Integrating XML with J2EE - Structure and Syntax of XML (
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In this section, you will explore the syntax of XML and understand what is
meant by a well-formed document.
Note - You will often encounter the terms "well-formed" and "valid"
applied to XML documents. These are not the same. A well-formed document is
structurally and syntactically correct (the XML conforms to the XML language
definition, that is all tags have a correctly nested corresponding end tag, all
attributes are quoted, only valid characters have been used, and so on), whereas
a valid document is also semantically correct (the XML conforms to some external
definition stored in an XML Schema or Document Type Definition). A document can
be well-formed but may not be valid.
The best way to become familiar with the syntax of XML is to write an XML
document. To check your XML, you will need access to an XML-aware browser or
another XML validator. The XML-aware browser or XML validator will allow you to
ensure that the XML is well-formed. If the XML references an XML Schema or
Document Type Definition (more on these later) the validator can also check that
the XML is valid.
An XML browser includes an XML parser. To get the browser to check the syntax
and structure of your XML document, simply use the browser to open the XML file.
Well-formed XML will be displayed in a structured way (with indentation). If the
XML is not well-formed, an appropriate error message will be given.
Tip - An easy way to validate XML is to use an XML aware browser. The
latest versions of most popular browsers are now XML aware. You can download
validating XML parsers from Sun Microsystems at http://www.sun.com/software/xml/developers/multischema/ and
the Microsoft Developers Network at http://msdn.microsoft.com/downloads/samples/internet/xml/xml_validator/.
There are numerous other XML validators and XML editors vailable from the
Internet.
HTML and XML
At first glance, XML looks very similar to HTML. An XML document consists of
elements that have a start and end tag, just like HTML. In fact, Listing 16.1 is
both well-formed HTML and XML.
Listing 16.1 Example XML and HTML
<html>
<head><title>Web Page</title></head>
<body>
<h1>Teach Yourself J2EE in 21 Days</h1>
<p>Now you have seen the web page – buy the book</p>
</body>
</html>
An XML document is only well-formed if there are no syntax errors. If you are
familiar with HTML, you will be aware that many browsers are lenient with poorly
formed HTML documents. Missing end tags and even missing sections will often be
ignored and therefore unnoticed until the page is displayed in a more rigorous
browser, and fails to display correctly.
XML differs from HTML in that a missing end tag will always cause an error.
We will now look at XML syntax so you can understand what is going on.
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 This chapter is from Teach Yourself
J2EE in 21 Days, second edition, by Martin Bond et. al. (Sams,
2004, ISBN: 0-672-32558-6). Check it out at your favorite bookstore today. Buy
this book now.
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