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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<T> implements Supplier<T> {private Supplier<T> supplier;private T result;private boolean resultSet;public CachedSupplier(Supplier<T> supplier) {this.supplier = supplier;}@Overridepublic synchronized T get() {if (resultSet == false) {result = supplier.get();resultSet = true;}return result;}}
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• 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<TotalHits> totalHitsSupplier;private final Supplier<TopDocs> topDocsSupplier;private final Supplier<Float> 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 collectortopDocsCollector = 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 chainfinal 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 collectortopDocsCollector = 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);}@OverrideCollector 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);}@Overridevoid postProcess(QuerySearchResult result) throws IOException {final TopDocsAndMaxScore topDocs = newTopDocs();result.topDocs(topDocs, sortAndFormats == null ? null : sortAndFormats.formats);}}
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• 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

doc

• CachedSupplier