Infobox file format
name = Extensible Markup Language
extension = .xml
mime = application/xml, text/xml (deprecated)
type code =
uniform type = public.xml
World Wide Web Consortium
container for =
contained by =
extended from = SGML
extended to =
XHTML, RSS, Atom, ...
standard = [http://www.w3.org/TR/2006/REC-xml-20060816/ 1.0 (Fourth Edition)] [http://www.w3.org/TR/2006/REC-xml11-20060816/ 1.1 (Second Edition)] The Extensible Markup Language (XML) is a general-purpose "specification" for creating custom
markup languages. [It is often said to be a markup language itself. This is incorrect.Fact|date=May 2008] It is classified as an extensible languagebecause it allows its users to define their own elements. Its primary purpose is to help information systems share structured data, particularly via the Internet,cite web | title=Extensible Markup Language (XML) 1.0 (Fourth Edition) - Origin and Goals | url=http://www.w3.org/TR/2006/REC-xml-20060816/#sec-origin-goals | first=Tim |last=Bray| coauthors=Jean Paoli, C. M. Sperberg-McQueen, Eve Maler, François Yergeau | month=September | year=2006 | publisher=World Wide Web Consortium | accessmonthday=October 29 | accessyear=2006 ] and it is used both to encode documents and to serialize data. In the latter context, it is comparable with other text-based serialization languages such as JSONand YAML. [JSON and YAML are among other alternative text-based formats commonly described as lighter-weight and less verbose in comparison to XML. See Critique of XML in this article.]
It started as a simplified subset of the
Standard Generalized Markup Language(SGML), and is designed to be relatively human-legible. By adding semantic constraints, application languages can be implemented in XML. These include XHTML, [XHTML is an attempt to simplify and improve the consistency of HTML, which was based on SGML.] RSS, MathML, GraphML, Scalable Vector Graphics, MusicXML, and thousands of others. Moreover, XML is sometimes used as the specification languagefor such application languages.
XML is recommended by the
World Wide Web Consortium(W3C). It is a fee-free open standard.The recommendation specifies both the lexical grammarand the requirements for parsing.
Well-formed and valid XML documents
There are two levels of correctness of an XML document:
* Well-formed. A well-formed document conforms to all of XML's syntax rules. For example, if a start-tag appears without a corresponding end-tag, it is not "well-formed". A document that is not well-formed is not considered to be XML; a "conforming parser" is not allowed to process it.
* Valid. A valid document additionally conforms to some semantic rules. These rules are either user-defined, or included as an
XML schema, especially DTD. For example, if a document contains an undefined element, then it is not "valid"; a "validating parser" is not allowed to process it.
Well-formed documents: XML syntax
As long as only well-formedness is required, XML is a generic framework for storing any amount of text or any data whose structure can be represented as a tree.The only indispensable syntactical requirement is that the document has exactly one root element (alternatively called the document element). This means that the text must be enclosed between a root start-tag and a corresponding end-tag.The following is a "well-formed" XML document:
The root element can be preceded by an optional XML declaration. This element states what version of XML is in use (normally 1.0); it may also contain information about
character encodingand external dependencies.
The specification "requires" that processors of XML support the pan-
Unicode character encodings UTF-8and UTF-16 (UTF-32 is not mandatory). The use of more limited encodings, such as those based on ISO/IEC 8859, is acknowledged and is widely used and supported.
Comments can be placed anywhere in the tree, including in the text if the content of the element is text or #PCDATA.
XML comments start with <!-- and end with -->. Two consecutive dashes (--) may not appear anywhere in the text of the comment.
In any meaningful application, additional markup is used to structure the contents of the XML document. The text enclosed by the root tags may contain an arbitrary number of XML elements.The basic syntax for one element is:The two instances of »element_name« are referred to as the start-tag and end-tag, respectively.Here, »Element Content« is some text which may again contain XML elements. So, a generic XML document contains a
tree-based data structure.Here is an example of a structured XML document:
Attribute values must always be quoted, using single or double quotes; and each attribute name must appear only once in any element.
XML requires that elements be properly nested — elements may never overlap, and so must be closed in the opposite order to which they are opened. For example, this fragment of code below cannot be part of a well-formed XML document because the "title" and "author" elements are closed in the wrong order:One way of writing the same information in a way which could be incorporated into a well-formed XML document is as follows:
XML provides special syntax for representing an element with empty content. Instead of writing a start-tag followed immediately by an end-tag, a document may contain an empty-element tag. An empty-element tag resembles a start-tag but contains a slash just before the closing angle bracket. The following three examples are equivalent in XML:An empty-element may contain attributes:
An entity in XML is a named body of data, usually text. Entities are often used to represent single characters that cannot easily be entered on the keyboard; they are also used to represent pieces of standard ("boilerplate") text that occur in many documents, especially if there is a need to allow such text to be changed in one place only.
Special characters can be represented either using entity references, or by means of
numeric character references. An example of a numeric character reference is "
&#x20AC;", which refers to the
Euro symbolby means of its Unicodecodepoint in hexadecimal.
