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XML Tutorial >> XML CDATA.


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In XML to read and update, create and manipulate an XML document, we will need an XML parser that will parse or validate all the XML document with standards of W3C.  The parser loads the document into our computer's memory and when the document is loaded, its data can be manipulated using the DOM Document Object Model. The DOM treats the XML document as a tree that's why we can see an XML document in web browser as a tree.

Now the only content inside a CDATA section of XML document will be ignored by the XML parser.  So when the parser parse the XML tags then it will also parse the content or text in between tags are also parsed by it.  As shown in example below.

XML for CDATA Example

<employee>
<firstName>Creston</firstName>
<lastName>Narvon</lastName>
<gender>m</gender>
<DOB>22/04/1976</DOB>
</employee>

On above XML code all the tags like <employee> <firstName> <lastName> <gender> and <DOB> will be parse by the XML parser and its content like Creston, Narvon, m and 22/04/1976 will also be parse by the XML parser.  Parser observe the tags and match them with XML standards. Like <firstName> is embeded in <employee> root tag and that's why in web browser we can see it as tree.

XML Parser Issues.

Some character are illegal in XML to use them like "<" less then sign and " quotes and grater then ">" sign & and sign, single quote ' or Apostrophes.  As we use the less then < sign in below example, it is not a valid XML document and it will be appear having an error in browser.

Invalid XML CDATA

<employee>;
   <firstName>"Creston"</firstName>
   <salary>salary is < 3100 </salary>
</employee>

On above example of invalid XML we can see we use " around the Creston and it is not a valid way to define content of XML tags. and in [Salary is < 3100] we used < sign it is also not allowed.  In  two character  are < and & sign are strictly illegal so when ever we will use these parser will not allow.  Solution for this is, we will use XML CDATA as an XML section and here we can use the < and & sign and others like > double quotes " and single quotes '.  

Valid XML CDATA Example

<script>
<![CDATA[
function checksalary(salary)
{
if (salary < 3100) than
   {
     return true
   }
else 
   {
     return false
   }
}
]]>
</script>

As we tried to used < sign on the example at the top of page of this section of XML Tutorial and XML parser generate the error now on above example we can use these characters as shown and parser will ignore them all.
Between the start of the section, "<![CDATA[" and the end of the section, "]]>", all character data is passed directly to the application. The only string that cannot occur in a CDATA section is "]]>". Comments are not recognized in a CDATA section. If present, the literal text "<!--comment-->" will be passed directly to the application. The character string ]]> is not allowed within a CDATA block as it would signal the end of the CDATA block. CDATA does not work in HTML.


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