this site is individual site for ueh students of information management faculty this site provides some students resources of it courses such as computer network data structure and algorithm enterprise resource planning

When a namespace prefix is declared, it remains in scope for: Attributes of the element where it is declared2. Child elements (and their attributes) of the element where it is declared.[r]

Welcome to:

XML Namespaces

Unit Objectives

After completing this unit, you should able to:

Describe the reasons for using namespaces

Describe the syntax used in namespaces

Define and illustrate an example using namespaces

Define problems with namespaces

List and define the best practices to use when using namespacesDescribe the status of namespaces in the industry

Consider the following XML document:

How does an application know that:

The first occurrence of title is a book title

The second occurrence of title is a person's title

Need a way to eliminate the ambiguity for the purpose of processing

Problem: Element and Attribute Names Can Be Ambiguous

No industry is an island, industries interact: who decides?

Naming standards down to the element/attribute level are too brittle

Use verbose element names, that is, bookTitle, courtesyTitle

Problem: naming becomes fundamentally difficult, there is no way to know if a name is already in use, further, the data and/or its model may not belong to the consuming application

Namespaces: The Big Idea

In concept, each element name and attribute name could be

expressed as: URI+name, for example, <title> might become:

<http://www.library.com/books:title>

There are two problems with this format:

1 It is not well-formed XML under the 1.0 specification

2 It is a lot of typing

If it were possible to create a synonym for the URI and replace occurrences of the URI with that synonym, the amount of typing would be reduced and, if handled correctly, the result would be compatible with XML 1.0

For example, specify books="http://www.library.com/books", and code the element as <books:title>

This concept forms the basis of the XML Namespace specification.

These URI qualifiers are called

Namespaces

XML Namespaces

The Namespace specification refers to these two-part names

as Qualified Names or QNames

For the purposes of XML namespaces, URIs are considered

identical when they match character for character If URIs are

different, they represent different Namespaces

Note: There is no network lookup associated with the use of URIs

in this specification, it is a lexical convention only

URIs are not checked by the processor to ensure they exist.

The Namespace specification deals with the mechanics of

associating a URI qualifier (aka namespace) with element and

attribute names to create two-part names that are unique and free of ambiguity

Qualified Names (QNames)

QNames are used in place of element and attribute names.QNames have a prefix and a local part - they look like this:

The syntax of a namespace declaration is:

< prefix : elementName xmlns: prefix = 'URI' />

The following example declares the namespace

http://www.library.com/books, assigns it a prefix of 'books'

and identifies the book element as a member of that namespace.

Unless the prefix is redefined on a nested element.

QNames are still required, the namespace is not assumed

Applying this technique, the previous example becomes:

The default may be respecified for each element scope

Nesting is respected, that is, respecification does not influence the outerscope containing the nested elements

Default namespaces don't apply to attributes Attributes have no namespace unless their names are qualified

Example - Default Namespaces

Documents with Multiple Namespaces

Document with three namespaces:

< book

xmlns= 'http://www.library.com/books'

xmlns: amazon='http://www.amazon.com/products' > < title >Tom Sawyer</ title >

Elements with No Namespace

What happens to the previous example with no default

The xmlns="" syntax resets the default namespace for the scope in

which it occurs The <isbn> element is not in a namespace.

There is no default null namespace

Attributes and Namespaces

Attributes are not affected by a default namespace declaration.Attributes on a single element must be unique

<invalid att="1" att="2" />

<invalid ns1:att ="1" ns2:att ="2" />

How does an XML parser deal with namespaces?

Needs the right API

SAX2

DOM Level 2

The parser simply reports the prefix, localName, and URI

associated with the element or attribute

It's up to your application to decide what to do

There are no validation rules associated with Namespaces - it depends on XMLSchema, DTD, or whatever grammar description language you are using

Namespace Processing

Example: Use of Namespaces

Composition of a particular airplane in an airline fleet:

<united:airplanes xmlns="http://www.wingco.com"

xmlns:united="http://www.ual.com"

xmlns:boeing="http://www.boeing.com"

xmlns:pratt="http://www.prattandwhitney.com" xmlns:goodyear="http://www.goodyear.com" xmlns:airbus="http://www.airbus.com"

Problems with Namespaces

Namespace recommendation after XML 1.0

DTDs don't integrate well

Must use QNames as element names in DTDs (remember, ":" is legal in an element name)

If the prefix changes, the DTD must also change

Testing the equality of namespaces is not handled by the parser.You have to test equality of URIs, you cannot just test equality of prefixes

Best Practices

When to use namespaces

When the data requires uniqueness for application processing.When the need to combine a schema [TBD] with other grammars

is necessary

Performance implications

Namespace processing may slow down the parser and/or

increases memory use

Don't use relative URIs for namespace identifiers

Pick the default namespace carefully

Don't declare more than one prefix for a namespace URI

Be careful with attributes when using namespaces

Collect the namespace declarations in one place, preferable near the top of the document

Checkpoint Questions

1 Which is true of XML namespaces?

(Select all that apply)

a They are stored in an Internet-based registry

b They are associated with URIs

c They are integrated with DTDs

d They are integrated with XML Schema

2 An XML namespace prefix (Select all that apply):

a Links to a schema definition

b Is scoped to the element where it is defined

c Is short hand for a URI

d Can stand for more than one namespace

3 Default namespaces apply to:

a Elements

b Attributes

c Elements and attributes

d Neither elements nor attributes

Unit Summary

Having completed this unit, you should understand:The reasons for using namespaces

The syntax used in namespaces

The use of default namespaces

The interaction between namespaces and attributesProblems with namespaces

Best practices regarding namespaces

Status of namespace technology