Extensible Markup Language (XML) is a set of rules for encoding documents in machine-readable form. It is defined in the XML 1.0 Specification produced by the W3C, and several other related specifications, all gratis
Gratis
Gratis is the process of providing goods or services without compensation. It is often referred to in English as "free of charge" or "complimentary"...
An open standard is a standard that is publicly available and has various rights to use associated with it, and may also have various properties of how it was designed . There is no single definition and interpretations vary with usage....
s.
The design goals of XML emphasize simplicity, generality, and usability over the Internet
Internet
The Internet is a global system of interconnected computer networks that use the standard Internet protocol suite to serve billions of users worldwide...
. It is a textual data format with strong support via Unicode
Unicode
Unicode is a computing industry standard for the consistent encoding, representation and handling of text expressed in most of the world's writing systems...
for the languages of the world. Although the design of XML focuses on documents, it is widely used for the representation of arbitrary data structures, for example in web service
Web service
A Web service is a method of communication between two electronic devices over the web.The W3C defines a "Web service" as "a software system designed to support interoperable machine-to-machine interaction over a network". It has an interface described in a machine-processable format...
s.
Many application programming interfaces (APIs) have been developed that software developers use to process XML data, and several schema systems
XML schema
An XML schema is a description of a type of XML document, typically expressed in terms of constraints on the structure and content of documents of that type, above and beyond the basic syntactical constraints imposed by XML itself...
exist to aid in the definition of XML-based languages.
, hundreds of XML-based languages have been developed, including RSS
RSS
-Mathematics:* Root-sum-square, the square root of the sum of the squares of the elements of a data set* Residual sum of squares in statistics-Technology:* RSS , "Really Simple Syndication" or "Rich Site Summary", a family of web feed formats...
The name Atom applies to a pair of related standards. The Atom Syndication Format is an XML language used for web feeds, while the Atom Publishing Protocol is a simple HTTP-based protocol for creating and updating web resources.Web feeds allow software programs to check for updates published on a...
SOAP, originally defined as Simple Object Access Protocol, is a protocol specification for exchanging structured information in the implementation of Web Services in computer networks...
XHTML is a family of XML markup languages that mirror or extend versions of the widely-used Hypertext Markup Language , the language in which web pages are written....
. XML-based formats have become the default for many office-productivity tools, including Microsoft Office
Microsoft Office
Microsoft Office is a non-free commercial office suite of inter-related desktop applications, servers and services for the Microsoft Windows and Mac OS X operating systems, introduced by Microsoft in August 1, 1989. Initially a marketing term for a bundled set of applications, the first version of...
OpenOffice.org, commonly known as OOo or OpenOffice, is an open-source application suite whose main components are for word processing, spreadsheets, presentations, graphics, and databases. OpenOffice is available for a number of different computer operating systems, is distributed as free software...
The Open Document Format for Office Applications is an XML-based file format for representing electronic documents such as spreadsheets, charts, presentations and word processing documents....
Apple Inc. is an American multinational corporation that designs and markets consumer electronics, computer software, and personal computers. The company's best-known hardware products include the Macintosh line of computers, the iPod, the iPhone and the iPad...
iWork is an office suite of desktop applications created by Apple for the Mac OS X and iOS operating systems. The first version of iWork, iWork '05, was released in 2005. The suite originally bundled Keynote, a presentation program which had previously been sold as a standalone application, and...
.
Characters and escaping
XML documents consist entirely of characters from the Unicode
Unicode
Unicode is a computing industry standard for the consistent encoding, representation and handling of text expressed in most of the world's writing systems...
repertoire. Except for a small number of specifically excluded control characters, any character defined by Unicode may appear within the content of an XML document. The selection of characters that may appear within markup is somewhat more limited but still large.
XML includes facilities for identifying the encoding of the Unicode characters that make up the document, and for expressing characters that, for one reason or another, cannot be used directly.
Valid characters
Unicode code points in the following ranges are valid in XML 1.0 documents:
U+0009, U+000A, U+000D: these are the only C0 controls accepted in XML 1.0;
U+0020–U+D7FF, U+E000–U+FFFD: this excludes some (not all) non-characters in the BMP (all surrogates, U+FFFE and U+FFFF are forbidden);
U+10000–U+10FFFF: this includes all code points in supplementary planes, including non-characters.
XML 1.1 extends the set of allowed characters to include all the above, plus the remaining characters in the range U+0001–U+001F. At the same time, however, it restricts the use of C0 and C1 control characters other than U+0009, U+000A, U+000D, and U+0085 by requiring them to be written in escaped form (for example U+0001 must be written as  or its equivalent). In the case of C1 characters, this restriction is a backwards incompatibility; it was introduced to allow common encoding errors to be detected.
The code point U+0000 is the only character that is not permitted in any XML 1.0 or 1.1 document.
