Hi,
It is not straight forward but it is not complicated. You only need to know that the requests to your server are done with grpc and protobuf. You already have the protobuf configuration for your sever, you need to create the stub for the client with grpc:
Basically you need to compile the protocol buffer definition for java using the pluging for grpc (you probably need to compile grpc with support for java in order to create the plugin first).
Take a look first the the examples for python in the repository. The idea is the same bug using the plugin of grpc for java.