What does CDATA in XML mean

Question

I often find this strange CDATA tag in XML files    lt   CDATA some stuff   gt    I have observed that this CDATA tag always comes at the beginning  and then followed by some stuff   But sometimes it is used  sometimes it is not  I assume it is to mark that some stuff is the  data  that will be inserted after that  But what kind of data is some stuff  Isn t anything I write in XML tags some sort of data

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One big use-case  your xml includes a program  as data  e g  a web-page tutorial for Java    In that situation your data includes a big chunk of characters that include   amp   and   lt   but those characters aren t meant to be xml   Compare    lt example-code gt  while  x  amp lt  len  amp amp  amp amp   done        print   amp quot Still working   amp apos zzz amp apos   amp quot           x         lt  example-code gt    with   lt example-code gt  lt   CDATA  while  x  lt  len  amp  amp   done        print   Still working   zzzz             x           gt  lt  example-code gt    Especially if you are copy pasting this code from a file  or including it  in a pre-processor   it s nice to just have the characters you want in your xml file  w o confusing them with XML tags attributes   As  paary mentioned  other common uses include when you re embedding URLs that contain ampersands   Finally  even if the data only contains a few special characters but the data is very very long  the text of a chapter  say   it s nice to not have to be en de-coding those few entities as you edit your xml file    I suspect all the comparisons to comments are kinda misleading unhelpful

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CDATA stands for Character Data and it means that the data in between these strings includes data that could be interpreted as XML markup  but should not be   The key differences between CDATA and comments are    As Richard points out  CDATA is still part of the document  while a comment is not  In CDATA you cannot include the string    gt   CDEnd   while in a comment -- is invalid  Parameter Entity references are not recognized inside of comments    This means given these four snippets of XML from one well-formed document    lt  ENTITY MyParamEntity  Has been expanded  gt       lt  -- Within this comment I can use    gt  and other reserved characters like  lt   amp      and    but  MyParamEntity  will not be expanded  if I retrieve the text of this node it will contain  MyParamEntity  and not  Has been expanded   and I can t place two dashes next to each other  -- gt       lt   CDATA  Within this Character Data block I can use double dashes as much as I want  along with  lt    amp      and     and   MyParamEntity  will be expanded to the text  Has been expanded      however  I can t use the CEND sequence  If I need to use CEND I must escape one of the brackets or the greater-than sign using concatenated CDATA sections     gt       lt description gt An example of escaped CENDs lt  description gt   lt  -- This text contains a CEND    gt  -- gt   lt  -- In this first case we put the    at the end of the first CDATA block      and the  gt  in the second CDATA block -- gt   lt data gt  lt   CDATA This text contains a CEND      gt  lt   CDATA  gt    gt  lt  data gt   lt  -- In this second case we put a   at the end of the first CDATA block      and the   gt  in the second CDATA block -- gt   lt alternative gt  lt   CDATA This text contains a CEND     gt  lt   CDATA   gt    gt  lt  alternative gt

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It escapes a string that cannot be passed to XML as usual   Example   The string contains   amp   in it   You can not    lt FL val  Company Name  gt Dolce  amp  Gabbana lt  FL gt    Therefore  you must use CDATA    lt FL val  Company Name  gt   lt   CDATA  Dolce  amp  Gabbana    gt   lt  FL gt

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The data contained therein will not be parsed as XML  and as such does not need to be valid XML or can contain elements that may appear to be XML but are not

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From Wikipedia       In  an XML document or external parsed entity  a CDATA section is a   section of element content that is marked for the parser to interpret   as only character data  not markup       http   en wikipedia org wiki CDATA   Thus  text inside CDATA is seen by the parser but only as characters not as XML nodes

