You should be able to pass in true for assignable, and it will check for inheritance, assuming StringSerializer is a Serializer

https://docs.spring.io/spring-kafka/docs/current/api/org/springframework/kafka/support/serializer/DelegatingByTypeSerializer.html

I ran into a similar issue in Kotlin, but similar idea should apply:

DelegatingByTypeSerializer(
    mapOf(
        ByteArray::class.java to ByteArraySerializer(),
        KafkaMessage::class.java to KafkaMessageSerializer(),
    ),
    true
)