RSS Feed (xml)
1. What is XML?
XML is the Extensible Markup Language. It improves the functionality
of the Web by letting you identify your information in a more accurate,
flexible, and adaptable way. It is extensible because it is not
a fixed format like HTML (which is a single, predefined markup language).
Instead, XML is actually a meta language—a language for describing
other languages—which lets you design your own markup languages
for limitless different types of documents. XML can do this because
it’s written in SGML, the international standard meta language for
text document markup (ISO 8879).
2. What is a markup language?
A markup language is a set of words and symbols for describing
the identity of pieces of a document (for example ‘this is
a paragraph’, ‘this is a heading’, ‘this
is a list’, ‘this is the caption of this figure’,
etc). Programs can use this with a style sheet to create output
for screen, print, audio, video, Braille, etc.
Some markup languages (eg those used in word processors) only describe
appearances (’this is italics’, ‘this is bold’),
but this method can only be used for display, and is not normally
re-usable for anything else.
3. Where should I use XML?
Its goal is to enable generic SGML to be served, received, and
processed on the Web in the way that is now possible with HTML.
XML has been designed for ease of implementation and for interoperability
with both SGML and HTML.
Despite early attempts, browsers never allowed other SGML, only
HTML (although there were plugins), and they allowed it (even encouraged
it) to be corrupted or broken, which held development back for over
a decade by making it impossible to program for it reliably. XML
fixes that by making it compulsory to stick to the rules, and by
making the rules much simpler than SGML.
But XML is not just for Web pages: in fact it’s very rarely used
for Web pages on its own because browsers still don’t provide reliable
support for formatting and transforming it. Common uses for XML
include:
Information identification because you can define your own markup,
you can define meaningful names for all your information items.
Information storage because XML is portable and non-proprietary,
it can be used to store textual information across any platform.
Because it is backed by an international standard, it will remain
accessible and processable as a data format. Information structure
XML can therefore be used to store and identify any kind of (hierarchical)
information structure, especially for long, deep, or complex document
sets or data sources, making it ideal for an information-management
back-end to serving the Web. This is its most common Web application,
with a transformation system to serve it as HTML until such time
as browsers are able to handle XML consistently. Publishing the
original goal of XML as defined in the quotation at the start of
this section. Combining the three previous topics (identity, storage,
structure) means it is possible to get all the benefits of robust
document management and control (with XML) and publish to the Web
(as HTML) as well as to paper (as PDF) and to other formats (eg
Braille, Audio, etc) from a single source document by using the
appropriate stylesheets. Messaging and data transfer XML is also
very heavily used for enclosing or encapsulating information in
order to pass it between different computing systems which would
otherwise be unable to communicate. By providing a lingua franca
for data identity and structure, it provides a common envelope for
inter-process communication (messaging). Web services Building on
all of these, as well as its use in browsers, machine-processable
data can be exchanged between consenting systems, where before it
was only comprehensible by humans (HTML). Weather services, e-commerce
sites, blog newsfeeds, AJaX sites, and thousands of other data-exchange
services use XML for data management and transmission, and the web
browser for display and interaction.
4. Why is XML such an important development?
It removes two constraints which were holding back Web developments:
1. dependence on a single, inflexible document type (HTML) which
was being much abused for tasks it was never designed for;
2. the complexity of full SGML, whose syntax allows many powerful
but hard-to-program options.
XML allows the flexible development of user-defined document types.
It provides a robust, non-proprietary, persistent, and verifiable
file format for the storage and transmission of text and data both
on and off the Web; and it removes the more complex options of SGML,
making it easier to program for.
5. Describe the differences between XML and HTML.
It’s amazing how many developers claim to be proficient programming
with XML, yet do not understand the basic differences between XML
and HTML. Anyone with a fundamental grasp of XML should be able
describe some of the main differences outlined in the table below.
XML
User definable tags
Content driven
End tags required for well formed documents
Quotes required around attributes values
Slash required in empty tags
HTML
Defined set of tags designed for web display
Format driven
End tags not required
Quotes not required
Slash not required
6. Describe the role that XSL can play when dynamically
generating HTML pages from a relational database.
Even if candidates have never participated in a project involving
this type of architecture, they should recognize it as one of the
common uses of XML. Querying a database and then formatting the
result set so that it can be validated as an XML document allows
developers to translate the data into an HTML table using XSLT rules.
Consequently, the format of the resulting HTML table can be modified
without changing the database query or application code since the
document rendering logic is isolated to the XSLT rules.
7. What is SGML?
SGML is the Standard Generalized Markup Language (ISO 8879:1986),
the international standard for defining descriptions of the structure
of different types of electronic document. There is an SGML FAQ
from David Megginson at http://math.albany.edu:8800/hm/sgml/cts-faq.htmlFAQ;
and Robin Cover’s SGML Web pages are at http://www.oasis-open.org/cover/general.html.
For a little light relief, try Joe English’s ‘Not the SGML
FAQ’ at http://www.flightlab.com/~joe/sgml/faq-not.txtFAQ.
SGML is very large, powerful, and complex. It has been in heavy
industrial and commercial use for nearly two decades, and there
is a significant body of expertise and software to go with it.
XML is a lightweight cut-down version of SGML which keeps enough
of its functionality to make it useful but removes all the optional
features which made SGML too complex to program for in a Web environment.
8. Aren’t XML, SGML, and HTML all the same thing?
Not quite; SGML is the mother tongue, and has been used for describing
thousands of different document types in many fields of human activity,
from transcriptions of ancient Irish manuscripts to the technical
documentation for stealth bombers, and from patients’ clinical records
to musical notation. SGML is very large and complex, however, and
probably overkill for most common office desktop applications.
XML is an abbreviated version of SGML, to make it easier to use
over the Web, easier for you to define your own document types,
and easier for programmers to write programs to handle them. It
omits all the complex and less-used options of SGML in return for
the benefits of being easier to write applications for, easier to
understand, and more suited to delivery and interoperability over
the Web. But it is still SGML, and XML files may still be processed
in the same way as any other SGML file (see the question on XML
software).
HTML is just one of many SGML or XML applications—the one
most frequently used on the Web.
Technical readers may find it more useful to think of XML as being
SGML– rather than HTML++.
9. Who is responsible for XML?
XML is a project of the World Wide Web Consortium (W3C), and the
development of the specification is supervised by an XML Working
Group. A Special Interest Group of co-opted contributors and experts
from various fields contributed comments and reviews by email.
XML is a public format: it is not a proprietary development of any
company, although the membership of the WG and the SIG represented
companies as well as research and academic institutions. The v1.0
specification was accepted by the W3C as a Recommendation on Feb
10, 1998.
10. Why is XML such an important development?
It removes two constraints which were holding back Web developments:
1. dependence on a single, inflexible document type (HTML) which
was being much abused for tasks it was never designed for;
2. the complexity of full question A.4, SGML, whose syntax allows
many powerful but hard-to-program options.
XML allows the flexible development of user-defined document types.
It provides a robust, non-proprietary, persistent, and verifiable
file format for the storage and transmission of text and data both
on and off the Web; and it removes the more complex options of SGML,
making it easier to program for.
11. Give a few examples of types of applications that can
benefit from using XML.
There are literally thousands of applications that can benefit
from XML technologies. The point of this question is not to have
the candidate rattle off a laundry list of projects that they have
worked on, but, rather, to allow the candidate to explain the rationale
for choosing XML by citing a few real world examples. For instance,
one appropriate answer is that XML allows content management systems
to store documents independently of their format, which thereby
reduces data redundancy. Another answer relates to B2B exchanges
or supply chain management systems. In these instances, XML provides
a mechanism for multiple companies to exchange data according to
an agreed upon set of rules. A third common response involves wireless
applications that require WML to render data on hand held devices.
12. What is DOM and how does it relate to XML?
The Document Object Model (DOM) is an interface specification maintained
by the W3C DOM Workgroup that defines an application independent
mechanism to access, parse, or update XML data. In simple terms
it is a hierarchical model that allows developers to manipulate
XML documents easily Any developer that has worked extensively with
XML should be able to discuss the concept and use of DOM objects
freely. Additionally, it is not unreasonable to expect advanced
candidates to thoroughly understand its internal workings and be
able to explain how DOM differs from an event-based interface like
SAX.
13. What is SOAP and how does it relate to XML?
The Simple Object Access Protocol (SOAP) uses XML to define a protocol
for the exchange of information in distributed computing environments.
SOAP consists of three components: an envelope, a set of encoding
rules, and a convention for representing remote procedure calls.
Unless experience with SOAP is a direct requirement for the open
position, knowing the specifics of the protocol, or how it can be
used in conjunction with HTTP, is not as important as identifying
it as a natural application of XML.
14. Why not just carry on extending HTML?
HTML was already overburdened with dozens of interesting but incompatible
inventions from different manufacturers, because it provides only
one way of describing your information.
XML allows groups of people or organizations to question C.13, create
their own customized markup applications for exchanging information
in their domain (music, chemistry, electronics, hill-walking, finance,
surfing, petroleum geology, linguistics, cooking, knitting, stellar
cartography, history, engineering, rabbit-keeping, question C.19,
mathematics, genealogy, etc).
HTML is now well beyond the limit of its usefulness as a way of
describing information, and while it will continue to play an important
role for the content it currently represents, many new applications
require a more robust and flexible infrastructure.
15. Why should I use XML?
Here are a few reasons for using XML (in no particular order).
Not all of these will apply to your own requirements, and you may
have additional reasons not mentioned here (if so, please let the
editor of the FAQ know!).
* XML can be used to describe and identify information accurately
and unambiguously, in a way that computers can be programmed to
‘understand’ (well, at least manipulate as if they could
understand).
* XML allows documents which are all the same type to be created
consistently and without structural errors, because it provides
a standardized way of describing, controlling, or allowing/disallowing
particular types of document structure. [Note that this has absolutely
nothing whatever to do with formatting, appearance, or the actual
text content of your documents, only the structure of them.]
* XML provides a robust and durable format for information storage
and transmission. Robust because it is based on a proven standard,
and can thus be tested and verified; durable because it uses plain-text
file formats which will outlast proprietary binary ones.
* XML provides a common syntax for messaging systems for the exchange
of information between applications. Previously, each messaging
system had its own format and all were different, which made inter-system
messaging unnecessarily messy, complex, and expensive. If everyone
uses the same syntax it makes writing these systems much faster
and more reliable.
* XML is free. Not just free of charge (free as in beer) but free
of legal encumbrances (free as in speech). It doesn’t belong to
anyone, so it can’t be hijacked or pirated. And you don’t have to
pay a fee to use it (you can of course choose to use commercial
software to deal with it, for lots of good reasons, but you don’t
pay for XML itself).
