How to run self-defined function on Gatling?

Gatling (Open-Source)
1

Demo code:

val scn = scenario(“test”)
.exec(http(“host”)
.post("/rest/n/test")
.queryParam(“key”, “value”)
.check(jsonPath("$.fileKey").saveAs(“fileKey”)
)
.exec(
// I hope to use fileKey do something here
// call some Java function
)
.exec(http(“host2”)
.post("/rest/n/end")
.queryParam(“k”, “v”)
check(jsonPath("$.result").is(“OK”)
)

How should I do? thanks!