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A CDATA section is  a section of element content that is marked for the parser to interpret as only character data  not markup    Syntactically  it behaves similarly to a comment    lt exampleOfAComment gt   lt  --     Since this is a comment     I can use all sorts of reserved characters     like  gt   lt    and  amp      or write things like      lt foo gt  lt  bar gt      but my document is still well-formed  -- gt   lt  exampleOfAComment gt        but it is still part of the document    lt exampleOfACDATA gt   lt   CDATA      Since this is a CDATA section     I can use all sorts of reserved characters     like  gt   lt    and  amp      or write things like      lt foo gt  lt  bar gt      but my document is still well formed     gt   lt  exampleOfACDATA gt    Try saving the following as a  xhtml file  not  html  and open it using FireFox  not Internet Explorer  to see the difference between the comment and the CDATA section  the comment won t appear when you look at the document in a browser  while the CDATA section will    lt  xml version  1 0  encoding  UTF-8  standalone  no    gt   lt  DOCTYPE html PUBLIC  -  W3C  DTD XHTML 1 0 Transitional  EN   http   www w3 org TR xhtml1 DTD xhtml1-transitional dtd  gt   lt html xmlns  http   www w3 org 1999 xhtml  xml lang  en  lang  en   gt   lt head gt   lt title gt CDATA Example lt  title gt   lt  head gt   lt body gt    lt h2 gt Using a Comment lt  h2 gt   lt div id  commentExample  gt   lt  -- You won t see this in the document and can use reserved characters like  lt   gt   amp    -- gt   lt  div gt    lt h2 gt Using a CDATA Section lt  h2 gt   lt div id  cdataExample  gt   lt   CDATA  You will see this in the document and can use reserved characters like  lt   gt   amp       gt   lt  div gt    lt  body gt   lt  html gt      Something to take note of with CDATA sections is that they have no encoding  so there s no way to include the string    gt  in them  Any character data which contains    gt  will have to - as far as I know - be a text node instead  Likewise  from a DOM manipulation perspective you can t create a CDATA section which includes    gt    var myEl   xmlDoc getElementById  cdata-wrapper    myEl appendChild xmlDoc createCDATASection  This section cannot contain    gt        This DOM manipulation code will either throw an exception  in Firefox  or result in a poorly structured XML document  http   jsfiddle net 9NNHA

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It s used to contain data which could otherwise be seen as xml because it contains certain characters    This way the data inside will be displayed  but not interpreted

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Note that the CDATA construct is only needed if placing text directly in the XML text file  That is  you only need to use CDATA if hand typing or programmatically building the XML text directly  Any text entered using a DOM processor API or SimpleXML will be automatically escaped to prevent running foul of XML content rules  Notwithstanding that  there can be times where using CDATA can reduce the text size that would otherwise be produced with all entities encoded  such as for css in style tags or javascript in script tags  where many language constructs use characters in HTML XML  like  lt  and  gt

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I once had to use CDATA when my xml element needed to store HTML code  Something like   lt codearea gt     lt   CDATA      lt div gt   lt p gt  my para  lt  p gt   lt  div gt        gt   lt  codearea gt    So CDATA means it will ignore any character which could otherwise be interpreted as XML tag like  lt  and   etc

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The Cdata is a data which you may want to pass to an xml parser and still not interpreted as an xml    Say for eg  - You have an xml which has encapsulates question answer object   Such open fields can have any data which does not strictly fall under basic data type or xml defined custom data types   Like --Is this a correct tag for xml comment      --  You may have a requirement to pass it as it is without being interpreted by the xml parser as another child element  Here Cdata comes to your rescue   By declaring as Cdata you are telling the parser don t treat the data wrapped as an xml  though it may look like one

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Usually used for embedding custom data  like pictures or sound data within an XML document

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As another example of its use   If you have an RSS Feed  xml document  and want to include some basic HTML encoding in the display of the description  you can use CData to encode it    lt item gt     lt title gt Title of Feed Item lt  title gt     lt link gt  mylink article1 lt  link gt     lt description gt       lt   CDATA         lt p gt         lt a href   mylink article1  gt  lt img style  float  left  margin-right  5px   height  80  src   mylink image  alt     gt  lt  a gt        Author Names        lt br  gt  lt em gt Date lt  em gt         lt br  gt Paragraph of text describing the article to be displayed lt  p gt         gt     lt  description gt   lt  item gt    The RSS Reader pulls in the description and renders the HTML within the CDATA    Note - not all HTML tags work - I think it depends on the RSS reader you are using     And as a explanation for why this example uses CData  and not the appropriate pubData and dc creator tags   this is for website display using a RSS widget for which we have no real formatting control   This enables us to specify the height and position of the included image  format the author names and date correctly  and so forth  without the need for a new widget  It also means I can script this and not have to add them by hand

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CDATA stands for Character Data  You can use this to escape some characters which otherwise will be treated as regular XML  The data inside this will not be parsed  For example  if you want to pass a URL that contains  amp  in it  you can use CDATA to do it  Otherwise  you will get an error as it will be parsed as regular XML

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