Friday, December 05, 2008

Providing your own collator for Java DB

Now this is pretty cool - using Java pluggability, in Java DB you can provide your own collation. Knut Anders provides the details in his blog
... sometimes you may want to sort your strings based on some other rules than those in the languages supported by the JRE. The most common example is that users want case-insensitive string matching. Another example came up recently in this thread on derby-user, where a user wanted to modify the German collation rules to make Greek characters sort near their Latin equivalents ('α' near 'a', 'β' near 'b', and so on).

Derby currently (as of 10.4) doesn't allow you to define your own collation rules, it merely relies on the collators returned by java.text.Collator.getInstance(java.util.Locale). Fortunately, the JRE allows you to define new locales with their own collation rules. And it turns out that it's quite easy, just perform these three simple steps:
  • Create a class that extends java.text.spi.CollatorProvider and returns a collator that orders strings the way you want it to
  • Create a text file named META-INF/services/java.text.spi.CollatorProvider which contains one line with the name of your collator provider class
  • Put the compiled class file and the text file in a jar file which you drop into your JRE's lib/ext directory or in one of the directories specified by the java.ext.dirs property
Bada boom, bada bing.

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