Implementing Oauth2 from scratch - oauth-2.0

Question

I want to implement Oauth2 protocol from scratch for study purposes.
I'm following the Github guide after having created an App with a Client ID and Client Secret.
The two information sources are pretty simple and are:
https://developer.github.com/apps/building-oauth-apps/authorizing-oauth-apps/
https://gist.github.com/technoweenie/419219
In particular, I'm starting from the first step pasting on my browser:
https://github.com/login/oauth/authorize?client_id=&redirect_uri=http://localhost:8080/auth/temp&scope=user&state=&allow_signup=true
I have a Spring application listening on port 8080 (I don't want to use Spring Security because I want to implement the protocol from scratch) with the following and working endpoint exposed:
#RequestMapping("/auth/temp")
public String redirectAuth(HttpServletRequest request) {
//TODO implement next steps
return "here we are!";
}
but when I go to the github link I get a 404 not found error, as my localhost application wouldn't exist.
I expect the official guide has some mandatory information missing, such as some other endpoints which my application must expose in order to be queried, for instance, about the client secret.
So, what am I missing?

Answer 1

Github only supports the auth code oauth 2 flow. It might be helpful to read up on that.
I have a blog and a video on the auth code flow that may help.
Disclaimer: I work at and created them for Ping Identity, but I think
they'll be helpful even for your study purposes.
https://developer.pingidentity.com/en/blog/posts/2019/what-are-oauth-2-0-grant-types-part-1-authorization-code-flow.html
https://youtu.be/eg7I8x-u0sc

Answer 2

You haven't included your client_id in the authorisation url:
https://github.com/login/oauth/authorize?client_id=&redirect_uri=http://localhost:8080/auth/temp&scope=user&state=&allow_signup=true
The authorization server (GitHub) needs this value to identify the client you have registered.