LN 5 XML

Extensible Markup Language (XML)

• A meta-language that describes the content of the document

• Provides a portable method for encapsulating and describing data

• Defines markup languages

Java = portable programs so XML =

portable data

Tag or Grammar of the language

XML does not specify the tag or grammar of the language

Tag

• Markup tags that have meaning to a language processor

Grammar

Defines correct usage of a language’s tag

Tags in XML vs Tags in HTML

Tags in XML are defined by the author while tags in HTML are predefined by the W3C standard

XML Components

• Prolog

• Components of the document

Prolog

Defines the xml version, entity definitions, and DOCTYPE

Components of the document

• Tags and attributes

• CDATA (character data)

• Entities

• Processing instructions

• Comments

XML Elements

• XML tag with encapsulated data

• Element contents must be character data in the encoding character set, no binary data

Tag names rules:

• Case sensitive

• Start with a letter or underscore

• After first character, numbers, ‘-’ and ‘.’ are allowed

• Cannot contain whitespaces

• Avoid use of colon expect for indicating namespaces, discussed later

XML Element Attributes

• Provide metadata for the element

• Attribute names must adhere to the same rules as element names

• For every attribute, there must be a value, even if the value is an empty string

• No duplicate attributes within a single element

Document Entities

• Entities refer to a data item, typically text

• Entities are user definable

Well-formed vs Valid XML Documents

• An XML document can be well-formed if it follows basic syntax rules.

• An XML document is valid if its structure matches a Document Type Definition (DTD).

Commonly abused XML syntax rules:

• Element and attribute names must be legal XML names

• Characters < and & must be escaped as character entities when used in text

• Every element must be closed

• Attributes must have values and values must be delimited with quotation marks

• Every element except the root element must be the child of exactly one element

• Comments must be properly formed, in particular, a comment may not contain the string “--”

Two Solutions to validating XML documents

1. Document Type Definition (DTD)

2. XML Schema

Document Type Definition (DTD)

• Sequence of declarations enclosed in a DOCTYPE declaration or stored separately and referred to from a DOCTYPE

• Defines Structure of the Document

What DTD defines

• Allowable tags and their attributes

• Attribute values constraints

• Nesting of tags

• Number of occurrences for tags

• Entity definitions

Formal Public Identifier (FPI) has four parts:

1. Connection of DTD to a formal standard

2. Group responsible for the DTD

3. Description and type of document

4. Language used in the DTD

Types of Elements

• ANY

• EMPTY

• PCDATA

• elements

• mixed

Attribute Modifiers

• #IMPLIED

• #REQUIRED

• #FIXED 

• Default Value

Limitations of DTD

• DTD itself is not in XML format

• Does not express data types

• Does not allow one to specify a specific format for the data to appear in  

• No namespace support