The Postgres JDBC driver does not guarantee that batch inserts come back in the same order that you insert them (when you use RETURNING *). So, if you generate UUIDs server-side, you can't conveniently match them up with the records you just inserted. You're better off generating them in the app server first and then sending them to Postgres.
Plus, it's way better to generate a UUID in the app server and send it to the server as a string. The reason is that you can deal with the id as a plain string and don't have to deal with a non-standard datatype in your app.
