How to authenticate a backend-to-backend with Spring Boot / Keycloak

Looks like you are missing some configuration for authenication server. KeycloakRestTemplate used client ID, client secret, username and password to validate against the Keycloak server. You need to set the clientid, clientsecret, realm and authentication server url for KeycloakClientCredentialsRestTemplate like -

@Service
public class MyKeycloakClientCredentialsConfig {

    @Value("${keycloak.realm}")
    private String realm;

    @Value("${keycloak.auth-server-url}")
    private String authServerUrl;

    @Value("${keycloak.resource}")
    private String clientId;

    @Value("${keycloak.credentials.secret}")
    private String clientSecret;

    @Bean
    public KeycloakClientCredentialsRestTemplate createRestTemplate() {
        return new KeycloakClientCredentialsRestTemplate(getClientCredentialsResourceDetails(),
                new DefaultOAuth2ClientContext());
    }

    private ClientCredentialsResourceDetails getClientCredentialsResourceDetails() {
        String accessTokenUri = String.format("%s/realms/%s/protocol/openid-connect/token",
            authServerUrl, realm);
        List<String> scopes = new ArrayList<String>(0); // TODO introduce scopes

        ClientCredentialsResourceDetails clientCredentialsResourceDetails = 
                new ClientCredentialsResourceDetails();

        clientCredentialsResourceDetails.setAccessTokenUri(accessTokenUri);
        clientCredentialsResourceDetails.setAuthenticationScheme(AuthenticationScheme.header);
        clientCredentialsResourceDetails.setClientId(clientId);
        clientCredentialsResourceDetails.setClientSecret(clientSecret);
        clientCredentialsResourceDetails.setScope(scopes);

        return clientCredentialsResourceDetails;
    }

}

My resttemplate is like this-

public class SampleRestTemplate extends OAuth2RestTemplate {

    public KeycloakClientCredentialsRestTemplate(OAuth2ProtectedResourceDetails resource,
            OAuth2ClientContext context) {
        super(resource, context);
    }

}

its working perfectly for me.

OK, found the solution myself: I needed to set the switch "Service accounts enabled" button to ON in the client config for "backend-service2" inside keycloak.