Package org.apache.lucene.search

Class Collector

java.lang.Object
org.apache.lucene.search.Collector
Direct Known Subclasses:
CachingCollector, MultiCollector, PositiveScoresOnlyCollector, TimeLimitingCollector, TopDocsCollector, TotalHitCountCollector
public abstract class Collector extends Object

Expert: Collectors are primarily meant to be used to gather raw results from a search, and implement sorting or custom result filtering, collation, etc.

Lucene's core collectors are derived from Collector. Likely your application can use one of these classes, or subclass TopDocsCollector, instead of implementing Collector directly:

Collector decouples the score from the collected doc: the score computation is skipped entirely if it's not needed. Collectors that do need the score should implement the setScorer(org.apache.lucene.search.Scorer) method, to hold onto the passed Scorer instance, and call Scorer.score() within the collect method to compute the current hit's score. If your collector may request the score for a single hit multiple times, you should use ScoreCachingWrappingScorer.

NOTE: The doc that is passed to the collect method is relative to the current reader. If your collector needs to resolve this to the docID space of the Multi*Reader, you must re-base it by recording the docBase from the most recent setNextReader call. Here's a simple example showing how to collect docIDs into a BitSet:

 IndexSearcher searcher = new IndexSearcher(indexReader);
 final BitSet bits = new BitSet(indexReader.maxDoc());
 searcher.search(query, new Collector() {
   private int docBase;
 
   // ignore scorer
   public void setScorer(Scorer scorer) {
   }

   // accept docs out of order (for a BitSet it doesn't matter)
   public boolean acceptsDocsOutOfOrder() {
     return true;
   }
 
   public void collect(int doc) {
     bits.set(doc + docBase);
   }
 
   public void setNextReader(AtomicReaderContext context) {
     this.docBase = context.docBase;
   }
 });

Not all collectors will need to rebase the docID. For example, a collector that simply counts the total number of hits would skip it.

NOTE: Prior to 2.9, Lucene silently filtered out hits with score <= 0. As of 2.9, the core Collectors no longer do that. It's very unusual to have such hits (a negative query boost, or function query returning negative custom scores, could cause it to happen). If you need that behavior, use PositiveScoresOnlyCollector.

Since:
2.9

  • Constructor Details

    • Collector

      public Collector()

  • Method Details

    • setScorer

      public abstract void setScorer(Scorer scorer) throws IOException
      Called before successive calls to collect(int). Implementations that need the score of the current document (passed-in to collect(int)), should save the passed-in Scorer and call scorer.score() when needed.
      Throws:
      IOException

    • collect

      public abstract void collect(int doc) throws IOException
      Called once for every document matching a query, with the unbased document number.

      Note: The collection of the current segment can be terminated by throwing a CollectionTerminatedException. In this case, the last docs of the current AtomicReaderContext will be skipped and IndexSearcher will swallow the exception and continue collection with the next leaf.

      Note: This is called in an inner search loop. For good search performance, implementations of this method should not call IndexSearcher.doc(int) or IndexReader.document(int) on every hit. Doing so can slow searches by an order of magnitude or more.

      Throws:
      IOException

    • setNextReader

      public abstract void setNextReader(AtomicReaderContext context) throws IOException
      Called before collecting from each AtomicReaderContext. All doc ids in collect(int) will correspond to IndexReaderContext.reader(). Add AtomicReaderContext.docBase to the current IndexReaderContext.reader()'s internal document id to re-base ids in collect(int).
      Parameters:
      context - next atomic reader context
      Throws:
      IOException

    • acceptsDocsOutOfOrder

      public abstract boolean acceptsDocsOutOfOrder()
      Return true if this collector does not require the matching docIDs to be delivered in int sort order (smallest to largest) to collect(int).

      Most Lucene Query implementations will visit matching docIDs in order. However, some queries (currently limited to certain cases of BooleanQuery) can achieve faster searching if the Collector allows them to deliver the docIDs out of order.

      Many collectors don't mind getting docIDs out of order, so it's important to return true here.