I am trying to implement authentication with google into my app, with Spring Boot as the backend framework and React.js in the frontend.

Frontend:
Basically I have a button and on an onClick event, the user will be redirected to the following link https://accounts.google.com/o/oauth2/auth?client_id=${clientId}&redirect_uri=${redirectUri}&response_type=code&scope=openid%20profile email
where the client id represents the client id given in the google console after creating a credential.

const handleGoogleLogin = async () => {
        window.location.href = GOOGLE_AUTH_URL;
    };

Backend:
Apart from the usual SecurityConfiguration I have created this OAuth2Configuration class where I use a builder with all the client details.

@Configuration
public class OAuth2Configuration {

    @Value("${spring.security.oauth2.client.registration.google.client-id}")
    private String googleClientId;

    @Value("${spring.security.oauth2.client.registration.google.client-secret}")
    private String googleClientSecret;

    @Value("${spring.security.oauth2.client.registration.google.redirect-uri}")
    private String googleRedirectUri;



    @Bean
    public ClientRegistrationRepository clientRegistrationRepository() {
        return new InMemoryClientRegistrationRepository(
                ClientRegistration.withRegistrationId("google")
                        .clientId(googleClientId)
                        .clientSecret(googleClientSecret)
                        .clientAuthenticationMethod(ClientAuthenticationMethod.CLIENT_SECRET_BASIC)
                        .authorizationGrantType(AuthorizationGrantType.AUTHORIZATION_CODE)
                        .redirectUri(googleRedirectUri)
                        .scope("openid", "profile", "email")
                        .authorizationUri("https://accounts.google.com/o/oauth2/auth")
                        .tokenUri("https://oauth2.googleapis.com/token")
                        .userInfoUri("https://openidconnect.googleapis.com/v1/userinfo")
                        .userNameAttributeName(IdTokenClaimNames.SUB)
                        .jwkSetUri("https://www.googleapis.com/oauth2/v3/certs")
                        .clientName("Google")
                        .build()
        );
    }

    @Bean
    public OAuth2AccessTokenResponseClient<OAuth2AuthorizationCodeGrantRequest> accessTokenResponseClient() {
        return new DefaultAuthorizationCodeTokenResponseClient();
    }

}

From what I understand, the redirect uri’s from the backend should match the frontend.
I then make use of clientRegistrationRepository in my OAuth2Controller where i want to get the access token from the authorization code sent after the user redirects from the consent screen but I can’t find an effective approach to do so..

@GetMapping("/oauth2/redirect")
    public String getAccessToken(@RequestParam("code") String code) throws ServletException, IOException {
        ClientRegistration clientRegistration = clientRegistrationRepository.findByRegistrationId("google");
        String clientId = clientRegistration.getClientId();
        String clientSecret = clientRegistration.getClientSecret();
        String redirectUri = clientRegistration.getRedirectUri();
        String tokenEndpoint = "https://oauth2.googleapis.com/token";
        //...
}

Technically the code below does get the user access token from the temporary code + client details, but I imagine it can be optimized

HttpHeaders headers = new HttpHeaders();
        headers.setContentType(MediaType.APPLICATION_FORM_URLENCODED);

        String requestBody = "code=" + code +
                "&client_id=" + clientId +
                "&client_secret=" + clientSecret +
                "&redirect_uri=" + redirectUri +
                "&grant_type=authorization_code";

        HttpEntity<String> entity = new HttpEntity<>(requestBody, headers);

        ResponseEntity<String> response = new RestTemplate().postForEntity(tokenEndpoint, entity, String.class);
        JSONObject jsonResponse = new JSONObject(response.getBody());
        String accessToken = jsonResponse.getString("access_token");
        System.out.println("Access Token: " + accessToken);
        System.out.println(response.getBody());