error when using method calling db connection

hi am trying to connect to my db in java am having this error
my db is mongo
this is the code

public DBObject findByUsername(String username){
		 // DBCollection dbCollection = mongo.getDB("system").getCollection("users");
		  DBCollection dbCollection =  ConnectToDB.getDB("system").getCollection("users");
		  DBObject criteria = new BasicDBObject("username", username);
		  return dbCollection.findOne(criteria); 
		}

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my connection class is
public class ConnectToDB {
      

	private DB db = null;
	private Mongo mongoClient = null;
	private Properties properties = new Properties();
        private InputStream input = null;
        
	public ConnectToDB() {
	
		getDB();
		
	}
	
	
	private boolean connectToDB() throws Exception{
		input = new FileInputStream("dsnApp.properties");
                properties.load(input);
                
		  this.mongoClient = new Mongo(properties.getProperty("mongoDBipAddress"), Integer.valueOf(properties.getProperty("mongoDBport")));
                  
		  this.db = mongoClient.getDB("dsn");
		  		  
		  if(db != null)
		  {
			  return true;
		  }
		     
		 return false;
	}
	
	
	public DB getDB() {
		
		try {
			if(connectToDB())
			{
				return db;
			}
			else
			{
				connectToDB();
				return db;
			}
		} catch (Exception e) {
			
			System.out.println("Something went wrong while trying to connect to the database..");
			e.printStackTrace();
			return null;
		}
	}
  
}

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