Encoding detection
The Unicode character set can be encoded into bytes for storage or transmission in a variety of different ways, called "encodings". Unicode itself defines encodings that cover the entire repertoire; well-known ones include UTF-8
UTF-8
UTF-8 is a multibyte character encoding for Unicode. Like UTF-16 and UTF-32, UTF-8 can represent every character in the Unicode character set. Unlike them, it is backward-compatible with ASCII and avoids the complications of endianness and byte order marks...
and UTF-16. There are many other text encodings that pre-date Unicode, such as ASCII
ASCII
The American Standard Code for Information Interchange is a character-encoding scheme based on the ordering of the English alphabet. ASCII codes represent text in computers, communications equipment, and other devices that use text...
ISO/IEC 8859 is a joint ISO and IEC series of standards for 8-bit character encodings. The series of standards consists of numbered parts, such as ISO/IEC 8859-1, ISO/IEC 8859-2, etc. There are 15 parts, excluding the abandoned ISO/IEC 8859-12...
; their character repertoires in almost every case are subsets of the Unicode character set.
XML allows the use of any of the Unicode-defined encodings, and any other encodings whose characters also appear in Unicode. XML also provides a mechanism whereby an XML processor can reliably, without any prior knowledge, determine which encoding is being used. Encodings other than UTF-8 and UTF-16 will not necessarily be recognized by every XML parser.
An escape sequence is a series of characters used to change the state of computers and their attached peripheral devices. These are also known as control sequences, reflecting their use in device control. Some control sequences are special characters that always have the same meaning...
facilities for including characters which are problematic to include directly. For example:
The characters "<" and "&" are key syntax markers and may never appear in content outside of a CDATA
CDATA
The term CDATA, meaning character data, is used for distinct, but related, purposes in the markup languages SGML and XML. The term indicates that a certain portion of the document is general character data, rather than non-character data or character data with a more specific, limited...
section.
Some character encodings support only a subset of Unicode: for example, it is legal to encode an XML document in ASCII, but ASCII lacks code points for Unicode characters such as "é".
It might not be possible to type the character on the author's machine.
A glyph is an element of writing: an individual mark on a written medium that contributes to the meaning of what is written. A glyph is made up of one or more graphemes....
s that cannot be visually distinguished from other characters: examples are
A numeric character reference is a common markup construct used in SGML and other SGML-related markup languages such as HTML and XML. It consists of a short sequence of characters that, in turn, represent a single character from the Universal Character Set of Unicode...
. Consider the Chinese character "中", whose numeric code in Unicode is hexadecimal 4E2D, or decimal 20,013. A user whose keyboard offers no method for entering this character could still insert it in an XML document encoded either as 中 or 中. Similarly, the string "I <3 Jörg" could be encoded for inclusion in an XML document as "I <3 Jörg".
"�" is not permitted, however, because the null character
Null character
The null character , abbreviated NUL, is a control character with the value zero.It is present in many character sets, including ISO/IEC 646 , the C0 control code, the Universal Character Set , and EBCDIC...
is one of the control characters excluded from XML, even when using a numeric character reference. An alternative encoding mechanism such as Base64
Base64
Base64 is a group of similar encoding schemes that represent binary data in an ASCII string format by translating it into a radix-64 representation...
is needed to represent such characters.
Comments
Comments may appear anywhere in a document outside other markup. Comments cannot appear before the XML declaration. The string "--" (double-hyphen) is not allowed inside comments. Comments start with "<!--". The ampersand has no special significance within comments, so entity and character references are not recognized as such, and there is no way to represent characters outside the character set of the document encoding.
An example of a valid comment:
"<!-- no need to escape <code> & such in comments -->"
Unicode is a computing industry standard for the consistent encoding, representation and handling of text expressed in most of the world's writing systems...
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 less-than sign, "<"). Therefore, the following is a well-formed XML document, even though it includes both Chinese
Chinese character
Chinese characters are logograms used in the writing of Chinese and Japanese , less frequently Korean , formerly Vietnamese , or other languages...
The Cyrillic script or azbuka is an alphabetic writing system developed in the First Bulgarian Empire during the 10th century AD at the Preslav Literary School...
characters:
<俄语>данные
Well-formedness and error-handling
The XML specification defines an XML document as a text that is well-formed
Well-formed element
In web page design, and generally for all markup languages such as SGML, HTML, and XML, a well-formed element is one that is either*opened and subsequently closed,*an empty element, which in that case must be terminated,...
, i.e. it satisfies a list of syntax rules provided in the specification. The list is fairly lengthy; some key points are:
It contains only properly encoded legal Unicode characters.
None of the special syntax characters such as "<" and "&" appear except when performing their markup-delineation roles.
The begin, end, and empty-element tags that delimit the elements are correctly nested, with none missing and none overlapping.
The element tags are case-sensitive; the beginning and end tags must match exactly. Tag names cannot contain any of the characters !"#$%&'*+,/;<=>?@[\]^`{|}~, nor a space character, and cannot start with -, ., or a numeric digit.
There is a single "root" element that contains all the other elements.
The definition of an XML document excludes texts that contain violations of well-formedness rules; they are simply not XML. An XML processor that encounters such a violation is required to report such errors and to cease normal processing. This policy, occasionally referred to as draconian, stands in notable contrast to the behavior of programs that process HTML
HTML
HyperText Markup Language is the predominant markup language for web pages. HTML elements are the basic building-blocks of webpages....
