在Lucene中，可以通过使用Facets模块来实现分组统计。Facets模块提供了FacetField和FacetResult类来支持分组统计操作。
下面是一个简单的示例代码，演示了如何使用Facets模块来实现分组统计：
import org.apache.lucene.analysis.standard.StandardAnalyzer;
import org.apache.lucene.document.Document;
import org.apache.lucene.document.Field;
import org.apache.lucene.document.StringField;
import org.apache.lucene.facet.FacetField;
import org.apache.lucene.facet.Facets;
import org.apache.lucene.facet.FacetsCollector;
import org.apache.lucene.facet.FacetsConfig;
import org.apache.lucene.facet.LabelAndValue;
import org.apache.lucene.facet.sortedset.DefaultSortedSetDocValuesReaderState;
import org.apache.lucene.facet.sortedset.SortedSetDocValuesFacetCounts;
import org.apache.lucene.facet.sortedset.SortedSetDocValuesFacetField;
import org.apache.lucene.facet.sortedset.SortedSetDocValuesReaderState;
import org.apache.lucene.facet.taxonomy.TaxonomyFacetSumValueSource;
import org.apache.lucene.facet.taxonomy.directory.DirectoryTaxonomyReader;
import org.apache.lucene.facet.taxonomy.directory.DirectoryTaxonomyWriter;
import org.apache.lucene.facet.taxonomy.directory.NRTCachingDirectoryTaxonomyWriter;
import org.apache.lucene.facet.taxonomy.directory.OrdinalPolicy;
import org.apache.lucene.facet.taxonomy.directory.OrdinalPolicy.Indexer;
import org.apache.lucene.index.IndexWriter;
import org.apache.lucene.index.IndexWriterConfig;
import org.apache.lucene.index.Term;
import org.apache.lucene.index.IndexWriterConfig.OpenMode;
import org.apache.lucene.search.IndexSearcher;
import org.apache.lucene.search.MatchAllDocsQuery;
import org.apache.lucene.search.Query;
import org.apache.lucene.search.Sort;
import org.apache.lucene.search.SortField;
import org.apache.lucene.search.TopDocs;
import org.apache.lucene.store.Directory;
import org.apache.lucene.store.FSDirectory;
import org.apache.lucene.util.BytesRef;
import java.io.IOException;
import java.nio.file.Paths;
import java.util.HashMap;
import java.util.Map;
public class LuceneGroupByDemo {
public static void main(String[] args) throws IOException {
// 创建索引和分类目录
Directory indexDir = FSDirectory.open(Paths.get("index"));
Directory taxoDir = FSDirectory.open(Paths.get("taxonomy"));
// 配置索引和分类写入器
IndexWriterConfig indexWriterConfig = new IndexWriterConfig(new StandardAnalyzer());
indexWriterConfig.setOpenMode(OpenMode.CREATE_OR_APPEND);
IndexWriter indexWriter = new IndexWriter(indexDir, indexWriterConfig);
DirectoryTaxonomyWriter taxoWriter = new NRTCachingDirectoryTaxonomyWriter(taxoDir);
// 创建分类索引
OrdinalPolicy ordinalPolicy = new OrdinalPolicy.DirectPolicy();
SortedSetDocValuesReaderState state = new DefaultSortedSetDocValuesReaderState(indexWriter.getReader(), ordinalPolicy);
SortedSetDocValuesFacetField field = new SortedSetDocValuesFacetField("category", "Books", "Children's");
indexWriter.addDocument(state.facetDocValuesField(field));
field = new SortedSetDocValuesFacetField("category", "Books", "Fiction");
indexWriter.addDocument(state.facetDocValuesField(field));
field = new SortedSetDocValuesFacetField("category", "Books", "Non-fiction");
indexWriter.addDocument(state.facetDocValuesField(field));
indexWriter.commit();
// 创建分类读取器和FacetsConfig
DirectoryTaxonomyReader taxoReader = new DirectoryTaxonomyReader(taxoWriter);
FacetsConfig config = new FacetsConfig();
// 创建FacetsCollector和Facets
FacetsCollector facetsCollector = new FacetsCollector();
IndexSearcher searcher = new IndexSearcher(indexWriter.getReader());
// 执行查询
Query query = new MatchAllDocsQuery();
searcher.search(query, facetsCollector);
Facets facets = new SortedSetDocValuesFacetCounts(state, facetsCollector);
// 获取分组统计结果
String[] categories = {"Books"};
Map<String, Integer> categoryCounts = new HashMap<>();
for (String category : categories) {
FacetResult facetResult = facets.getTopChildren(10, category);
for (LabelAndValue labelAndValue : facetResult.labelValues) {
categoryCounts.put(labelAndValue.label, (int) labelAndValue.value);
}
}
// 打印分组统计结果
for (Map.Entry<String, Integer> entry : categoryCounts.entrySet()) {
System.out.println(entry.getKey() + ": " + entry.getValue());
}
// 关闭资源
indexWriter.close();
taxoWriter.close();
taxo