spring boot查询，仅从子文档数组中的一个字段获取最大值

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.data.mongodb.core.MongoTemplate;
import org.springframework.data.mongodb.core.aggregation.Aggregation;
import org.springframework.data.mongodb.core.aggregation.AggregationResults;
import org.springframework.data.mongodb.core.aggregation.GroupOperation;
import org.springframework.data.mongodb.core.aggregation.TypedAggregation;
import org.springframework.data.mongodb.core.query.Criteria;
import org.springframework.stereotype.Service;

import java.util.List;

@Service
public class MyService {

    private final MongoTemplate mongoTemplate;

    @Autowired
    public MyService(MongoTemplate mongoTemplate) {
        this.mongoTemplate = mongoTemplate;
    }

    public Integer getMaxValueFromSubdocumentField() {
        GroupOperation groupOperation = Aggregation.group().max("subdocumentField").as("maxValue");
        TypedAggregation<MyDocument> aggregation = Aggregation.newAggregation(MyDocument.class, groupOperation);

        AggregationResults<Result> result = mongoTemplate.aggregate(aggregation, Result.class);
        List<Result> resultList = result.getMappedResults();

        if (!resultList.isEmpty()) {
            return resultList.get(0).getMaxValue();
        }

        return null;
    }

    private static class Result {
        private Integer maxValue;

        public Integer getMaxValue() {
            return maxValue;
        }

        public void setMaxValue(Integer maxValue) {
            this.maxValue = maxValue;
        }
    }
}