* XML information can be manipulated programmatically (under machine
control), so XML documents can be pieced together from disparate
sources, or taken apart and re-used in different ways. They can
be converted into almost any other format with no loss of information.
* XML lets you separate form from content. Your XML file contains
your document information (text, data) and identifies its structure:
your formatting and other processing needs are identified separately
in a style sheet or processing system. The two are combined at output
time to apply the required formatting to the text or data identified
by its structure (location, position, rank, order, or whatever).
16. Can you walk us through the steps necessary to parse
XML documents?
Superficially, this is a fairly basic question. However, the point
is not to determine whether candidates understand the concept of
a parser but rather have them walk through the process of parsing
XML documents step-by-step. Determining whether a non-validating
or validating parser is needed, choosing the appropriate parser,
and handling errors are all important aspects to this process that
should be included in the candidate’s response.
17. Give some examples of XML DTDs or schemas that you
have worked with.
Although XML does not require data to be validated against a DTD,
many of the benefits of using the technology are derived from being
able to validate XML documents against business or technical architecture
rules. Polling for the list of DTDs that developers have worked
with provides insight to their general exposure to the technology.
The ideal candidate will have knowledge of several of the commonly
used DTDs such as FpML, DocBook, HRML, and RDF, as well as experience
designing a custom DTD for a particular project where no standard
existed.
18. Using XSLT, how would you extract a specific attribute
from an element in an XML document?
Successful candidates should recognize this as one of the most
basic applications of XSLT. If they are not able to construct a
reply similar to the example below, they should at least be able
to identify the components necessary for this operation: xsl:template
to match the appropriate XML element, xsl:value-of to select the
attribute value, and the optional xsl:apply-templates to continue
processing the document.
Extract Attributes from XML Data
Example 1.
Attribute Value:
19. When constructing an XML DTD, how do you create an
external entity reference in an attribute value?
Every interview session should have at least one trick question.
Although possible when using SGML, XML DTDs don’t support defining
external entity references in attribute values. It’s more important
for the candidate to respond to this question in a logical way than
than the candidate know the somewhat obscure answer.
20. How would you build a search engine for large volumes
of XML data?
The way candidates answer this question may provide insight into
their view of XML data. For those who view XML primarily as a way
to denote structure for text files, a common answer is to build
a full-text search and handle the data similarly to the way Internet
portals handle HTML pages. Others consider XML as a standard way
of transferring structured data between disparate systems. These
candidates often describe some scheme of importing XML into a relational
or object database and relying on the database’s engine for searching.
Lastly, candidates that have worked with vendors specializing in
this area often say that the best way the handle this situation
is to use a third party software package optimized for XML data.
21. What is the difference between XML and C or C++ or
Java? Updated
C and C++ (and other languages like FORTRAN, or Pascal, or Visual
Basic, or Java or hundreds more) are programming languages with
which you specify calculations, actions, and decisions to be carried
out in order:
mod curconfig[if left(date,6) = "01-Apr",
t.put "April googlel!",
f.put days('31102005','DDMMYYYY') -
days(sdate,'DDMMYYYY')
" more shopping days to Samhain"];
XML is a markup specification language with which you can design
ways of describing information (text or data), usually for storage,
transmission, or processing by a program. It says nothing about
what you should do with the data (although your choice of element
names may hint at what they are for):
y=”226″/>
Fasteners Ltd
and replacement of this part.
On its own, an SGML or XML file (including HTML) doesn’t do anything.
It’s a data format which just sits there until you run a program
which does something with it.
22. Does XML replace HTML?
No. XML itself does not replace HTML. Instead, it provides an alternative
which allows you to define your own set of markup elements. HTML
is expected to remain in common use for some time to come, and the
current version of HTML is in XML syntax. XML is designed to make
the writing of DTDs much simpler than with full SGML. (See the question
on DTDs for what one is and why you might want one.)
23. Do I have to know HTML or SGML before I learn XML?
No, although it’s useful because a lot of XML terminology and practice
derives from two decades’ experience of SGML.
Be aware that ‘knowing HTML’ is not the same as ‘understanding
SGML’. Although HTML was written as an SGML application, browsers
ignore most of it (which is why so many useful things don’t work),
so just because something is done a certain way in HTML browsers
does not mean it’s correct, least of all in XML.
24. What does an XML document actually look like (inside)?
The basic structure of XML is similar to other applications of
SGML, including HTML. The basic components can be seen in the following
examples. An XML document starts with a Prolog:
1. The XML Declaration which specifies that this is an XML document;
2. Optionally a Document Type Declaration which identifies the type
of document and says where the Document Type Description (DTD) is
stored;
The Prolog is followed by the document instance:
1. A root element, which is the outermost (top level) element (start-tag
plus end-tag) which encloses everything else: in the examples below
the root elements are conversation and titlepage;
2. A structured mix of descriptive or prescriptive elements enclosing
the character data content (text), and optionally any attributes
(’name=value’ pairs) inside some start-tags.
XML documents can be very simple, with straightforward nested markup
of your own design:
off!
Or they can be more complicated, with a Schema or question C.11,
Document Type Description (DTD) or internal subset (local DTD changes
in [square brackets]), and an arbitrarily complex nested structure:
]>
decoration is hand-colored, presumably
by the author –>
style=”italic”>Vitam capias
Or they can be anywhere between: a lot will depend on how you want
to define your document type (or whose you use) and what it will
be used for. Database-generated or program-generated XML documents
used in e-commerce is usually unformatted (not for human reading)
and may use very long names or values, with multiple redundancy
and sometimes no character data content at all, just values in attributes:
ORDER-UPDATE-ISSUE=”193E22C2-EAF3-11D9-9736-CAFC705A30B3″
ORDER-UPDATE-DATE=”2005-07-01T15:34:22.46″ ORDER-UPDATE-DESTINATION=”6B197E02-EAF3-11D9-85D5-997710D9978F”
ORDER-UPDATE-ORDERNO=”8316ADEA-EAF3-11D9-9955-D289ECBC99F3″>
25. How does XML handle white-space in my documents?
All white-space, including linebreaks, TAB characters, and normal
spaces, even between ’structural’ elements where no
text can ever appear, is passed by the parser unchanged to the application
(browser, formatter, viewer, converter, etc), identifying the context
in which the white-space was found (element content, data content,
or mixed content, if this information is available to the parser,
eg from a DTD or Schema). This means it is the application’s responsibility
to decide what to do with such space, not the parser’s:
* insignificant white-space between structural elements (space which
occurs where only element content is allowed, ie between other elements,
where text data never occurs) will get passed to the application
(in SGML this white-space gets suppressed, which is why you can
put all that extra space in HTML documents and not worry about it)
* significant white-space (space which occurs within elements which
can contain text and markup mixed together, usually mixed content
or PCDATA) will still get passed to the application exactly as under
SGML. It is the application’s responsibility to handle it correctly.
The parser must inform the application that white-space has occurred
in element content, if it can detect it. (Users of SGML will recognize
that this information is not in the ESIS, but it is in the Grove.)
text
In the example above, the application will receive all the pretty-printing
linebreaks, TABs, and spaces between the elements as well as those
embedded in the chapter title. It is the function of the application,
not the parser, to decide which type of white-space to discard and
which to retain. Many XML applications have configurable options
to allow programmers or users to control how such white-space is
handled.
26. Which parts of an XML document are case-sensitive?
All of it, both markup and text. This is significantly different
from HTML and most other SGML applications. It was done to allow
markup in non-Latin-alphabet languages, and to obviate problems
with case-folding in writing systems which are caseless.
* Element type names are case-sensitive: you must follow whatever
combination of upper- or lower-case you use to define them (either
by first usage or in a DTD or Schema). So you can’t say …:
upper- and lower-case must match; thus ,
,
and are three different element types;
* For well-formed XML documents with no DTD, the first occurrence
of an element type name defines the casing;
* Attribute names are also case-sensitive, for example the two width
attributes in
(if they occurred in the same file) are separate attributes, because
of the different case of width and WIDTH;
* Attribute values are also case-sensitive. CDATA values (eg Url=”MyFile.SGML”)
always have been, but NAME types (ID and IDREF attributes, and token
list attributes) are now case-sensitive as well;
* All general and parameter entity names (eg A), and your
data content (text), are case-sensitive as always.
27. How can I make my existing HTML files work in XML?
Either convert them to conform to some new document type (with
or without a DTD or Schema) and write a stylesheet to go with them;
or edit them to conform to XHTML. It is necessary to convert existing
HTML files because XML does not permit end-tag minimisation (missing
, etc), unquoted attribute values, and a number of other SGML shortcuts
which have been normal in most HTML DTDs. However, many HTML authoring
tools already produce almost (but not quite) well-formed XML.
You may be able to convert HTML to XHTML using the Dave Raggett’s
HTML Tidy program, which can clean up some of the formatting mess
left behind by inadequate HTML editors, and even separate out some
of the formatting to a stylesheet, but there is usually still some
hand-editing to do.
28. Is there an XML version of HTML?
Yes, the W3C recommends using XHTML which is ‘a reformulation
of HTML 4 in XML 1.0′. This specification defines HTML as
an XML application, and provides three DTDs corresponding to the
ones defined by HTML 4.* (Strict, Transitional, and Frameset). The
semantics of the elements and their attributes are as defined in
the W3C Recommendation for HTML 4. These semantics provide the foundation
for future extensibility of XHTML. Compatibility with existing HTML
browsers is possible by following a small set of guidelines (see
the W3C site).
29. If XML is just a subset of SGML, can I use XML files
directly with existing SGML tools?
Yes, provided you use up-to-date SGML software which knows about
the WebSGML Adaptations TC to ISO 8879 (the features needed to support
XML, such as the variant form for EMPTY elements; some aspects of
the SGML Declaration such as NAMECASE GENERAL NO; multiple attribute
token list declarations, etc).
An alternative is to use an SGML DTD to let you create a fully-normalised
SGML file, but one which does not use empty elements; and then remove
the DocType Declaration so it becomes a well-formed DTDless XML
file. Most SGML tools now handle XML files well, and provide an
option switch between the two standards.
30. Can XML use non-Latin characters?
Yes, the XML Specification explicitly says XML uses ISO 10646,
the international standard character repertoire which covers most
known languages. Unicode is an identical repertoire, and the two
standards track each other. The spec says (2.2): ‘All XML
processors must accept the UTF-8 and UTF-16 encodings of ISO 10646…’.
There is a Unicode FAQ at http://www.unicode.org/faq/FAQ.
UTF-8 is an encoding of Unicode into 8-bit characters: the first
128 are the same as ASCII, and higher-order characters are used
to encode anything else from Unicode into sequences of between 2
and 6 bytes. UTF-8 in its single-octet form is therefore the same
as ISO 646 IRV (ASCII), so you can continue to use ASCII for English
or other languages using the Latin alphabet without diacritics.
