I have this hint in the logs of the my Flink 1.15 app for ‘java.time.Duration’ and ‘java.util.HashMap’:

Class class <*> cannot be used as a POJO type because not all fields are valid POJO fields, and must be processed as GenericType. Please read the Flink documentation on "Data Types & Serialization" for details of the effect on performance.

Is there a Flink compatible class for Duration and HashMap?