XML

Chapter 15: XML

CS 80: Internet Programming

Instructor: Mark Edmonds

XML describes data

Remember the above!
Stands for "Extensible Markup Language"
- XML is a meta-language meaning it is not a language itself, but rather a language for building languages

XML describes data

HTML is a sort of "variation" of XML, though it technically is not XML
- XHTML is a version of HTML that does adhere to actual XML rules
- So, we've seen something very similar before

XML describes data

What's the point?
- XML allows us to describe data in a strict, organized, but flexible manner
- This means we can create specific markup languages for any sort of data
  - We'd need to parse the data for it to be meaningful, but XML is a building block

XML describes data

Consider we have the following data:
- John 10 Bill 15 Judy 25
- What does this data mean?
  - I have no idea from just the above
  - If I put this on the internet, no one else will know what it means either

XML describes data

XML allows us to share data efficiently

Consider the following

            <!-- XML representing a family - notice the explicit structure -->
        <family>
          <member>
            <name>John</name>
            <age>10</age>
          </member>
          <member>
            <name>Bill</name>
            <age>15</age>
          </member>
          <member>
            <name>Judy</name>
            <age>25</age>
          </member>
        </family>

XML describes data

This data makes a lot more sense!
- The initial data had a lot of implicit information that we have made explicit through XML

XML Concepts

Why care?
- XML makes data formats portable and application independent
  - Which makes them a very good idea for the internet!
  - Application independent means I don't need the application using the data to understand the data (contrast a format like Word document to a .txt)

XML Concepts

Specify the document's structure
Consist of the element's name in angle brackets
Example: <data>

XML Concepts

XML elements have start and end tags
- Start tag proceeds as above, e.g. <data>
- End tag has a backslash (\ after the <, e.g. </data>
  - End tags can be shorthanded in the starting tag by place a forward slash / before the closing < of the opening tag. e.g. with <data/> as the start tag
- Looks familiar!

XML Concepts

Every XML document contains one root element, which contains all other elements
- Similar to <html>

XML Concepts

XML-based markup languages are called XML vocabularies
- Provide a mechanism to describe data in a standardized, structured way.
- Examples: XHTML, MathML (math), VoiceXML (speech), XBRL (financial data)
- Why do XML vocabularies matter?
  - Large companies often employ their own XML vocabulary to describe their data internally
  - They provide a standard for data markup using a standard data format (e.g. if you can read XML, a XML vocabulary will be easier to understand than a proprietary data format)

XML Concepts

XML documents have the extension .xml and are readable by any text editor
XML is just a data format; it does not contain styling
- Devices are responsible for how a XML is rendered
- However, Extensible Stylesheet Language allows you specify rendering on different platforms

XML Parsing

Because we are specifying a data markup, we need a way to understand the format
XML parsers read XML
- Now that we have covered DOM, think about what your browser does to load a .html file into the DOM tree (it has to parse it!)

XML Parsing

Basic XML Rules:
1. Single root element
2. A start and end tag for each element
3. Properly nested tags
4. Case sensitive
- Following these rules means the document is well-formed

XML Parsing

Basic XML Rules:
1. Single root element
2. A start and end tag for each element
3. Properly nested tags
4. Case sensitive
Which of these rules does HTML break?
- 2 and 4

XML Validation

Some parsers can also validate the XML's adhere to a particular markup
Relies on a Document Type Definition (DTD) or a Schema
- These documents describe the proper document structure
- Think of these like a grammar for what forms a valid XML document using this data markup

XML Validation

A validating parser reads the XML and makes sure that it follows the structure defined in the DTD or Schema
- If the document is well-formed XML and adheres to the DTD/Schema, then it is valid
- Otherwise, the document is invalid
- Note that a document may be well-formed XML but may not be a valid document

Example: `article.xml`

<?xml version="1.0"?>
        <!-- Fig. 15.2: article.xml -->
        <!-- Article structured with XML -->
        <article>
          <title>Simple XML</title>
          <date>July 4, 2007</date>
          <author>
            <firstName>John</firstName>
            <lastName>Doe</lastName>
          </author>
          <summary>XML is pretty easy.</summary>
          <content>This chapter presents examples that use XML.</content>
        </article>

Writing XML

The first line, <?xml version="1.0"?> declares the document as a XML document
- Similar to <!DOCTYPE HTML>
- NO characters must be before the XML declaration
XML Comments are identical to HTML comments
The first XML element is the root node; it's closing tag should be the last tag in the document

Writing XML

XML Element Names
- Can contain letters, digits, underscores, hyphens, and periods.
- Must start with an underscore or letter
- Must not begin with any case-combination of "xml" as these are reserved for XML
Nesting XML elements is identical to nesting HTML elements
- Must still be careful about proper nesting

