Recently I ran into a GPL license related problem and I heard many people claiming many different things. I made this post to get this cleared up for myself.

Situation There is a program which is licensed under GPL V3. This program has an API, which is also licensed under GPL V3. This program can load external jar files (plugins) which depend on the public GPL V3 API. So, the program actually "starts" my program (my plugin).

The GPL FAQ says to following.

If a library is released under the GPL (not the LGPL), does that mean that any software which uses it has to be under the GPL or a GPL-compatible license? (#IfLibraryIsGPL)

Yes, because the software as it is actually run includes the library.

My question So, if I include a GPL library into my project, I have to license my project under GPL as well. However, I am not including any code from the GPL V3 API. I am calling code from the API, but I have not copied any code of the API into my own project. Neither is any code shaded into the jar. Do I have to license my project under GPL as well? Or may I choose my own license?


This is a difficult, and often-asked, question. IANAL.

Essentially it comes down to how closely linked your code is to this API. If it is clear that they are two completely separate programs, then you're ok. If your program makes calls back and forth and shares "data structures", then they could be considered the same program.

I wrote this answer and this answer recently which both touch on this topic. I'm sure there are plenty others around here too.

One thing to consider is what GNU say on this topic themselves, particularly this FAQ answer. In the end it's going to be down to interpretation - that of judges if it ever goes to court.

But what I would recommend in your instance is just getting in touch with the authors of Bukkit. Tell them what you want to do and how you're doing it. If their interpretation is that the programs are not connected, great - you've pretty much got a green light. But if they interpret it differently, then - legalities aside, ethics at the very least - would say that you would need to also GPL your code.

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