Connecting flight checked bag

Since the customs comes under the control of each member country, you have to go through customs only at final airport. A very rare exception is if the final airport is small enough not to have any customs facility (domestic only), you may have to recheck bags.

You will go through the immigration at Germany (first point of entry in Schengen area) anyway. In any case, confirm with your airline when checking in for the flight.

If your final destination is not contained in the List of international Union airports PDF for Spain, then it is likely your registered baggage will be checked at the first EU airport.

Based on the list for Germany, many regional (i.e. domestic) airports have the facilities for customs controls that can be carried out on registered baggage.

Since Barcelona is listed, your baggage will be transfered to that flight when you have a single ticket.

In Frankfurt you will go through Immigration and customs controls. Should your registered baggage contain anything that needs to be declared, that declaration should be done in Frankfurt (where you are entering the customs area).

Taxation and Customs Union - Travelling by air
How do these controls operate in practice ?
That depends on your situation:
...
5. Are you taking a flight between a non-EU airport and an EU airport with a change of plane in another EU airport? (e.g. Tokyo - Copenhagen - Amsterdam with a change of aircraft in Copenhagen)
You will get off the first plane in Copenhagen where your hand baggage is liable to be checked by customs. Meanwhile, your registered baggage, which will have been given a normal label (no green edges) in Tokyo, will be transferred from the baggage hold of the first plane to that of the second). On arrival in Amsterdam , your hand baggage will not, in theory, be liable to be checked by customs, whereas your registered baggage may be. If the second EU airport is not equipped for air traffic with third countries (See, for information, the list of international Union airports), your registered baggage will be liable to be checked in the first EU airport.

From: list of international Union airports PDF in Germany (some of which no longer exist):

• Flughafen Augsburg
• Flughafen Berlin-Schönefeld
• Flughafen Berlin-Tegel (closed)
• Zentralflughafen Berlin-Tempelhof (closed)
• Flughafen Bremen
• Flughafen Cochstedt
• Flughafen Dortmund
• Flughafen Dresden
• Flughafen Düsseldorf
• Landeplatz Egelsbach (Hessen)
• Flughafen Erfurt
• Flughafen Frankfurt Main
• Flugplatz Friedrichshafen
• Flughafen Hamburg
• Flughafen Hannover
• Flughafen Karlsruhe/Baden-Baden
• Flughafen Köln/Bonn
• Flughafen Leipzig/Halle
• Flughafen Mönchengladbach
• Flughafen München
• Flughafen Münster/Osnabrück
• Flughafen Nürnberg
• Flughafen Paderborn/Lippstadt
• Flughafen Rostock-Laage
• Flughafen Saarbrücken
• Flughafen Stuttgart

List of airports in Germany - Wikipedia

The following airports in Germany (based on the Wikipedia image) are not contained in the list of Union airports:

• Barth, Braunschweig, Cuxhaven, Heringsdorf, Ingolstadt, Kassel, Magdeburg, Memmingen, Neubrandenburg, Niederrhein, Oberpfaffenhofen, Siegerland, Sylt and Zweibrücken

Should your final destination be one of these airports, then assume that your registered baggage will be checked at the first EU airport. You should ask your airline for details.

Sources:

• Taxation and Customs Union - Travelling by air
  • List of international Union airports (PDF) dated 2022

  This list comprises all Union airports which, having been so authorized by the competent authorities of the Member State concerned, are approved for air traffic with third countries and where, in particular, customs controls can be carried out on baggage.

• List of airports in Germany - Wikipedia

Barcelona is a main international airport, your luggage will be checked through.

Of course you should check that it is labeled right when you hand it in.

Your short layover is not exceptionally short, so unless your incoming plane is delayed you should make it and see your luggage in Barcelona. There you will pass through the channels, red, green and possibly blue. You should take the green if nothing to declare or the red one when you have things to declare. Your luggage tag will show you started outside the EU and customs officers can easily see that. Blue, if there, is for people who travel within the European Union, so not you.