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:

• TopDocsCollector is an abstract base class that assumes you will retrieve the top N docs, according to some criteria, after collection is done.
• TopScoreDocCollector is a concrete subclass TopDocsCollector and sorts according to score + docID. This is used internally by the IndexSearcher search methods that do not take an explicit Sort. It is likely the most frequently used collector.
• TopFieldCollector subclasses TopDocsCollector and sorts according to a specified Sort object (sort by field). This is used internally by the IndexSearcher search methods that take an explicit Sort.
• TimeLimitingCollector, which wraps any other Collector and aborts the search if it's taken too much time.
• PositiveScoresOnlyCollector wraps any other Collector and prevents collection of hits whose score is <= 0.0

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

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

Constructor Summary

Constructors

Constructor and Description
Collector()

Method Summary

Methods

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

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

Collector

public Collector()

Method Detail

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.