The theory is, go through the constructor. However, some objects are designed to go through several steps before reaching the desired state.
If GSON must deserialize {…, state:”CONFIRMED”}, it needs to call new Transaction(account1, account2, amount), then .setState(STARTED) then .setState(PENDING) then .setState(PAID) then .setState(CONFIRMED) ? That’s the theory of the constructor and mutation methods guarding the state, so that it is physically impossible to reach a wrong state.
There is a convention that deserialization is an exception to this theory: It should be able to restore the object as-is, after for example a transfer over the wire. So it was conventionally enabled to set final variables of the object, but only at initialization and only for its own good. It was assumed that, even though GSON could reach a state that was unachievable through normal means, it was, after all, the role of the programmer to add the right annotations to avoid this.
So how do we do it now?
Maybe I don't know of your use case, but GSON/Jackson/Json type classes are strictly data that should only represent the data coming over the wire. If you need to further manipulate that data it sounds like the classes have too much responsibility.
The JEP says:
> the developers of serialization libraries should serialize and deserialize objects using the sun.reflect.ReflectionFactory class, which is supported for this purpose. Its deserialization methods can mutate final fields even if called from code in modules that are not enabled for final field mutation.
I don't know enough about the details here to say if that's sufficient, but I imagine that it at least should be, or if it's not, it will be improved to the point where it can be.
The JEP also says:
> The sun.reflect.ReflectionFactory class only supports deserialization of objects whose classes implement java.io.Serializable.
In my experience, most classes being deserialized by libraries like GSON do not implement Serializable. Implementing Serializable is mostly done by classes which want to be serialized and deserialized through Java's native serialization format (which is used by nothing outside Java, unlike cross-platform formats like JSON or CBOR).