Fragment.create() requires mutable map - which is unusable when used with Kotlin

[huberchrigu]

I use Fragment and FragmentsRendering in Kotlin. If I call something like that:

FragmentsRendering
    .with("myView", mapOf("modelParam" to "modelValue"))
    .build()

I get following error:

java.lang.UnsupportedOperationException: null
at java.base/java.util.Collections$SingletonMap.putIfAbsent(Collections.java:5291) ~[na:na]
Suppressed: reactor.core.publisher.FluxOnAssembly$OnAssemblyException:
Error has been observed at the following site(s):
*__checkpoint ⇢ Handler MyController#getRequest(Id, Continuation) [DispatcherHandler]
Original Stack Trace:
at java.base/java.util.Collections$SingletonMap.putIfAbsent(Collections.java:5291) ~[na:na]
at org.springframework.web.reactive.result.view.Fragment.lambda$mergeAttributes$0(Fragment.java:97) ~[spring-webflux-6.2.5.jar:6.2.5]
at java.base/java.util.concurrent.ConcurrentHashMap.forEach(ConcurrentHashMap.java:1603) ~[na:na]
at org.springframework.web.reactive.result.view.Fragment.mergeAttributes(Fragment.java:97) ~[spring-webflux-6.2.5.jar:6.2.5]
at org.springframework.web.reactive.result.view.ViewResolutionResultHandler.renderFragment(ViewResolutionResultHandler.java:378) ~[spring-webflux-6.2.5.jar:6.2.5]
at org.springframework.web.reactive.result.view.ViewResolutionResultHandler.lambda$handleResult$0(ViewResolutionResultHandler.java:298) ~[spring-webflux-6.2.5.jar:6.2.5]
...

I found that Spring Webflux tries to add other parameters (in my case from a @ControllerAdvice) to the map. I have to migrate my model to a mutable map like

FragmentsRendering
    .with("myView", mapOf("modelParam" to "modelValue").toMutableMap() as Map<String, Any>)
    .build()

It would be nice if Spring Webflux could ensure that the map is mutable. Or if the builder is similar to Rendering and keeps its model map internal. Or at the very least, this contract should be documented at every function that accepts the model map.

[huberchrigu]

Another option would be to provide a Kotlin extension for building fragments. There it could be handled specifically for immutable maps.

[huberchrigu]

Wow, that was quick! Thanks a lot!