, which are designed to produce a reasonable result even in the presence of severe markup errors. XML's policy in this area has been criticized as a violation of Postel's law ("Be conservative in what you send; be liberal in what you accept").
Schemas and validation
In addition to being well-formed, an XML document may be valid. This means that it contains a reference to a Document Type Definition (DTD)
Document Type Definition
Document Type Definition is a set of markup declarations that define a document type for SGML-family markup languages...
, and that its elements and attributes are declared in that DTD and follow the grammatical rules for them that the DTD specifies.
XML processors are classified as validating or non-validating depending on whether or not they check XML documents for validity. A processor that discovers a validity error must be able to report it, but may continue normal processing.
An XML schema is a description of a type of XML document, typically expressed in terms of constraints on the structure and content of documents of that type, above and beyond the basic syntactical constraints imposed by XML itself...
or grammar. Since the initial publication of XML 1.0, there has been substantial work in the area of schema languages for XML. Such schema languages typically constrain the set of elements that may be used in a document, which attributes may be applied to them, the order in which they may appear, and the allowable parent/child relationships.
In the Standard Generalized Markup Language , an entity is a primitive data type, which associates a string with either a unique alias or an SGML reserved word . Entities are foundational to the organizational structure and definition of SGML documents...
for publishing characters.
DTDs define a document type rather than the types used by a namespace, thus grouping all constraints for a document in a single collection.
The Institute of Electrical and Electronics Engineers defines the term feature in IEEE 829 as "A distinguishing characteristic of a software item ." - Feature-rich :...
xmlns tagged XML namespaces are used for providing uniquely named elements and attributes in an XML document. They are defined in a W3C recommendation. An XML instance may contain element or attribute names from more than one XML vocabulary...
.
They lack expressiveness. XML DTDs are simpler than SGML DTDs and there are certain structures that cannot be expressed with regular grammars. DTDs only support rudimentary datatypes.
They lack readability. DTD designers typically make heavy use of parameter entities (which behave essentially as textual macros), which make it easier to define complex grammars, but at the expense of clarity.
In computing, a regular expression provides a concise and flexible means for "matching" strings of text, such as particular characters, words, or patterns of characters. Abbreviations for "regular expression" include "regex" and "regexp"...
syntax, inherited from SGML, to describe the schema. Typical XML APIs such as SAX
Simple API for XML
SAX is an event-based sequential access parser API developed by the XML-DEV mailing list for XML documents. SAX provides a mechanism for reading data from an XML document that is an alternative to that provided by the Document Object Model...
do not attempt to offer applications a structured representation of the syntax, so it is less accessible to programmers than an element-based syntax may be.
Two peculiar features that distinguish DTDs from other schema types are the syntactic support for embedding a DTD within XML documents and for defining entities, which are arbitrary fragments of text and/or markup that the XML processor inserts in the DTD itself and in the XML document wherever they are referenced, like character escapes.
DTD technology is still used in many applications because of its ubiquity.
An XML schema is a description of a type of XML document, typically expressed in terms of constraints on the structure and content of documents of that type, above and beyond the basic syntactical constraints imposed by XML itself...
language, described by the W3C as the successor of DTDs, is XML Schema, often referred to by the initialism
Acronym and initialism
Acronyms and initialisms are abbreviations formed from the initial components in a phrase or a word. These components may be individual letters or parts of words . There is no universal agreement on the precise definition of the various terms , nor on written usage...
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 and allow for more detailed constraints on an XML document's logical structure. XSDs also use an XML-based format, which makes it possible to use ordinary XML tools to help process them.
In computing, RELAX NG is a schema language for XML, based on Murata Makoto's RELAX and James Clark's TREX. A RELAX NG schema specifies a pattern for the structure and content of an XML document...
The Organization for the Advancement of Structured Information Standards is a global consortium that drives the development, convergence and adoption of e-business and web service standards...
and is now also an ISO/IEC International Standard (as part of DSDL). RELAX NG schemas may be written in either an XML based syntax or a more compact non-XML syntax; the two syntaxes are isomorphic and James Clark's Trang conversion tool can convert between them without loss of information. 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 datatype framework plug-ins; a RELAX NG schema author, for example, can require values in an XML document to conform to definitions in XML Schema Datatypes.
In computer programming, an assertion is a predicate placed in a program to indicate that the developer thinks that the predicate is always true at that place.For example, the following code contains two assertions:...
about the presence or absence of patterns in an XML document. It typically uses XPath
XPath
XPath is a language for selecting nodes from an XML document. In addition, XPath may be used to compute values from the content of an XML document...
expressions.
ISO DSDL and other schema languages
The ISO DSDL (Document Schema Description Languages) standard brings together a comprehensive set of small schema languages, each targeted at specific problems. DSDL includes RELAX NG
RELAX NG
In computing, RELAX NG is a schema language for XML, based on Murata Makoto's RELAX and James Clark's TREX. A RELAX NG schema specifies a pattern for the structure and content of an XML document...
