如何在spring boot中使用mongodb实现group这样的分组统计查询呢？
比如：统计中国每个省的同名人数,数据内容如下：

{ "country" : "china", "province" : "sh", "userid" : "a" }  
{  "country" : "china", "province" : "sh", "userid" : "b" } 
{  "country" : "china", "province" : "sh", "userid" : "c" }  
{  "country" : "china", "province" : "bj", "userid" : "da" }  
{  "country" : "china", "province" : "bj", "userid" : "fa" }  

统计结果为：[{"country" : "china", "province" : "sh", "count" : 3 },{"country" : "china", "province" : "bj", "count" : 2 }]
使用aggregate([{ $group: {"_id": { "country" : "$country", "province": "$province" , "uid" : "$userid" } } } , { $group: {"_id": { "country" : "$_id.country", "province": "$_id.province" }, count : { $sum : 1 } } }]) mongdb正确获取到数据，那怎么使用java 在springboot中实现呢？