XML Fundamentals

1
XML Fundamentals
2
XML Documents and XML Files

• An XML document contains text, never binary data
• An XML document can be opened with any program
  that knows how to read a text file
• It is usual to give a .xml extension file name
• MIME media type application/xml or text/xml

ltpersongt Alan Turing lt/persongt
3
Elements, Tags, and Character Data

• The previous example is composed of a single
  element named person
• Start-tag ltpersongt
• End-tag lt/persongt
• Everything between start-tag and end-tag is
  called content
• Content encompasses real information
• Whitespace is part of the content, though many
  applications will choose to ignore it
• ltpersongt and lt/persongt are markup
• Alan Turing and its surrounding whitespace are
  character data

4
Tag Syntax

• Like HTML tags
• Start-tags begin with lt and end-tags begin with
  lt/
• Both of start-tags and end-tags are followed by
  the name of the element and are closed by gt
• You are allowed to make up new XML tags
• Tag names generally reflect the type of content
  inside the element, not how that content will be
  formatted
• Case sensitivity
• ltPersongt ? ltPERSONgt ? ltpersongt

5
Empty Element

• Empty element elements that have no content
• For the value of their attributes
• Example
• ltemail hrefmailtogtlt/emailgt
• Shorthand notation
• ltemail hrefmailto /gt

6
XML Trees

• Elements can contain other elements that in turn
  can contain text or elements and so on
• Start and end tags must always be balanced and
  children are always completed enclosed in their
  parents
• ltnamegtltfnamegtJacklt/fnamegtltlnamegtSmithltnamegtlt/lname
  gt
• Illegal
• Parent Child Sibling
• Each element (except the root element) has
  exactly one parent element
• An XML document is a tree of elements
• Root (document) element the first element in the
  document and the element that contains all other
  elements

7
A Tree Diagram for Example 2-2
8
Tree of the Address Book
addressbook
entry
entry
name
address
tel
tel
email
name
tel
email
fname
lname
street
region
postal-code
locality
country
9
Mixed Content

• The dichotomy between elements that contain only
  character data and elements that contain only
  child elements is common in data-oriented XML
  documents
• Mixed content some elements may contain
  sub-elements and raw data
• Common in XML documents containing articles,
  essays, stories, books, novels, reports, web
  pages document-oriented applications
• Example 2-3

10
Attributes

• Attach additional information to elements
• An attribute is a name-value pair attached to an
  elements start-tag
• One element can have more than one attribute
• Name and value are separated by and optional
  whitespace
• Attribute value is enclosed in double or single
  quotation marks
• lttel preferredtruegt03-5712121lt/telgt
• Attribute order is not significant
• Example 2-4

ltperson born1912-06-23 died1954-06-07gt
Alan Turing lt/persongt
11
Attributes and Elements

• When and whether one should use child elements or
  attributes to hold information?
• Attributes are for metadata about the element,
  while elements are for the information itself
• Each element may have no more than one attribute
  with a given name
• The value of attribute is simply a text string
  limited in structure
• An element-based structure is a lot more flexible
  and extensible
• If you are designing your own XML vocabulary, it
  is up to you to decide when to use which

12
XML Names

• Rules for naming elements, attributes
• May contain essentially any alphanumeric
  character and non-english letters, numbers, and
  ideograms
• May contain underscore(_), period (.), and hyphen
  (-)
• XML may not contain whitespace of any kind
• All names beginning with the string xml (in any
  combination of case) are reserved for
  standardization in W3C XML-related specifications
• Start with either letters. ideograms and
  underscore (_)
• No limit to the name length

13
XML Names (Cont.)

• HTML elements in XML are always in uppercase
• XML elements are frequently written in lowercase
• When a name consists of several words, the words
  are usually separated by a hyphen (-)
• address-book
• OR
• The first letter of each word in XML elements are
  frequently in uppercase and no separation
  character
• AddressBook

14
Entity References

• What if the character data inside an element
  contains lt ?
• Entity reference when an application parses an
  XML document, it replaces the entity reference
  with the actual characters to which the entity
  reference refers
• Entity references are markups
• XML predefines 5 entity references you can
  define more
• lt the less-than sign
• amps the ampersand ()
• gt the greater-than sign
• quot the straight, double quotation marks (")
• apos the straight single quote (')
• Example 2-5

15
CDATA Sections

• What if your character data have a lot of lt, ,
  ', "
• Enclose the character data in a CDATA section
• lt!CDATA .. gt
• Everything inside a CDATA section is treated as
  raw character data not markup
• The only thing that cannot appear in a CDATA
  section is the CDATA section end delimiter gt
• Example 2-6

16
Comments

• XML documents can be commented so that coauthors
  can leave notes for each other and themselves
• Begin with lt!-- and end with the first occurrence
  of --gt
• The double hyphen -- should not appear anywhere
  inside the comment until the closing --gt
• Comments may appear anywhere in the character
  data of a document
• Comments may appear before or after the root
  element
• Comments may not appear inside a tag or inside
  another comment
• Comments are strictly for making the raw source
  code of an XML document more legible to human
  readers

17
The XML Declaration

• XML documents should (but not have to) begin with
  an XML declaration
• The XML declaration must be the first thing in
  the document
• It must not be preceded by any comments,
  whitespace
• Example 2-7
• An XML declaration specifies encoding and
  standalone
• Encoding specify the character set used in the
  XML document
• Default to Unicode/UTF8
• Standalone if the value is "no", then an
  application may be required to read an external
  DTD to determine the proper values for parts of
  the document

18
Rules for Well-Formed XML

• Rule 1 Mandatory closing tags
• The set of tags is unlimited but all container
  tags must have end tags
• Example of legal XML
• ltpersongtltnamegt Hao-Ren Ke lt/namegtlttitlegt
  Associate Professor lt/titlegtltagegt 25
  lt/agegtlt/persongt
• Rule 2 There must be exactly one root element

19
Rules for Well-Formed XML (Cont.)

• Rule 3 Proper element nesting
• All tags must be nested correctly. Like HTML, XML
  can intermix tags and text, but tags may not
  overlap each other.
• Legal XML
• ltpersongt Hao-Ren Ke is an ltrolegt
  pioneerlt/rolegtforltservicegtComputerized
  Interlibrary Loanlt/servicegt in Taiwanlt/persongt
• Illegal XML
• ltpersongtltnamegtClavenlt/namegtltkeypointgtlthdgtXML
  provides a data buslt/hdgtlt/persongtltmoregtlt/moregtlt/
  keypointgt

20
Rules for Well-Formed XML (Cont.)

• Rule 4 Attribute values must be single or double
  quoted
• Legal
• lttag attributevaluegt
• lttag attributevaluegt
• Illegal
• ltfont size6gt
• Rule 5 An element may not have two attributes
  with the same name
• Rule 6 Comments and processing instructions may
  not appear inside tags
• Rule 7 No unescaped lt or signs may occur in
  the character data of an element or attributes

21
Rules for Well-Formed XML

• Rule 8 Empty elements must be written in an
  abbreviated form using special XML syntax.
• Legal
• ltBR /gt
• ltHR /gt
• ltTITLEgtlt/TITLEgt is equivalent to ltTITLE/gt
• Illegal
• ltBRgt
• ltHRgt

22
Four Common Errors

• Forget End Tags
• Forget that XML is Case Sensitive
• Introduce Spaces in the Name of Element
• Forget the Quotes for Attribute Value