In markup languages, Schematron is a rule-based validation language for making assertions about the presence or absence of patterns in XML trees...
assertion language, and languages for defining datatypes, character repertoire constraints, renaming and entity expansion, and namespace-based routing
Routing
Routing is the process of selecting paths in a network along which to send network traffic. Routing is performed for many kinds of networks, including the telephone network , electronic data networks , and transportation networks...
of 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
Publishing
Publishing is the process of production and dissemination of literature or information—the activity of making information available to the general public...
.
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 the infoset augmentation facility and attribute defaults. RELAX NG and Schematron intentionally do not provide these.
Related specifications
A cluster of specifications closely related to XML have been developed, starting soon after the initial publication of XML 1.0. It is frequently the case that the term "XML" is used to refer to XML together with one or more of these other technologies which have come to be seen as part of the XML core.
xmlns tagged XML namespaces are used for providing uniquely named elements and attributes in an XML document. They are defined in a W3C recommendation. An XML instance may contain element or attribute names from more than one XML vocabulary...
s enable the same document to contain XML elements and attributes taken from different vocabularies, without any naming collision
Naming collision
A naming collision is a circumstance where two or more identifiers in a given namespace or a given scope cannot be unambiguously resolved, and such unambiguous resolution is a requirement of the underlying system.- Example: XML element names :...
s occurring. Although XML Namespaces are not part of the XML specification itself, virtually all XML software also supports XML Namespaces.
XML Base is a World Wide Web Consortium recommended facility for defining base URIs for parts of XML documents.XML Base recommendation was adopted on 2001-06-27....
defines the xml:base attribute, which may be used to set the base for resolution of relative URI references within the scope of a single XML element.
XML Information Set is a W3C specification describing an abstract data model of an XML document in terms of a set of information items...
or XML infoset describes an abstract data model for XML documents in terms of information items. The infoset is commonly used in the specifications of XML languages, for convenience in describing constraints on the XML constructs those languages allow.
xml:id Version 1.0 asserts that an attribute named xml:id functions as an "ID attribute" in the sense used in a DTD.
XPath is a language for selecting nodes from an XML document. In addition, XPath may be used to compute values from the content of an XML document...
defines a syntax named XPath expressions which identifies one or more of the internal components (elements, attributes, and so on) included in an XML document. XPath is widely used in other core-XML specifications and in programming libraries for accessing XML-encoded data.
XSLT is a declarative, XML-based language used for the transformation of XML documents. The original document is not changed; rather, a new document is created based on the content of an existing one. The new document may be serialized by the processor in standard XML syntax or in another format,...
is a language with an XML-based syntax that is used to transform XML documents into other XML documents, HTML, or other, unstructured formats such as plain text or RTF. XSLT is very tightly coupled with XPath, which it uses to address components of the input XML document, mainly elements and attributes.
XSL Formatting Objects, or XSL-FO, is a markup language for XML document formatting which is most often used to generate PDFs. XSL-FO is part of XSL , a set of W3C technologies designed for the transformation and formatting of XML data. The other parts of XSL are XSLT and XPath...
, or XSL-FO, is a markup language for XML document formatting which is most often used to generate PDFs.
- Features :XQuery provides the means to extract and manipulate data from XML documents or any data source that can be viewed as XML, such as relational databases or office documents....
is an XML-oriented query language strongly rooted in XPath and XML Schema. It provides methods to access, manipulate and return XML, and is mainly conceived as a query language for XML database
XML database
An XML database is a data persistence software system that allows data to be stored in XML format. This data can then be queried, exported and serialized into the desired format.Two major classes of XML database exist:...
XML Signature defines an XML syntax for digital signatures and is defined in the W3C recommendation . Functionally, it has much in common with PKCS#7 but is more extensible and geared towards signing XML documents...
defines syntax and processing rules for creating digital signatures on XML content.
XML Encryption, also known as XML-Enc, is a specification, governed by a W3C recommendation, that defines how to encrypt the contents of an XML element....
defines syntax and processing rules for encrypting
Encryption
In cryptography, encryption is the process of transforming information using an algorithm to make it unreadable to anyone except those possessing special knowledge, usually referred to as a key. The result of the process is encrypted information...
XML content.
Some other specifications conceived as part of the "XML Core" have failed to find wide adoption, including XInclude
XInclude
XInclude is a generic mechanism for merging XML documents, by writing inclusion tags in the "main" document to automatically include other documents or parts thereof. The resulting document becomes a single composite XML Information Set...
XML Linking Language, or XLink, is an XML markup language and W3C specification that provides methods for creating internal and external links within XML documents, and associating metadata with those links.-The XLink specification:...
XPointer is a system for addressing components of XML based internet media.XPointer is divided among four specifications: a "framework" which forms the basis for identifying XML fragments, a positional element addressing scheme, a scheme for namespaces, and a scheme for XPath-based addressing...
.
Use on the Internet
XML has come into common use for the interchange of data over the Internet. RFC 3023 gives rules for the construction of Internet Media Types
Internet media type
An Internet media type, originally called a MIME type after MIME and sometimes a Content-type after the name of a header in several protocols whose value is such a type, is a two-part identifier for file formats on the Internet.The identifiers were originally defined in RFC 2046 for use in email...
for use when sending XML. It also defines the types "application/xml" and "text/xml", which say only that the data are in XML, and nothing about its semantics
Semantics
Semantics is the study of meaning. It focuses on the relation between signifiers, such as words, phrases, signs and symbols, and what they stand for, their denotata....