Note that UTF-8 is incompatible with ISO 8859-1 (ISO Latin-1) after
code point 127 decimal (the end of ASCII).
UTF-16 is an encoding of Unicode into 16-bit characters, which lets
it represent 16 planes. UTF-16 is incompatible with ASCII because
it uses two 8-bit bytes per character (four bytes above U+FFFF).
31. What’s a Document Type Definition (DTD) and where do
I get one?
A DTD is a description in XML Declaration Syntax of a particular
type or class of document. It sets out what names are to be used
for the different types of element, where they may occur, and how
they all fit together. (A question C.16, Schema does the same thing
in XML Document Syntax, and allows more extensive data-checking.)
For example, if you want a document type to be able to describe
Lists which contain Items, the relevant part of your DTD might contain
something like this:
This defines a list as an element type containing one or more items
(that’s the plus sign); and it defines items as element types containing
just plain text (Parsed Character Data or PCDATA). Validators read
the DTD before they read your document so that they can identify
where every element type ought to come and how each relates to the
other, so that applications which need to know this in advance (most
editors, search engines, navigators, and databases) can set themselves
up correctly. The example above lets you create lists like:
(The indentation in the example is just for legibility while editing:
it is not required by XML.)
A DTD provides applications with advance notice of what names and
structures can be used in a particular document type. Using a DTD
and a validating editor means you can be certain that all documents
of that particular type will be constructed and named in a consistent
and conformant manner.
DTDs are not required for processing the tip in question Bwell-formed
documents, but they are needed if you want to take advantage of
XML’s special attribute types like the built-in ID/IDREF cross-reference
mechanism; or the use of default attribute values; or references
to external non-XML files (’Notations’); or if you simply
want a check on document validity before processing.
There are thousands of DTDs already in existence in all kinds of
areas (see the SGML/XML Web pages for pointers). Many of them can
be downloaded and used freely; or you can write your own (see the
question on creating your own DTD. Old SGML DTDs need to be converted
to XML for use with XML systems: read the question on converting
SGML DTDs to XML, but most popular SGML DTDs are already available
in XML form.
The alternatives to a DTD are various forms of question C.16, Schema.
These provide more extensive validation features than DTDs, including
character data content validation.
32. Does XML let me make up my own tags?
No, it lets you make up names for your own element types. If you
think tags and elements are the same thing you are already in considerable
trouble: read the rest of this question carefully.
33. How do I create my own document type?
Document types usually need a formal description, either a DTD
or a Schema. Whilst it is possible to process well-formed XML documents
without any such description, trying to create them without one
is asking for trouble. A DTD or Schema is used with an XML editor
or API interface to guide and control the construction of the document,
making sure the right elements go in the right places.
Creating your own document type therefore begins with an analysis
of the class of documents you want to describe: reports, invoices,
letters, configuration files, credit-card verification requests,
or whatever. Once you have the structure correct, you write code
to express this formally, using DTD or Schema syntax.
34. How do I write my own DTD?
You need to use the XML Declaration Syntax (very simple: declaration
keywords begin with
It says that there shall be an element called Shopping-List and
that it shall contain elements called Item: there must be at least
one Item (that’s the plus sign) but there may be more than one.
It also says that the Item element may contain only parsed character
data (PCDATA, ie text: no further markup).
Because there is no other element which contains Shopping-List,
that element is assumed to be the ‘root’ element, which
encloses everything else in the document. You can now use it to
create an XML file: give your editor the declarations:
(assuming you put the DTD in that file). Now your editor will let
you create files according to the pattern:
It is possible to develop complex and powerful DTDs of great subtlety,
but for any significant use you should learn more about document
systems analysis and document type design. See for example Developing
SGML DTDs: From Text to Model to Markup (Maler and el Andaloussi,
1995): this was written for SGML but perhaps 95% of it applies to
XML as well, as XML is much simpler than full SGML—see the
list of restrictions which shows what has been cut out.
Warning
Incidentally, a DTD file never has a DOCTYPE Declaration in it:
that only occurs in an XML document instance (it’s what references
the DTD). And a DTD file also never has an XML Declaration at the
top either. Unfortunately there is still software around which inserts
one or both of these.
35. Can a root element type be explicitly declared in the
DTD?
No. This is done in the document’s Document Type Declaration, not
in the DTD.
36. I keep hearing about alternatives to DTDs. What’s a
Schema?
The W3C XML Schema recommendation provides a means of specifying
formal data typing and validation of element content in terms of
data types, so that document type designers can provide criteria
for checking the data content of elements as well as the markup
itself. Schemas are written in XML Document Syntax, like XML documents
are, avoiding the need for processing software to be able to read
XML Declaration Syntax (used for DTDs).
There is a separate Schema FAQ at http://www.schemavalid.com.
The term ‘vocabulary’ is sometimes used to refer to
DTDs and Schemas together. Schemas are aimed at e-commerce, data
control, and database-style applications where character data content
requires validation and where stricter data control is needed than
is possible with DTDs; or where strong data typing is required.
They are usually unnecessary for traditional text document publishing
applications.
Unlike DTDs, Schemas cannot be specified in an XML Document Type
Declaration. They can be specified in a Namespace, where Schema-aware
software should pick it up, but this is optional:
…
More commonly, you specify the Schema in your processing software,
which should record separately which Schema is used by which XML
document instance.
In contrast to the complexity of the W3C Schema model, Relax NG
is a lightweight, easy-to-use XML schema language devised by James
Clark (see http://relaxng.org/) with development hosted by OASIS.
It allows similar richness of expression and the use of XML as its
syntax, but it provides an additional, simplified, syntax which
is easier to use for those accustomed to DTDs.
37. How do I get XML into or out of a database?
Ask your database manufacturer: they all provide XML import and
export modules to connect XML applications with databases. In some
trivial cases there will be a 1:1 match between field names in the
database table and element type names in the XML Schema or DTD,
but in most cases some programming will be required to establish
the desired match. This can usually be stored as a procedure so
that subsequent uses are simply commands or calls with the relevant
parameters.
In less trivial, but still simple, cases, you could export by writing
a report routine that formats the output as an XML document, and
you could import by writing an XSLT transformation that formatted
the XML data as a load file.
38. Can I encode mathematics using XML?Updated
Yes, if the document type you use provides for math, and your users’
browsers are capable of rendering it. The mathematics-using community
has developed the MathML Recommendation at the W3C, which is a native
XML application suitable for embedding in other DTDs and Schemas.
It is also possible to make XML fragments from other DTDs, such
as ISO 12083 Math, or OpenMath, or one of your own making. Browsers
which display math embedded in SGML existed for many years (eg DynaText,
Panorama, Multidoc Pro), and mainstream browsers are now rendering
MathML. David Carlisle has produced a set of stylesheets for rendering
MathML in browsers. It is also possible to use XSLT to convert XML
math markup to LATEX for print (PDF) rendering, or to use XSL:FO.
Please note that XML is not itself a programming language, so concepts
such as arithmetic and if-statements (if-then-else logic) are not
meaningful in XML documents.
39. How will XML affect my document links?
The linking abilities of XML systems are potentially much more
powerful than those of HTML, so you’ll be able to do much more with
them. Existing href-style links will remain usable, but the new
linking technology is based on the lessons learned in the development
of other standards involving hypertext, such as TEI and HyTime,
which let you manage bidirectional and multi-way links, as well
as links to a whole element or span of text (within your own or
other documents) rather than to a single point. These features have
been available to SGML users for many years, so there is considerable
experience and expertise available in using them. Currently only
Mozilla Firefox implements XLink.
The XML Linking Specification (XLink) and the XML Extended Pointer
Specification (XPointer) documents contain the details. An XLink
can be either a URI or a TEI-style Extended Pointer (XPointer),
or both. A URI on its own is assumed to be a resource; if an XPointer
follows it, it is assumed to be a sub-resource of that URI; an XPointer
on its own is assumed to apply to the current document (all exactly
as with HTML).
An XLink may use one of #, ?, or |. The # and ? mean the same as
in HTML applications; the | means the sub-resource can be found
by applying the link to the resource, but the method of doing this
is left to the application. An XPointer can only follow a #.
The TEI Extended Pointer Notation (EPN) is much more powerful than
the fragment address on the end of some URIs, as it allows you to
specify the location of a link end using the structure of the document
as well as (or in addition to) known, fixed points like IDs. For
example, the linked second occurrence of the word ‘XPointer’
two paragraphs back could be referred to with the URI (shown here
with linebreaks and spaces for clarity: in practice it would of
course be all one long string):
http://xml.silmaril.ie/faq.xml#ID(hypertext)
.child(1,#element,’answer’)
.child(2,#element,’para’)
.child(1,#element,’link’)
This means the first link element within the second paragraph within
the answer in the element whose ID is hypertext (this question).
Count the objects from the start of this question (which has the
ID hypertext) in the XML source:
1. the first child object is the element containing the question
();
2. the second child object is the answer (the element);
3. within this element go to the second paragraph;
4. find the first link element.
Eve Maler explained the relationship of XLink and XPointer as follows:
XLink governs how you insert links into your XML document, where
the link might point to anything (eg a GIF file); XPointer governs
the fragment identifier that can go on a URL when you’re linking
to an XML document, from anywhere (eg from an HTML file).
[Or indeed from an XML file, a URI in a mail message, etc…Ed.]
David Megginson has produced an xpointer function for Emacs/psgml
which will deduce an XPointer for any location in an XML document.
XML Spy has a similar function.
40. How does XML handle metadata?
Because XML lets you define your own markup languages, you can
make full use of the extended hypertext features of XML (see the
question on Links) to store or link to metadata in any format (eg
using ISO 11179, as a Topic Maps Published Subject, with Dublin
Core, Warwick Framework, or with Resource Description Framework
(RDF), or even Platform for Internet Content Selection (PICS)).
There are no predefined elements in XML, because it is an architecture,
not an application, so it is not part of XML’s job to specify how
or if authors should or should not implement metadata. You are therefore
free to use any suitable method. Browser makers may also have their
own architectural recommendations or methods to propose.
41. Can I use JavaScript, ActiveX, etc in XML files?
This will depend on what facilities your users’ browsers implement.
XML is about describing information; scripting languages and languages
for embedded functionality are software which enables the information
to be manipulated at the user’s end, so these languages do not normally
have any place in an XML file itself, but in stylesheets like XSL
and CSS where they can be added to generated HTML.
XML itself provides a way to define the markup needed to implement
scripting languages: as a neutral standard it neither encourages
not discourages their use, and does not favour one language over
another, so it is possible to use XML markup to store the program
code, from where it can be retrieved by (for example) XSLT and re-expressed
in a HTML script element.
