That's absolutely solid feedback and if you're doing only a handful of field mappings, I'd agree that this pattern could be used. However, as mappings grow, you tent to end up with a whole bunch of constants, something that can be hard to maintain, or you simply have an overeager developer who thinks it's a good idea to refactor the code into static final int TWO = 2, which of course leads you right back to the original problem.

When grouping the constants together, you make it more clear how these constants tie into your domain model and you make it easier for maintainers to read and extend the code.

Personally I wouldn't call an enum boilerplate, since it is quite small and efficient, and I'd rather take that over Constants.java with a group of constants I cannot easily understand.