. The use of "text/xml" has been criticized as a potential source of encoding problems and is now in the process of being deprecated. RFC 3023 also recommends that XML-based languages be given media types beginning in "application/" and ending in "+xml"; for example "application/svg+xml" for SVG.
Further guidelines for the use of XML in a networked context may be found in RFC 3470, also known as IETF BCP 70; this document is very wide-ranging and covers many aspects of designing and deploying an XML-based language.
XML can also form a crucial part of a databaseless design.
Programming interfaces
The design goals of XML include, "It shall be easy to write programs which process XML documents." Despite this, the XML specification contains almost no information about how programmers might go about doing such processing. The XML Infoset specification provides a vocabulary to refer to the constructs within an XML document, but also does not provide any guidance on how to access this information. A variety of APIs for accessing XML have been developed and used, and some have been standardized.
Existing APIs for XML processing tend to fall into these categories:
Stream-oriented APIs accessible from a programming language, for example SAX
Simple API for XML
SAX is an event-based sequential access parser API developed by the XML-DEV mailing list for XML documents. SAX provides a mechanism for reading data from an XML document that is an alternative to that provided by the Document Object Model...
Streaming API for XML is an application programming interface to read and write XML documents, originating from the Java programming language community.Traditionally, XML APIs are either:...
.
Tree-traversal APIs accessible from a programming language, for example DOM.
XML data binding refers to a means of representing information in an XML document as an object in computer memory. This allows applications to access the data in the XML from the object rather than using the DOM or SAX to retrieve the data from a direct representation of the XML itself.An XML data...
, which provides an automated translation between an XML document and programming-language objects.
XSLT is a declarative, XML-based language used for the transformation of XML documents. The original document is not changed; rather, a new document is created based on the content of an existing one. The new document may be serialized by the processor in standard XML syntax or in another format,...
- Features :XQuery provides the means to extract and manipulate data from XML documents or any data source that can be viewed as XML, such as relational databases or office documents....
.
Stream-oriented facilities require less memory and, for certain tasks which are based on a linear traversal of an XML document, are faster and simpler than other alternatives. Tree-traversal and data-binding APIs typically require the use of much more memory, but are often found more convenient for use by programmers; some include declarative retrieval of document components via the use of XPath expressions.
XSLT is designed for declarative description of XML document transformations, and has been widely implemented both in server-side packages and Web browsers. XQuery overlaps XSLT in its functionality, but is designed more for searching of large XML database
XML database
An XML database is a data persistence software system that allows data to be stored in XML format. This data can then be queried, exported and serialized into the desired format.Two major classes of XML database exist:...
SAX is an event-based sequential access parser API developed by the XML-DEV mailing list for XML documents. SAX provides a mechanism for reading data from an XML document that is an alternative to that provided by the Document Object Model...
In computer science, lexical analysis is the process of converting a sequence of characters into a sequence of tokens. A program or function which performs lexical analysis is called a lexical analyzer, lexer or scanner...
In computer programming, event-driven programming or event-based programming is a programming paradigm in which the flow of the program is determined by events—i.e., sensor outputs or user actions or messages from other programs or threads.Event-driven programming can also be defined as an...
interface in which a document is read serially and its contents are reported as callback
Callback (computer science)
In computer programming, a callback is a reference to executable code, or a piece of executable code, that is passed as an argument to other code. This allows a lower-level software layer to call a subroutine defined in a higher-level layer....
In object-oriented programming, a method is a subroutine associated with a class. Methods define the behavior to be exhibited by instances of the associated class at program run time...
s 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.
Pull parsing
Pull parsing 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
Recursive descent parser
A recursive descent parser is a top-down parser built from a set of mutually-recursive procedures where each such procedure usually implements one of the production rules of the grammar...
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 StAX
StAX
Streaming API for XML is an application programming interface to read and write XML documents, originating from the Java programming language community.Traditionally, XML APIs are either:...
Java is a programming language originally developed by James Gosling at Sun Microsystems and released in 1995 as a core component of Sun Microsystems' Java platform. The language derives much of its syntax from C and C++ but has a simpler object model and fewer low-level facilities...
PHP is a general-purpose server-side scripting language originally designed for web development to produce dynamic web pages. For this purpose, PHP code is embedded into the HTML source document and interpreted by a web server with a PHP processor module, which generates the web page document...
The .NET Framework is a software framework that runs primarily on Microsoft Windows. It includes a large library and supports several programming languages which allows language interoperability...
.
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
XML Namespace
xmlns tagged XML namespaces are used for providing uniquely named elements and attributes in an XML document. They are defined in a W3C recommendation. An XML instance may contain element or attribute names from more than one XML vocabulary...
, 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..
The Document Object Model is a cross-platform and language-independent convention for representing and interacting with objects in HTML, XHTML and XML documents. Aspects of the DOM may be addressed and manipulated within the syntax of the programming language in use...
