Comma-delimited request params not working with Kotlin data class - spring-boot

Question

I have an endpoint which can accept dozens of request params. It would be nice to collect all these params in a data class. Additionally, the request params must be separated by commas
path?param=1,2
to keep backward compatibility.
Assume we have this endpoint:
#GetMapping("path")
fun someFun(#RequestParam param: Set<Int> = emptySet()
...other 11 params
)
I created a data class to collect all the request params:
data class ClusteredParams(val param: Set<Int> = emptySet()
...other 11 params
)
So the endpoint looks like the following:
#GetMapping("path")
fun someFun(param: ClusteredParams)
When I called path?param=1,2 I got:
"error": "Failed to convert value of type java.lang.String[] to required type java.util.Set; nested exception is java.lang.NumberFormatException: For input string: "1,2"",
"field": "param",
"rejectedValue": "1,2"
When I call path?param=1&param=2 everything is fine. This problem does not exist when ClusteredParams class is written in Java.
public class ClusteredParams {
private Set<Int> param;
...other 11 params
getters and setters
}

Answer 1

I don't know the answer, but I did the comparison what works and when:
in Java:
You can either call ?path=1,2 and ?path=1&path=2 no matter if your request param is declared using #RequestParam path: List<Int> inside signature of controller method, or it is a field of external object that's used to group all request params of controller method.
In Kotlin:
if you declare your request param inside of controller method signature, as a #RequestParam path: List<Int> or whatever else, you can call it using both methods, so:
?path=1,2 and ?path=1&path=2
If you however extract it to separate data class, and then expect this class to be populated in controller method, you can only do the ?path=1&path=2, because in the case of ?path=1,2 you get:
org.springframework.validation.BindException because it tries to parse entire '1,2' instead of split it first.
I hope now it is more clear, does anyone have idea why this happens?

Answer 2

Instead of val use var inside data class body:
data class ClusteredParams(
...other 11 params
) {
var param: Set<Int> = emptySet()
}