XML Namespaces

Suppose we want to use the use "subject" in multiple ways: one for subjects in high school, the other for subjects in medical schools
```
            <subject>Geometry</subject>
        <subject>Radiology</subject>
```
We have an ambiguity in our data format as we probably don't want to mix high school and medical school subjects!
- So we need a way to add additional categorical/hierarchical information

XML Namespaces

Namespaces allow us to give more specific scope to an XML element
- The namespace itself is called a namespace prefix and is followed by a colon (:) before the XML element name

For our example

            <highschool:subject>Geometry</highschool:subject>
        <medicalschool:subject>Radiology</medicalschool:subject>

XML Namespaces

The xmlns defines a namespace
- Syntax xmlns:prefix="URI"
- URI can be anything, it is just supposed to be a uniform resource identifier
- Can be Uniform Resource Name (URN) or Uniform Resource Locator (URL)
  - URN's are a series of names separated with colons
    - E.g. urn:schooltypes
- No namespace prefix should begin with xml (it is reserved)

Example: `namespaces.xml`

<?xml version="1.0"?>
        <!-- Fig. 15.5: namespace.xml -->
        <!-- Demonstrating namespaces -->
        <text:directory xmlns:text="urn:deitel:textInfo" xmlns:image="urn:deitel:imageInfo">
          <text:file filename="book.xml">
            <text:description>A book list</text:description>
          </text:file>
          <image:file filename="funny.jpg">
            <image:description>A funny picture</image:description>
            <image:size width="200" height="100" />
          </image:file>
        </text:directory>

Default Namespaces

Specifying xmlns = "URI" specifies a default namespace for the entire document

Example: `default_namespaces.xml`

<?xml version="1.0"?>
        <!-- Fig. 15.6: defaultnamespace.xml -->
        <!-- Using default namespaces -->
        <directory xmlns="urn:deitel:textInfo" xmlns:image="urn:deitel:imageInfo">
          <file filename="book.xml">
            <description>A book list</description>
          </file>
          <image:file filename="funny.jpg">
            <image:description>A funny picture</image:description>
            <image:size width="200" height="100" />
          </image:file>
        </directory>

Default Namespaces

Notice the difference between the two versions
- They have the same semantic meaning, but one contains significantly less manual tagging of elements!
- Use a default namespace if want every element to be in a namespace and have a namespace that is particularly common

DTD

A method for defining a grammar for validating XML
Reasonably simple to follow, but it's an aging implementation. Schema is more powerful and more intuitive once you know XML
Follows Extended Backus-Naur Form (EBNF) grammar

DTD

Follows Extended Backus-Naur Form (EBNF) grammar
- Basically a list of production rules for what makes up a valid document
- E.g. a sentence is a SUBJECT followed by a PREDICATE, but also has many optional arguments
- A context-free grammar (CGF) to recursively write rules to generate patterns
  - Technically, English is not a context-free grammar
  - For more about CFG's look into Alan Turing and Noam Chomsky's work, a branch of computer science called Automata Theory!

XML Schema

Allows us to validate an XML document
- Why do we need to specify this?
  - XML is a meta-language, so there's nothing to validate by default. We define a language to validate, so we must define the validation as well
    - Think if you were writing your own programming language; you'd have to write a "syntax validator" to validate that a program contained valid syntax

XML Schema

Used by validating parsers to validate documents
Documents that conform the to schema are valid, documents that do not conform to the schema are invalid
Schema documents have the extension .xsd
- Can validate schema at www.xmlforasp.net/SchemaValidator.aspx
Let's start with an example

Example: `book.xml`

<?xml version="1.0"?>
        <!-- Fig. 15.9: book.xml -->
        <!-- Book list marked up as XML -->
        <deitel:books xmlns:deitel="http://www.deitel.com/booklist">
          <book>
            <title>Visual Basic 2010 How to Program</title>
          </book>
          <book>
            <title>Visual C# 2010 How to Program, 4/e</title>
          </book>
          <book>
            <title>Java How to Program, 9/e</title>
          </book>
          <book>
            <title>C++ How to Program, 8/e</title>
          </book>
          <book>
            <title>Internet and World Wide Web How to Program, 5/e</title>
          </book>
        </deitel:books>