An entity reference is a
placeholderthat represents that entity. It consists of the entity's name preceded by an ampersand("
&") and followed by a
;"). XML has five
&amp;(& or "ampersand")
&lt;(< or "less than")
&gt;(> or "greater than")
&apos;(' or "apostrophe")
&quot;(" or "quotation mark")
Here is an example using a predeclared XML entity to represent the ampersand in the name "AT&T":Additional entities (beyond the predefined ones) can be declared in the document's Document Type Definition (DTD). A basic example of doing so in a minimal internal DTD follows. Declared entities can describe single characters or pieces of text, and can reference each other.
When viewed in a suitable browser, the XML document above appears as:
Copyright © 2006, XYZ Enterprises
Numeric character references
Numeric character references look like entity references, but instead of a name, they contain the "
#" character followed by a number. The number (in decimal or "
hexadecimal) represents a Unicode code point. Unlike entity references, they are neither predeclared nor do they need to be declared in the document's DTD. They have typically been used to represent characters that are not easily encodable, such as an Arabic character in a document produced on a European computer. The ampersand in the "AT&T" example could also be escaped like this (decimal 38 and hexadecimal 26 both represent the Unicode code point for the "&" character):
Similarly, in the previous example, notice that "&#xA9;" is used to generate the “©” symbol.
numeric character references.
In XML, a well-formed document must conform to the following rules, among others:
* Non-empty elements are delimited by both a start-tag and an end-tag.
* Empty elements may be marked with an empty-element (self-closing) tag, such as
<IAmEmpty />. This is equal to
* All attribute values are quoted with either single (') or double (") quotes. Single quotes close a single quote and double quotes close a double quote.
* Tags may be nested but must not overlap. Each non-root element must be completely contained in another element.
* The document complies with its declared character encoding. The encoding may be declared or implied externally, such as in "Content-Type" headers when a document is transported via HTTP, or internally, using explicit markup at the very beginning of the document. When no such declaration exists, a Unicode encoding is assumed, as defined by a Unicode Byte Order Mark before the document's first character. If the mark does not exist, UTF-8 encoding is assumed.
Element names are case-sensitive. For example, the following is a well-formed matching pair::
</Step>whereas this is not:
By carefully choosing the names of the XML elements one may convey the meaning of the data in the markup. This increases human readability while retaining the rigor needed for software parsing.
Choosing meaningful names implies the
semanticsof elements and attributes to a human reader without reference to external documentation. However, this can lead to verbosity, which complicates authoringand increases file size.
It is relatively simple to verify that a document is well-formed or validated XML, because the rules of well-formedness and validation of XML are designed for portability of tools. The idea is that any tool designed to work with XML files will be able to work with XML files written in any XML language (or XML application). Here are some examples of ways to verify XML documents:
* load it into an XML-capable browser, such as Firefox or
* use a tool like xmlwf (usually bundled with expat)
* parse the document, for instance in Ruby: irb> require "rexml/document" irb> include REXML irb> doc = Document.new(File.new("test.xml")).root
Valid documents: XML semantics
By leaving the names, allowable hierarchy, and meanings of the elements and attributes open and definable by a customizable "schema or DTD", XML provides a syntactic foundation for the creation of purpose-specific, XML-based markup languages. The general syntax of such languages is rigid — documents must adhere to the general rules of XML, ensuring that all XML-aware software can at least read and understand the relative arrangement of information within them. The schema merely supplements the syntax rules with a set of constraints. Schemas typically restrict element and attribute names and their allowable containment hierarchies, such as only allowing an element named 'birthday' to contain one element named 'month' and one element named 'day', each of which has to contain only character data. The constraints in a schema may also include
data typeassignments that affect how information is processed; for example, the 'month' element's character data may be defined as being a month according to a particular schema language's conventions, perhaps meaning that it must not only be formatted a certain way, but also must not be processed as if it were some other type of data.
An XML document that complies with a particular schema/DTD, in addition to being well-formed, is said to be valid.
An XML schema is a description of a type of XML document, typically expressed in terms of
constraintson the structure and content of documents of that type, above and beyond the basic constraints imposed by XML itself. A number of standard and proprietary XML schema languages have emerged for the purpose of formally expressing such schemas, and some of these languages are XML-based, themselves.
Before the advent of generalised data description languages such as SGML and XML, software designers had to define special
file formats or small languages to share data between programs. This required writing detailed specificationsand special-purpose parsers and writers.
XML's regular structure and strict parsing rules allow software designers to leave parsing to standard tools, and since XML provides a general,
data model-oriented framework for the development of application-specific languages, software designers need only concentrate on the development of rules for their data, at relatively high levels of abstraction.
Well-tested tools exist to
validatean XML document "against" a schema: the tool automatically verifies whether the document conforms to constraints expressed in the schema. Some of these validation tools are included in XML parsers, and some are packaged separately.
Other usages of schemas exist: XML editors, for instance, can use schemas to support the editing process (by suggesting valid elements and attributes names, etc).
The oldest schema format for XML is the
Document Type Definition(DTD), inherited from SGML. While DTD support is ubiquitous due to its inclusion in the XML 1.0 standard, it is seen as limited for the following reasons:
* It has no support for newer
features of XML, most importantly namespaces.
