xml. dcs – swc 2 data vs. information we often use the terms data and information interchangeably...

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

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• 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

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• 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

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• 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…

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• 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…

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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”

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• 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?

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• 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 ” ”

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• 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!

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• 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

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• 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

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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 <b>very</b> good

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Markup languages

• The markings <b> and </b> 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!

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What is XML…?

• eXtensible

• Markup

• Language

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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

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XML

• A product defined in XML:

<product>

<name>Milk</name>

<price>4.95</price>

<weight>1000</weight>

</product>

Start the Product description

End the Product description

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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

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XML

<product>

<name>Milk</name>

<price currency=”DKK”>4.95</price>

<weight unit=”gram”>1000</weight>

</product>

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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

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XML

<product name=”Milk” price=”4.95” weight=”1000”/>

• Tempting, but not in the spirit of XML…

• Harder to process by recipient

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XML

• The general structure of an XML document is then– An XML declaration:

<?xml version=”1.0”?>

– A root element containing the data<products>

</products>

– Inside the root element; all the child elements

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XML

<?xml=”version 1.0”?>

<products><product>

<name>Milk</name>



</product>

<product>

<name>Orange Juice</name>



</product>

...

</products>

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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)

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products

product product

name price weight

Milk 4.95 1000

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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

...

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• 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

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products

product product

name price weight

Milk 4.95 1000

/products/product[1]/weight

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• 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

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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

...

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• 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…

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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

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Document doc = builder.newDocument();

// Now doc contains an empty DOM tree

...

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• 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);

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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

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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!

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• 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

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private Element createTextElement(String name, String text)

{

Text t = doc.createTextNode(text);

Element e = doc.createElement(name);

e.appendChild(t);

return e;

}

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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;

}

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private Element createProducts(ArrayList<Product> pList)

{

Element e = doc.createElement(”products”);

for (product p : pList)

{

e.appendChild(createProduct(p));

}

return e;

}

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Document doc = builder.newDocument;

// Now doc contains an empty DOM tree

ArrayList<Product> pList = ...;

Element root = createProducts(pList);

doc.appendChild(root);


• 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”…

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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);

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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

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• 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

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• Examples of DTD rules:– <!ELEMENT products (product*)> - a

products element must contain zero or more elements of type product

– <!ELEMENT product (name, price, weight)> - a product element must have the children: one name, one price, one weight, in that order

– <!ELEMENT name (#PCDATA)> - a name element must have a child of type text

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• 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…

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• 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

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<xsd:complexType name=”product”>

<xsd:sequence>

<xsd:element name=”name” type=xsd:string>

<xsd:element name=”price” type=xsd:float>

<xsd:element name=”weight” type=xsd:integer>

</xsd:sequence>

</xsd:complexType>

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

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• 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 (!)– …?


• 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…

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xml. dcs – swc 2 data vs. information we often use the terms data and information interchangeably...

Documents

structure of data

terms data

metadata explicit

kinds of metadata

gooddcs swcdcs swc

readabledcs swcdcs swc

communicatedcs swcdcs

contextdcs swcdcs swc