Example: `book.xsd`

<?xml version = "1.0"?>
        <!-- Fig. 15.10: book.xsd
        -->
        <!-- Simple W3C XML Schema document -->
        <!-- 
        The first xmlns defines the namespace for this document, which is a schema.
        xmlns:deitel defines a namespace of "dietel", used to differentiate between names used for the XML schema and names used by our schema
        targetNamespace defines which namespace will use this schema for validation
        -->
        <schema xmlns = "http://www.w3.org/2001/XMLSchema"
          xmlns:deitel = "http://www.deitel.com/booklist"
          targetNamespace = "http://www.deitel.com/booklist">
          <!-- declaring an element named "books" and its schema type, "BooksType" in the "dietel" namespace -->
          <element name = "books" type = "deitel:BooksType"/>
          <!-- declare the complext type "BooksType" used with the "books" element -->
          <complexType name = "BooksType">
            <!-- sequence specifies the order in which child elements must appear -->
            <sequence>
              <!-- declare an element names "book" of type "SingleBookType" that must occur at least once and can occur an infinite amount of times -->
              <element name = "book" type = "deitel:SingleBookType"
                minOccurs = "1" maxOccurs = "unbounded"/>
            </sequence>
          </complexType>
          <!-- declare the "SingleBookType" complex type used with the "book" element -->
          <complexType name = "SingleBookType">
            <sequence>
              <!-- specify that the "title" element is a string -->
              <element name = "title" type = "string"/>
            </sequence>
          </complexType>
        </schema>

XML Schema

In the schema, we have two namespaces
- One for the schema itself, xmlns, which can be used to validate the schema
- The second, xmlns:deitel, which is used to define names created by us
Our targetNamespace is the URI of the XML vocabulary that this schema defines

Schema Attributes

Name corresponds to the element's name and type specifies the element's type
Types:
- XML has predefined types, or you can create user-defined types

Schema Attributes

There are two categories of types:
1. Simple types: a basic type. Cannot contain attributes or child elements
2. Complex types: a complex type. Can contain attributes or child elements
- Complex types may have simple content or complex content. Both can contain attributes, but only complex content contain child elements. Simple content must extend or restrict a base user or XML type

XML Types

Example: `laptop.xml`

<?xml version="1.0"?>
        <!-- Fig. 15.13: laptop.xml
        -->
        <!-- Laptop components marked up as XML -->
        <!-- declare a laptop computer with manufacturer "IBM" -->
        <computer:laptop xmlns:computer="http://www.deitel.com/computer" manufacturer="IBM">
          <processor model="Centrino">Intel</processor>
          <monitor>17</monitor>
          <CPUSpeed>2.4</CPUSpeed>
          <RAM>256</RAM>
        </computer:laptop>

Example: `laptop.xsd`

<?xml version = "1.0"?>
        <!-- Fig. 15.12: computer.xsd -->
        <!-- W3C XML Schema document
        -->
        <schema xmlns = "http://www.w3.org/2001/XMLSchema"
          xmlns:computer = "http://www.deitel.com/computer"
          targetNamespace = "http://www.deitel.com/computer">
          <!-- declare a simple type of "gigahertz"-->
          <simpleType name = "gigahertz">
            <!-- sepcify a restriction on the base type decimal -->
            <restriction base = "decimal">
              <!-- set minimum value -->
              <minInclusive value = "2.1"/>
            </restriction>
          </simpleType>
          <!-- declare a complex type of CPU -->
          <complexType name = "CPU">
            <!-- create simple content -->
            <simpleContent>
              <!-- here we "extend" the simple content to contain a string -->
              <extension base = "string">
                <!-- set the name and the type of CPU -->
                <attribute name = "model" type = "string"/>
              </extension>
            </simpleContent>
          </complexType>
          <!-- declare a complex type "portable" -->
          <complexType name = "portable">
            <!-- All specifies that each child element must be included -->
            <all>
              <!-- declare elements and their type -->
              <element name = "processor" type = "computer:CPU"/>
              <element name = "monitor" type = "int"/>
              <element name = "CPUSpeed" type = "computer:gigahertz"/>
              <element name = "RAM" type = "int"/>
            </all>
            <!-- declare an attribute for the manufacturer of the laptop -->
            <attribute name = "manufacturer" type = "string"/>
          </complexType>
          <!-- declare a single laptop element -->
          <element name = "laptop" type = "computer:portable"/>
        </schema>

Chapter 15: XML

XML describes data

XML describes data

XML describes data

XML describes data

XML describes data

XML describes data

XML Concepts

XML Concepts

XML Concepts

XML Concepts

XML Concepts

XML Concepts

XML Parsing

XML Parsing

XML Parsing

XML Validation

XML Validation

Example: article.xml

Writing XML

Writing XML

XML Namespaces

XML Namespaces

XML Namespaces

Example: namespaces.xml

Default Namespaces

Example: default_namespaces.xml

Default Namespaces

DTD

DTD

XML Schema

XML Schema

Example: book.xml

Example: book.xsd

XML Schema

Schema Attributes

Schema Attributes

XML Types

XML Types

Example: laptop.xml

Example: laptop.xsd

Example: `article.xml`

Example: `namespaces.xml`

Example: `default_namespaces.xml`

Example: `book.xml`

Example: `book.xsd`

Example: `laptop.xml`

Example: `laptop.xsd`