Depends on where you start and if the journey towards Spain is part of the experience.

The journey can be done fully reservation-free from Ventimiglia. It only takes longer:

- TER Ventimiglia - Nice

- TER Nice - Marseille

- TER Marseille - Montpellier - Portbou (or via Avignon) -> an Intercités (10-20€) to Narbonne could save an hour or two

- R Portbou - Barcelona

Don't forget to add the hidden costs when flying: 2 x 10€ or so to get to and from the airport, 40€ extra for luggage, etc.

You could fly one-way to Portugal and take the train afterwards (or vice-versa).

Be aware that (unfortunately) not all Spanish reservations are available online. I'd recommend to get all of them at the first Spanish station you encounter.

From Lione you can catch a Flixbus directly to Barcelona/Alicante.
If you buy a ticket on time, it is cheaper and you don’t have to suffer the expensive and long way from France to Spain, at the border the connections available are not the best.
Ofc, this option is good if you’re not really interested in travelling in south france.

Edit, there is now a direct train (10 euros fees) Lyon-Barcelona, 5 hours. I think it is a good option now.

Milan → Geneve → Lyon → Barcelona 

You’ll need to crush at least one night in Lyon, but in my opinion it is worth the stop.