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Effective XML• XML Developers Network of
the Capital District• Elliotte Rusty Harold• [email protected]• http://
www.cafeconleche.org/
Part I: Syntax
Item 1: Include an XML declaration<?xml version="1.0" encoding="UTF-8"?>
• Optional, but treat as required• Specifies version, character set, and
encoding• Very important for detecting
encoding• Identifies XML when file and media
type information is unavailable or unreliable
Item 3: Stay with XML 1.0
• XML 1.1:• New name characters• C0 control characters• C1 control characters • NEL• Undeclare namespace prefixes
• Incompatible with• Most XML parsers• W3C and RELAX NG schema languages• XOM, JDOM
Part II: Structure
The XML Stack
Item 14: Allow All XML syntax
• CDATA sections• Entity references• Processing instructions• Comments• Numeric character references• Document type declarations• Different ways of representing the
same core content; not different information
Item 9: Distinguish text from markup• A DocBook element<programlisting><![CDATA[<value> <double>28657</double></value>]]></programlisting>
• The content is:<value> <double>28657</double></value>
• This is the same:<programlisting><value> <double>28657</double> </value></programlisting>
The reverse problem
• Tools that create XML from strings:• Tree-based editors like <Oxygen/> or XML Spy
• WYSIWYG applications like OpenOffice Writer
• Programming APIs such as DOM, JDOM, and XOM
• The tool automatically escapes reserved characters like <, >, or &.
• Just because something looks like an XML tag does not mean it is an XML tag.
Item 10: White space matters
• Parsers report all white space in element content, including boundary white space
• An xml:space attribute is for the client application only, not the parser
• White space in attribute values is normalized
• Parsers do not report white space in the prolog, epilog, the document type declaration, and tags.
Item 11: Make structure explicit through markup• Bad
<Transaction>Withdrawal 2003 12 15 200.00</Transaction>
• Better<Transaction type="withdrawal"> <Date>2003-12-15</Date> <Amount>200.00</Amount></Transaction>
Item 12: Store metadata in attributes• Material the reader doesn’t want
to see• URLs• IDs• Styles• Revision dates• Authors name
• No substructure• Revision tracking• Citations
• No multiple elements
Item 13: Remember mixed content
• Narrative documents• Record-like documents• The RSS problem<item> <title>Xerlin 1.3 released</title> <description> Xerlin 1.3, an open source XML Editor written in Java, has been released. Users can extend the application via custom editor interfaces for specific DTDs. New features in version 1.3 include XML Schema support, WebDAV capabilities, and various user interface enhancements. Java 1.2 or later is required. </description><link>http://www.cafeconleche.org/#news2003April7</link></item>
What you really want is this:<description> <p><a href="http://www.xerlin.org"><strong>Xerlin 1.3</strong></a>,an open source XML Editor written in Java, has been released. Users can extend the application via custom editor interfaces for specific DTDs. New features in version 1.3 include:</p> <ul> <li>XML Schema support</li> <li>WebDAV capabilities</li> <li>Various user interface enhancements</li> </ul> <p>Java 1.2 or later is required.</p> </description>
What people do is this:<description><p><a href="http://www.xerlin.org"><strong>Xerlin 1.3</strong></a>, an open source XML Editor written in Java, has been released. Users can extend the application via custom editor interfaces for specific DTDs. New features in version 1.3 include:</p> <ul> <li>XML Schema support</li> <li>WebDAV capabilities</li> <li>Various user interface enhancements</li> </ul> <p>Java 1.2 or later is required.</p> </description>
Item 16: Prefer URLs to unparsed entities and notations• URLs are simple and well
understood• Notations and unparsed entities
are confusing and little used• URLs don’t require the DTD to be
read• Many APIs don’t even support
notations and unparsed entities
Part III: Semantics
Item 17: Use processing instructions for process-specific content
• For a very particular, even local, process
• Describes how a particular process acts on the data in the document
• Does not describe or add to the content itself
• A unit that can be treated in isolation
• Content is not XML-like.• Applies to the entire document
Processing instructions are not appropriate when:• Content is closely related to the
content of the document itself.• Structure extends beyond a single
processing instruction• Needs to be validated.
Item 18: Include all information in instance documents• Not all parsers read the DTD• Especially browsers• Beware
• Default attribute values• Parsed entity references• XInclude• ID type dependence (XPath, DOM,
etc.)
Item 19: Encode binary data using quoted printable and/or Base64
• Quoted printable works well for mostly text
• Base-64 for non-text data• Can you link to the data with a URL
instead?
Item 20-22: Use namespaces for modularity and extensibility• Not hard; simple cases can use one
default namespace• http URIs are normally preferred• DTD validation is tricky• Code to namespace URIs, not
prefixes• Avoid namespace prefixes in
element content and attribute values
Item 23: Reuse XHTML for generic narrative content
Item 24: Choose the right schema language for the job• DTDs• The W3C XML Schema Language• RELAX NG• Schematron
Item 25: Pretend there's no such thing as the PSVI• Post Schema Validation Infoset• Adds types like int and gYear to
elements• Often not specific enough• Element/attribute names are types
Item 28: Use only what you need• You need
• Well-formed XML 1.0• A parser
• You probably need:• Namespaces
• You may not need:• DTDs• Schemas• XInclude• WS-Kitchen-Sink• etc.
