spring boot 1.5.x with spring security 4 vs. spring boot 2.1.3 with spring security 5

SpringBoot15Application.java

@EnableOAuth2Sso
@RestController
@SpringBootApplication
public class OAuth2DemoApplication_1_5 {

    @Value("#{ @environment['security.oauth2.resource.server'] }")
    private String resourceServerUrl;

    private OAuth2ProtectedResourceDetails resource;

    public OAuth2DemoApplication_1_5(OAuth2ProtectedResourceDetails resource) {
        this.resource = resource;
    }

    public static void main(String[] args) {
        SpringApplication.run(OAuth2DemoApplication_1_5.class, args);
    }

    @GetMapping("/")
    String home(@AuthenticationPrincipal OAuth2Authentication authentication) {
        return "Hello " + authentication.getName();
    }

    @GetMapping("/api")
    String api(@AuthenticationPrincipal OAuth2Authentication authentication) {
        return tokenRelayTemplate(authentication).getForObject(resourceServerUrl + "/api", String.class);
    }

    private OAuth2RestTemplate tokenRelayTemplate(OAuth2Authentication authentication) {
        OAuth2AuthenticationDetails details = (OAuth2AuthenticationDetails) authentication.getDetails();
        OAuth2ClientContext context = new DefaultOAuth2ClientContext(new DefaultOAuth2AccessToken(details.getTokenValue()));

        return new OAuth2RestTemplate(resource, context);
    }
}

SpringBoot21ApplicationBAD.java

@RestController
@SpringBootApplication
public class OAuth2DemoApplication_2_1 {

    @Value("#{ @environment['spring.security.oauth2.resource.server'] }")
    private String resourceServerUrl;

    private OAuth2AuthorizedClientService oAuth2AuthorizedClientService;

    public OAuth2DemoApplication_2_1(OAuth2AuthorizedClientService oAuth2AuthorizedClientService) {
        this.oAuth2AuthorizedClientService = oAuth2AuthorizedClientService;
    }

    public static void main(String[] args) {
        SpringApplication.run(OAuth2DemoApplication_2_1.class, args);
    }

    @GetMapping("/")
    String home(@AuthenticationPrincipal OidcUser user) {
        return "Hello " + user.getFullName();
    }

    @GetMapping("/api")
    String api(@AuthenticationPrincipal OAuth2AuthenticationToken oauthToken) {
        OAuth2AuthorizedClient client = oAuth2AuthorizedClientService.loadAuthorizedClient(
            oauthToken.getAuthorizedClientRegistrationId(), oauthToken.getName()
        );
        return tokenRelayTemplate(client.getAccessToken().getTokenValue())
            .getForObject(resourceServerUrl + "/api", String.class);
    }

    private RestTemplate tokenRelayTemplate(String accessToken) {
        RestTemplate restTemplate = new RestTemplate();
        restTemplate.getInterceptors().add(getBearerTokenInterceptor(accessToken));
        return restTemplate;
    }

    private ClientHttpRequestInterceptor getBearerTokenInterceptor(String accessToken) {
        return new ClientHttpRequestInterceptor() {
            @Override
            public ClientHttpResponse intercept(
                    HttpRequest request, byte[] bytes, ClientHttpRequestExecution execution
            ) throws IOException {
                request.getHeaders().add("Authorization", "Bearer " + accessToken);
                return execution.execute(request, bytes);
            }
        };
    }
}

SpringBoot21ApplicationGOOD.java

@RestController
@SpringBootApplication
public class OAuth2DemoApplication_2_1 {

    @Value("#{ @environment['spring.security.oauth2.resource.server'] }")
    private String resourceServerUrl;

    private WebClient webClient;

    public OAuth2DemoApplication_2_1(WebClient webClient) {
        this.webClient = webClient;
    }

    public static void main(String[] args) {
        SpringApplication.run(OAuth2DemoApplication_2_1.class, args);
    }

    @GetMapping("/")
    String home(@AuthenticationPrincipal OidcUser user) {
        return "Hello " + user.getFullName();
    }

    @GetMapping("/api")
    String api() {
        // thanks to WebClientConfig, the access token will be 
        // included in the request automatically
        return this.webClient
            .get()
            .uri(this.resourceServerUrl + "/api")
            .retrieve()
            .bodyToMono(String.class)
            .block();
    }
}

WebClientConfig.java

@Configuration
public class WebClientConfig {
    @Bean
    WebClient webClient(
        ClientRegistrationRepository clientRegistrations, OAuth2AuthorizedClientRepository authorizedClients
    ) {
        ServletOAuth2AuthorizedClientExchangeFilterFunction oauth2 =
            new ServletOAuth2AuthorizedClientExchangeFilterFunction(clientRegistrations, authorizedClients);
        oauth2.setDefaultOAuth2AuthorizedClient(true);
        oauth2.setDefaultClientRegistrationId("okta");
        return WebClient.builder()
            .apply(oauth2.oauth2Configuration())
            .build();
    }
}