Spring Messaging batch payload incompatible with Kotlin

[samyempcc]

When using batched messaging with Kafka, if the message listener is using Kotlin List<Message<*>> in the listener, the ClassUtils.isAssignable is checking if the method is "compatible" with java.util.List and does not work with kotlin's default List:
org.springframework.messaging.handler.annotation.support.PayloadMethodArgumentResolver.resolveArgument

Kotlin by default uses List from kotlin.collections and the reflection test are not assignable.

Expected to have it work with kotlin's collection identical to the Java version.

[sdeleuze]

I think Kotlin lists are expected to be java.util.List as well, see this code sample.

Could you please provide a reproducer?

[samyempcc]

I will try to put a full reproducer once I get some time. Meanwhile this workaround fixed it for me:
@KafkaListener() fun batchListener(records: java.util.List<Message<*>?>) {..}
This returns the list of GenericMessage.

When using Kotlin's default List like:
@KafkaListener() fun batchListener(records: List<Message<*>?>) {..}
the records would not be a list of Message, but a list of the payload inside the message.

[sdeleuze]

I would be really interested by a repro to understand what happens here. When using Kotlin List, what types do you get here for targetClass and payloadClass?

[spring-projects-issues]

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[samyempcc]

I would be really interested by a repro to understand what happens here. When using Kotlin List, what types do you get here for targetClass and payloadClass?

Digging further, I saw the root cause is actually in: https://github.com/spring-projects/spring-kafka/blob/main/spring-kafka/src/main/java/org/springframework/kafka/listener/adapter/MessagingMessageListenerAdapter.java#L682

with Kotlin the paramType resolves to:
? extends org.springframework.messaging.Message<?>

while for java, it is:
org.springframework.messaging.Message<?>

The rest of the code flow in that method ends up assigning the payload content based on the test for paramType and ends up behaving differently for kotlin.

I have a working code to reproduce the issue given a local kafka running on port 9092:

https://github.com/samyem/spring-messaging-issue29963/blob/master/src/main/kotlin/com/example/issue29963/service/BatchTest.kt#L35

[sdeleuze]

Thanks for digging into that. Indeed as discusses in #22313, Kotlin lists involve declaration site-variance so extra care should be taken for this use case. The related code seems to be on Spring Kafka side so please open a related issue on https://github.com/spring-projects/spring-kafka/issues. cc @garyrussell