).getFirstChild().getNodeValue());
}
System.out.println("*************************");
expression = "/Employees/Employee[@type='admin']/firstname";
System.out.println(expression);
nodeList = (NodeList) xPath.compile(expression).evaluate(xmlDocument, XPathConstants.NODESET);
for (int i = 0; i < nodeList.getLength(); i++) {
System.out.println(nodeList.item(i).getFirstChild().getNodeValue());
}
System.out.println("*************************");
expression = "/Employees/Employee[@emplid='2222']";
System.out.println(expression);
Node node = (Node) xPath.compile(expression).evaluate(xmlDocument, XPathConstants.NODE);
if(null != node) {
nodeList = node.getChildNodes();
for (int i = 0;null!=nodeList && i < nodeList.getLength(); i++) {
Node nod = nodeList.item(i);
if(nod.getNodeType() == Node.ELEMENT_NODE)
System.out.println(nodeList.item(i).getNodeName() + " : " + nod.getFirstChild().getNodeValue());
}
}
System.out.println("*************************");
expression = "/Employees/Employee[age>40]/firstname";
nodeList = (NodeList) xPath.compile(expression).evaluate(xmlDocument, XPathConstants.NODESET);
System.out.println(expression);
for (int i = 0; i < nodeList.getLength(); i++) {
System.out.println(nodeList.item(i).getFirstChild().getNodeValue());
}
System.out.println("*************************");
expression = "/Employees/Employee[1]/firstname";
System.out.println(expression);
nodeList = (NodeList) xPath.compile(expression).evaluate(xmlDocument, XPathConstants.NODESET);
for (int i = 0; i < nodeList.getLength(); i++) {
System.out.println(nodeList.item(i).getFirstChild().getNodeValue());
}
System.out.println("*************************");
expression = "/Employees/Employee[position() <= 2]/firstname";
System.out.println(expression);
nodeList = (NodeList) xPath.compile(expression).evaluate(xmlDocument, XPathConstants.NODESET);
for (int i = 0; i < nodeList.getLength(); i++) {
System.out.println(nodeList.item(i).getFirstChild().getNodeValue());
}
System.out.println("*************************");
expression = "/Employees/Employee[last()]/firstname";
System.out.println(expression);
nodeList = (NodeList) xPath.compile(expression).evaluate(xmlDocument, XPathConstants.NODESET);
for (int i = 0; i < nodeList.getLength(); i++) {
System.out.println(nodeList.item(i).getFirstChild().getNodeValue());
}
System.out.println("*************************");
} catch (FileNotFoundException e) {
e.printStackTrace();
} catch (SAXException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
} catch (ParserConfigurationException e) {
e.printStackTrace();
} catch (XPathExpressionException e) {
e.printStackTrace();
}
}
}
That’s all folks 
If you’re having a hard time getting your kid to sleep in the evening and reading from the old fashioned storybook doesn’t seem to do the job, this new invention might be what you need. The Smart PJs are interactive pajamas with built-in bedtime stories, promising to make that whole ‘going to bed’ ritual more fun for you and your child.
The Smart PJs are advertized as the world’s first and only interactive pajamas. They are covered in a multi-colored dot pattern which can be scanned with a smartphone or other tablet device equipped with specific apps, bringing up various bedtime stories or animal pictures on the screen.
Every pattern generates a different code, which in turn brings up a unique story. Parents have the option of either reading that story to their child or listening to it played back. It is not yet known whether it will be possible to add any new stories to the Smart PJs after all the codes are scanned.
Apparently the system will work with several specific applications that the developers are working on and will not require any Internet connectivity once the apps are downloaded. At the moment, there are two such apps available.
The interactive pajamas were invented by realtor Juan Murdoch, who based his concept on the QR codes we see on virtually any product nowadays. As for criticism that the invention may make it worse for a generation of children already fascinated and almost addicted to technology, Murdoch dismissed any concerns. He insisted that beyond the use of mobile tech and apps, what matters the most is that Smart PJs will get parents to spend more time with their children.
We admit the idea of interactive pajamas is cool, as what kid wouldn’t like going around in PJs that have built-in stories? As for the effects such technology may have, in the end it depends on how the parents will use it. If you decide to give the Smart PJs a try, they are available in four sizes for both boys and girls and are available to be purchased online, for $25.
The post Smart PJs Have Built-in Bedtime Stories appeared first on TechBeat.
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