Tuesday, December 18, 2012

Generating Alphanumeric Random Strings

In one of my previous blog post, I had mentioned about the excellent Apache Commons RandomNumberGenerator utility class that is very handy for common use.

Sometimes, we need to generate an alphanumeric ramdom string for specific use cases. For e.g. file names, registry keys, etc. The Apache Commons module has one more class called RandomStringUtils for this.
If you are looking for a more simpler copy-n-paste code, then the following snippet should suffice for most non-secure requirements.

static final String AB = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";
static Random rnd = new Random();

String randomString( int len ) 
   StringBuilder sb = new StringBuilder( len );
   for( int i = 0; i < len; i++ ) 
      sb.append( AB.charAt( rnd.nextInt(AB.length()) ) );
   return sb.toString();

Monday, December 03, 2012

Ruminating on Schematron and XML Validation

Schematron is a rule-based XML validation language. It can be used for making assertions on patterns in XML nodes. For e.g. if a person (element) has "Mr" as prefix (attribute), then gender (child element) should be "male". If the assertion fails, an error message that is supplied by the author of the schema can be displayed.

Thus Schematron is capable of expressing complex constraints that cannot be expressed using XML Schema or DTD. Using XML schemas, you can only put constraints on the document structure and basic datatype validation.  The following sites give good intro about this language.


So how do we apply Schematron schema to validate XML document instances? Given below are the simple steps one should follow:
  1. Create a schematron schema file
  2. Use a meta-stylesheet to convert this schema file into a XSLT stylesheet. ( A meta-stylesheet is a stylesheet which generates other stylesheets)
  3. The above generated XSLT stylesheet can be used as a XML validator against the XML instance document (using a XSLT transformation engine)
  4. The output of the transformation would be a XML document with validation errors.
Since the fundamental technologies used as XML Schema, XSLT and XPath; most of the XML processing APIs of Java/.NET can be used for the same.

Difference between XSL, XSL-FO and XSLT

The XSL (Extensible StyleSheet Language) specification has a long history and has gone through multiple revisions. Many folks are confused between the difference between XSL, XSL-FO and XSLT.

Found a good link that explains the history of XSL - http://www.dpawson.co.uk/xsl/sect1/history.html
W3Schools has a good explanation - snippet below:

XSL-FO is Formally Named XSL !!  Why this confusion? Is XSL-FO and XSL the same thing?
Yes it is, but we will give you an explanation:
Styling is both about transforming and formatting information. When the World Wide Web Consortium (W3C) made their first XSL Working Draft, it contained the language syntax for both transforming and formatting XML documents.
Later, the Working Group at W3C split the original draft into separate Recommendations:

  •     XSLT, a language for transforming XML documents
  •     XSL or XSL-FO, a language for formatting XML documents
  •     XPath, a language for navigating in XML documents