Class Collector
- Direct Known Subclasses:
CachingCollector
,MultiCollector
,PositiveScoresOnlyCollector
,TimeLimitingCollector
,TopDocsCollector
,TotalHitCountCollector
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 subclassTopDocsCollector
and sorts according to score + docID. This is used internally by theIndexSearcher
search methods that do not take an explicitSort
. It is likely the most frequently used collector.TopFieldCollector
subclassesTopDocsCollector
and sorts according to a specifiedSort
object (sort by field). This is used internally by theIndexSearcher
search methods that take an explicitSort
.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
-
Constructor Summary
Constructors -
Method Summary
Modifier and TypeMethodDescriptionabstract boolean
Returntrue
if this collector does not require the matching docIDs to be delivered in int sort order (smallest to largest) tocollect(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 eachAtomicReaderContext
.abstract void
Called before successive calls tocollect(int)
.
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Constructor Details
-
Collector
public Collector()
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Method Details
-
setScorer
Called before successive calls tocollect(int)
. Implementations that need the score of the current document (passed-in tocollect(int)
), should save the passed-in Scorer and call scorer.score() when needed.- Throws:
IOException
-
collect
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 currentAtomicReaderContext
will be skipped andIndexSearcher
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)
orIndexReader.document(int)
on every hit. Doing so can slow searches by an order of magnitude or more.- Throws:
IOException
-
setNextReader
Called before collecting from eachAtomicReaderContext
. All doc ids incollect(int)
will correspond toIndexReaderContext.reader()
. AddAtomicReaderContext.docBase
to the currentIndexReaderContext.reader()
's internal document id to re-base ids incollect(int)
.- Parameters:
context
- next atomic reader context- Throws:
IOException
-
acceptsDocsOutOfOrder
public abstract boolean acceptsDocsOutOfOrder()Returntrue
if this collector does not require the matching docIDs to be delivered in int sort order (smallest to largest) tocollect(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 theCollector
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.
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