8.4. Customizing Lucene's scoring formula

Lucene allows the user to customize its scoring formula by extending org.apache.lucene.search.Similarity. The abstract methods defined in this class match the factors of the follownig formula calculating the score of query q for document d:

score(q,d) = coord(q,d) · queryNorm(q) · ∑_{t in q} ( tf(t in d) · idf(t)² · t.getBoost() · norm(t,d) )

It is beyond the scope of this manual to explain this formula in more detail. Please refer to Similarity's Javadocs for more information.

Hibernate Search provides two ways to modify Lucene's similarity calculation. First you can set the default similarity by specifying the fully specified classname of your Similarity implementation using the property hibernate.search.similarity. The default value is org.apache.lucene.search.DefaultSimilarity. Additionally you can override the default similarity on class level using the @Similarity annotation. As an exmaple, let's assume it is not important how often a term appears in a document. Documents with a single occurrence of the term should be scored the same as documents with multiple occurrences. In this case your custom implementation of the method tf(float freq) should return 1.0.