title: Dynamic entry while posting a json body in jmeter
body: <p>I been trying to write jmeter load test for a SAML secured web-service. So for i have a http request sampler which gets the access code and stores in a variable named access_code. But the web-service accepts post request in the form:</p>
  
  <p>api.service.edu/api/authentication with body data  as { "code":"${access_code}","redirect_uri":"some site"} .</p>
  
  <p>but whenever I tried running the jmeter , my sampler gives the following error :</p>
  
  <p>Thread Name: Basic App Usage Flow 1-1
  Sample Start: 2018-11-07 21:08:50 EST
  Load time: 1209
  Connect Time: 0
  Latency: 1208
  Size in bytes: 370
  Sent bytes:0
  Headers size in bytes: 324
  Body size in bytes: 46
  Sample Count: 1
  Error Count: 1
  Data type ("text"|"bin"|""): text
  Response code: 500
  Response message: Internal Server Error</p>
  
  <p>HTTPSampleResult fields:
  ContentType: application/json; charset=utf-8
  DataEncoding: utf-8</p>
  
  <p>is it because of the way I am parsing the access_code? if so how can I parse a dynamic value via a json post request?.</p>