Item 29: Always use a parser• Can’t use regular expressions:• Detecting encoding• Comments and processing instructions that
contain tags• CDATA sections• Unexpected placement of spaces and line
breaks within tags• Default attribute values• Character and entity references• Malformed documents• Internal DTD Subset
• Why not?• Unfamiliarity with parsers• Too slow
Item 30: Layer Functionalitybook.xml
XInclude
XSLT Transform to
XHTML
finished_book.xml
preface.xml
xmlsyntax.xml
XSLT Transform to
HTML
XSLT Transform to
XSL-FO
XSLT Transform to
Extract
SAX Program that extracts
examples
16 more chapters...
finished_book.xml
Valid?
book.xhtml book.html book.fo chapter1.xmlchapter1.xmlchapter2.xml
fop
book.pdf
chapters 1 to 17.xml
Example Source Code
Files
XSLT Transform to
XSL-FO
chapter1.xmlchapter2.xmlchapters 1 to 17.fo
xmlprotocols.xml
Yes
Print Error MessageNo
fop
chapter1.xmlchapter2.xmlchapters 1 to 17.pdf
Item 31-33: Program to standard APIs• Easier to deploy in Java 1.4/1.5• Different implementations have
different performance characteristics
• SAX is fast• DOM interoperates• Semi-standard:• JDOM• XOM
• Bleeding edge• StAX• JAXB
Item 34: Read the complete DTD• Be conservative in what you
generate; liberal in what you accept
• Important content from DTD:• Default attribute values• Namespace declarations• Entity references
Item 35: Navigate with XPath
• More robust against unexpected structure
• Allow optimization by engine• Easier to code; enhanced
programmer productivity
Item 36: Serialize XML with XML
Item 37: Validate inside your program with schemas
Part IV: Implementation
Item 38: Write documents in Unicode
• Prefer UTF-8• Smaller in English• ASCII compatible
• Normalization• É, ü, ì and so forth• NFC• ICU
Item 40: Avoid Vendor Lockin; Beware• Opaque, binary data used in place
of marked up text. • Over-abbreviated, inobvious
names like F17354 and grgyt • APIs that hide the XML• Products that focus on the
"Infoset”• Alternate serializations of XML• Patented formats
Item 41: Hang on to your relational database
Item 42: Document Namespaces with RDDL
<!DOCTYPE html PUBLIC "-//XML-DEV//DTD XHTML RDDL 1.0//EN" "http://www.rddl.org/rddl-xhtml.dtd"><html xmlns="http://www.w3.org/1999/xhtml" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:rddl="http://www.rddl.org/"><head> <title>MegaBank Statement Markup Language (MBSML)</title></head><p>This is the XML namespace for the <ahref="http://developer.megabank.com/xml/">MegaBank Statement Markup Language</a>.</p><rddl:resource xlink:type="simple" xlink:href="http://developer.megabank.com/xml/spec.html" xlink:role="http://www.w3.org/TR/html4/" xlink:arcrole ="http://www.rddl.org/purposes#normative-reference"> <p> The <a href="http://developer.megabank.com/xml/spec.html">MegaBank Statement Markup Language Specification 1.0</a> </p></rddl:resource></body></html>
Item 43: Preprocess XSLT on the server side
Item 44: Serve XML+CSS to the client• Supported by• Safari• IE 5.0 and later• Mozilla• Netscape 6 and later• Konqueror• Opera• Firefox• Omniweb
Item 45: Pick the correct MIME type• application/xml• Not text/xml!• Don't use charset• application/mathml+xml• image/svg+xml• application/xslt+xml
Item 46: TagSoup Your HTML
Item 47: Catalog common resources
<?xml version="1.0"?><catalog xmlns= "urn:oasis:names:tc:entity:xmlns:xml:catalog">
<public publicId= "-//OASIS//DTD DocBook XML V4.2//EN" uri= "file:///opt/xml/docbook/docbookx.dtd"/>
</catalog>
Item 50: Compress if space is a problem
//output OutputStream fout = new FileOutputStream("data.xml.gz"); OutputStream out = new GZipOutputStream(fout); OutputFormat format = new OutputFormat(document); XMLSerializer output = new XMLSerializer(out, format); output.serialize(doc); // input InputStream fin = new FileInputStream("data.xml.gz"); InputStream in = new GZipInputStream(fin); DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance(); DocumentBuilder parser = factory.newDocumentBuilder(); Document doc = parser.parse(in); S // work with the document...
To Learn More
• This Presentation: http://cafeconleche.org/slides/albany/effectivexml
• Effective XML: 50 Specific Ways to Improve Your XML Documents• Elliotte Rusty Harold• Addison-Wesley, 2003• ISBN 0-321-15040-6• $44.99• http://cafeconleche.org/books/
effectivexml