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 <= 0.0Collector 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
.
Constructor and Description |
---|
Collector() |
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) . |
public abstract void setScorer(Scorer scorer) throws IOException
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.IOException
public abstract void collect(int doc) throws IOException
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.
IOException
public abstract void setNextReader(AtomicReaderContext context) throws IOException
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)
.context
- next atomic reader contextIOException
public abstract boolean acceptsDocsOutOfOrder()
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.
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