org.apache.lucene.search

## Class CachingCollector

- java.lang.Object
- - org.apache.lucene.search.FilterCollector
  - - org.apache.lucene.search.CachingCollector

- All Implemented Interfaces:
    Collector

      public abstract class CachingCollector
      extends FilterCollector

  Caches all docs, and optionally also scores, coming from a search, and is then able to replay them to another collector. You specify the max RAM this class may use. Once the collection is done, call `isCached()`. If this returns true, you can use `replay(Collector)` against a new collector. If it returns false, this means too much RAM was required and you must instead re-run the original search.

  NOTE: this class consumes 4 (or 8 bytes, if scoring is cached) per collected document. If the result set is large this can easily be a very substantial amount of RAM!

  See the Lucene `modules/grouping` module for more details including a full code example.

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

- - ### Field Summary

    - ### Fields inherited from class org.apache.lucene.search.FilterCollector

      `in`

  - ### Method Summary

    All Methods Static Methods Instance Methods Abstract Methods Concrete Methods
    Modifier and Type | Method and Description
    --- | ---
    `static CachingCollector` | `create(boolean cacheScores, double maxRAMMB)` Creates a `CachingCollector` which does not wrap another collector.
    `static CachingCollector` | `create(Collector other, boolean cacheScores, double maxRAMMB)` Create a new `CachingCollector` that wraps the given collector and caches documents and scores up to the specified RAM threshold.
    `static CachingCollector` | `create(Collector other, boolean cacheScores, int maxDocsToCache)` Create a new `CachingCollector` that wraps the given collector and caches documents and scores up to the specified max docs threshold.
    `boolean` | `isCached()` Return true is this collector is able to replay collection.
    `abstract void` | `replay(Collector other)` Replays the cached doc IDs (and scores) to the given Collector.

    - ### Methods inherited from class org.apache.lucene.search.FilterCollector

      `getLeafCollector, needsScores, toString`

    - ### Methods inherited from class java.lang.Object

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

- - ### Method Detail

    - #### create

          public static CachingCollector create(boolean cacheScores,
                                                double maxRAMMB)

      Creates a `CachingCollector` which does not wrap another collector. The cached documents and scores can later be `replayed`.

    - #### create

          public static CachingCollector create(Collector other,
                                                boolean cacheScores,
                                                double maxRAMMB)

      Create a new `CachingCollector` that wraps the given collector and caches documents and scores up to the specified RAM threshold.
      Parameters:
        `other` - the Collector to wrap and delegate calls to.
        `cacheScores` - whether to cache scores in addition to document IDs. Note that this increases the RAM consumed per doc
        `maxRAMMB` - the maximum RAM in MB to consume for caching the documents and scores. If the collector exceeds the threshold, no documents and scores are cached.

    - #### create

          public static CachingCollector create(Collector other,
                                                boolean cacheScores,
                                                int maxDocsToCache)

      Create a new `CachingCollector` that wraps the given collector and caches documents and scores up to the specified max docs threshold.
      Parameters:
        `other` - the Collector to wrap and delegate calls to.
        `cacheScores` - whether to cache scores in addition to document IDs. Note that this increases the RAM consumed per doc
        `maxDocsToCache` - the maximum number of documents for caching the documents and possible the scores. If the collector exceeds the threshold, no documents and scores are cached.

    - #### isCached

          public final boolean isCached()

      Return true is this collector is able to replay collection.

    - #### replay

          public abstract void replay(Collector other)
                               throws IOException

      Replays the cached doc IDs (and scores) to the given Collector. If this instance does not cache scores, then Scorer is not set on `other.setScorer` as well as scores are not replayed.
      Throws:
        `IllegalStateException` - if this collector is not cached (i.e., if the RAM limits were too low for the number of documents + scores to cache).
        `IllegalArgumentException` - if the given Collect's does not support out-of-order collection, while the collector passed to the ctor does.
        `IOException`

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