To restore from the savepoints in previous versions, Flink now keeps all classes whose serialization is changed and put them in a separated package ("migration").
When deserializing the old savepoints, flink will look up correct descriptors (ObjectStreamClass) for these classes, without using those ones written in serialized data. The implementation however is problematic when there exist missing field descriptors in the serialized data.
When serializing an object, Java will only write the descriptors of those non-null fields. But when we look up class descriptors with given classes, all fields will be put into the descriptors. As a result, we will deserialize the savepoints with incorrect descriptors, leading to serialization exceptions.
A simple resolution is to update the name of read descriptors using Reflections, without using different descriptors.