* It lacks expressiveness. Certain formal aspects of an XML document cannot be captured in a DTD.
* It uses a custom non-XML syntax, inherited from
SGML, to describe the schema.
DTD is still used in many applications because it is considered the easiest to read and write.
XML schemalanguage, described by the W3C as the successor of DTDs, is XML Schema, or more informally referred to by the initialism for XML Schema instances, XSD (XML Schema Definition). XSDs are far more powerful than DTDs in describing XML languages. They use a rich datatyping system, allow for more detailed constraints on an XML document's logical structure, and must be processed in a more robust validation framework. XSDs also use an XML-based format, which makes it possible to use ordinary XML tools to help process them, although XSD implementations require much more than just the ability to read XML.
Another popular schema language for XML is
RELAX NG. Initially specified by OASIS, RELAX NG is now also an ISO international standard (as part of DSDL). It has two formats: an XML based syntax and a non-XML compact syntax. The compact syntax aims to increase readability and writability but, since there is a well-defined way to translate the compact syntax to the XML syntax and back again by means of James Clark's [http://www.thaiopensource.com/relaxng/trang.html Trang conversion tool] , the advantage of using standard XML tools is not lost. RELAX NG has a simpler definition and validation framework than XML Schema, making it easier to use and implement. It also has the ability to use datatypeframework plug-ins; a RELAX NG schema author, for example, can require values in an XML document to conform to definitions in XML Schema Datatypes.
ISO DSDL and other schema languages
DSDL(Document Schema Description Languages) standard brings together a comprehensive set of small schema languages, each targeted at specific problems. DSDL includes RELAX NGfull and compact syntax, Schematronassertion language, and languages for defining datatypes, character repertoire constraints, renaming and entity expansion, and namespace-based routingof document fragments to different validators. DSDL schema languages do not have the vendor support of XML Schemas yet, and are to some extent a grassroots reaction of industrial publishers to the lack of utility of XML Schemas for publishing.
Some schema languages not only describe the structure of a particular XML format but also offer limited facilities to influence processing of individual XML files that conform to this format. DTDs and XSDs both have this ability; they can for instance provide attribute defaults. RELAX NG and Schematron intentionally do not provide these; for example the
XML supports the direct use of almost any Unicode character in element names, attributes, comments, character data, and processing instructions (other than the ones that have special symbolic meaning in XML itself, such as the open corner bracket, "<"). Therefore, the following is a well-formed XML document, even though it includes both Chinese and Cyrillic characters:
Displaying XML on the web
Generally, generic XML documents do not carry information about how to display the data. [The XML specification does not (by itself) indicate rules for formatting and layout. This is in contrast to the HTML specification (for example) which does indicate such rules. Nevertheless, some XML documents may indicate graphical layout, display or formatting based on some other specification. An example is SVG, which is a modularized language for describing two-dimensional vector and mixed vector/raster graphics in XML.] Without using CSS or
XSLT, a generic XML document is rendered as raw XML text by most web browsers. Some display it with 'handles' (e.g. + and - signs in the margin) that allow parts of the structure to be expanded or collapsed with mouse-clicks.
In order to style the rendering in a browser with CSS, the XML document must include a reference to the
Note that this is different from specifying such a stylesheet in HTML, which uses the
XSLT( XSL Transformations) can be used to alter the format of XML data, either into HTML or other formats that are suitable for a browser to display.
client-sideXSLT, the following processing instruction is required in the XML:
Client-side XSLT is supported by many
web browsers. Alternatively, one may use XSLT to convert XML into a displayable format "on the server" rather than being dependent on the end-user's browser capabilities. The end-user is not aware of what has gone on 'behind the scenes'; all they see is well-formatted, displayable data.
See the XSLT article for an example of server-side XSLT in action.
XPathmakes it possible to refer to individual parts of an XML document. This provides random access to XML data for other technologies, including XSLT, XSL-FO, XQuery etc. XPath expressions can refer to all or part of the text, data and values in XML elements, attributes, processing instructions, comments etc. They can also access the names of elements and attributes. XPaths can be used in both valid and well-formed XML, with and without defined namespaces.
XIncludedefines the ability for XML files to include all or part of an external file. When processing is complete, the final XML infoset has no XInclude elements, but instead has copied the documents or parts thereof into the final infoset. It uses XPath to refer to a portion of the document for partial inclusions.
XQueryis to XML and XML Databases what SQLand PL/SQLare to relational databases: ways to access, manipulate and return XML.
XML Namespaces enable the same document to contain XML elements and attributes taken from different vocabularies, without any naming collisions occurring.
XML Signaturedefines the syntax and processing rules for creating digital signatureson XML content.
XML Encryptiondefines the syntax and processing rules for encrypting XML content.
XPointeris a system for addressing components of XML-based internet media.
