Error thrown in custom handler fails

[Sam-Kruglov]

If my handler throws an error, then DataFetcherHandlerMethod (line 239) returns Mono.error(..), and then AnnotatedControllerExceptionResolver (line 241) tries to convert it to Mono again, which gives ClassCastException

spring-graphql/spring-graphql/src/main/java/org/springframework/graphql/data/method/annotation/support/AnnotatedControllerExceptionResolver.java

Lines 239 to 241 in 381c9f3

	Object result = exceptionHandler.invoke(env, arguments);
	Mono<List<GraphQLError>> errorsMono = methodReturnValueAdapter.adapt(result, exception);

Specifically, in my case I see I threw an error from my handler @GraphQlExceptionHandler fun handle(...): List<GraphQLError> {...} that ended up here:

doInvoke:129, InvocableHandlerMethodSupport (org.springframework.graphql.data.method)
validateAndInvoke:142, DataFetcherHandlerMethod (org.springframework.graphql.data.method.annotation.support)
invoke:125, DataFetcherHandlerMethod (org.springframework.graphql.data.method.annotation.support)
invokeExceptionHandler:239, AnnotatedControllerExceptionResolver (org.springframework.graphql.data.method.annotation.support)
resolveException:211, AnnotatedControllerExceptionResolver (org.springframework.graphql.data.method.annotation.support)

After that, the Mono.error with my exception is attempted to be cast to the return type of my handler here:

spring-graphql/spring-graphql/src/main/java/org/springframework/graphql/data/method/annotation/support/AnnotatedControllerExceptionResolver.java

Lines 444 to 449 in 381c9f3

	ReturnValueAdapter forCollection = (result, returnType, ex) ->
	(result != null) ?
	Mono.just((result instanceof List) ?
	(List<GraphQLError>) result :
	new ArrayList<>((Collection<GraphQLError>) result)) :
	Mono.empty();

Line 446 evaluates to false because it is instance of Mono and it tried to cast it to Collection<GraphQLError> and then fails.

And finally, I don't even get to see my actual error that got thrown in my exception handler in the logs, all I can see are these:

42:57.187 WARN --- [xec-7] a.s.AnnotatedControllerExceptionResolver: Failure while handling exception with *handler signature*
java.lang.ClassCastException...
42:57.189 WARN --- [xec-7] a.s.AnnotatedControllerExceptionResolver: Failure while handling exception with *handler signature*
java.lang.ClassCastException... --- why print the same log AGAIN?
42:57.190 WARN  --- [xec-7] s.g.e.ExceptionResolversExceptionHandler: Failure while resolving *resolver signature*
jakarta.validation.ConstraintViolationException... ---- error that got thrown in the code and that I was trying to handle

[bclozel]

Thanks for the analysis but you are one step ahead of us. Can you show a minimal error handler code snippet that would trigger this so we can have a look?

[Sam-Kruglov]

Just throw any error. I believe it doesn't matter how you get to the exception handler, only matters that the handler itself throws an error.

@GraphQlExceptionHandler
fun handle(ex: ConstraintViolationException, env: DataFetchingEnvironment): List<GraphQLError> {
   throw IllegalStateException("oops")
}