application.yaml

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spring:
  security:
    oauth2:
      client:
        provider:
          <provider-name>:
            issuer-uri: <issuer-uri implementing OIDC>
        registration:
          <provider-name>:
            client-id: <client-id>
            client-secret: <client-secret>
            scope: <comma separated scopes>
            authorization-grant-type: client_credentials
            
OR

spring:
  security:
    oauth2:
      client:
        provider:
          <provider-name>:
            token-uri: <token-uri of provider implementing OIDC>
        registration:
          <provider-name>:
            client-id: <client-id>
            client-secret: <client-secret>
            scope: <comma separated scopes>
            authorization-grant-type: client_credentials

build.gradle

plugins {
	id 'org.springframework.boot' version '2.3.1.RELEASE'
	id 'io.spring.dependency-management' version '1.0.9.RELEASE'
	id 'java'
}
//Relevant dependencies
dependencies {
	implementation 'org.springframework.boot:spring-boot-starter-webflux'
	implementation 'org.springframework.boot:spring-boot-starter-oauth2-client'
}

sample request

webClient.get()
        .uri(<protected resource uri which you want to access>)
        .attributes(clientRegistrationId(<The Provider name specified under registration in app yaml>))
        .retrieve()
        .bodyToMono(String.class)
        .map(string
            -> "Retrieved using Client Credentials Grant Type: " + string)
        .subscribe(LOGGER::info);

Spring WebFlux Security- Client config - OAuth2.0 Client Credentials Flow.md

This gist describes the configuration required for Spring reactive WebClient to make a call to an OAuth2 protected resource through OAuth2.0 Client Credentials Grant Type Flow.

Assumption is that the Authorization Server supports OpenId Connect 1.0 specifications.

websecurityconfiguration.java

@EnableWebFluxSecurity
public class WebSecurityConfiguration {
  @Bean
  public ReactiveOAuth2AuthorizedClientManager authorizedClientManager(
      ReactiveClientRegistrationRepository clientRegistrationRepository,
      ReactiveOAuth2AuthorizedClientService authorizedClientService) {

    ReactiveOAuth2AuthorizedClientProvider authorizedClientProvider =
        ReactiveOAuth2AuthorizedClientProviderBuilder.builder()
            .clientCredentials()
            .build();

    AuthorizedClientServiceReactiveOAuth2AuthorizedClientManager authorizedClientManager =
        new AuthorizedClientServiceReactiveOAuth2AuthorizedClientManager(
            clientRegistrationRepository, authorizedClientService);
    authorizedClientManager.setAuthorizedClientProvider(authorizedClientProvider);

    return authorizedClientManager;
  }
  @Bean
  public WebClient webClient(ReactiveOAuth2AuthorizedClientManager authorizedClientManager) {
    ServerOAuth2AuthorizedClientExchangeFilterFunction oauth =new ServerOAuth2AuthorizedClientExchangeFilterFunction(authorizedClientManager);
    return WebClient.builder().filter(oauth).build();
  }
	
  @Bean
  public SecurityWebFilterChain configure(ServerHttpSecurity http) {
	return http
		.oauth2Client()
		.and()
		.build();
  }
}