XML files may be served with a variety of Media types. RFC 3023 defines the types "application/xml" and "text/xml", which say only that the data is in XML, and nothing about its semantics. The use of "text/xml" has been criticized as a potential source of encoding problems but is now in the process of being deprecated. [ [http://lists.xml.org/archives/xml-dev/200407/msg00208.html xml-dev - Fw: An I-D for text/xml, application/xml, etc ] ] RFC 3023 also recommends that XML-based languages be given media types beginning in "application/" and ending in "+xml"; for example "application/atom+xml" for Atom. This page discusses further
XML and MIME.
Processing XML files
More recent and emerging techniques for processing XML files are:
* Pull Parsing
* Non-Extractive Parsing (i.e. in-situ parsing)
* Data binding
Simple API for XML (SAX)
SAX is a lexical, event-driven interface in which a document is read serially and its contents are reported as "
callbacks" to various methods on a handler object of the user's design. SAX is fast and efficient to implement, but difficult to use for extracting information at random from the XML, since it tends to burden the application author with keeping track of what part of the document is being processed. It is better suited to situations in which certain types of information are always handled the same way, no matter where they occur in the document.
DOM is an
interface-oriented Application Programming Interfacethat allows for navigation of the entire document as if it were a tree of "Node" objects representing the document's contents. A DOM document can be created by a parser, or can be generated manually by users (with limitations). Data types in DOM Nodes are abstract; implementations provide their own programminglanguage-specific bindings. DOM implementations tend to be memoryintensive, as they generally require the entire document to be loaded into memory and constructed as a tree of objects before access is allowed.DOM is supported in Java by several packages that usually come with the standard libraries. As the DOM specification is regulated by the World Wide Web Consortium, the main interfaces (Node, Document, etc.) are in the package org.w3c.dom.*, as well as some of the events and interfaces for other capabilities like serialization (output). The package com.sun.org.apache.xml.internal.serialize.* provides the serialization (output capacities) by implementing the appropriate interfaces, while the javax.xml.parsers.* package parses data to create DOM XML documents for manipulation. [ [http://java.sun.com/javase/6/docs/api Java Platform SE 6 ] ]
Transformation engines and filters
A filter in the
Extensible Stylesheet Language(XSL) family can transform an XML file for displaying or printing.
XSL-FOis a declarative, XML-based page layoutlanguage. An XSL-FO processor can be used to convert an XSL-FO document into another non-XML format, such as
XSLTis a declarative, XML-based document transformation language. An XSLT processor can use an XSLT "stylesheet" as a guide for the conversion of the data tree represented by one XML document into another tree that can then be serialized as XML, HTML, plain text, or any other format supported by the processor.
XQueryis a W3C language for querying, constructing and transforming XML data.
XPathis a DOM-like node tree data modeland path expressionlanguage for selecting data within XML documents. XSL-FO, XSLT and XQuery all make use of XPath. XPath also includes a useful function library.
Pull parsing [ [http://www.xml.com/pub/a/2005/07/06/tr.html Push, Pull, Next!] by Bob DuCharme, at XML.com] treats the document as a series of items which are read in sequence using the Iterator design pattern. This allows for writing of recursive-descent parsers in which the structure of the code performing the parsing mirrors the structure of the XML being parsed, and intermediate parsed results can be used and accessed as local variables within the methods performing the parsing, or passed down (as method parameters) into lower-level methods, or returned (as method return values) to higher-level methods. Examples of pull parsers include
StAXin the Java programming language, SimpleXML in PHP and System.Xml.XmlReader in .NET.
A pull parser creates an iterator that sequentially visits the various elements, attributes, and data in an XML document. Code which uses this 'iterator' can test the current item (to tell, for example, whether it is a start or end element, or text), and inspect its attributes (local name, namespace, values of XML attributes, value of text, etc.), and can also move the iterator to the 'next' item. The code can thus extract information from the document as it traverses it. The recursive-descent approach tends to lend itself to keeping data as typed local variables in the code doing the parsing, while SAX, for instance, typically requires a parser to manually maintain intermediate data within a stack of elements which are parent elements of the element being parsed. Pull-parsing code can be more straightforward to understand and maintain than SAX parsing code.
Non-extractive XML Processing API
Non-extractive XML Processing API is a new and emerging category of parsers that aim to overcome the fundamental limitations of DOM and SAX. The most representative is
VTD-XML, which abolishes the object-oriented modeling of XML hierarchy and instead uses 64-bit Virtual Token Descriptors (encoding offsets, lengths, depths, and types) of XML tokens. VTD-XML's approach enables a number of interesting features/enhancements, such as high performance, low memory usage [ [http://www.javaworld.com/javaworld/jw-03-2006/jw-0327-simplify.html Simplify XML processing with VTD-XML - Java World ] ] , ASIC implementation [http://www.ximpleware.com/wp_SUN.pdf] , incremental update [ [http://www.javaworld.com/javaworld/jw-07-2006/jw-0724-vtdxml.html Cut, paste, split, and assemble XML documents with VTD-XML - Java World ] ] , and native XML indexing [ [http://vtd-xml.sf.net/persistence.html VTD+XML format spec] ] [ [http://xml.sys-con.com/read/453082.htm Index XML documents with VTD-XML] ] .