The user interface, in the industrial design field of human–machine interaction, is the space where interaction between humans and machines occurs. The goal of interaction between a human and a machine at the user interface is effective operation and control of the machine, and feedback from the...
An application programming interface is a source code based specification intended to be used as an interface by software components to communicate with each other...
that allows for navigation of the entire document as if it were a tree of node
Node (computer science)
A node is a record consisting of one or more fields that are links to other nodes, and a data field. The link and data fields are often implemented by pointers or references although it is also quite common for the data to be embedded directly in the node. Nodes are used to build linked, often...
In computer science, an object is any entity that can be manipulated by the commands of a programming language, such as a value, variable, function, or data structure...
s 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 programming language-specific bindings
Language binding
In computing, a binding from a programming language to a library or OS service is an API providing that service in the language.Many software libraries are written in systems programming languages such as C or C++...
In psychology, memory is an organism's ability to store, retain, and recall information and experiences. Traditional studies of memory began in the fields of philosophy, including techniques of artificially enhancing memory....
intensive, as they generally require the entire document to be loaded into memory and constructed as a tree of objects before access is allowed.
XML data binding refers to a means of representing information in an XML document as an object in computer memory. This allows applications to access the data in the XML from the object rather than using the DOM or SAX to retrieve the data from a direct representation of the XML itself.An XML data...
, where XML data are made available as a hierarchy of custom, strongly typed classes, in contrast to the generic objects created by a Document Object Model
Document Object Model
The Document Object Model is a cross-platform and language-independent convention for representing and interacting with objects in HTML, XHTML and XML documents. Aspects of the DOM may be addressed and manipulated within the syntax of the programming language in use...
parser. This approach simplifies code development, and in many cases allows problems to be identified at compile time rather than run-time. Example data binding systems include the Java Architecture for XML Binding
Java Architecture for XML Binding
Java Architecture for XML Binding allows Java developers to map Java classes to XML representations. JAXB provides two main features: the ability to marshal Java objects into XML and the inverse, i.e. to unmarshal XML back into Java objects...
(JAXB) and XML Serialization in .NET.
XML as data type
XML is beginning to appear as a first-class data type in other languages. The ECMAScript for XML
E4X
ECMAScript for XML is a programming language extension that adds native XML support to ECMAScript . The goal is to provide an alternative to DOM interfaces that uses a simpler syntax for accessing XML documents. It also offers a new way of making XML visible...
ECMAScript is the scripting language standardized by Ecma International in the ECMA-262 specification and ISO/IEC 16262. The language is widely used for client-side scripting on the web, in the form of several well-known dialects such as JavaScript, JScript, and ActionScript.- History :JavaScript...
/JavaScript language explicitly defines two specific objects (XML and XMLList) for JavaScript, which support XML document nodes and XML node lists as distinct objects and use a dot-notation specifying parent-child relationships. E4X is supported by the Mozilla
Mozilla
Mozilla is a term used in a number of ways in relation to the Mozilla.org project and the Mozilla Foundation, their defunct commercial predecessor Netscape Communications Corporation, and their related application software....
ActionScript is an object-oriented language originally developed by Macromedia Inc. . It is a dialect of ECMAScript , and is used primarily for the development of websites and software targeting the Adobe Flash Player platform, used on Web pages in the form of...
, but has not been adopted more universally. Similar notations are used in Microsoft's LINQ
LINQ
Linq is a word-based card game from Endless Games, introduced at the American International Toy Fair in 2005.Game play requires at least four players, two of whom are dealt cards with the same word, while the others receive blanks. The goal is to gain points by correctly naming the players with...
implementation for Microsoft .NET 3.5 and above, and in Scala (which uses the Java VM). The open-source xmlsh application, which provides a Linux-like shell with special features for XML manipulation, similarly treats XML as a data type, using the <[ ]> notation. The Resource Description Framework
Resource Description Framework
The Resource Description Framework is a family of World Wide Web Consortium specifications originally designed as a metadata data model...
defines a data type rdf:XMLLiteral to hold wrapped, canonical XML
Canonical XML
Canonical XML is a profile or subset of XML. Any XML document can be converted to Canonical XML, thus normalizing away specific kinds of minor differences while remaining an XML document...
.
History
XML is an application profile of SGML (ISO 8879).
The versatility of SGML for dynamic information display was understood by early digital media publishers in the late 1980s prior to the rise of the Internet. By the mid-1990s some practitioners of SGML had gained experience with the then-new World Wide Web
World Wide Web
The World Wide Web is a system of interlinked hypertext documents accessed via the Internet...
, and believed that SGML offered solutions to some of the problems the Web was likely to face as it grew. Dan Connolly
Dan Connolly
Dan Connolly received a B.S. in Computer Science from the University of Texas at Austin in 1990. His research interests include investigating the value of formal descriptions of chaotic systems like the Web, particularly in the consensus-building process, and the Semantic Web.He became involved...
added 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 Bosak
Jon Bosak
Jon Bosak led the creation of the XML specification at the W3C. From 1996–2008, he worked for Sun Microsystems.-XML:Tim Bray, who was one of the editors of the XML specification, has this to say in his note on Bosak in his annotated version of the specification:In a 1999 posting to the xml-dev...
developed a charter and recruited collaborators. Bosak was well connected in the small community of people who had experience both in SGML and the Web.