Server-side script embedding, like PHP or ASP, can be used with
the relevant server to modify the XML code on the fly, as the document
is served, just as they can with HTML. Authors should be aware,
however, that embedding server-side scripting may mean the file
as stored is not valid XML: it only becomes valid when processed
and served, so care must be taken when using validating editors
or other software to handle or manage such files. A better solution
may be to use an XML serving solution like Cocoon, AxKit, or PropelX.
42. Can I use Java to create or manage XML files?
Yes, any programming language can be used to output data from any
source in XML format. There is a growing number of front-ends and
back-ends for programming environments and data management environments
to automate this. Java is just the most popular one at the moment.
There is a large body of middleware (APIs) written in Java and other
languages for managing data either in XML or with XML input or output.
43. How do I execute or run an XML file?
You can’t and you don’t. XML itself is not a programming language,
so XML files don’t ‘run’ or ‘execute’. XML
is a markup specification language and XML files are just data:
they sit there until you run a program which displays them (like
a browser) or does some work with them (like a converter which writes
the data in another format, or a database which reads the data),
or modifies them (like an editor).
If you want to view or display an XML file, open it with an XML
editor or an question B.3, XML browser.
The water is muddied by XSL (both XSLT and XSL:FO) which use XML
syntax to implement a declarative programming language. In these
cases it is arguable that you can ‘execute’ XML code,
by running a processing application like Saxon, which compiles the
directives specified in XSLT files into Java bytecode to process
XML.
44. How do I control formatting and appearance?
In HTML, default styling was built into the browsers because the
tagset of HTML was predefined and hardwired into browsers. In XML,
where you can define your own tagset, browsers cannot possibly be
expected to guess or know in advance what names you are going to
use and what they will mean, so you need a stylesheet if you want
to display formatted text.
Browsers which read XML will accept and use a CSS stylesheet at
a minimum, but you can also use the more powerful XSLT stylesheet
language to transform your XML into HTML—which browsers, of
course, already know how to display (and that HTML can still use
a CSS stylesheet). This way you get all the document management
benefits of using XML, but you don’t have to worry about your readers
needing XML smarts in their browsers.
45. How do I use graphics in XML?
Graphics have traditionally just been links which happen to have
a picture file at the end rather than another piece of text. They
can therefore be implemented in any way supported by the XLink and
XPointer specifications (see question C.18, ‘How will XML
affect my document links?’), including using similar syntax
to existing HTML images. They can also be referenced using XML’s
built-in NOTATION and ENTITY mechanism in a similar way to standard
SGML, as external unparsed entities.
However, the SVG specification (see the tip below, by Peter Murray-Rust)
lets you use XML markup to draw vector graphics objects directly
in your XML file. This provides enormous power for the inclusion
of portable graphics, especially interactive or animated sequences,
and it is now slowly becoming supported in browsers.
The XML linking specifications for external images give you much
better control over the traversal and activation of links, so an
author can specify, for example, whether or not to have an image
appear when the page is loaded, or on a click from the user, or
in a separate window, without having to resort to scripting.
XML itself doesn’t predicate or restrict graphic file formats: GIF,
JPG, TIFF, PNG, CGM, EPS, and SVG at a minimum would seem to make
sense; however, vector formats (EPS, SVG) are normally essential
for non-photographic images (diagrams).
You cannot embed a raw binary graphics file (or any other binary
[non-text] data) directly into an XML file because any bytes happening
to resemble markup would get misinterpreted: you must refer to it
by linking (see below). It is, however, possible to include a text-encoded
transformation of a binary file as a CDATA Marked Section, using
something like UUencode with the markup characters ], & and
> removed from the map so that they could not occur as an erroneous
CDATA termination sequence and be misinterpreted. You could even
use simple hexadecimal encoding as used in PostScript. For vector
graphics, however, the solution is to use SVG (see the tip below,
by Peter Murray-Rust).
Sound files are binary objects in the same way that external graphics
are, so they can only be referenced externally (using the same techniques
as for graphics). Music files written in MusiXML or an XML variant
of SMDL could however be embedded in the same way as for SVG.
The point about using entities to manage your graphics is that you
can keep the list of entity declarations separate from the rest
of the document, so you can re-use the names if an image is needed
more than once, but only store the physical file specification in
a single place. This is available only when using a DTD, not a Schema.
46. How do I include one XML file in another?
This works exactly the same as for SGML. First you declare the
entity you want to include, and then you reference it by name:
]>
…blah blah…
&chap1;
&chap2;
&chap3;
&chap4;
&chap5;
The difference between this method and the one used for including
a DTD fragment (see question D.15, ‘How do I include one DTD
(or fragment) in another?’) is that this uses an external
general (file) entity which is referenced in the same way as for
a character entity (with an ampersand).
The one thing to make sure of is that the included file must not
have an XML or DOCTYPE Declaration on it. If you’ve been using one
for editing the fragment, remove it before using the file in this
way. Yes, this is a pain in the butt, but if you have lots of inclusions
like this, write a script to strip off the declaration (and paste
it back on again for editing).
47. What is parsing and how do I do it in XML
Parsing is the act of splitting up information into its component
parts (schools used to teach this in language classes until the
teaching profession collectively caught the anti-grammar disease).
‘Mary feeds Spot’ parses as
1. Subject = Mary, proper noun, nominative case
2. Verb = feeds, transitive, third person singular, present tense
3. Object = Spot, proper noun, accusative case
In computing, a parser is a program (or a piece of code or API that
you can reference inside your own programs) which analyses files
to identify the component parts. All applications that read input
have a parser of some kind, otherwise they’d never be able to figure
out what the information means. Microsoft Word contains a parser
which runs when you open a .doc file and checks that it can identify
all the hidden codes. Give it a corrupted file and you’ll get an
error message.
XML applications are just the same: they contain a parser which
reads XML and identifies the function of each the pieces of the
document, and it then makes that information available in memory
to the rest of the program.
While reading an XML file, a parser checks the syntax (pointy brackets,
matching quotes, etc) for well-formedness, and reports any violations
(reportable errors). The XML Specification lists what these are.
Validation is another stage beyond parsing. As the component parts
of the program are identified, a validating parser can compare them
with the pattern laid down by a DTD or a Schema, to check that they
conform. In the process, default values and datatypes (if specified)
can be added to the in-memory result of the validation that the
validating parser gives to the application.
The example above parses as: 1. Element person identified with Attribute
corpid containing abc123 and Attribute birth containing 1960-02-31
and Attribute gender containing female containing …
2. Element name containing …
3. Element forename containing text ‘Judy’ followed
by …
4. Element surname containing text ‘O’Grady’
(and lots of other stuff too).
As well as built-in parsers, there are also stand-alone parser-validators,
which read an XML file and tell you if they find an error (like
missing angle-brackets or quotes, or misplaced markup). This is
essential for testing files in isolation before doing something
else with them, especially if they have been created by hand without
an XML editor, or by an API which may be too deeply embedded elsewhere
to allow easy testing.
48. When should I use a CDATA Marked Section?
You should almost never need to use CDATA Sections. The CDATA mechanism
was designed to let an author quote fragments of text containing
markup characters (the open-angle-bracket and the ampersand), for
example when documenting XML (this FAQ uses CDATA Sections quite
a lot, for obvious reasons). A CDATA Section turns off markup recognition
for the duration of the section (it gets turned on again only by
the closing sequence of double end-square-brackets and a close-angle-bracket).
Consequently, nothing in a CDATA section can ever be recognised
as anything to do with markup: it’s just a string of opaque characters,
and if you use an XML transformation language like XSLT, any markup
characters in it will get turned into their character entity equivalent.
If you try, for example, to use:
some text with in it.
in the expectation that the embedded markup would remain untouched,
it won’t: it will just output
some text with markup in it.
In other words, CDATA Sections cannot preserve the embedded markup
as markup. Normally this is exactly what you want because this technique
was designed to let people do things like write documentation about
markup. It was not designed to allow the passing of little chunks
of (possibly invalid) unparsed HTML embedded inside your own XML
through to a subsequent process—because that would risk invalidating
the output.
As a result you cannot expect to keep markup untouched simply because
it looked as if it was safely ‘hidden’ inside a CDATA
section: it can’t be used as a magic shield to preserve HTML markup
for future use as markup, only as characters.
49. How can I handle embedded HTML in my XML
Apart from using CDATA Sections, there are two common occasions
when people want to handle embedded HTML inside an XML element:
1. when they have received (possibly poorly-designed) XML from somewhere
else which they must find a way to handle;
2. when they have an application which has been explicitly designed
to store a string of characters containing <>
* Provide templates for the handling of that markup in your XSLT
transformation or whatever software you use which simply replicates
what was there, eg
50. What are the special characters in XML
For normal text (not markup), there are no special characters:
just make sure your document refers to the correct encoding scheme
for the language and/or writing system you want to use, and that
your computer correctly stores the file using that encoding scheme.
See the question on non-Latin characters for a longer explanation.
If your keyboard will not allow you to type the characters you want,
or if you want to use characters outside the limits of the encoding
scheme you have chosen, you can use a symbolic notation called ‘entity
referencing’. Entity references can either be numeric, using
the decimal or hexadecimal Unicode code point for the character
(eg if your keyboard has no Euro symbol (€) you can type €);
or they can be character, using an established name which you declare
in your DTD (eg ) and then use as € in your document. If you
are using a Schema, you must use the numeric form for all except
the five below because Schemas have no way to make character entity
declarations. If you use XML with no DTD, then these five character
entities are assumed to be predeclared, and you can use them without
declaring them: <
The less-than character (<) starts element markup (the first character of a start-tag or an end-tag). &
The ampersand character (>) starts entity markup (the first
character of a character entity reference).>
The greater-than character (>) ends a start-tag or an end-tag.
"
The double-quote character (”) can be symbolised with this
character entity reference when you need to embed a double-quote
inside a string which is already double-quoted.
‘
The apostrophe or single-quote character (’) can be symbolised with
this character entity reference when you need to embed a single-quote
or apostrophe inside a string which is already single-quoted.
If you are using a DTD then you must declare all the character entities
you need to use (if any), including any of the five above that you
plan on using (they cease to be predeclared if you use a DTD). If
you are using a Schema, you must use the numeric form for all except
the five above because Schemas have no way to make character entity
declarations.
51. Do I have to change any of my server software to work
with XML?
The only changes needed are to make sure your server serves up
.xml, .css, .dtd, .xsl, and whatever other file types you will use
as the correct MIME content (media) types.
The details of the settings are specified in RFC 3023. Most new
versions of Web server software come preset.
If not, all that is needed is to edit the mime-types file (or its
equivalent: as a server operator you already know where to do this,
right?) and add or edit the relevant lines for the right media types.
