Access session value in gatling checks - load-testing

Question

I use gatling to send data to an ActiveMQ. The payload is generated in a separate method. The response should also be validated. However, how can I access the session data within the checks
check(bodyString.is()) or simpleCheck(...)? I have also thought about storing the current payload in a separate global variable, but I don't know if this is the right approach. My code's setup looks like this at the moment:
val scn = scenario("Example ActiveMQ Scenario")
.exec(jms("Test").requestReply
.queue(...)
.textMessage{ session => val message = createPayload(); session.set("payload", payload); message}
.check(simpleCheck{message => customCheck(message, ?????? )})) //access stored payload value, alternative: check(bodystring.is(?????)
def customCheck(m: Message, string: String) = {
// check logic goes here
}

Answer 1

Disclaimer: providing example in Java as you don't seem to be a Scala developper, so Java would be a better fit for you (supported since Gatling 3.7).
The way you want to do things can't possibly work.
.textMessage(session -> {
String message = createPayload();
session.set("payload", payload);
return message;
}
)
As explained in the documentation, Session is immutable, so in a function that's supposed to return the payload, you can't also return a new Session.
What you would have to do it first store the payload in the session, then fetch it:
.exec(session -> session.set("payload", createPayload()))
...
.textMessage("#{payload}")
Regarding writing your check, simpleCheck doesn't have access to the Session. You have to use check(bodyString.is()) and pass a function to is, again as explained in the documentation.