Another form of XML Processing API is data binding, where XML data is made available as a custom, strongly typed programming language data structure, in contrast to the interface-oriented DOM. Example data binding systems include the Java
Architecturefor XML Binding ( JAXB) [http://java.sun.com/xml/jaxb/] or C++CodeSynthesis XSD [http://www.artima.com/cppsource/xml_data_binding.html] [http://www.codesynthesis.com/products/xsd/] .
Specific XML applications and editors
The native file format of
OpenOffice.org, AbiWord, and Apple's iWorkapplications is XML. Some parts of Microsoft Office2007 are also able to edit XML files with a user-supplied schema (but not a DTD), and Microsoft has released a file format compatibility kit for Office 2003 that allows previous versions of Office to save in the new XML based format. There are dozens of other XML editors available.
The versatility of
SGMLfor dynamic information display was understood by early digital media publishers in the late 1980s prior to the rise of the Internet. cite web | title=A conversation with Tim Bray: Searching for ways to tame the world’s vast stores of information | url=http://www.acmqueue.com/modules.php?name=Content&pa=showpage&pid=282 | first=Tim |last=Bray | month=February | year=2005 | publisher=Association for Computing Machinery's "Queue site" | accessmonthday=April 16 | accessyear=2006 ] cite book | title=Interactive multimedia | chapter=Publishers, multimedia, and interactivity | publisher= Cobb Group | id=ISBN 1-55615-124-1 | year=1988] By the mid-1990s some practitioners of SGML had gained experience with the then-new World Wide Web, and believed that SGML offered solutions to some of the problems the Web was likely to face as it grew. Dan Connollyadded SGML to the list of W3C's activities when he joined the staff in 1995; work began in mid-1996 when Sun Microsystems engineer Jon Bosakdeveloped a charter and recruited collaborators. Bosak was well connected in the small community of people who had experience both in SGML and the Web.
XML was compiled by a
working groupof eleven members, [The working group was originally called the "Editorial Review Board." The original members and seven who were added before the first edition was complete, are listed at the end of the first edition of the XML Recommendation, at http://www.w3.org/TR/1998/REC-xml-19980210.] supported by an (approximately) 150-member Interest Group. Technical debate took place on the Interest Group mailing list and issues were resolved by consensus or, when that failed, majority vote of the Working Group. A record of design decisions and their rationales was compiled by Michael Sperberg-McQueenon December 4, 1997. [ [http://www.w3.org/XML/9712-reports.html Reports From the W3C SGML ERB to the SGML WG And from the W3C XML ERB to the XML SIG] ] James Clark served as Technical Lead of the Working Group, notably contributing the empty-element " " syntax and the name "XML". Other names that had been put forward for consideration included "MAGMA" (Minimal Architecture for Generalized Markup Applications), "SLIM" (Structured Language for Internet Markup) and "MGML" (Minimal Generalized Markup Language). The co-editors of the specification were originally Tim Brayand Michael Sperberg-McQueen. Halfway through the project Bray accepted a consulting engagement with Netscape, provoking vociferous protests from Microsoft. Bray was temporarily asked to resign the editorship. This led to intense dispute in the Working Group, eventually solved by the appointment of Microsoft's Jean Paolias a third co-editor.
The XML Working Group never met face-to-face; the design was accomplished using a combination of email and weekly teleconferences. The major design decisions were reached in twenty weeks of intense work between July and November 1996, when the first Working Draft of an XML specification was published. [ [http://www.w3.org/TR/WD-xml-961114.html Extensible Markup Language (XML) ] ] Further design work continued through 1997, and XML 1.0 became a
W3CRecommendation on February 10, 1998.
XML 1.0 achieved the Working Group's goals of Internet usability, general-purpose usability, SGML compatibility, facilitation of easy development of processing software, minimization of optional features, legibility, formality, conciseness, and ease of authoring.Like its antecedent SGML, XML allows for some redundant syntactic constructs and includes repetition of element identifiers. In these respects, terseness was not considered essential in its structure.
XML is a profile of an ISO standard
SGML, and most of XML comes from SGML unchanged. From SGML comes the separation of logical and physical structures (elements and entities), the availability of grammar-based validation (DTDs), the separation of data and metadata (elements and attributes), mixed content, the separation of processing from representation (processing instructions), and the default angle-bracket syntax. Removed were the SGML Declaration (XML has a fixed delimiter set and adopts Unicodeas the document character set).
Other sources of technology for XML were the
Text Encoding Initiative(TEI), which defined a profile of SGML for use as a 'transfer syntax'; HTML, in which elements were synchronous with their resource, the separation of document character set from resource encoding, the xml:lang attribute, and the HTTPnotion that metadata accompanied the resource rather than being needed at the declaration of a link; and the Extended Reference Concrete Syntax (ERCS), from which XML 1.0's naming rules were taken, and which had introduced hexadecimal numeric character references and the concept of references to make available all Unicode characters.
Ideas that developed during discussion which were novel in XML, were the algorithm for encoding detection and the encoding header, the processing instruction target, the xml:space attribute, and the new close delimiter for empty-element tags.