A working group is an interdisciplinary collaboration of researchers working on new research activities that would be difficult to develop under traditional funding mechanisms . The lifespan of the WG can last anywhere between a few months and several years...
of eleven members, 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-McQueen
Michael Sperberg-McQueen
C. M. "Michael" Sperberg-McQueen is an American markup specialist. He was co-editor of the Extensible Markup Language 1.0 spec , and chair of the XML Schema working group....
James Clark, is the author of groff and expat and has done much work with open-source software and XML. Born in London, and educated at Charterhouse and Merton College, Oxford, Clark has lived in Bangkok, Thailand since 1995, and is now a permanent resident...
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 Bray
Tim Bray
Timothy William Bray is a Canadian software developer and entrepreneur. He co-founded Open Text Corporation and Antarctica Systems. Bray was Director of Web Technologies at Sun Microsystems from early 2004 to early 2010. Since then he has served as a Developer Advocate at Google, focusing on...
C. M. "Michael" Sperberg-McQueen is an American markup specialist. He was co-editor of the Extensible Markup Language 1.0 spec , and chair of the XML Schema working group....
. 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 Paoli
Jean Paoli
Jean Paoli was one of the inventors of XML. Along with Tim Bray and Michael Sperberg-McQueen, Paoli co-edited the XML 1.0 recommendation for the World Wide Web Consortium starting in 1997 and until at least 2008....
as 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 a short burst of intense work between August and November 1996, when the first Working Draft of an XML specification was published. Further design work continued through 1997, and XML 1.0 became a W3C Recommendation on February 10, 1998.
Sources
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 instruction
Processing Instruction
A Processing Instruction is an SGML and XML node type, which may occur anywhere in the document, intended to carry instructions to the application....
s), and the default angle-bracket syntax. Removed were the SGML Declaration (XML has a fixed delimiter set and adopts Unicode
Unicode
Unicode is a computing industry standard for the consistent encoding, representation and handling of text expressed in most of the world's writing systems...
A character encoding system consists of a code that pairs each character from a given repertoire with something else, such as a sequence of natural numbers, octets or electrical pulses, in order to facilitate the transmission of data through telecommunication networks or storage of text in...
The Text Encoding Initiative is a text-centric community of practice in the academic field of digital humanities. The community runs a mailing list, meetings and conference series, and maintains a technical standard, a wiki and a toolset....
(TEI), which defined a profile of SGML for use as a "transfer syntax"; and HTML
HTML
HyperText Markup Language is the predominant markup language for web pages. HTML elements are the basic building-blocks of webpages....
, in which elements were synchronous with their resource, document character sets were separate from resource encoding, the xml:lang attribute was invented, and (like HTTP) metadata accompanied the resource rather than being needed at the declaration of a link. The Extended Reference Concrete Syntax (ERCS) project of the SPREAD (Standardization Project Regarding East Asian Documents) project of the ISO-related China/Japan/Korea Document Processing expert group was the basis of XML 1.0's naming rules; SPREAD also introduced hexadecimal numeric character references and the concept of references to make available all Unicode characters. To support ERCS, XML and HTML better, the SGML standard IS 8879 was revised in 1996 and 1998 with WebSGML Adaptations. The XML header followed that of ISO HyTime
HyTime
HyTime is a markup language that is an "application" of SGML. HyTime defines a set of hypertext-oriented element types that, in effect, supplement SGML and allow SGML document authors to build hypertext and multimedia presentations in a standardized way.HyTime is an international standard...
.
Ideas that developed during discussion which were novel in XML included 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. The notion of well-formedness as opposed to validity (which enables parsing without a schema) was first formalized in XML, although it had been implemented successfully in the Electronic Book Technology "Dynatext" software; the software from the University of Waterloo New Oxford English Dictionary Project; the RISP LISP SGML text processor at Uniscope, Tokyo; the US Army Missile Command IADS hypertext system; Mentor Graphics Context; Interleaf and Xerox Publishing System.
Versions
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 fifth edition, as published on November 26, 2008. 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. The main changes are to enable the use of line-ending characters used on EBCDIC
EBCDIC
Extended Binary Coded Decimal Interchange Code is an 8-bit character encoding used mainly on IBM mainframe and IBM midrange computer operating systems....
platforms, and the use of scripts and characters absent from Unicode 3.2. XML 1.1 is not very widely implemented and is recommended for use only by those who need its unique features.
Prior to its fifth edition release, XML 1.0 differed from XML 1.1 in having stricter requirements for characters available for use in element and attribute names and unique identifiers: in the first four editions of XML 1.0 the characters were exclusively enumerated using a specific version of the Unicode
Unicode
Unicode is a computing industry standard for the consistent encoding, representation and handling of text expressed in most of the world's writing systems...
standard (Unicode 2.0 to Unicode 3.2.) The fifth edition substitutes the mechanism of XML 1.1, which is more future-proof but reduces redundancy
Redundancy (information theory)
Redundancy in information theory is the number of bits used to transmit a message minus the number of bits of actual information in the message. Informally, it is the amount of wasted "space" used to transmit certain data...
