JAVA,PROTOBUF - how to implement dynamic deserialization/serialization knowing the object's class name?

Question

Java

Good evening
I am very bad at JAVA, please do not kick much.
There is a Packet package class in it 2 messages (in, out), the first bytes contain the length of the package and the package identifier (through which I get the class name for the package from the config) then the body the data itself.
PROTOBUF has a parseFrom method to convert a byte array to an object.
Problem - the class for the object to refuse is through
- Class.forName("Packet.In")
- Class.forName("Packet$In")
At the moment I don't know the intricacies of these 2 variations
If someone has already encountered such a problem , I will be very grateful for possible solutions.
Perhaps I didn’t read something on the dock, but so far I can’t find a solution for dynamic data parsing.
Thanks to all.