https://github.com/spring-projects/spring-framework/blob/5b1ab31559798df83f1e8d54d2b754f12c69c14e/spring-webmvc/src/main/kotlin/org/springframework/web/servlet/function/RouterFunctionDsl.kt#L650-L655

https://github.com/spring-projects/spring-framework/blob/5b1ab31559798df83f1e8d54d2b754f12c69c14e/spring-webflux/src/main/kotlin/org/springframework/web/reactive/function/server/CoRouterFunctionDsl.kt#L531-L539

  1. Function filterFunction takes serverRequest and function handler as parameters. (I just name it second parameter of filterFunction as handler.)
  2. Function handler also takes serverRequest as parameter.
  3. These DSLs define handler.
  4. Function handlers that these DSLs defined are now using mistakenly filterFunction's serverRequest, not their own serverRequest parameters, which makes their own parameters meaningless.

This bug only exists in Kotlin DSL.

You can see java codes have no problem as follows.

https://github.com/spring-projects/spring-framework/blob/01e50fb60ad0d82f96fe4df6054ed6029e2f6a12/spring-webmvc/src/main/java/org/springframework/web/servlet/function/HandlerFilterFunction.java#L46

https://github.com/spring-projects/spring-framework/blob/01e50fb60ad0d82f96fe4df6054ed6029e2f6a12/spring-webflux/src/main/java/org/springframework/web/reactive/function/server/HandlerFilterFunction.java#L48

In java you can override filter something like

ServerRequest newRequest = someOperation(request)
return next.handle(newRequest)

but in Kotlin DSL, even if you write 

filter { serverRequest, handlerFunction ->
    val newServerRequest = someOperation(serverRequest)
    handlerFunction(newServerRequest)
}

handlerFunction will use original serverRequest, not the newServerRequest

Comment From: sdeleuze

Good catch, to be backported on 5.2.x as well. I will be able to merge the proposed fix with additional tests asked here, thanks.

Comment From: ShindongLee

@sdeleuze I added tests. Thank you !