4. JSON
• JSON: JavaScript Object Notation.
• JSON is syntax for storing and exchanging text
information.
• JSON is smaller than XML, and faster and easier to
parse.
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5. JSON
Maps to two universal structures
1. An unordered collection of name value pairs:
{"firstName”:"Nicole”,"lastName": "Kidman”}
2. An ordered list of values
["Monday”, "Tuesday”, "Wednesday”]
Source: Theresa Velden
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7. JSON syntax
• An object is an unordered set of name/value pairs
•
•
•
•
The pairs are enclosed within braces, { }
There is a colon between the name and the value
Pairs are separated by commas
Example: { "firstName":"John" , "lastName":"Doe" }
• An array is an ordered collection of values
• The values are enclosed within brackets, [ ]
• Values are separated by commas
• Example: [ "html", xml", "css" ]
14. Introduction to XML
• Why is XML important?
• simple open non-proprietary widely accepted data
exchange format
• XML is like HTML but
• no fixed set of tags
− X = “extensible”
• no fixed semantics (c.q. representation) of tags
− representation determined by separate ‘style sheet’
− semantics determined by application
• no fixed structure
− user-defined schemas
16. Elements of an XML Document
• Global structure
• Mandatory first line
<?xml version ="1.0"?>
• A single root element
<order>
. . .
</order>
• Elements have a recursive structure
• Tags are chosen by author;
<item>, <itemId>, <itemName>
• Opening tag must have a matching closing tag
<item></item>, <a><b></b></a>
17. Elements of an XML Document
• The content of an element is a sequence of:
− Elements
<item> … </item>
− Text
Jan Vijs
− Processing Instructions
<! . . . !>
− Comments
<!– This is a comment --!>
• Empty elements can be abbreviated:
<item/> is shorthand for
<item></item>
18. Elements of an XML Document
• Elements can have attributes
<Title Value="Student List"/>
<PersonList Type="Student" Date="2004-12-12">
. . .
</Personlist>
Attribute_name = “Value”
Attribute name can only occur once
Value is always quoted text (even numbers)
19. Elements of an XML Document
• Text and elements can be freely mixed
<Course ID=“2ID45”>
The course <fullname>Database
Technology</fullname> is lectured
by <title>dr.</title>
<fname>George</fname>
<sname>Fletcher</sname>
</Course>
• The order between elements is considered important
• Order between attributes is not
20. Well-formedness
• We call an XML-document well-formed iff
• it has one root element;
• elements are properly nested;
• any attribute can only occur once in a given opening
tag and its value must be quoted.
• Check for instance at:
http://www.w3schools.com/xml/xml_validator.asp
22. Parsing XML
• Goal
• Read XML files into data structures in programming
languages
• Possible strategies
• Parse into generic tree structure (DOM)
• Parse as sequence of events (SAX)
• Automatically parse to language-specific objects (JAXB)
23. DOM
• A DOM document is an object containing all the
information of an XML document
• It is composed of a tree (DOM tree) of nodes
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24. DOM parsers
Different types of nodes
Document node
Element node
Text node
Attribute node
Processing instruction node
…….
30. DOM tree example
Example code fragment:
Node order= doc.getFirstChild();
order
item
CustomerId
_123456789
item
CustomerId
_333445555
item
CustomerId
_999887777
31. DOM tree example
Example code fragment:
Node order= doc.getFirstChild();
NodeList items= order.getChildNodes();
order
item
CustomerId
_123456789
item
CustomerId
_333445555
item
CustomerId
_999887777
32. DOM tree example
Example code fragment:
NodeList items=
doc.getElementsByTagName("item")
order
item
CustomerId
_123456789
item
CustomerId
_333445555
item
CustomerId
_999887777
33. Main features of DOM parsers
• A DOM parser creates an internal structure in memory
which is a DOM document object
• Client applications get the information of the original
XML document by invoking methods on this Document
object or on other objects it contains
• DOM parser is tree-based
34. Parsing XML in Java
• Package javax.xml.parsers
• Provides classes allowing the processing of XML
documents.