. The approach taken in the fifth edition of XML 1.0 and in all editions of XML 1.1 is that only certain characters are forbidden in names, and everything else is allowed, in order to accommodate the use of suitable name characters in future versions of Unicode. In the fifth edition, XML names may contain characters in the Balinese
Balinese script
The Balinese alphabet is an abugida that was used to write the Balinese language, an Austronesian language spoken by about three million people on the Indonesian island of Bali. The use of the Balinese script has mostly been replaced by the Roman alphabet. Although it is learned in school, few...
The Phoenician alphabet, called by convention the Proto-Canaanite alphabet for inscriptions older than around 1050 BC, was a non-pictographic consonantal alphabet, or abjad. It was used for the writing of Phoenician, a Northern Semitic language, used by the civilization of Phoenicia...
scripts among many others which have been added to Unicode since Unicode 3.2.
Almost any Unicode code point can be used in the character data and attribute values of an XML 1.0 or 1.1 document, even if the character corresponding to the code point is not defined in the current version of Unicode. In character data and attribute values, XML 1.1 allows the use of more control character
Control character
In computing and telecommunication, a control character or non-printing character is a code point in a character set, that does not in itself represent a written symbol.It is in-band signaling in the context of character encoding....
s than XML 1.0, but, for "robustness", most of the control characters introduced in XML 1.1 must be expressed as numeric character references (and #x7F through #x9F, which had been allowed in XML 1.0, are in XML 1.1 even required to 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 has been discussion of an XML 2.0, although no organization has announced plans for work on such a project. XML-SW (SW for skunkworks
Skunkworks project
A skunkworks project is one typically developed by a small and loosely structured group of people who research and develop a project primarily for the sake of radical innovation. The term typically refers to technology projects, and originated with Skunk Works, an official alias for the Lockheed...
), 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 namespace
XML Namespace
xmlns tagged XML namespaces are used for providing uniquely named elements and attributes in an XML document. They are defined in a W3C recommendation. An XML instance may contain element or attribute names from more than one XML vocabulary...
XML Base is a World Wide Web Consortium recommended facility for defining base URIs for parts of XML documents.XML Base recommendation was adopted on 2001-06-27....
XML Information Set is a W3C specification describing an abstract data model of an XML document in terms of a set of information items...
(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
Fast Infoset
Fast Infoset is an international standard that specifies a binary encoding format for the XML Information Set as an alternative to the XML document format...
for their own binary infoset to avoid confusion (see ITU-T Rec. X.891 | ISO/IEC 24824-1).
Criticism
XML and its extensions have regularly been criticized for verbosity and complexity. Mapping the basic tree model of XML to type system
Type system
A type system associates a type with each computed value. By examining the flow of these values, a type system attempts to ensure or prove that no type errors can occur...
s of programming languages or databases can be difficult, especially when XML is used for exchanging highly structured data between applications, which was not its primary design goal. Other criticisms attempt to refute the claim that XML is a self-describing language (though the XML specification itself makes no such claim). JSON
JSON
JSON , or JavaScript Object Notation, is a lightweight text-based open standard designed for human-readable data interchange. It is derived from the JavaScript scripting language for representing simple data structures and associative arrays, called objects...
YAML is a human-readable data serialization format that takes concepts from programming languages such as C, Perl, and Python, and ideas from XML and the data format of electronic mail . YAML was first proposed by Clark Evans in 2001, who designed it together with Ingy döt Net and Oren Ben-Kiki...
are frequently proposed as alternatives; both focus on representing structured data, rather than narrative documents.
The following tables compare XML compatibility and support for a number of layout engines. Please see the individual products' articles for further information. This article is not all-inclusive or necessarily up-to-date...
Binary XML refers to any specification which defines the compact representation of XML in a binary format. While there are several competing formats, none has been widely adopted by a standards organization or accepted as a de facto standard...
WAP Binary XML is a binary representation of XML. It was developed by the WAP Forum and is now maintained by the Open Mobile Alliance as a standard to allow XML documents to be transmitted in a compact manner over mobile networks and proposed as an addition to the World Wide Web Consortium's...
Struxt is a human-readable data format designed to be structurally equivalent to XML yet representationally similar to C-style programming languages.'Struxt stands for "Structured Text".- Features :...
In computer security, a billion laughs attack is a type of denial-of-service attack which is aimed at parsers of XML documents.It's also referred to as an XML bomb or as an exponential entity expansion attack....
This is a comparison of data serialization formats, different ways to convert complex objects to sequences of bits. It does not include markup languages used exclusively as document file formats.-Overview:*a. The current default format is binary....
Richard Alan Jelliffe is an Australian programmer and standards activist , particularly associated with web standards, markup languages, internationalization and schema languages. He is the founder and Chief Technical Officer of Topologi Pty. Ltd, an XML tools vendor in Sydney...
Jon Bosak led the creation of the XML specification at the W3C. From 1996–2008, he worked for Sun Microsystems.-XML:Tim Bray, who was one of the editors of the XML specification, has this to say in his note on Bosak in his annotated version of the specification:In a 1999 posting to the xml-dev...
