Vars.get returns a null value - groovy

Question

def signValue = '${signature_value}.${timestamp}.${signature_value}'
def token_secret = '${APP_CLIENT_SECRET}'
log.info("token is " + signValue)
def signingKey = new javax.crypto.spec.SecretKeySpec(signValue.getBytes(),"HmacSHA256");
def mac = javax.crypto.Mac.getInstance("HmacSHA256")
mac.init(signingKey);
def hmac = mac.doFinal(token_secret.getBytes());
def result = hmac.encodeBase64().toString()
---- I want to use the above "result" variable into a Http sampler request body------
---- I tried many possible ways but I end up is getting value as null or some error--
//${__groovy(vars.get("result"))}
//vars.put("signature", vars.get(result))
I've been trying to extract the value of the variable "result" and use it in HTTP sampler results. But I ended up getting a null value or some other error. Anyone could help me to sort out this problem.
Thanks!

Answer 1

I don't get the point, result should be a String object, is it not what you are expecting to have?
Anyway, signValue and token_secret perhaps could be different from your expectations: using single quotes instead of double quotes you are not using GStrings (e.g. the value of token_secret will be always exactly '${APP_CLIENT_SECRET}', regardless of the value of APP_CLIENT_SECRET)

Answer 2

Change this line:
vars.put("signature", vars.get(result))
to this:
vars.put("signature", result)
Also don't inline JMeter variables into Groovy scripts like this:
def token_secret = '${APP_CLIENT_SECRET}'
use vars shorthand instead:
def token_secret = vars.get('APP_CLIENT_SECRET')
Because:
It conflicts with Groovy GStrings
For best performance it's recommended to cache compiled scripts (enabled by default)
and in this mode JMeter resolves only first value and caches it which means you will get the same value for every iteration
More information:
JSR223 Sampler Documentation
Apache Groovy - Why and How You Should Use It

Answer 3

Put vars the result:
vars.put("signature", hmac.encodeBase64().toString());
And in HTTP use ${signature}