Spring - How to create a custom Access and Refresh OAuth2 Token?

I know those token spring generates a UUID formatted string. One of my concerns is that it's not really "unique"; it is possible for the UUID to create a token exactly the same as a previous one (of course the odds are VERY small but still possible).

I'm using a database to store my user's token and I'm not sure if Spring checks if the token already exists before creating one in the database?

My second question is : Is it possible to create my own token instead of the UUID format, I'd like to have a more "unique" token like the current timestamp with the user's ID and username and then hash everything and that will be my token instead of 49784c38-43b1-.....

I already have a custom TokenEnhancer that I use to add custom info when returning the token to the client but how can I create a custom token before saving it in my database?

Thanks for you help!