In some servers (eg Apache), individual content providers or directory
owners may also be able to change the MIME types for specific file
types from within their own directories by using directives in a
.htaccess file. The media types required are:
* text/xml for XML documents which are ‘readable by casual
users’;
* application/xml for XML documents which are ‘unreadable
by casual users’;
* text/xml-external-parsed-entity for external parsed entities such
as document fragments (eg separate chapters which make up a book)
subject to the readability distinction of text/xml;
* application/xml-external-parsed-entity for external parsed entities
subject to the readability distinction of application/xml;
* application/xml-dtd for DTD files and modules, including character
entity sets.
The RFC has further suggestions for the use of the +xml media type
suffix for identifying ancillary files such as XSLT (application/xslt+xml).
If you run scripts generating XHTML which you wish to be treated
as XML rather than HTML, they may need to be modified to produce
the relevant Document Type Declaration as well as the right media
type if your application requires them to be validated.
51. I’m trying to understand the XML Spec: why does it
have such difficult terminology?
For implementation to succeed, the terminology needs to be precise.
Design goal eight of the specification tells us that ‘the
design of XML shall be formal and concise’. To describe XML,
the specification therefore uses formal language drawn from several
fields, specifically those of text engineering, international standards
and computer science. This is often confusing to people who are
unused to these disciplines because they use well-known English
words in a specialised sense which can be very different from their
common meanings—for example: grammar, production, token, or
terminal.
The specification does not explain these terms because of the other
part of the design goal: the specification should be concise. It
doesn’t repeat explanations that are available elsewhere: it is
assumed you know this and either know the definitions or are capable
of finding them. In essence this means that to grok the fullness
of the spec, you do need a knowledge of some SGML and computer science,
and have some exposure to the language of formal standards.
Sloppy terminology in specifications causes misunderstandings and
makes it hard to implement consistently, so formal standards have
to be phrased in formal terminology. This FAQ is not a formal document,
and the astute reader will already have noticed it refers to ‘element
names’ where ‘element type names’ is more correct;
but the former is more widely understood.
52. Can I still use server-side inclusions?
Yes, so long as what they generate ends up as part of an XML-conformant
file (ie either valid or just well-formed).
Server-side tag-replacers like shtml, PHP, JSP, ASP, Zope, etc store
almost-valid files using comments, Processing Instructions, or non-XML
markup, which gets replaced at the point of service by text or XML
markup (it is unclear why some of these systems use non-HTML/XML
markup). There are also some XML-based preprocessors for formats
like XVRL (eXtensible Value Resolution Language) which resolve specialised
references to external data and output a normalised XML file.
53. Can I (and my authors) still use client-side inclusions?
The same rule applies as for server-side inclusions, so you need
to ensure that any embedded code which gets passed to a third-party
engine (eg calls to SQL, VB, Java, etc) does not contain any characters
which might be misinterpreted as XML markup (ie no angle brackets
or ampersands). Either use a CDATA marked section to avoid your
XML application parsing the embedded code, or use the standard <, and & character entity references instead.
54. How can I include a conditional statement in my XML?
You can’t: XML isn’t a programming language, so you can’t say things
like
If you need to make an element optional, based on some internal
or external criteria, you can do so in a Schema. DTDs have no internal
referential mechanism, so it isn’t possible to express this kind
of conditionality in a DTD at the individual element level.
It is possible to express presence-or-absence conditionality in
a DTD for the whole document, by using parameter entities as switches
to include or ignore certain sections of the DTD based on settings
either hardwired in the DTD or supplied in the internal subset.
Both the TEI and Docbook DTDs use this mechanism to implement modularity.
Alternatively you can make the element entirely optional in the
DTD or Schema, and provide code in your processing software that
checks for its presence or absence. This defers the checking until
the processing stage: one of the reasons for Schemas is to provide
this kind of checking at the time of document creation or editing.
55. I have to do an overview of XML for my manager/client/investor/advisor.
What should I mention?
* XML is not a markup language. XML is a ‘metalanguage’,
that is, it’s a language that lets you define your own markup languages
(see definition).
* XML is a markup language [two (seemingly) contradictory statements
one after another is an attention-getting device that I'm fond of],
not a programming language. XML is data: is does not ‘do’
anything, it has things done to it.
* XML is non-proprietary: your data cannot be held hostage by someone
else.
* XML allows multi-purposing of your data.
* Well-designed XML applications most often separate ‘content’
from ‘presentation’. You should describe what something
is rather what something looks like (the exception being data content
which never gets presented to humans).
Saying ‘the data is in XML’ is a relatively useless
statement, similar to saying ‘the book is in a natural language’.
To be useful, the former needs to specify ‘we have used XML
to define our own markup language’ (and say what it is), similar
to specifying ‘the book is in French’.
A classic example of multipurposing and separation that I often
use is a pharmaceutical company. They have a large base of data
on a particular drug that they need to publish as:
* reports to the FDA;
* drug information for publishers of drug directories/catalogs;
* ‘prescribe me!’ brochures to send to doctors;
* little pieces of paper to tuck into the boxes;
* labels on the bottles;
* two pages of fine print to follow their ad in Reader’s Digest;
* instructions to the patient that the local pharmacist prints out;
* etc.
Without separation of content and presentation, they need to maintain
essentially identical information in 20 places. If they miss a place,
people die, lawyers get rich, and the drug company gets poor. With
XML (or SGML), they maintain one set of carefully validated information,
and write 20 programs to extract and format it for each application.
The same 20 programs can now be applied to all the hundreds of drugs
that they sell.
In the Web development area, the biggest thing that XML offers is
fixing what is wrong with HTML:
* browsers allow non-compliant HTML to be presented;
* HTML is restricted to a single set of markup (’tagset’).
If you let broken HTML work (be presented), then there is no motivation
to fix it. Web pages are therefore tag soup that are useless for
further processing. XML specifies that processing must not continue
if the XML is non-compliant, so you keep working at it until it
complies. This is more work up front, but the result is not a dead-end.
If you wanted to mark up the names of things: people, places, companies,
etc in HTML, you don’t have many choices that allow you to distinguish
among them. XML allows you to name things as what they are:
at
gives you a flexibility that you don’t have with HTML:
Charles Goldfarb worked atIBM<
With XML you don’t have to shoe-horn your data into markup that
restricts your options.
56. What is the purpose of XML namespaces?
XML namespaces are designed to provide universally unique names
for elements and attributes. This allows people to do a number of
things, such as:
* Combine fragments from different documents without any naming
conflicts. (See example below.)
* Write reusable code modules that can be invoked for specific elements
and attributes. Universally unique names guarantee that such modules
are invoked only for the correct elements and attributes.
* Define elements and attributes that can be reused in other schemas
or instance documents without fear of name collisions. For example,
you might use XHTML elements in a parts catalog to provide part
descriptions. Or you might use the nil attribute defined in XML
Schemas to indicate a missing value.
As an example of how XML namespaces are used to resolve naming conflicts
in XML documents that contain element types and attributes from
multiple XML languages, consider the following two XML documents:
and:
Each document uses a different XML language and each language defines
an Address element type. Each of these Address element types is
different — that is, each has a different content model, a different
meaning, and is interpreted by an application in a different way.
This is not a problem as long as these element types exist only
in separate documents. But what if they are combined in the same
document, such as a list of departments, their addresses, and their
Web servers? How does an application know which Address element
type it is processing?
One solution is to simply rename one of the Address element types
– for example, we could rename the second element type IPAddress.
However, this is not a useful long term solution. One of the hopes
of XML is that people will standardize XML languages for various
subject areas and write modular code to process those languages.
By reusing existing languages and code, people can quickly define
new languages and write applications that process them. If we rename
the second Address element type to IPAddress, we will break any
code that expects the old name.
A better answer is to assign each language (including its Address
element type) to a different namespace. This allows us to continue
using the Address name in each language, but to distinguish between
the two different element types. The mechanism by which we do this
is XML namespaces.
(Note that by assigning each Address name to an XML namespace, we
actually change the name to a two-part name consisting of the name
of the XML namespace plus the name Address. This means that any
code that recognizes just the name Address will need to be changed
to recognize the new two-part name. However, this only needs to
be done once, as the two-part name is universally unique.
57. What is an XML namespace?
An XML namespace is a collection of element type and attribute
names. The collection itself is unimportant — in fact, a reasonable
argument can be made that XML namespaces don’t actually exist as
physical or conceptual entities . What is important is the name
of the XML namespace, which is a URI. This allows XML namespaces
to provide a two-part naming system for element types and attributes.
The first part of the name is the URI used to identify the XML namespace
– the namespace name. The second part is the element type or attribute
name itself — the local part, also known as the local name. Together,
they form the universal name.
This two-part naming system is the only thing defined by the XML
namespaces recommendation.
58. Does the XML namespaces recommendation define anything
except a two-part naming system for element types and attributes?
No.
This is a very important point and a source of much confusion, so
we will repeat it:
THE XML NAMESPACES RECOMMENDATION DOES NOT DEFINE ANYTHING EXCEPT
A TWO-PART NAMING SYSTEM FOR ELEMENT TYPES AND ATTRIBUTES.
In particular, they do not provide or define any of the following:
* A way to merge two documents that use different DTDs.
* A way to associate XML namespaces and schema information.
* A way to validate documents that use XML namespaces.
* A way to associate element type or attribute declarations in a
DTD with an XML namespace.
58. What do XML namespaces actually contain?
XML namespaces are collections of names, nothing more. That is,
they contain the names of element types and attributes, not the
elements or attributes themselves. For example, consider the following
document.
The element type name A and the attribute name C are in the http://www.google.org/
namespace because they are mapped there by the google prefix. The
element type name B and the attribute name D are not in any XML
namespace because no prefix maps them there. On the other hand,
the elements A and B and the attributes C and D are not in any XML
namespace, even though they are physically within the scope of the
http://www.google.org/ namespace declaration. This is because XML
namespaces contain names, not elements or attributes.
XML namespaces also do not contain the definitions of the element
types or attributes. This is an important difference, as many people
are tempted to think of an XML namespace as a schema, which it is
not.
59. Are the names of all element types and attributes in
some XML namespace?
No.
If an element type or attribute name is not specifically declared
to be in an XML namespace — that is, it is unprefixed and (in the
case of element type names) there is no default XML namespace —
then that name is not in any XML namespace. If you want, you can
think of it as having a null URI as its name, although no “null”
XML namespace actually exists. For example, in the following, the
element type name B and the attribute names C and E are not in any
XML namespace:
60. Do XML namespaces apply to entity names, notation names,
or processing instruction targets?
No.
XML namespaces apply only to element type and attribute names. Furthermore,
in an XML document that conforms to the XML namespaces recommendation,
entity names, notation names, and processing instruction targets
must not contain colons.
