Get from Barcelona airport to city center

I have always used a taxi. It''s not a long ride.

Though we took a bus, it was much earlier in the day. Since you are arriving at 12am, I would suggest a taxi. That way you can go directly to your hostel, you won't need to walk from the bus stop.

Here's some info that might help: https://www.barcelona.com/barcelona_city_guide/all_about/barcelona_airport_bcn/from_barcelona_airport_to_city_centre

Make sure your hostel knows you are coming and will let you in at that hour.

There is a bus that goes from the airport and drops you off in Plaza Catalunya. I found it very convenient, especially since my hotel was in the area.

Edit: Didn't see the part where you were arriving at 12 AM. Although the buses should still be running, you may want to take a taxi in that case.

Yes, a taxi would work, but if you are on a budget, you could check out Aerobus Barcelona. You can get your ticket online ahead of time or buy from the driver when you get there, it runs 24 hours a day, and stops at four different places, with the last being Placa Catalunya, before returning to the airport. You can get round-trip or one-way tickets very easily. I have used this service a couple of times and it's quite convenient. When you leave the luggage pick-up area of the airport, you have to go downstairs on the right side to the transportation area, and the blue Aerobuses are right there.

Whatever you choose, I think it will be fine. I always feel safer in Barcelona at night than I do in broad daylight in many cities in the US. It pays to be cautious, though...

The local folks don't start eating dinner till around 10 PM, so there will be a lot more folks walking around than you would expect. Still, at that hour, even I would be tempted by a taxi.