Should one make constants in singletons static in java?

A final field in a singleton class should be constant (assuming immutability) regardless of whether it is static, as there cannot be multiple instances of the class holding different values.

Are there reasons to still declare the field as static, or not to do so?

One reason to make it static is clear declaration of intent. But are there nuances regarding memory management that could make one practice best?

To my understanding, what differs is that in the non-static case, the reference to the object assigned to the field is stored in the class instance in heap space, while in the static case it's stored in metaspace, but I don't know which one (if any) is to be preferred.

Edit: A user noted that the implementation of the singleton may make a difference. This question came up regarding Spring components, where manually creating multiple instances could be done, but will not happen in practice.

I would say that you are on to the answer already, it's heap vs meta space. For the non-static version, the data needs to be in both, it needs to be in metaspace all the time, and copied over to the (singelton) instance of the object once it's instantiated.

If it's small data, like a single primitive value, it can be stored directly in the instruction memory itself if declared static final, and are then likely to be accessed within a single clock cycle during execution. But who knows, maybe the GIT compiler can achieve this anyway. 🤷

If it is a lagre dataset, declaring it static final avoid a data copy (or more if the singelton intention is overridden using serialization or reflection) since only the version in metaspace is needed.

So I would say no reason to not have it declared static final. It will probably not do any harm. A byte saved is a byte earned. 😄