I saw that questions get downvoted at Stack Overflow when they are like "Which java frameworks provide RPC functionality?", because they are off-topic.
Is there a Stack Exchange community where I legitimately can ask such a question?
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You can ask on Software Recommendations. However:
It could very well be that you needs can be met, but it might take a bit of work on your part. Answers will go into this. It could also be that nothing available meets your specific needs, so make sure to put only what's absolutely essential as essential.
In short, your job is to limit (as much as possible) the number of answers that would actually apply.
Just look around prior to asking, I'm pretty sure a few Java framework questions have been asked. If they're totally different from what you hope to get out of something, it's fine to ask.
And yeah, you better read this, and make sure you follow the links it contains :)