Under Article 368 PC, growing and selling marijuana are illegal in Barcelona. However, growing weed for medical purposes, for teaching and research is allowed in Spain. Marijuana for personal use is not considered as criminal offense but smoking it in public is considered as misdemeanor and thus, it is punishable by fines. In this case, the weed will also be confiscated. Marijuana for personal consumption is acceptable in Barcelona. Even growing for medical purposes is permitted by the law but the police are given the authority to check and determine if you are really cultivating for personal usage or for commercial purposes.
It’s a public secret, but there is a legal way to purchase proper marijuana in Barcelona. There is one legal coffee shop in the center of Barcelona, were you can stay and chill out. You have to be a member and you can only sign up when referred by another member. 
You can smoke the proper weed and eat hash space cookies either at the coffee shop or at your hotel or whatever. The only requirement is that you are at least 18 of age and bring a valid ID.