Class CollationAttributeFactory

  • public class CollationAttributeFactory
    extends AttributeSource.AttributeFactory

    Converts each token into its CollationKey, and then encodes the bytes as an index term.

    WARNING: Make sure you use exactly the same Collator at index and query time -- CollationKeys are only comparable when produced by the same Collator. Since RuleBasedCollators are not independently versioned, it is unsafe to search against stored CollationKeys unless the following are exactly the same (best practice is to store this information with the index and check that they remain the same at query time):

    1. JVM vendor
    2. JVM version, including patch version
    3. The language (and country and variant, if specified) of the Locale used when constructing the collator via Collator.getInstance(java.util.Locale).
    4. The collation strength used - see Collator.setStrength(int)

    The ICUCollationAttributeFactory in the analysis-icu package uses ICU4J's Collator, which makes its version available, thus allowing collation to be versioned independently from the JVM. ICUCollationAttributeFactory is also significantly faster and generates significantly shorter keys than CollationAttributeFactory. See for key generation timing and key length comparisons between ICU4J and java.text.Collator over several languages.

    CollationKeys generated by java.text.Collators are not compatible with those those generated by ICU Collators. Specifically, if you use CollationAttributeFactory to generate index terms, do not use ICUCollationAttributeFactory on the query side, or vice versa.