61. Who can create an XML namespace?
Anybody can create an XML namespace — all you need to do is assign
a URI as its name and decide what element type and attribute names
are in it. The URI must be under your control and should not be
being used to identify a different XML namespace, such as by a coworker.
(In practice, most people that create XML namespaces also describe
the element types and attributes whose names are in it — their
content models and types, their semantics, and so on. However, this
is not part of the process of creating an XML namespace, nor does
the XML namespace include or provide a way to discover such information.)
62. Do I need to use XML namespaces?
Maybe, maybe not.
If you don’t have any naming conflicts in the XML documents you
are using today, as is often the case with documents used inside
a single organization, then you probably don’t need to use XML namespaces.
However, if you do have conflicts today, or if you expect conflicts
in the future due to distributing your documents outside your organization
or bringing outside documents into your organization, then you should
probably use XML namespaces.
Regardless of whether you use XML namespaces in your own documents,
it is likely that you will use them in conjunction with some other
XML technology, such as XSL, XHTML, or XML Schemas. For example,
the following XSLT (XSL Transformations) stylesheet uses XML namespaces
to distinguish between element types defined in XSLT and those defined
elsewhere:
xmlns:xsl=”http://www.w3.org/1999/XSL/Transform”>
63. What is the relationship between XML namespaces and
the XML 1.0 recommendation?
Although the XML 1.0 recommendation anticipated the need for XML
namespaces by noting that element type and attribute names should
not include colons, it did not actually support XML namespaces.
Thus, XML namespaces are layered on top of XML 1.0. In particular,
any XML document that uses XML namespaces is a legal XML 1.0 document
and can be interpreted as such in the absence of XML namespaces.
For example, consider the following document:
If this document is processed by a namespace-unaware processor,
that processor will see two elements whose names are google:A and
google:B. The google:A element has an attribute named xmlns:google
and the google:B element has an attribute named google:C. On the
other hand, a namespace-aware processor will see two elements with
universal names {http://www.google.org}A and {http://www.google.org}B.
The {http://www.google.org}A does not have any attributes; instead,
it has a namespace declaration that maps the google prefix to the
URI http://www.google.org. The {http://www.google.org}B element
has an attribute named {http://www.google.org}C.
Needless to say, this has led to a certain amount of confusion.
One area of confusion is the relationship between XML namespaces
and validating XML documents against DTDs. This occurs because the
XML namespaces recommendation did not describe how to use XML namespaces
with DTDs. Fortunately, a similar situation does not occur with
XML schema languages, as all of these support XML namespaces.
The other main area of confusion is in recommendations and specifications
such as DOM and SAX whose first version predates the XML namespaces
recommendation. Although these have since been updated to include
XML namespace support, the solutions have not always been pretty
due to backwards compatibility requirements. All recommendations
in the XML family now support XML namespaces.
64. What is the difference between versions 1.0 and 1.1
of the XML namspaces recommendation?
There are only two differences between XML namespaces 1.0 and XML
namespaces 1.1:
* Version 1.1 adds a way to undeclare prefixes. For more information,
see question 4.7.
* Version 1.1 uses IRIs (Internationalized Resource Identifiers)
instead of URIs. Basically, URIs are restricted to a subset of ASCII
characters, while IRIs allow much broader use of Unicode characters.
For complete details, see section 9 of Namespaces in XML 1.1.
NOTE: As of this writing (February, 2003), Namespaces in XML 1.1
is still a candidate recommendation and not widely used. PART II:
DECLARING AND USING XML NAMESPACES
65. How do I declare an XML namespace in an XML document?
To declare an XML namespace, you use an attribute whose name has
the form:
xmlns:prefix
–OR–
xmlns
These attributes are often called xmlns attributes and their value
is the name of the XML namespace being declared; this is a URI.
The first form of the attribute (xmlns:prefix) declares a prefix
to be associated with the XML namespace. The second form (xmlns)
declares that the specified namespace is the default XML namespace.
For example, the following declares two XML namespaces, named http://www.google.com/ito/addresses
and http://www.google.com/ito/servers. The first declaration associates
the addr prefix with the http://www.google.com/ito/addresses namespace
and the second declaration states that the http://www.google.com/ito/servers
namespace is the default XML namespace.
xmlns=”http://www.google.com/ito/servers”>
NOTE: Technically, xmlns attributes are not attributes at all —
they are XML namespace declarations that just happen to look like
attributes. Unfortunately, they are not treated consistently by
the various XML recommendations, which means that you must be careful
when writing an XML application.
For example, in the XML Information Set (http://www.w3.org/TR/xml-infoset),
xmlns “attributes” do not appear as attribute information
items. Instead, they appear as namespace declaration information
items. On the other hand, both DOM level 2 and SAX 2.0 treat namespace
attributes somewhat ambiguously. In SAX 2.0, an application can
instruct the parser to return xmlns “attributes” along
with other attributes, or omit them from the list of attributes.
Similarly, while DOM level 2 sets namespace information based on
xmlns “attributes”, it also forces applications to manually
add namespace declarations using the same mechanism the application
would use to set any other attributes.
66. Where can I declare an XML namespace?
You can declare an XML namespace on any element in an XML document.
The namespace is in scope for that element and all its descendants
unless it is overridden.
67. Can I use an attribute default in a DTD to declare
an XML namespace?
Yes.
For example, the following uses the FIXED attribute xmlns:google
on the A element type to associate the google prefix with the http://www.google.org/
namespace. The effect of this is that both A and B are in the http://www.google.org/
namespace.
xmlns:google CDATA #FIXED "http://www.google.org/">
]>
IMPORTANT: You should be very careful about placing XML namespace
declarations in external entities (external DTDs), as non-validating
parsers are not required to read these. For example, suppose the
preceding DTD was placed in an external entity (google.dtd) and
that the document was processed by a non-validating parser that
did not read google.dtd. This would result in a namespace error
because the google prefix was never declared:
68. Do the default values of xmlns attributes declared
in the DTD apply to the DTD?
No.
Declaring a default value of an xmlns attribute in the DTD does
not declare an XML namespace for the DTD. (In fact, no XML namespace
declarations apply to DTDs.) Instead, these defaults (declarations)
take effect only when the attribute is instantiated on an element.
For example:
]>
For more information, see question 7.2. (Note that an earlier version
of MSXML (the parser used by Internet Explorer) did use fixed xmlns
attribute declarations as XML namespace declarations, but that this
was removed in MSXML 4.
69. How do I override an XML namespace declaration that
uses a prefix?
To override the prefix used in an XML namespace declaration, you
simply declare another XML namespace with the same prefix. For example,
in the following, the google prefix is associated with the http://www.google.org/
namespace on the A and B elements and the http://www.bar.org/ namespace
on the C and D elements. That is, the names A and B are in the http://www.google.org/
namespace and the names C and D are in the http://www.bar.org/ namespace.
In general, this leads to documents that are confusing to read
and should be avoided.
70. How do I override a default XML namespace declaration?
To override the current default XML namespace, you simply declare
another XML namespace as the default. For example, in the following,
the default XML namespace is the http://www.google.org/ namespace
on the A and B elements and the http://www.bar.org/ namespace on
the C and D elements. That is, the names A and B are in the http://www.google.org/
namespace and the names C and D are in the http://www.bar.org/ namespace.
Using multiple default XML namespaces can lead to documents that
are confusing to read and should be done carefully.
71. How do I undeclare an XML namespace prefix?
In version 1.0 of the XML namespaces recommendation, you cannot
“undeclare” an XML namespace prefix. It remains in scope
until the end of the element on which it was declared unless it
is overridden. Furthermore, trying to undeclare a prefix by redeclaring
it with an empty (zero-length) name (URI) results in a namespace
error. For example:
In version 1.1 of the XML namespaces recommendation [currently
a candidate recommendation -- February, 2003], you can undeclare
an XML namespace prefix by redeclaring it with an empty name. For
example, in the above document, the XML namespace declaration xmlns:google=”"
is legal and removes the mapping from the google prefix to the http://www.google.org
URI. Because of this, the use of the google prefix in the google:D
element results in a namespace error.
71. How do I undeclare the default XML namespace?
To “undeclare” the default XML namespace, you declare
a default XML namespace with an empty (zero-length) name (URI).
Within the scope of this declaration, unprefixed element type names
do not belong to any XML namespace. For example, in the following,
the default XML namespace is the http://www.google.org/ for the
A and B elements and there is no default XML namespace for the C
and D elements. That is, the names A and B are in the http://www.google.org/
namespace and the names C and D are not in any XML namespace.
72. Why are special attributes used to declare XML namespaces?
I don’t know the answer to this question, but the likely reason
is that the hope that they would simplify the process of moving
fragments from one document to another document. An early draft
of the XML namespaces recommendation proposed using processing instructions
to declare XML namespaces. While these were simple to read and process,
they weren’t easy to move to other documents. Attributes, on the
other hand, are intimately attached to the elements being moved.
Unfortunately, this hasn’t worked as well as was hoped. For example,
consider the following XML document:
Simply using a text editor to cut the fragment headed by the
element from one document and paste it into another document results
in the loss of namespace information because the namespace declaration
is not part of the fragment — it is on the parent element ()
– and isn’t moved.
74. How do I use prefixes to refer to element type and
attribute names in an XML namespace?
75. How do I use the default XML namespace to refer to
element type names in an XML namespace?
76. How do I use the default XML namespace to refer to
attribute names in an XML namespace?
77. When should I use the default XML namespace instead
of prefixes?
When elements whose names are in multiple XML namespaces are interspersed,
default XML namespaces definitely make a document more difficult
to read and prefixes should be used instead. For example:
In some cases, default namespaces can be processed faster than
namespace prefixes, but the difference is certain to be negligible
in comparison to total processing time.
78. What is the scope of an XML namespace declaration?
The scope of an XML namespace declaration is that part of an XML
document to which the declaration applies. An XML namespace declaration
remains in scope for the element on which it is declared and all
of its descendants, unless it is overridden or undeclared on one
of those descendants.
For example, in the following, the scope of the declaration of the
http://www.google.org/ namespace is the element A and its descendants
(B and C). The scope of the declaration of the http://www.bar.org/
namespace is only the element C.
79. Does the scope of an XML namespace declaration include
the element it is declared on?
Yes.
For example, in the following, the names B and C are in the http://www.bar.org/
namespace, not the http://www.google.org/ namespace. This is because
the declaration that associates the google prefix with the http://www.bar.org/
namespace occurs on the B element, overriding the declaration on
the A element that associates it with the http://www.google.org/
namespace.
Similarly, in the following, the names B and C are in the http://www.bar.org/
namespace, not the http://www.google.org/ namespace because the
declaration declaring http://www.bar.org/ as the default XML namespace
occurs on the B element, overriding the declaration on the A element.
A final example is that, in the following, the attribute name D
is in the http://www.bar.org/ namespace.
