...

This generation is not currently exposed to any of partition assignors. It will be exposed as part of the assignor.assign(...) call in ConsumerCoordinator.java:

// current call
Map<String, Assignment> assignment = assignor.assign(metadata.fetch(), subscriptions);

// new call
Map<String, Assignment> assignment = assignor.assign(metadata.fetch(), subscriptions, latestGeneration().generationId); 

Note that when this call is made the group is rebalancing and, therefore, the call to generation() will return null. That is why a new latestGeneration() method is created to return the latest generation irrespective of the group status.

Compatibility, Deprecation, and Migration Plan

...