org.apache.lucene.search

Interface LeafCollector

All Known Implementing Classes:

FilterLeafCollector, SimpleCollector, TopDocsCollector, TopFieldCollector, TopScoreDocCollector, TotalHitCountCollector

public interface LeafCollector

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() {

   public LeafCollector getLeafCollector(AtomicReaderContext context)
       throws IOException {
     final int docBase = context.docBase;
     return new LeafCollector() {

       // ignore scorer
       public void setScorer(Scorer scorer) throws IOException {
       }

       public void collect(int doc) throws IOException {
         bits.set(docBase + doc);
       }

       // accept docs out of order (for a BitSet it doesn't matter)
       public boolean acceptsDocsOutOfOrder() {
         return true;
       }
          
     };
   }
      
 });
 

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

WARNING: This API is experimental and might change in incompatible ways in the next release.

Method Summary

Methods

Modifier and Type	Method and Description
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).
void	collect(int doc) Called once for every document matching a query, with the unbased document number.
void	setScorer(Scorer scorer) Called before successive calls to collect(int).

Method Detail

setScorer

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

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

acceptsDocsOutOfOrder

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.