Instead I added ConfigBuilder to make Config object immutable:
https://github.com/msgpack/msgpack-java/commit/b4497cba403765929b827d5d82af39197e469bb2

I'm not sure which is preferred in general; JavaBeans-like getters/setters or builder pattern. But, for now, I think using a builder is sufficient for forbidding to change the configuration after creating an unpacker instance.