1. XML

2. DCS – SWC 2 Data vs. Information We often use the terms data and information interchangeably More precisely, data is some ”value” of a certain type, like –33 –”High Street 7” –false Data comes without a context

3. DCS – SWC 3 Data vs. Information When we provide a context for the data, the data ( + the context) becomes information, like: –The age of Alan Wake is 33 years –John Peterson lives at High Street 7 –Is Petra Wilson married? false Data + Context = Information

4. DCS – SWC 4 Data vs. Information We could also denote the context as ”data about the data” This is often referred to as meta-data Information is thus composed of: –Data –Meta-data

5. DCS – SWC 5 Data vs. Information This is – more or less – how we structure our communication with each other –The age of Alan Wake is 33 years –John Peterson lives at High Street 7 –Is Petra Wilson married? false Meta-data and data One part is not very useful without the other part…

6. DCS – SWC 6 Data vs. Information Of course, we are often somewhat ”implicit” when we communicate: –He is 22 years (who…?) –My dog is named Kaya (what kind of dog…?) –John is ill (Who is John, what illness…?) We sometimes assume part of the context implicitly, otherwise it would be very tedious to communicate…

7. DCS – SWC 7 Transmitting information When computers transmit information, they can also be more or less implicit A method call is a kind of data transmis- sion, which is highly implicit: CalculateFactorial(int n) n is ”the number for which we want to calculate the factorial”

8. DCS – SWC 8 Transmitting information Suppose a program needs to receive information about some product A product has –A name –A price –A weight How can we transmit this information to the program?

9. DCS – SWC 9 Transmitting information Perhaps just put the data into a file: ”Milk 4.95 1000” The meaning being: –The name of the product –The price of the product (in kroner) –The weight of the product (in grams) –Each element separated by a ” ”

10. DCS – SWC 10 Transmitting information The program can then just read the file, and ”decode” the data However, this assumes that sender and receiver of the data have agreed about how to interpret the file content!

11. DCS – SWC 11 Transmitting information Advantages –A compact format, no space wasted –Fast to process Disadvantages –Static, hard to change –Receiver and sender tied to each other –What about other recipients? –Not humanly readable

12. DCS – SWC 12 Transmitting information Main problem: Meta-data is ”encoded” in the receiving program Probably better to make meta-data explicit, to overcome disadvantages Use a ”markup language” to include meta- data in the transmission

13. DCS – SWC 13 Markup languages In a markup language, we can ”mark” data in a way which conveys the context We mark the data with meta-data An example of a markup language is HTML (HyperText Markup Language): This is very good

14. DCS – SWC 14 Markup languages The markings and are markings (tags) indicating that some meta-data should be applied to the data between the tags – write it in bold In HTML, tags are used for formatting and structure of ”documents”, not for defining structure of data as such Enter XML!

15. DCS – SWC 15 What is XML…? eXtensible Markup Language

16. DCS – SWC 16 XML XML can be seen as a genera- lisation of HTML – tags can be used for everything! All kinds of meta-data can be included as tags in XML Important! XML does not define anything about presentation of data

17. DCS – SWC 17 XML A product defined in XML: Milk 4.95 1000 Start the Product description End the Product description

18. DCS – SWC 18 XML XML is highly recursive Inside a definition, we can have a number of ”child” definitions At some point, the definitions only contains data, like ”Milk” A definition can also have attributes associated with it

19. DCS – SWC 19 XML Milk 4.95 1000

20. DCS – SWC 20 XML When to use attributes vs a child element Attributes should not be data in itself, it should be information about some data element Not a strict rule… When in doubt, use child elements

21. DCS – SWC 21 XML Tempting, but not in the spirit of XML… Harder to process by recipient

22. DCS – SWC 22 XML The general structure of an XML document is then –An XML declaration: –A root element containing the data –Inside the root element; all the child elements

23. DCS – SWC 23 XML Milk 4.95 1000 Orange Juice 8.95 500...

24. DCS – SWC 24 Processing XML documents How do we process an XML document, in order to retrieve data from it? We apply an XML parser to the document The XML parser transforms the XML document into a tree structure The tree structure follows the Document Object Model (DOM)

25. DCS – SWC 25 Processing XML documents products product namepriceweight Milk4.951000

26. DCS – SWC 26 Processing XML documents DocumentBuilderFactory fac = DocumentBuilderFactory.newInstance(); DocumentBuilder builder = fac.newDocumentBuilder(); String fileName =...; File xmlFile = new File(fileName); Document doc = builder.parse(xmlFile); // Now doc contains the DOM tree...

27. DCS – SWC 27 Processing XML documents Given a tree following the DOM standard, we can address various elements in the tree, using the XPath syntax –XPath describes a single node in the tree, or a set of nodes –Syntax similar to directory paths

28. DCS – SWC 28 Processing XML documents products product namepriceweight Milk4.951000 /products/product[1]/weight

29. DCS – SWC 29 Processing XML documents Other XPath constructions: –count(/products/product) – get the number of product instances –/products/product[1]/weight/@unit – get the value of the attribute unit –name(/products/product[1]/*[1]) – get the name of the first child of the first product

30. DCS – SWC 30 Processing XML documents XPathFactory xpfac = XPathFactory.newInstance(); XPath path = xpfac.newXPath();... String result = path.evaluate(”/products/product[1]/price”,doc); // Now result contains the price of the first product...

