Congrats on making the system work to your benefit.

When I traveled to Paris via Amsterdam, we went through customs in Amsterdam and then in Paris. However, since our luggage had been checked through to Paris from Detroit, the customs stop in Amsterdam was easy.

On the way back surely you can;t be going through JFK twice - that is madness. And why are you listing Tokyo's airport?

And your question really has two parts:

1) Immigration - you entering the country

2) Customs - your luggage entering the country

You only need to worry about the first - which can take a while if several large international flights land at once.

Going you will go through Immigration in Barcelona (in London you MUST stay airside - do not follow signs for Immigration but for connecting flights). In Barcelona after Immigration you will pick up your luggage (if you checked any) and then walk out through Customs. You just walk through the Green (nothing to declare) lane.

On the way back you will go through Immigration at JFK (may take up to 30/45 minutes on line) then get your luggage and then go through Customs. There will be an agent there and you just hand them the customs card (showing any purchases you made) and they most likely will just let you walk right through. Every so often they pull someone out of line and open their bags.

Be very careful you have NO food with you/in your bags. The sniffer beagles will spot this immediately (even though they are really trained to look for other substances) and your bags will be torn apart - and you delayed.

On entry to Spain you should understand that Immigration officials are trained to look for potential illegal immigrants - and so look very carefully at any young people. Usually they just ask purpose of the visit (vacation) and how long you will be there (a week - or whatever). But with younger people they often ask you what hotel you will be staying at (you should know the name and address) and ask you to demonstrate you have enough money for the trip - cash or credit/debit cards.

They do have the right to send you back home on the next flight if they don't like your answers - so be ready with them and be polite. (They don't want to be mean - but don't want to be fooled either.)