There are two current versions of XML. The first, "XML 1.0", was initially defined in 1998. It has undergone minor revisions since then, without being given a new version number, and is currently in its fourth edition, as published on August 16, 2006. It is widely implemented and still recommended for general use. The second, "XML 1.1", was initially published on February 4, 2004, the same day as XML 1.0 Third Edition, and is currently in its second edition, as published on August 16, 2006. It contains features — some contentious — that are intended to make XML easier to use in certain cases [cite web |url=http://www.w3.org/TR/xml11/#sec-xml11 |title=Extensible Markup Language (XML) 1.1 (Second Edition) - Rationale and list of changes for XML 1.1 |accessdate=2006-12-21 |publisher=W3C] - mainly enabling the use of line-ending characters used on
EBCDICplatforms, and the use of scripts and characters absent from Unicode 2.0. XML 1.1 is not very widely implemented and is recommended for use only by those who need its unique features. [cite book
last = Harold
first = Elliotte Rusty
title = Effective XML
publisher = Addison-Wesley
year = 2004
pages = 10-19
url = http://www.cafeconleche.org/books/effectivexml/
isbn = 0321150406]
XML 1.0 and XML 1.1 differ in the requirements of characters used for element and attribute names: XML 1.0 only allows characters which are defined in Unicode 2.0, which includes most world scripts, but excludes those which were added in later Unicode versions. Among the excluded scripts are Mongolian, Cambodian,
Amharic, Burmese, and others.
Almost any Unicode character can be used in the character data and attribute values of an XML 1.1 document, even if the character is not defined, aside from having a code point, in the current version of Unicode. The approach in XML 1.1 is that only certain characters are forbidden, and everything else is allowed, whereas in XML 1.0, only certain characters are explicitly allowed, thus XML 1.0 cannot accommodate the addition of characters in future versions of Unicode.
In character data and attribute values, XML 1.1 allows the use of more
control characters than XML 1.0, but, for "robustness", most of the control characters introduced in XML 1.1 must be expressed as numeric character references. Among the supported control characters in XML 1.1 are two line break codes that must be treated as whitespace. Whitespace characters are the only control codes that can be written directly.
There are also discussions on an XML 2.0, although it remains to be seen Vague|date=March 2008 if such will ever come about. [http://www.textuality.com/xml/xmlSW.html XML-SW] (SW for
skunk works), written by one of the original developers of XML, contains some proposals for what an XML 2.0 might look like: elimination of DTDs from syntax, integration of namespaces, XML Baseand XML Information Set("infoset") into the base standard.
The World Wide Web Consortium also has an XML Binary Characterization Working Group doing preliminary research into use cases and properties for a binary encoding of the XML infoset. The working group is not chartered to produce any official standards. Since XML is by definition text-based, ITU-T and ISO are using the name "
Fast Infoset[http://asn1.elibel.tm.fr/xml/finf.htm] " for their own binary infoset to avoid confusion (see ITU-T Rec. X.891 | ISO/IEC 24824-1).
In October 2005 the small company
Scientigopublicly asserted that two of its patents, US patent|5842213 and US patent|6393426, apply to the use of XML. The patents cover the "modeling, storage and transfer [of data] in a particular "non-hierarchical", non-integrated neutral form", according to their applications, which were filed in 1997 and 1999. Scientigo CEO Doyal Bryantexpressed a desire to "monetize" the patents but stated that the company was "not interested in having us against the world." He said that Scientigo was discussing the patents with several large corporations. [ [http://news.com.com/Small+company+makes+big+claims+on+XML+patents/2100-1014_3-5905949.html Small company makes big claims on XML patents - CNET News.com ] ]
XML users and independent experts responded to Scientigo's claims with widespread skepticism and criticism. Some derided the company as a
patent troll. Tim Braydescribed any claims that the patents covered XML as "ridiculous on the face of it". [ [http://blogs.zdnet.com/BTL/?p=2052 XML co-inventor Bray responds to patent assault | Between the Lines | ZDNet.com ] ]
= Critique of XML =
Commentators have offered various critiques of XML, suggesting circumstances where XML provides both advantages and potential disadvantages.(See e.g., [http://www.w3.org/TR/NOTE-xml-ql/ XML-QL Proposal discussing XML benefits] , [http://www.25hoursaday.com/weblog/PermaLink.aspx?guid=dada27bf-2af0-400d-94c9-5575546f5664 When to use XML] , [http://c2.com/cgi/wiki?XmlSucks "XML Sucks" on c2.com] , [http://www.xml.com/pub/a/2001/05/02/champion.html Daring to Do Less with XML] )]
Advantages of XML
* It supports
Unicode, allowing almost any information in any written human language to be communicated.
* It can represent common
computer science data structures: records, lists and trees.
self-documentingformat describes structureand field names as well as specific values.
* The strict
syntaxand parsingrequirements make the necessary parsing algorithms extremely simple, efficient, and consistent.
* XML is heavily used as a format for
document storageand processing, both online and offline.
* It is based on
* It can be updated incrementally.
* It allows validation using schema languages such as
XSDand Schematron, which makes effective unit-testing, firewalls, acceptance testing, contractual specification and software construction easier.
* The hierarchical structure is suitable for most (but not all) types of documents.
* It is platform-independent, thus relatively immune to changes in technology.
