In what version(s) of Spring for Apache Kafka are you seeing this issue?
Since 4.0.0

Describe the bug
The ProducerFactory and ConsumerFactory interface type parameters do not allow instantiating implementing classes with nullable types when using Kotlin. For example the following Kotlin code to create a producer that allows sending tombstones does not compile:

val properties = mapOf(...)
val producer = DefaultKafkaProducerFactory<String, String?>(properties).createProducer()

This is an issue because now the factory has a nullability specification (non-nullable) while the producer/consumer created by the factory still allows sending/consuming tombstones.

We encountered this issue while implementing a KafkaTestUtilities class and we created a sample project to reproduce it here. Each following step is represented in a dedicated commit.

• (Step 1) We created a test that fails due to a NullPointerException: denniseffing/spring-kafka-nullability-issues@86ce5c54:src/test/kotlin/com/example/kafka_nullability_issues/ReproducingNullabilityIssueTest.kt#28-37
• (Step 2) To fix it we change the types of our utility methods: denniseffing/spring-kafka-nullability-issues@66d6fa12 but now the code doesn't compile anymore because the type parameters do not accept a nullable type
• (Step 3) We implemented a workaround relying on manual casts: denniseffing/spring-kafka-nullability-issues@989c1f29

To Reproduce

See sample project and follow along the three commits.

Expected behavior

ProducerFactory and ConsumerFactory and their implementations should allow nullable type parameters, i.e.

public interface ProducerFactory<K, V extends @Nullable Object> { ... }
public interface ConsumerFactory<K, V extends @Nullable Object> { ... }

This would allow the Step 2 commit in the reproducible sample to compile and execute all tests successfully.

Sample

https://github.com/denniseffing/spring-kafka-nullability-issues