"Breaking" change on 2.7.0 - ConcurrentHashMap vs HashMap

[piresrui]

Just wanted to share an issue we ran into.

While upgrading a service running Spring Boot 2.3.X to 2.5.X, we ran into a NullPointerException when creating a DefaultKafkaProducerFactory.

@Bean
public ProducerFactory<String, String> producerFactory() {
	return new DefaultKafkaProducerFactory<>(producerConfigs(), new StringSerializer(), new StringSerializer());
}

This did not happen before the update. We narrowed it down to a change from HashMap to ConcurrentHashMap on the constructor of the DefaultKafkaProducerFactory. Turns out our configs had null values, which is acceptable to pass to a HashMap constructor but not to a ConcurrentHashMap.

This was changed here. We also couldn't find anything documenting this change (outside of the commit message).

Bit of a random bug, but maybe it'll help someone else. Solution was obviously to not have null values in our config map.