* Forward and backward compatibility are relatively easy to maintain despite changes in DTD or Schema.
* Its predecessor,
SGML, has been in use since 1986, so there is extensive experience and software available.
Disadvantages of XML
* XML syntax is redundant or large relative to binary representations of similar data,cite book
last = Harold
first = Elliotte Rusty
title = Processing XML with Java(tm): a guide to SAX, DOM, JDOM, JAXP, and TrAX
publisher = Addison-Wesley
year = 2002
id = 0201771861
ref = Reference-Rusty-2002-aXML documents are too verbose compared with binary equivalents.] especially with tabular data.
* The redundancy may affect application efficiency through higher storage, transmission and processing costs.cite book
last = Harold
first = Elliotte Rusty
title = XML in a Nutshell: A Desktop Quick Reference
publisher = O'Reilly
year = 2002
id = 0596002920
ref = Reference-Rusty-2002-b XML documents are very verbose and searching is inefficient forhigh-performance largescale database applications.] However, the
Binary XMLeffort strives to alleviate these problems by using a binary representation for the XML document. For example, the Java reference implementation of the Fast Infosetstandard parsing speed is better by a factor 10 compared to Java Xerces, and by a factor 4 compared to the [http://piccolo.sourceforge.net/ Piccolo driver] , one of the fastest Java-based XML parser [https://fi.dev.java.net/reports/parsing/report.html] .]
* XML syntax is verbose, especially for human readers, relative to other alternative 'text-based' data transmission formats.cite book
last = Bierman
first = Gavin
title = Database Programming Languages: 10th international symposium, DBPL 2005 Trondheim, Norway
publisher = Springer
year = 2005
id = 3540309519XML syntax is too verbose for human readers in for certain applications.Proposes a dual syntax for human readability.] Although many purportedly"less verbose" text formats actually cite XML asboth inspiration and prior art.See e.g., http://yaml.org/spec/current.html,http://innig.net/software/sweetxml/index.html,http://www.json.org/xml.html.]
hierarchical modelfor representation is limited in comparison to an object orientedgraph.A hierarchical model only gives a fixed, monolithic view of the tree structure. For example, either actors under movies, or movies under actors, but not both.] cite book
last = Lim
first = Ee-Peng
title = Digital Libraries: People, Knowledge, and Technology
publisher = Springer
year = 2002
id = 3540002618Discusses some of the limitation with fixed hierarchy. Proceedings of the 5th International Conference on Asian Digital Libraries, ICADL 2002, held in Singapore in December 2002. ]
* Expressing overlapping (non-hierarchical) node relationships requires extra effort.cite book
last = Searle
first = Leroy F.
title = Voice, text, hypertext: emerging practices in textual studies
publisher = University of Washington Press
year = 2004
id = 0295983051 Proposes an alternative system for encoding overlapping elements. ]
* XML namespaces are problematic to use and namespace support can be difficult to correctly implement in an XML parser.(See e.g., http://www-128.ibm.com/developerworks/library/x-abolns.html )]
* XML is commonly depicted as "
self-documenting" but this depiction ignores critical ambiguities.cite web
title = The Myth of Self-Describing XML
url = http://www.oceaninformatics.biz/publications/e2.pdf
accessdate = 2007-05-12] [(See e.g.,
Use–mention distinction, Naming collision, Polysemy)]
* The distinction between content and attributes in XML seems unnatural to some and makes designing XML data structures harder.cite web
title = Does XML Suck?
url = http://xmlsucks.org/but_you_have_to_use_it_anyway/does-xml-suck.html
accessdate = 2007-12-15(See "8. Complexity: Attributes and Content")]
* Transformations, even identity transforms, result in changes to format (whitespace, attribute ordering, attribute quoting, whitespace around attributes, newlines). These problems can make
diff-ing the XML source very difficult except via Canonical XML.
* Encourages non-relational data structures (data non-normalized)
* XML is very concrete and highly non-canonical. It introduces a very strong coupling between the actual representation chosen and the processing program (unlike relational storage and SQL)Fact|date=September 2008
XML in business world
* XBRL (eXtensible Business Reporting Language) based on XML is widely used in financial world. Since more and more investors and regulators call for financial transparency, XBRL technology could facilitate the process of transferring data, as well as for business reporting. Japanese banks are exemplified by using XBRL during their daily business.
* Web publishing uses XML to have one single source for creating and updating all content, which significantly saves time and reduce cost for companies when printing documents. In addition, governments in the world use XML for large documentation and printing maps. Airplane and Car manufactures may use XML technology to print maintenance booklets.
