If you're wanting to use the token to access user-owned resources at Facebook (e.g., the user's timeline, friends, etc) there's no way to get a token without involving the user. That's on purpose so that you aren't allowed to fetch data or write data to Facebook without getting the user's permission to do so.
But if that's what you want, then you must redirect the user to Facebook and back. This is OAuth 2's authorization code grant. Spring Social's ConnectController can help you with this.
You might be able to do it via the JS API, but it's been awhile since I've done it that way and I know a few things have changed at Facebook in that regard. And it probably relies on OAuth 2 Implicit Grant (which involves a redirect).
There is another way to obtain an access token without redirecting: Via OAuth 2's client credentials grant. But I don't think that's what you want. Such a token can only be used to work with non-user resources (such as Facebook's Insights API). But if that's what you want, then you can use OAuth2Template's authenticateClient() method.
Again, to be perfectly clear, it is very much by design that you can't obtain a user access token without involving the user. And with Facebook, that will require redirects.