MongoDB Java错误:管道阶段规范对象必须恰好包含一个字段

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

public class MongoDBPipelineExample {
    public static void main(String[] args) {
        // 连接到MongoDB数据库
        MongoClient mongoClient = MongoClients.create("mongodb://localhost:27017");
        MongoDatabase database = mongoClient.getDatabase("mydb");
        MongoCollection<Document> collection = database.getCollection("mycollection");

        // 构建管道操作
        List<Document> pipeline = new ArrayList<>();
        Document matchStage = new Document("$match", new Document("field", "value"));
        pipeline.add(matchStage);

        // 执行管道操作
        AggregateIterable<Document> result = collection.aggregate(pipeline);

        // 处理结果
        for (Document document : result) {
            System.out.println(document.toJson());
        }

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