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本文主要研究一下Elasticsearch的CachedSupplier

CachedSupplier

elasticsearch-7.0.1/server/src/main/java/org/elasticsearch/common/util/CachedSupplier.java

public final class CachedSupplier
    
      implements Supplier
     
       {    private Supplier
      
        supplier;    private T result;    private boolean resultSet;    public CachedSupplier(Supplier
       
         supplier) {        this.supplier = supplier;    }    @Override    public synchronized T get() {        if (resultSet == false) {            result = supplier.get();            resultSet = true;        }        return result;    }}复制代码
       
      
     
    

• CachedSupplier实现了Supplier接口，它包装了一个supplier，其get方法只调用一次原始supplier的get方法并缓存其结果，下次调用直接返回缓存的结果

实例

elasticsearch-7.0.1/server/src/main/java/org/elasticsearch/search/query/TopDocsCollectorContext.java

abstract static class SimpleTopDocsCollectorContext extends TopDocsCollectorContext {        private static TopDocsCollector
     createCollector(@Nullable SortAndFormats sortAndFormats, int numHits,                @Nullable ScoreDoc searchAfter, int hitCountThreshold) {            if (sortAndFormats == null) {                return TopScoreDocCollector.create(numHits, searchAfter, hitCountThreshold);            } else {                return TopFieldCollector.create(sortAndFormats.sort, numHits, (FieldDoc) searchAfter, hitCountThreshold);            }        }        protected final @Nullable SortAndFormats sortAndFormats;        private final Collector collector;        private final Supplier
    
      totalHitsSupplier;        private final Supplier
     
       topDocsSupplier;        private final Supplier
      
        maxScoreSupplier;        /**         * Ctr         * @param reader The index reader         * @param query The Lucene query         * @param sortAndFormats The query sort         * @param numHits The number of top hits to retrieve         * @param searchAfter The doc this request should "search after"         * @param trackMaxScore True if max score should be tracked         * @param trackTotalHitsUpTo True if the total number of hits should be tracked         * @param hasFilterCollector True if the collector chain contains at least one collector that can filters document         */        private SimpleTopDocsCollectorContext(IndexReader reader,                                              Query query,                                              @Nullable SortAndFormats sortAndFormats,                                              @Nullable ScoreDoc searchAfter,                                              int numHits,                                              boolean trackMaxScore,                                              int trackTotalHitsUpTo,                                              boolean hasFilterCollector) throws IOException {            super(REASON_SEARCH_TOP_HITS, numHits);            this.sortAndFormats = sortAndFormats;            final TopDocsCollector
        topDocsCollector;            if (trackTotalHitsUpTo == SearchContext.TRACK_TOTAL_HITS_DISABLED) {                // don't compute hit counts via the collector                topDocsCollector = createCollector(sortAndFormats, numHits, searchAfter, 1);                topDocsSupplier = new CachedSupplier<>(topDocsCollector::topDocs);                totalHitsSupplier = () -> new TotalHits(0, TotalHits.Relation.GREATER_THAN_OR_EQUAL_TO);            } else {                // implicit total hit counts are valid only when there is no filter collector in the chain                final int hitCount = hasFilterCollector ? -1 : shortcutTotalHitCount(reader, query);                if (hitCount == -1) {                    topDocsCollector = createCollector(sortAndFormats, numHits, searchAfter, trackTotalHitsUpTo);                    topDocsSupplier = new CachedSupplier<>(topDocsCollector::topDocs);                    totalHitsSupplier = () -> topDocsSupplier.get().totalHits;                } else {                    // don't compute hit counts via the collector                    topDocsCollector = createCollector(sortAndFormats, numHits, searchAfter, 1);                    topDocsSupplier = new CachedSupplier<>(topDocsCollector::topDocs);                    totalHitsSupplier = () -> new TotalHits(hitCount, TotalHits.Relation.EQUAL_TO);                }            }            MaxScoreCollector maxScoreCollector = null;            if (sortAndFormats == null) {                maxScoreSupplier = () -> {                    TopDocs topDocs = topDocsSupplier.get();                    if (topDocs.scoreDocs.length == 0) {                        return Float.NaN;                    } else {                        return topDocs.scoreDocs[0].score;                    }                };            } else if (trackMaxScore) {                maxScoreCollector = new MaxScoreCollector();                maxScoreSupplier = maxScoreCollector::getMaxScore;            } else {                maxScoreSupplier = () -> Float.NaN;            }            this.collector = MultiCollector.wrap(topDocsCollector, maxScoreCollector);        }        @Override        Collector create(Collector in) {            assert in == null;            return collector;        }        TopDocsAndMaxScore newTopDocs() {            TopDocs in = topDocsSupplier.get();            float maxScore = maxScoreSupplier.get();            final TopDocs newTopDocs;            if (in instanceof TopFieldDocs) {                TopFieldDocs fieldDocs = (TopFieldDocs) in;                newTopDocs = new TopFieldDocs(totalHitsSupplier.get(), fieldDocs.scoreDocs, fieldDocs.fields);            } else {                newTopDocs = new TopDocs(totalHitsSupplier.get(), in.scoreDocs);            }            return new TopDocsAndMaxScore(newTopDocs, maxScore);        }        @Override        void postProcess(QuerySearchResult result) throws IOException {            final TopDocsAndMaxScore topDocs = newTopDocs();            result.topDocs(topDocs, sortAndFormats == null ? null : sortAndFormats.formats);        }    }复制代码
      
     
    

• SimpleTopDocsCollectorContext的构造器使用CachedSupplier创建了topDocsSupplier；之后newTopDocs方法会调用topDocsSupplier.get()来获取TopDocs

小结

CachedSupplier实现了Supplier接口，它包装了一个supplier，其get方法只调用一次原始supplier的get方法并缓存其结果，下次调用直接返回缓存的结果

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