XML eXtensible Markup Language Jan 13 2019 HTML and XML I XML stands for eXtensible Markup Language HTML is used to mark up text so it can be displayed to users XML is used to mark up data so it can be processed by computers HTML describes both structure e g p h2 em and appearance e g br font i HTML uses a fixed unchangeable set of tags XML describes only content or meaning In XML you make up your own tags 2 HTML and XML II HTML and XML look similar because they are both SGML languages SGML Standard Generalized Markup Language Both HTML and XML use elements enclosed in tags e g body This is an element body Both use tag attributes e g font face Verdana size 1 color red Both use entities lt gt amp quot apos More precisely HTML is defined in SGML XML is a very small subset of SGML 3 HTML and XML III HTML is for humans HTML describes web pages You don t want to see error messages about the web pages you visit Browsers ignore and or correct as many HTML errors as they can so HTML is often sloppy XML is for computers XML describes data The rules are strict and errors are not allowed In this way XML is like a programming language Current versions of most browsers can display XML However browser support of XML is spotty at best 4 XML related technologies DTD Document Type Definition and XML Schemas are used to define legal XML tags and their attributes for particular purposes CSS Cascading Style Sheets describe how to display HTML or XML in a browser XSLT eXtensible Stylesheet Language Transformations and XPath are used to translate from one form of XML to another DOM Document Object Model SAX Simple API for XML and JAXP Java API for XML Processing are all APIs for XML parsing 5 Example XML document xml version 1 0 weatherReport date 7 14 97 date city North Place city state NX state country USA country High Temp high scale F 103 high Low Temp low scale F 70 low Morning morning Partly cloudy Hazy morning Afternoon afternoon Sunny amp hot afternoon Evening evening Clear and Cooler evening weatherReport From XML A Primer by Simon St Laurent 6 Overall structure An XML document may start with one or more processing instructions PIs or directives xml version 1 0 xml stylesheet type text css href ss css Following the directives there must be exactly one tag called the root element containing all the rest of the XML weatherReport weatherReport 7 XML building blocks Aside from the directives an XML document is built from elements high in high scale F 103 high tags in pairs high scale F 103 high attributes high scale F 103 high entities afternoon Sunny amp hot afternoon character data which may be parsed processed as XML this is the default unparsed all characters stand for themselves 8 Elements and attributes Attributes and elements are somewhat interchangeable Example using just elements name first David first last Matuszek last name Example using attributes name first David last Matuszek name You will find that elements are easier to use in your programs this is a good reason to prefer them Attributes often contain metadata such as unique IDs Generally speaking browsers display only elements values enclosed by tags not tags and attributes 9 Well formed XML Every element must have both a start tag and an end tag e g name name But empty elements can be abbreviated break XML tags are case sensitive XML tags may not begin with the letters xml in any combination of cases Elements must be properly nested e g not b i bold and italic b i Every XML document must have one and only one root element The values of attributes must be enclosed in single or double quotes e g time unit days Character data cannot contain or 10 Entities Five special characters must be written as entities amp for lt for gt for quot for apos for almost always necessary almost always necessary not usually necessary necessary inside double quotes necessary inside single quotes These entities can be used even in places where they are not absolutely required These are the only predefined entities in XML 11 XML declaration The XML declaration looks like this xml version 1 0 encoding UTF 8 standalone yes The XML declaration is not required by browsers but is required by most XML processors so include it If present the XML declaration must be first not even whitespace should precede it Note that the brackets are and version 1 0 is required this is the only version so far encoding can be UTF 8 ASCII or UTF 16 Unicode or something else or it can be omitted standalone tells whether there is a separate DTD 12 Processing instructions PIs Processing Instructions may occur anywhere in the XML document but usually first A PI is a command to the program processing the XML document to handle it in a certain way XML documents are typically processed by more than one program Programs that do not recognize a given PI should just ignore it General format of a PI target instructions Example xml stylesheet type text css href mySheet css 13 Comments This is a comment in both HTML and XML Comments can be put anywhere in an XML document Comments are useful for Explaining the structure of an XML document Commenting out parts of the XML during development and testing Comments are not elements and do not have an end tag The blanks after and before are optional The character sequence cannot occur in the comment The closing bracket must be Comments are not displayed by browsers but can be seen by anyone who looks at the source code 14 CDATA By default all text inside an XML document is parsed You can force text to be treated as unparsed character data by enclosing it in CDATA Any characters even and can occur inside a CDATA Whitespace inside a CDATA is usually preserved The only real restriction is that the character sequence cannot occur inside a CDATA CDATA is useful when your text has a lot of illegal characters for example if your XML document contains some HTML text 15 Names in XML Names as used for tags and attributes must begin with a letter or underscore and can consist of Letters both Roman English and foreign Digits both Roman and foreign dot hyphen underscore colon should be used only for namespaces Combining characters and extenders not used in English 16 Namespaces Recall that DTDs are used to define the tags that can be used in an XML document An XML document may reference more than one DTD Namespaces are a way to specify which DTD defines a given tag XML like Java uses qualified names This helps to avoid collisions between names Java myObject
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