If you've come across a redirect_uri_mismatch error before when using Spring Boot and OAuth 2, then this solution is for you.
Solution: A redirect_uri_mismatch error occurs when the redirect URL defined for your application at the authorization service doesn't match the value of parameter "redirect_uri" passed by your request.
When integrating with Spring Boot, the default value of redirect_uri is set to "<ip>:<port>/login".
In the above configuration, we set a custom redirect URI as "http://localhost:9090/callback".