Java代码的MongoDB聚合命令

import com.mongodb.MongoClient;
import com.mongodb.client.MongoCollection;
import com.mongodb.client.MongoDatabase;
import org.bson.Document;

import java.util.Arrays;

public class MongoDBAggregationExample {
    public static void main(String[] args) {
        // 连接到MongoDB数据库
        MongoClient mongoClient = new MongoClient("localhost", 27017);
        MongoDatabase database = mongoClient.getDatabase("mydb");

        // 获取要进行聚合操作的集合
        MongoCollection<Document> collection = database.getCollection("mycollection");

        // 构建聚合命令
        Document matchStage = new Document("$match", new Document("age", new Document("$gte", 18)));
        Document groupStage = new Document("$group", new Document("_id", "$gender").append("count", new Document("$sum", 1)));
        Document sortStage = new Document("$sort", new Document("count", -1));
        Document limitStage = new Document("$limit", 5);

        // 执行聚合命令
        collection.aggregate(Arrays.asList(matchStage, groupStage, sortStage, limitStage))
                .forEach(document -> System.out.println(document.toJson()));

        // 关闭MongoDB连接
        mongoClient.close();
    }
}