How to count occurances of special character in a string in mongodb using springboot?

In a mongo collection there are around 120k documents. For each document I have to check how many occurrences of special character '@' is in message field.

Below is the document in json format

[{
"_id":356,
"message":"ty@78i@gh8"
}]

If it contains more than 2 @. I will perform some java function. I tried the below solution --

db.test.aggregate( [
  { 
      $project: { 
          _id: 0, 
          message: 1, 
          count: { 
              $subtract: [ 
                  { $size: { $split: [ "$message", "@" ] } }, 1 
              ] 
          } 
      } 
  }
] )

Java equavilent code --

DBObject project = new BasicDBObject("_id",0).append(message, 1)
          .append(count, new BasicDBObject("$submit", new BasicDBObject("size", 
           new BasicDBObject("$split", "$messgae").append(@))));
      

Java equivalent code is actually incomplete because I don't know how can I aggregate and then traverse for each document.