• SAX (Simple API for XML)
• DOM (Document Object Model)
36. Representing objects in Java
public class Item {
String id;
String name;
String quantity;
public Item(String id, String name, String quantity) {
this.id = id;
this.name = name;
this.quantity = quantity;
}
public Item(){}
+ getters and setters
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37. Representing objects in Java
public class Order {
private String date;
private String customerId;
private String customerName;
private ArrayList<Item> items;
public Order(String date, String customerId, String customerName) {
this.date = date;
this.customerId = customerId;
this.customerName = customerName;
items=new ArrayList<Item>();
}
…
}
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38. DOM parser example
public void parse(String fileName) throws Exception{
DocumentBuilderFactory factory =
DocumentBuilderFactory.newInstance();
DocumentBuilder builder = factory.newDocumentBuilder();
Document doc = builder.parse(fileName);
NodeList itemList = doc.getElementsByTagName("item");
for (int i = 0; i < itemList.getLength(); i++)
{
Node itemNode = itemList.item(i);
Item item=getItem(itemNode);
}
}
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39. DOM tree example
Example code fragment:
NodeList items=
doc.getElementsByTagName("item")
order
item
CustomerId
_123456789
item
CustomerId
_333445555
item
CustomerId
_999887777
40. DOM parser example
public Item getItem (Node n){
NodeList itemElements = n.getChildNodes();
Item item = new Item();
for (int j = 0; j < itemElements.getLength(); j++)
{
Node node=itemElements.item(j);
if (node.getNodeName().equalsIgnoreCase("itemId"))
item.setId(node.getTextContent());
else if (node.getNodeName().equalsIgnoreCase("itemName"))
item.setName(node.getTextContent());
else if (node.getNodeName().equalsIgnoreCase("quantity"))
item.setQuantity(node.getTextContent());
}
return item;
}
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41. JAXB
• JAXB: Java API for XML Bindings
• Defines an API for automatically representing XML
schema as collections of Java classes.
42. Annotations markup
•
•
•
•
@XmlAttribute to designate a field as an attribute
@XmlRootElement to designate the document root element.
@XmlElement to designate a field as a node element
@XmlElementWrapper to specify the element that encloses a
repeating series of elements
• Note that you should specify only the getter method as
@XmlAttribute or @XmlElement.
• Jaxb oddly treats both the field and the getter method as
independent entities
44. Order example
import javax.xml.bind.annotation.*;
import java.util.*;
@XmlRootElement
public class Order {
@XmlElement
private String date;
@XmlElement
private String customerId;
@XmlElement
private String customerName;
@XmlElement
private List<Item> items;
public Order() {
this.items=new ArrayList<Item>();
}
45. Marshalling
• marshalling
• the process of producing an XML document from Java objects
• unmarshalling
• the process of producing a content tree from an XML document
• JAXB only allows you to unmarshal valid XML documents
• JAXB only allows you to marshal valid content trees into XML
46. Marshalling example
public String toXmlString(){
try{
JAXBContext context=JAXBContext.newInstance(Order.class);
Marshaller m = context.createMarshaller();
m.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, Boolean.TRUE);
ByteArrayOutputStream b=new ByteArrayOutputStream();
m.marshal(this,b);
return b.toString();
}catch (Exception e){
e.printStackTrace();
return null;
}
}
47. Unmarshalling example
public Order fromXmlString(String s){
try{
JAXBContext jaxbContext = JAXBContext.newInstance(Order.class);
Unmarshaller jaxbUnmarshaller = jaxbContext.createUnmarshaller();
Order order = (Order) jaxbUnmarshaller.unmarshal(new StreamSource( new
StringReader(s)));
return order;
}catch (Exception e){
e.printStackTrace();
return null;
}
}
48. Test transformation
public static void main(String args[]){
Order o=new Order("1 March 2013", "123", "Katrien");
o.getItems().add(new Item("1", "iPhone 5", 2));
o.getItems().add(new Item("2", "Nokia Lumia 800", 2));
System.out.println(o.toXmlString());
}
