When to check entity uniqueness in java?

Question

Java

Hello.

When in java to check the uniqueness of an entity (custom ConstraintValidator)?
At the time of receiving the DTO from the form (when it checks for completeness and pattern matching for all fields) or when the model is saved?

If you check when saving the model, then you have to catch the TransactionSystemException and get through the wilds to which field was non-unique.

Or am I doing it all wrong? :)