One consequence of XML namespace declarations applying to the elements
they occur on is that they actually apply before they appear. Because
of this, software that processes qualified names should be particularly
careful to scan the attributes of an element for XML namespace declarations
before deciding what XML namespace (if any) an element type or attribute
name belongs to.
80. If an element or attribute is in the scope of an XML
namespace declaration, is its name in that namespace?
Not necessarily.
When an element or attribute is in the scope of an XML namespace
declaration, the element or attribute’s name is checked to see if
it has a prefix that matches the prefix in the declaration. Whether
the name is actually in the XML namespace depends on whether the
prefix matches. For example, in the following, the element type
names A, B, and D and the attribute names C and E are in the scope
of the declaration of the http://www.google.org/ namespace. While
the names A, B, and C are in that namespace, the names D and E are
not.
81. What happens when an XML namespace declaration goes
out of scope?
When an XML namespace declaration goes out of scope, it simply
no longer applies. For example, in the following, the declaration
of the http://www.google.org/ namespace does not apply to the C
element because this is outside its scope. That is, it is past the
end of the B element, on which the http://www.google.org/ namespace
was declared.
In addition to the declaration no longer applying, any declarations
that it overrode come back into scope. For example, in the following,
the declaration of the http://www.google.org/ namespace is brought
back into scope after the end of the B element. This is because
it was overridden on the B element by the declaration of the http://www.bar.org/
namespace.
abcd
82. What happens if no XML namespace declaration is in
scope?
If no XML namespace declaration is in scope, then any prefixed
element type or attribute names result in namespace errors. For
example, in the following, the names google:A and google:B result
in namespace errors.
In the absence of an XML namespace declaration, unprefixed element
type and attribute names do not belong to any XML namespace. For
example, in the following, the names A and B are not in any XML
namespace.
83. Can multiple XML namespace declarations be in scope
at the same time?
Yes, as long as they don’t use the same prefixes and at most one
of them is the default XML namespace. For example, in the following,
the http://www.google.org/ and http://www.bar.org/ namespaces are
both in scope for all elements:
One consequence of this is that you can place all XML namespace
declarations on the root element and they will be in scope for all
elements. This is the simplest way to use XML namespaces.
84. How can I declare XML namespaces so that all elements
and attributes are in their scope?
XML namespace declarations that are made on the root element are
in scope for all elements and attributes in the document. This means
that an easy way to declare XML namespaces is to declare them only
on the root element.
85. Does the scope of an XML namespace declaration ever
include the DTD?
No.
XML namespaces can be declared only on elements and their scope
consists only of those elements and their descendants. Thus, the
scope can never include the DTD.
86. Can I use XML namespaces in DTDs?
Yes and no.
In particular, DTDs can contain qualified names but XML namespace
declarations do not apply to DTDs .
This has a number of consequences. Because XML namespace declarations
do not apply to DTDs:
1. There is no way to determine what XML namespace a prefix in a
DTD points to. Which means…
2. Qualified names in a DTD cannot be mapped to universal names.
Which means…
3. Element type and attribute declarations in a DTD are expressed
in terms of qualified names, not universal names. Which means…
4. Validation cannot be redefined in terms of universal names as
might be expected.
This situation has caused numerous complaints but, as XML namespaces
are already a recommendation, is unlikely to change. The long term
solution to this problem is an XML schema language: all of the proposed
XML schema languages provide a mechanism by which the local name
in an element type or attribute declaration can be associated with
an XML namespace. This makes it possible to redefine validity in
terms of universal names.
87. Do XML namespace declarations apply to DTDs?
No.
In particular, an xmlns attribute declared in the DTD with a default
is not an XML namespace declaration for the DTD.. (Note that an
earlier version of MSXML (the parser used by Internet Explorer)
did use such declarations as XML namespace declarations, but that
this was removed in MSXML 4.
88. Can I use qualified names in DTDs?
Yes.
For example, the following is legal:
However, because XML namespace declarations do not apply to DTDs
, qualified names in the DTD cannot be converted to universal names.
As a result, qualified names in the DTD have no special meaning.
For example, google:A is just google:A — it is not A in the XML
namespace to which the prefix google is mapped.
The reason qualified names are allowed in the DTD is so that validation
will continue to work.
89. Can the content model in an element type declaration
contain element types whose names come from other XML namespaces?
Yes and no.
The answer to this question is yes in the sense that a qualified
name in a content model can have a different prefix than the qualified
name of the element type being declared. For example, the following
is legal:
The answer to this question is no in the sense that XML namespace
declarations do not apply to DTDs so the prefixes used in an element
type declaration are technically meaningless. In particular, they
do not specify that the name of a certain element type belongs to
a certain namespace. Nevertheless, the ability to mix prefixes in
this manner is crucial when: a) you have a document whose names
come from multiple XML namespaces , and b) you want to construct
that document in a way that is both valid and conforms to the XML
namespaces recommendation .
90. Can the attribute list of an element type contain attributes
whose names come from other XML namespaces?
Yes and no.
For example, the following is legal:
91. How can I construct an XML document that is valid and
conforms to the XML namespaces recommendation?
In answering this question, it is important to remember that:
* Validity is a concept defined in XML 1.0,
* XML namespaces are layered on top of XML 1.0 , and
* The XML namespaces recommendation does not redefine validity,
such as in terms of universal names .
Thus, validity is the same for a document that uses XML namespaces
and one that doesn’t. In particular, with respect to validity:
* xmlns attributes are treated as attributes, not XML namespace
declarations.
* Qualified names are treated like other names. For example, in
the name google:A, google is not treated as a namespace prefix,
the colon is not treated as separating a prefix from a local name,
and A is not treated as a local name. The name google:A is treated
simply as the name google:A.
Because of this, XML documents that you might expect to be valid
are not. For example, the following document is not valid because
the element type name A is not declared in the DTD, in spite of
the fact both google:A and A share the universal name {http://www.google.org/}A:
xmlns:google CDATA #FIXED "http://www.google.org/"
xmlns CDATA #FIXED "http://www.google.org/">
]>
Similarly, the following is not valid because the xmlns attribute
is not declared in the DTD:
xmlns:bar CDATA #FIXED "http://www.google.org/">
]>
92. How can I allow the prefixes in my document to be different
from the prefixes in my DTD?
This is legal because the * modifier directly follows the closing
parenthesis.
This document is valid because the declarations for p, s, and nsdecl
in the DTD set p and s to “” and nsdecl to “xmlns”.
That is, after replacing the p, s, and nsdecl parameter entities,
the DTD is as follows. Notice that both the DTD and document use
the element type names A, B, and C and the attribute names D and
E.
xmlns CDATA “http://www.google.org/”>
But what if the document wants to use a different prefix, such
as google? In this case, the document must override the declarations
of the p and s entities in its internal DTD. That is, it must declare
these entities so that they use google as a prefix (followed by
a colon) and a suffix (preceded by a colon). For example:
]>
In this case, the internal DTD is read before the external DTD,
so the values of the p and s entities from the document are used.
Thus, after replacing the p, s, and nsdecl parameter entities, the
DTD is as follows. Notice that both the DTD and document use the
element type names google:A, google:B, and google:C and the attribute
names google:D and E.
E CDATA #REQUIRED>
93. How can I validate an XML document that uses XML namespaces?
When people ask this question, they usually assume that validity
is different for documents that use XML namespaces and documents
that don’t. In fact, it isn’t — it’s the same for both. Thus, there
is no difference between validating a document that uses XML namespaces
and validating one that doesn’t. In either case, you simply use
a validating parser or other software that performs validation.
94. If I start using XML namespaces, do I need to change
my existing DTDs?
Probably. If you want your XML documents to be both valid and conform
to the XML namespaces recommendation, you need to declare any xmlns
attributes and use the same qualified names in the DTD as in the
body of the document.
If your DTD contains element type and attribute names from a single
XML namespace, the easiest thing to do is to use your XML namespace
as the default XML namespace. To do this, declare the attribute
xmlns (no prefix) for each possible root element type. If you can
guarantee that the DTD is always read , set the default value in
each xmlns attribute declaration to the URI used as your namespace
name. Otherwise, declare your XML namespace as the default XML namespace
on the root element of each instance document.
If your DTD contains element type and attribute names from multiple
XML namespaces, you need to choose a single prefix for each XML
namespace and use these consistently in qualified names in both
the DTD and the body of each document. You also need to declare
your xmlns attributes in the DTD and declare your XML namespaces.
As in the single XML namespace case, the easiest way to do this
is add xmlns attributes to each possible root element type and use
default values if possible.
95. How do I create documents that use XML namespaces?
The same as you create documents that don’t use XML namespaces.
If you’re currently using Notepad on Windows or emacs on Linux,
you can continue using Notepad or emacs. If you’re using an XML
editor that is not namespace-aware, you can also continue to use
that, as qualified names are legal names in XML documents and xmlns
attributes are legal attributes. And if you’re using an XML editor
that is namespace-aware, it will probably provide features such
as automatically declaring XML namespaces and keeping track of prefixes
and the default XML namespace for you.
96. How can I check that a document conforms to the XML
namespaces recommendation?
Unfortunately, I know of no software that only checks for conformance
to the XML namespaces recommendation. It is possible that some namespace-aware
validating parsers (such as those from DataChannel (Microsoft),
IBM, Oracle, or Sun) check XML namespace conformance as part of
parsing and validating. Thus, you might be able to run your document
through such parsers as a way of testing conformance.
Note that writing an application to check conformance to the XML
namespaces recommendation is not as easy as it might seem. The problem
is that most parsers do not make DTD information available to the
application, so it might not be possible to check conformance in
the DTD. Also note that writing a SAX 1.0 application that checks
conformance in the body of the document (as opposed to the DTD)
should be an easy thing to do.
97. Can I use the same document with both namespace-aware
and namespace-unaware applications?
Yes.
This situation is quite common, such as when a namespace-aware application
is built on top of a namespace-unaware parser. Another common situation
is when you create an XML document with a namespace-unaware XML
editor but process it with a namespace-aware application.
Using the same document with both namespace-aware and namespace-unaware
applications is possible because XML namespaces use XML syntax.
That is, an XML document that uses XML namespaces is still an XML
document and is recognized as such by namespace-unaware software.
The only thing you need to be careful about when using the same
document with both namespace-aware and namespace-unaware applications
is when the namespace-unaware application requires the document
to be valid. In this case, you must be careful to construct your
document in a way that is both valid and conforms to the XML namespaces
recommendation. (It is possible to construct documents that conform
to the XML namespaces recommendation but are not valid and vice
versa.)
98. What software is needed to process XML namespaces?
From a document author’s perspective, this is generally not a relevant
question. Most XML documents are written in a specific XML language
and processed by an application that understands that language.
