Spring Boot 3.4.x Oauth2 Client using RestClient, no reactive dependency

Prior to Spring boot 3.4.x(backed by Spring security 6.4.x) the spring framework worked quite well with Oauth2 Clients but that was not just perfect.

A developer who had no use of webflux/ reactor libraries in general, had to unnecessarily include them without actually needing them otherwise.

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Spring came up Spring boot 3.4.x around 21st novemeber 2024.

It removes the need of the Webflux/reactor, etc. to be part of the project now.

This blog is kinda a POC for that bit. Plus added bonus of the various possible customisations focusing around mutual/client authentication aka MTLS(or MASSL)

Important ➫ this covers the client credentials grant type only for the sake of simplicity. But can prove to be an important reference for other variants of grant types.

Prerequisites

➫ A fair enough capable machine.
➫ JDK 21 (17 should also be fine)
➫ Maven, IDE
➫ Some knowledge on TLS, MTLS.
➫ OPENSSL or STEP CLI
➫ Basic knowledge to clone Git repos :-)
➫ Concept around Oauth2 should also be clear.