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Extensible Markup Language (XML) is a general-purpose specification for creating custom markup languages. It is classified as an extensible language because it allows its users to define their own elements. Its primary purpose is to facilitate the sharing of structured data across different information systems, particularly via the Internet, and it is used both to encode documents and to serialize data.
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:
<book>This is a book…. </book>
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 encoding and external dependencies.
<?xml version="1.0" encoding="UTF-8"?>
Here is an example of a structured XML document:
<recipe name="bread" prep_time="5 mins" cook_time="3 hours">
<title>Basic bread</title>
<ingredient amount="3" unit="cups">Flour</ingredient>
<ingredient amount="0.25" unit="ounce">Yeast</ingredient>
<ingredient amount="1.5" unit="cups" state="warm">Water</ingredient>
<ingredient amount="1" unit="teaspoon">Salt</ingredient>
<instructions>
<step>Mix all ingredients together.</step>
<step>Knead thoroughly.</step>
<step>Cover with a cloth, and leave for one hour in warm room.</step>
<step>Knead again.</step>
<step>Place in a bread baking tin.</step>
<step>Cover with a cloth, and leave for one hour in warm room.</step>
<step>Bake in the oven at 350°F for 30 minutes.</step>
</instructions>
</recipe>
Entity references
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 "€", which refers to the Euro symbol by means of its Unicode codepoint in hexadecimal.
An entity reference is a placeholder that represents that entity. It consists of the entity’s name preceded by an ampersand ("&") and followed by a semicolon (";"). XML has five predeclared entities:
& & ampersand
< < less than
> > greater than
' ‘ apostrophe
" " quotation mark
Here is an example using a predeclared XML entity to represent the ampersand in the name "AT&T":
<company_name>AT&T</company_name>
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.
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE example [
<!ENTITY copy "©">
<!ENTITY copyright-notice "Copyright © 2006, hurricanesoftwares.com">
]>
<example>
©right-notice;
</example>
When viewed in a suitable browser, the XML document above appears as:
<example> Copyright © 2006, Hurricanesoftwares.com </example>
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 "x"-prefixed 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):
<company_name>AT&T</company_name>
<company_name>AT&T</company_name>
Well-formed documents
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 <IAmEmpty></IAmEmpty>.
* 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> … </Step>
whereas this is not
<Step> … </step>
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 semantics of elements and attributes to a human reader without reference to external documentation. However, this can lead to verbosity, which complicates authoring and increases file size.
Automatic verification
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). One example of using an independent tool follows:
* load it into an XML-capable browser, such as Firefox or Internet Explorer
* 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
Displaying XML on the web
XML documents do not carry information about how to display the data. Without using CSS or XSL, 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 stylesheet:
<?xml-stylesheet type="text/css" href="myStyleSheet.css"?>
Note that this is different from specifying such a stylesheet in HTML, which uses the <link> element.
Extensible Stylesheet Language (XSL) can be used to alter the format of XML data, either into HTML or other formats that are suitable for a browser to display.
To specify client-side XSL Transformation (XSLT), the following processing instruction is required in the XML:
<?xml-stylesheet type="text/xsl" href="myTransform.xslt"?>
Client-side XSLT is supported by many web browsers. Alternatively, one may use XSL 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.
Processing XML files
Three traditional techniques for processing XML files are:
* Using a programming language and the SAX API.
* Using a programming language and the DOM API.
* Using a transformation engine and a filter
More recent and emerging techniques for processing XML files are:
* Pull 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
DOM is an interface-oriented Application Programming Interface that 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 programming language-specific bindings. DOM implementations tend to be 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. 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.
Transformation engines and filters
A filter in the Extensible Stylesheet Language (XSL) family can transform an XML file for displaying or printing.
* XSL-FO is a declarative, XML-based page layout language. An XSL-FO processor can be used to convert an XSL-FO document into another non-XML format, such as PDF.
* XSLT is 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.
* XQuery is a W3C language for querying, constructing and transforming XML data.
* XPath is a DOM-like node tree data model and path expression language 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
Pull parsing [6] 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 StAX in 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.
Data binding
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 Architecture for XML Binding (JAXB).
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, ASIC implementation, incremental update, and native XML indexing .
Specific XML applications and editors
The native file format of OpenOffice.org, AbiWord, and Apple’s iWork applications is XML. Some parts of Microsoft Office 2007 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.
Advantages of XML
* It is text-based.
* 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.
* Its self-documenting format describes structure and field names as well as specific values.
* The strict syntax and parsing requirements make the necessary parsing algorithms extremely simple, efficient, and consistent.
* XML is heavily used as a format for document storage and processing, both online and offline.
* It is based on international standards.
* It can be updated incrementally.
* It allows validation using schema languages such as XSD and 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.
* An element fragment of a well-formed XML document is also a well-formed XML document.
Disadvantages of XML
* XML syntax is redundant or large relative to binary representations of similar data, especially with tabular data.
* The redundancy may affect application efficiency through higher storage, transmission and processing costs.
* XML syntax is verbose, especially for human readers, relative to other alternative ‘text-based’ data transmission formats.
* The hierarchical model for representation is limited in comparison to an object oriented graph.
* Expressing overlapping (non-hierarchical) node relationships requires extra effort.
* XML namespaces are problematic to use and namespace support can be difficult to correctly implement in an XML parser.
* XML is commonly depicted as "self-documenting" but this depiction ignores critical ambiguities.
* The distinction between content and attributes in XML seems unnatural to some and makes designing XML data structures harder