If the language uses an XML namespace, then the application will
already use that namespace — there is no need for any special XML
namespace software.
99. How do I use XML namespaces with Internet Explorer
5.0 and/or the MSXML parser?
WARNING! The following applies only to earlier versions of MSXML.
It does not apply to MSXML 4, which is the currently shipping version
[July, 2002].
An early version of the MSXML parser, which was shipped as part
of Internet Explorer 5.0, required that every XML namespace prefix
used in an element type or attribute declaration had to be “declared”
in the attribute declaration for that element type. This had to
be done with a fixed xmlns attribute declaration. For example, the
following was accepted by MSXML and both xmlns:google attributes
were required:
MSXML returned an error for the following because the second google
prefix was not “declared”:
The reason for this restriction was so that MSXML could use universal
names to match element type and attribute declarations to elements
and attributes during validation. Although this would have simplified
many of the problems of writing documents that are both valid and
conform to the XML namespaces recommendation some users complained
about it because it was not part of the XML namespaces recommendation.
In response to these complaints, Microsoft removed this restriction
in later versions, which are now shipping. Ironically, the idea
was later independently derived as a way to resolve the problems
of validity and namespaces. However, it has not been implemented
by anyone.
100. How do applications process documents that use XML
namespaces?
Applications process documents that use XML namespaces in almost
exactly the same way they process documents that don’t use XML namespaces.
For example, if a namespace-unaware application adds a new sales
order to a database when it encounters a Sales Order element, the
equivalent namespace-aware application does the same. The only difference
is that the namespace-aware application:
* Might need to check for xmlns attributes and parse qualified names.
Whether it does this depends on whether such processing is already
done by lower-level software, such as a namespace-aware DOM implementation.
* Uses universal (two-part) names instead of local (one-part) names.
For example, the namespace-aware application might add a new sales
order in response to an {http://www.google.com/ito/sales}SalesOrder
element instead of a Sales Order element.
101. How do I use XML namespaces with SAX 1.0?
The easiest way to use XML namespaces with SAX 1.0 is to use John
Cowan’s Namespace SAX Filter (see http://www.ccil.org/~cowan/XML).
This is a SAX filter that keeps track of XML namespace declarations,
parses qualified names, and returns element type and attribute names
as universal names in the form:
URI^local-name
For example:
http://www.google.com/ito/sales^SalesOrder
Your application can then base its processing on these longer names.
For example, the code:
public void startElement(String elementName, AttributeList attrs)
throws SAXException
{
…
if (elementName.equals(”SalesOrder”))
{
// Add new database record.
}
…
}
might become:
public void startElement(String elementName, AttributeList attrs)
throws SAXException
{
…
if (elementName.equals(”http://www.google.com/sales^SalesOrder”))
{
// Add new database record.
}
…
}
or:
public void startElement(String elementName, AttributeList attrs)
throws SAXException
{
…
// getURI() and getLocalName() are utility functions
// to parse universal names.
if (getURI(elementName).equals(”http://www.foo.com/ito/sales”))
{
if (getLocalName(elementName).equals(”SalesOrder”))
{
// Add new database record.
}
}
…
}
If you do not want to use the Namespace SAX Filter, then you will
need to do the following in addition to identifying element types
and attributes by their universal names:
* In startElement, scan the attributes for XML namespace declarations
before doing any other processing. You will need to maintain a table
of current prefix-to-URI mappings (including a null prefix for the
default XML namespace).
* In startElement and endElement, check whether the element type
name includes a prefix. If so, use your mappings to map this prefix
to a URI. Depending on how your software works, you might also check
if the local part of the qualified name includes any colons, which
are illegal.
* In startElement, check whether attribute names include a prefix.
If so, process as in the previous point.
102. How do I use XML namespaces with SAX 2.0?
SAX 2.0 primarily supports XML namespaces through the following
methods: * startElement and endElement in the ContentHandler interface
return namespace names (URIs) and local names as well as qualified
names. * getValue, getType, and getIndex in the Attributes interface
can retrieve attribute information by namespace name (URI) and local
name as well as by qualified name.
103. How do I use XML namespaces with DOM level 2?
// Check the local name.
// getNodeName() is a DOM level 1 method.
if (elementNode.getNodeName().equals(”SalesOrder”))
{
// Add new database record.
}
might become the following namespace-aware code:
// Check the XML namespace name (URI).
// getNamespaceURI() is a DOM level 2 method.
String SALES_NS = “http://www.foo.com/ito/sales”;
if (elementNode.getNamespaceURI().equals(SALES_NS))
{
// Check the local name.
// getLocalName() is a DOM level 2 method.
if (elementNode.getLocalName().equals(”SalesOrder”))
{
// Add new database record.
}
}
Note that, unlike SAX 2.0, DOM level 2 treats xmlns attributes
as normal attributes.
104. Can an application process documents that use XML
namespaces and documents that don’t use XML namespaces?
Yes.
This is a common situation for generic applications, such as editors,
browsers, and parsers, that are not wired to understand a particular
XML language. Such applications simply treat all element type and
attribute names as qualified names. Those names that are not mapped
to an XML namespace — that is, unprefixed element type names in
the absence of a default XML namespace and unprefixed attribute
names — are simply processed as one-part names, such as by using
a null XML namespace name (URI).
Note that such applications must decide how to treat documents that
do not conform to the XML namespaces recommendation. For example,
what should the application do if an element type name contains
a colon (thus implying the existence of a prefix), but there are
no XML namespace declarations in the document? The application can
choose to treat this as an error, or it can treat the document as
one that does not use XML namespaces, ignore the “error”,
and continue processing.
105. Can an application be both namespace-aware and namespace-unaware?
Yes.
However, there is generally no reason to do this. The reason is
that most applications understand a particular XML language, such
as one used to transfer sales orders between companies. If the element
type and attribute names in the language belong to an XML namespace,
the application must be namespace-aware; if not, the application
must be namespace-unaware.
For a few applications, being both namespace-aware and namespace-unaware
makes sense. For example, a parser might choose to redefine validity
in terms of universal names and have both namespace-aware and namespace-unaware
validation modes. However, such applications are uncommon.
106. What does a namespace-aware application do when it
encounters an error?
The XML namespaces recommendation does not specify what a namespace-aware
application does when it encounters a document that does not conform
to the recommendation. Therefore, the behavior is application-dependent.
For example, the application could stop processing, post an error
to a log and continue processing, or ignore the error.
PART III: NAMES, PREFIXES, AND URIs
107. What is a qualified name?
A qualified name is a name of the following form. It consists of
an optional prefix and colon, followed by the local part, which
is sometimes known as a local name.
prefix:local-part
–OR–
local-part
For example, both of the following are qualified names. The first
name has a prefix of serv; the second name does not have a prefix.
For both names, the local part (local name) is Address.
serv:Address
Address
In most circumstances, qualified names are mapped to universal
names.
108. What characters are allowed in a qualified name?
The prefix can contain any character that is allowed in the Name
[5] production in XML 1.0 except a colon. The same is true of the
local name. Thus, there can be at most one colon in a qualified
name — the colon used to separate the prefix from the local name.
109. Where can qualified names appear?
Qualified names can appear anywhere an element type or attribute
name can appear: in start and end tags, as the document element
type, and in element type and attribute declarations in the DTD.
For example:
]>
Qualified names cannot appear as entity names, notation names,
or processing instruction targets.
110. Can qualified names be used in attribute values?
Yes, but they have no special significance. That is, they are not
necessarily recognized as such and mapped to universal names. For
example, the value of the C attribute in the following is the string
“foo:D”, not the universal name {http://www.foo.org/}D.
In spite of this, there is nothing to stop an application from
recognizing a qualified name in an attribute value and processing
it as such. This is being done in various technologies today. For
example, in the following XML Schemas definition, the attribute
value xsd:string identifies the type of the foo attribute as the
universal name {http://www.w3.org/1999/XMLSchema}string.
There are two potential problems with this. First, the application
must be able to retrieve the prefix mappings currently in effect.
Fortunately, both SAX 2.0 and DOM level 2 support this capability.
Second, any general purpose transformation tool, such as one that
writes an XML document in canonical form and changes namespace prefixes
in the process, will not recognize qualified names in attribute
values and therefore not transform them correctly. Although this
may be solved in the future by the introduction of the QName (qualified
name) data type in XML Schemas, it is a problem today.
111. How are qualified names mapped to names in XML namespaces?
If a qualified name in the body of a document (as opposed to the
DTD) includes a prefix, then that prefix is used to map the local
part of the qualified name to a universal name — that is, a name
in an XML namespace. For example, in the following, the prefix foo
is used to map the local names A, B, and C to names in the http://www.foo.org/
namespace:
If a qualified name in the body of a document does not include
a prefix and a default XML namespace is in scope then one of two
things happens. If the name is used as an element tag, it is mapped
to a name in the default XML namespace. If it is used as an attribute
name, it is not in any XML namespace. For example, in the following,
A and B are in the http://www.foo.org/ namespace and C is not in
any XML namespace:
If a qualified name in the body of a document does not include
a prefix and no default XML namespace is in scope, then that name
is not in any XML namespace. For example, in the following, A, B,
and C are not in any XML namespace:
abcd
112. How are universal names represented?
113. Are universal names universally unique?
114. What is an XML namespace prefix?
115. What characters are allowed in an XML namespace prefix?
116. Can I use the same prefix for more than one XML namespace?
117. What happens if there is no prefix on an element type
name?
118. What does the URI used as an XML namespace name point
to?
119. What is an XML namespace name?
120. Can I resolve the URI used as an XML namespace name?
121. Can I use a relative URI as a namespace name?
Yes. However, such usage is deprecated, so you should never do
it.
123. How do I install the XPointer processor?
124. What is server-side XPointer?
GET /myTripleStore HTTP/1.1
Host: www.myorg.org
Response:
HTTP/1.1 200 Ok
Content-Type: application/rdf+xml
This request asks for the entire triple store, serialized as RDF/XML.
125. What about non-XML resources?
126. What XPointer schemes are supported in this release?
127. How do I configure an XPointer processor?
128. How do integrate XPointer into my application?
129. How do I implement an application-specific XPointer
scheme?
130. How do I support very large resources?
133. What are the valid values for xlink:actuate and xlink:show?
1. XLink can be attached with any element. Hyperlinks in HTML can
be attached to only an ANCHOR element.
2. XLink can refer to a specific location in XML document by name
or context with the help of XPointer. HTML ANCHOR does
not have capability to point to specific location within an html
document.
3. XLink / XML links can be multidirectional. HTML links are unidirectional.
4. HTML links are activated when user clicks on them. XLink has
option of activating automatically when XML document is processed.
135. What three essential components of security does the
XML Signatures provide?
136. XLink Processing and Conformance
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