31. DCS – SWC 31 Processing XML documents In general, we will convert an XML document into a number of Java objects We map XML data to Java classes Up to us to define proper classes to store the data – XML does not know about classes, data is ”objects” Each element in an XML document is like an instance field, not a class…

32. DCS – SWC 32 Creating XML documents In addition to processing given XML documents, we often wish to program- matically produce XML documents For this purpose, we again use the Document- Builder classes

33. DCS – SWC 33 Creating XML documents DocumentBuilderFactory fac = DocumentBuilderFactory.newInstance(); DocumentBuilder builder = fac.newDocumentBuilder(); Document doc = builder.newDocument(); // Now doc contains an empty DOM tree...

34. DCS – SWC 34 Creating XML documents We must now insert node elements into the tree, corresponding to the structure of the data Fundamental methods are: –createElement(String name); –setAttribute(String name,String value); –createTextNode(String text); –appendChild(Element e);

35. DCS – SWC 35 Creating XML documents createElement(String name); Creates an empty new element, with the given name Is called on the document object On a new element, we will –Set value of attributes –Add child elements, or –Add text nodes

36. DCS – SWC 36 Creating XML documents appendChild(Element e); Is itself called on an element Appends the element e as a child on itself This is how we create the structure for the tree!

37. DCS – SWC 37 Creating XML documents The previous methods are enough to create a DOM tree Usually, we combine the methods into ”helper methods”, designed to insert a certain type of element Helper methods will often call other helper methods, depending on tree structure

38. DCS – SWC 38 Creating XML documents private Element createTextElement(String name, String text) { Text t = doc.createTextNode(text); Element e = doc.createElement(name); e.appendChild(t); return e; }

39. DCS – SWC 39 Creating XML documents private Element createProduct(Product p) { Element e = doc.createElement(”product”); e.appendChild(createTextElement(”name”, p.getName())); e.appendChild(createTextElement(”price”, p.getPrice())); e.appendChild(createTextElement(”weight”, p.getWeight())); return e; }

40. DCS – SWC 40 Creating XML documents private Element createProducts(ArrayList pList) { Element e = doc.createElement(”products”); for (product p : pList) { e.appendChild(createProduct(p)); } return e; }

41. DCS – SWC 41 Creating XML documents DocumentBuilderFactory fac = DocumentBuilderFactory.newInstance(); DocumentBuilder builder = fac.newDocumentBuilder(); Document doc = builder.newDocument; // Now doc contains an empty DOM tree ArrayList pList =...; Element root = createProducts(pList); doc.appendChild(root);

42. Creating XML documents Final step – convert the completed DOM tree to a string (which could then be displayed on screen or written to a file) Requires a bit of ”black maigic”… DCS – SWC 42

43. DCS – SWC 43 Creating XML documents DOMImplementation impl = doc.getImplementation(); DOMImplementationLS implLS = (DOMImplementationLS) impl.getFeature(”LS”, ”3.0”); LSSerializer ser = implLS.createLSSerializer(); ser.getDomConfig().setParameter(”format-pretty-print”, true); String str = ser.writeToString(doc);

44. DCS – SWC 44 Validating XML documents It will often be convenient to know if an XML document obeys certain rules about its content Can e.g make processing easier – do not need to include error handling Specification of such rules can be done in various ways

45. DCS – SWC 45 Validating XML documents Original way – use a DTD DTD – Document Type Definition A DTD is a sequence of rules describing –The valid attributes for each element type –The valid child elements for each element type

46. DCS – SWC 46 Validating XML documents Examples of DTD rules: – - a products element must contain zero or more elements of type product – - a product element must have the children: one name, one price, one weight, in that order – - a name element must have a child of type text

47. DCS – SWC 47 Validating XML documents In order to validate an XML document against a DTD, the DTD must be specified –Can be included in the XML document –Can be referenced NOTE: Validation is optional, it is up to us to do it…

48. DCS – SWC 48 Validating XML documents A more modern way of validating XML documents is by using an XSD XSD – XML Schema Definition Provides a more general framework for specification of the document format Is itself written in XML Comes closer to actual class definitions

49. DCS – SWC 49 Validating XML documents

50. Transforming XML documents A common task is to transform data given in XML format to ”something else”… Reading/writing XML in Java transforms the data to an in-memory object model This is a ”programmatic” transformation, we can also imagine more static or declarative transformations DCS – SWC 50

51. DCS – SWC 51 Transforming XML documents Such a transformation can be specified by a so-called XSLT (XSL Transformation) Specifies a transformation from the XML document into….anything! –A Word document –A HTML page –Java code (!) –…?

52. Transforming XML documents Example: A complex electronic device is described in XML We wish to create a software model of the device, with classes, interfaces, etc., to enable software simulation of the device The transformation from XML to Java code could be done by an XSLT Input: XML, Output: Java code… DCS – SWC 52