首先，我们在数据库中，mongodb的聚合查询是这样写。

db.getCollection('parking_record').aggregate(

{$match : {"appId" : "2e1800b22ae70600", "leaveTime" : {"$gt" : ISODate("2017-07-12T00:00:00"), "$lt" : ISODate("2017-07-13T00:00:00")}}},

{$group : {"_id" : "$leaveMethod", "count" : {$sum : 1}}},

{$sort : {"_id" : 1}}

)

在java类中，应该怎样呢？这是我写的其中一个方法。

(首先要导入mongodb的java驱动包mongo-java-driver-3.2.2.jar)

/**

* 根据日期统计离场方式

* @param app_id 插件ID

* @param beginDate 开始日期

* @param endDate 结束日期

* @return {"ManualLeave":2,"AutoLeave":4}

* @throws Exception

*/

public String aggregateLeaveMethodByDate(String app_id, Date beginDate, Date endDate) throws Exception {

MongoCollection collection = PluginMongo.instance().getDatabase().getCollection(MongoCollectionName.PARKING_RECORD);

Document sub_match = new Document();

sub_match.put("appId", app_id);

sub_match.put("leaveTime", new Document("$gt", beginDate).append("$lt", endDate));

Document sub_group = new Document();

sub_group.put("_id", "$leaveMethod");

sub_group.put("count", new Document("$sum", 1));

Document match = new Document("$match", sub_match);

Document group = new Document("$group", sub_group);

Document sort = new Document("$sort", new Document("_id", 1));

List aggregateList = new ArrayList();

aggregateList.add(match);

aggregateList.add(group);

aggregateList.add(sort);

JSONObject ret_obj = new JSONObject();

AggregateIterable resultset = collection.aggregate(aggregateList);

MongoCursor cursor = resultset.iterator();

try {

while(cursor.hasNext()) {

Document item_doc = cursor.next();

int leaveMethod = item_doc.getInteger("_id", 0);

int count = item_doc.getInteger("count", 0);

LeaveMethodEnum leaveMethodVal = LeaveMethodEnum.fromType(leaveMethod);

ret_obj.put(leaveMethodVal.name(), count);

}

} finally {

cursor.close();

}

return ret_obj.toJSONString();

}上面的只有matche，group等几个常用，project，limit等类似，可以参考上面的。

aggregate的相关sql知识可以参考菜鸟教程：http://www.runoob.com/mongodb/mongodb-aggregate.html