* Web searching- Since XML does not have fixed tags, the user is free to define the type of information in their work which means that other users will find their work easily when search on the Web. [http://www.sfu.ca/~ksc13/xml2.html]
In addition to the
ISOstandards mentioned above, other related document include
* ISO/IEC 8825-4:2002 "Information technology -- ASN.1 encoding rules: XML Encoding Rules (XER)"
* ISO/IEC 8825-5:2004 "Information technology -- ASN.1 encoding rules: Mapping W3C XML schema definitions into ASN.1"
* ISO/IEC 9075-14:2006 "Information technology -- Database languages -- SQL -- Part 14: XML-Related Specifications (SQL/XML)"
* ISO 10303-28:2007 "Industrial automation systems and integration -- Product data representation and exchange -- Part 28: Implementation methods: XML representations of EXPRESS schemas and data, using XML schemas"
* ISO/IEC 13250-3:2007 "Information technology -- Topic Maps -- Part 3: XML syntax"
* ISO/IEC 13522-5:1997 "Information technology -- Coding of multimedia and hypermedia information -- Part 5: Support for base-level interactive applications"
* ISO/IEC 13522-8:2001 "Information technology -- Coding of multimedia and hypermedia information -- Part 8: XML notation for ISO/IEC 13522-5"
* ISO/IEC 18056:2007 "Information technology -- Telecommunications and information exchange between systems -- XML Protocol for Computer Supported Telecommunications Applications (CSTA) Phase III"
* ISO/IEC 19503:2005 "Information technology -- XML Metadata Interchange (XMI)"
* ISO/IEC 19776-1:2005 "Information technology -- Computer graphics, image processing and environmental data representation -- Extensible 3D (X3D) encodings -- Part 1: Extensible Markup Language (XML) encoding
* ISO/IEC 22537:2006 "Information technology -- ECMAScript for XML (E4X) specification"
* ISO 22643:2003 "Space data and information transfer systems -- Data entity dictionary specification language (DEDSL) -- XML/DTD Syntax"
* ISO/IEC 23001-1:2006 "Information technology -- MPEG systems technologies -- Part 1: Binary MPEG format for XML"
* ISO 24531:2007 "Intelligent transport systems -- System architecture, taxonomy and terminology -- Using XML in ITS standards, data registries and data dictionaries"
*CDATA section, the mechanism for including non-markup text in XML
Comparison of layout engines (XML)
Extensible Binary Meta Language
Extensible Metadata Platform(XMP), used in graphics applications
General purpose markup language
List of XML markup languages
Single source publishing
Universal Business Language
XML Data Binding
XML Information Set
*XML processing APIs:
XML query language
**XML Forms Architecture
XML Certification Program
* W3C XML standards:
** DOM (the XML processing "reference model").
** XML Protocol:
** XSL and XSLT
LGMLLinguistics Markup Language
Notes and references
* [http://www.w3.org/XML/ W3C XML homepage]
* [http://www.w3.org/TR/REC-xml The XML 1.0 specification]
* [http://www.w3.org/TR/xml11 The XML 1.1 specification]
* [http://www.altova.com/altovaxml.html AltovaXML] royalty-free parser from
Altova, also included in XMLSpy, MapForce, and StyleVision
* [http://www.allegrosoft.com/romxml.html RomXML] Embedded XML commercial toolkit written in ANSI-C.
* [http://www.philo.de/xml/ XDOM] open-source XML parser (and DOM and XPath implementation) in Delphi/Kylix.
* [http://www.grinninglizard.com/tinyxml/ TinyXml] Simple and small C++ XML parser.
* [http://uszla.me.uk/FoX FoX] fully validating XML parser library, written in Fortran.
* [http://www.intel.com/software/xml Intel_XSS] XML parsing, validation, XPath, XSLT.
* [http://www.sgmlsource.com/history/AnnexA.htm Introduction to Generalized Markup] by
* [http://www.developer.com/xml/article.php/10929_3583081_1 Making Mistakes with XML] by
*Annex A of ISO 8879:1986 (SGML)
* [http://www.mind-to-mind.com/library/papers/multilingual/multilingual-www.html The Multilingual WWW] by Gavin Nicol
* [http://xml.ascc.net/en/utf-8/ercsretro.html Retrospective on Extended Reference Concrete Syntax] by
* [http://www.xeml.net/family.html XML Based languages]
* [http://www.asiaing.com/essential-xml-quick-reference.html Essential XML Quick Reference]
* [http://www.xml.com/pub/a/w3j/s3.bosak.html XML, Java and the Future of the Web] by
* [http://www.w3schools.com/xml/default.asp XML tutorials in w3schools]
* [http://xml.gov/ XML.gov]
* [http://www-128.ibm.com/developerworks/library/x-think38.html Thinking XML: The XML decade] by Uche Ogbuji
* [http://drmacros-xml-rants.blogspot.com/2006/11/xml-ten-year-aniversary.html XML: Ten year anniversary] by Elliot Kimber
* [http://2006.xmlconference.org/proceedings/162/presentation.html Closing Keynote, XML 2006] by
* [http://www.oreillynet.com/xml/blog/2003/02/five_years_later_xml.html Five years later, XML...] by Simon St. Laurent
* [http://open.itworld.com/4934/xml-fallacies-nlstipsm-080122/page_1.html 23 XML fallacies to watch out for] by Sean McGrath
* [http://www.w3.org/2008/02/xml10-pressrelease W3C XML is Ten!] , XML 10 years press release
*cite journal |id= SSRN|900616 |author=Lawrence A. Cunningham|title=Language, Deals and Standards: The Future of XML Contracts|journal=Washington University Law Review|year=2005
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