I want to be able to log in through the social networks at the front. All you need is to end up with a server (Java server) that I received something like a composite user id from the pair "social network" - "go on the social network", which will be used for the session.
In general, there is nothing complicated in the task, social networks have api. The only difficulty is a lot of social networks, the work is monotonous and boring)
Is there any ready-made free solution on this topic? To pretty and in two lines) Googled a bit - firebase has something like that, but the solution is not free.