JMeter BeanShell Assertion: Getting error when convert String to Long

1. Inlining JMeter variables into code-based scripts is not something recommended so go for 2nd approach.

2. How do you know that exactly String is being returned from the database all the time? It easily can be any other object type, in fact any of described in the Mapping SQL and Java Types article. The way more safe approach will be something like:

   if (vars.getObject("balance_after_credit_from_db") instanceof String) {
       long balance_after_credit = Long.parseLong(vars.get("balance_after_credit_from_db"));
   } 
   else {
       log.error("Unexpected \balance_after_credit_from_db\" variable type");
       log.error("Expected: String, Actual: " + vars.getObject("balance_after_credit_from_db").getClass().getName());
       throw new Exception("Unexpected variable type");
   }
   

   So in case of non-String JDBC query result you will be able to see the relevant message in jmeter.log file

   

See Debugging JDBC Sampler Results in JMeter article for more information on working with the entities coming from databases in JMeter tests