Get matched terms from Lucene query

Question

Given a Lucene search query like: +(letter:A letter:B letter:C) +(style:Capital), how can I tell which of the three letters actually matched any given document?

Answer 1

I basically used the same approach as @L.B, but updated it for usage for the newest Lucene Version 7.4.0. Note: FuzzyQuery now supports .setRewriteMethod (that's why I removed the if).

I also included handling for BoostQuerys and saved the words that were found by Lucene in a HashSet to avoid duplicates instead of the Terms.

private void saveHitWordInList(Query query, IndexSearcher indexSearcher,
    int docId, HashSet hitWords) throws IOException {
  if (query instanceof TermQuery)
    if (indexSearcher.explain(query, docId).isMatch())
      hitWords.add(((TermQuery) query).getTerm().toString().split(":")[1]);
  if (query instanceof BooleanQuery) {
    for (BooleanClause clause : (BooleanQuery) query) {
      saveHitWordInList(clause.getQuery(), indexSearcher, docId, hitWords);
    }
  }

  if (query instanceof MultiTermQuery) {
    ((MultiTermQuery) query)
        .setRewriteMethod(MultiTermQuery.SCORING_BOOLEAN_REWRITE);
    saveHitWordInList(query.rewrite(indexSearcher.getIndexReader()),
        indexSearcher, docId, hitWords);
  }

  if (query instanceof BoostQuery)
    saveHitWordInList(((BoostQuery) query).getQuery(), indexSearcher, docId,
        hitWords);
}