how to make an auto generated id in spring boot code example

Example: without increment mvc spring

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.data.mongodb.core.FindAndModifyOptions;
import org.springframework.data.mongodb.core.MongoTemplate;
import org.springframework.data.mongodb.core.query.Criteria;
import org.springframework.data.mongodb.core.query.Query;
import org.springframework.data.mongodb.core.query.Update;
import org.springframework.stereotype.Service;
import sample.data.mongo.models.IdGenerator;
import sample.data.mongo.repository.IdGeneratorRepository;

@Service
public class IdGeneratorService {

  @Autowired
  private IdGeneratorRepository idGeneratorRepository;

  @Autowired
  private MongoTemplate mongoTemplate;

  public long generateId(String key) {

    Query query = new Query();
    // key = identified_by_Id;
    Criteria criteria = new Criteria("identifier").is(key);
    query.addCriteria(criteria);

    Update update = new Update();
    update.inc("counter", 1);

    FindAndModifyOptions options = new FindAndModifyOptions();
    options.upsert(true);
    options.returnNew(true);

    IdGenerator idGenerator = mongoTemplate
        .findAndModify(query, update, options, IdGenerator.class);

    return idGenerator.getCounter();
  }
}