It turns impossible to use Java dynamic proxy objects in the job definition (for example, as a MapFunction implementation).
During deserialization of the job graph, the default implementation of ObjectInputStream.resolveProxyClass(..) is used, which is not using the custom class loader (to look into the submitted jar) and therefore throws ClassNotFoundException.
Looks like in order to address this, InstantiationUtil.ClassLoaderObjectInputStream needs to provide custom implementation of resolveProxyClass(..) method as well (in addition to resolveClass(..)).
In order to reproduce the issue, run the attached SocketWindowWordCount Flink app. It's a slight variation of the canonical [SocketWindowWordCount
example|https://ci.apache.org/projects/flink/flink-docs-master/tutorials/local_setup.html] with a dynamic proxy implementation of the flat map transformation.