Spring Boot starter - RestTemplate

Question

I'm currently trying to write a Spring Boot starter that would automatically authenticate our microservices with an API gateway and include the access token in the headers for all outgoing requests (towards the gateway).
I'm creating a RestTemplate bean and giving it our custom interceptor, but my problem is, by doing this, I prevent other teams (who would be using this starter) to use their own RestTemplate config, since they would have to define the same bean leading to multiple beans existing.

@Bean
public RestTemplate restTemplate(){
  RestTemplate restTemplate = new RestTemplate();
  List<ClientHttpRequestInterceptor> interceptors = restTemplate.getInterceptors();
  if (interceptors.isEmpty()){
    interceptors = new ArrayList<>();
  }
  interceptors.add(clientCredentialsAuthInterceptor());
  restTemplate.setInterceptors(interceptors);
  return restTemplate;
}

Is there another way of intercepting all outgoing requests or make the RestTemplate further configurable?

Answer 1

Not tested but it may give you a start point:

// Create an interface that users of your dependency 
// can implement which provides interceptors
public interface RestTemplateAuthInterceptorProvider {

  // This interface provides interceptors, so they can add as many as they want
  List<ClientHttpRequestInterceptor> provideInterceptor();
}

import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
// define a conditional default implementation of your interceptor provider
@Bean
@ConditionalOnMissingBean(RestTemplateAuthInterceptorProvider.class)
public RestTemplateAuthInterceptorProvider restTemplateAuthInterceptorProvider() {
  return () -> ... // implement your auth interceptor and return
}

// In your actual rest template creation use method argument injection
// If other teams implement the RestTemplateAuthInterceptorProvider interface
// conditional case above will be false and your implementation will not interfere
// If they dont implement RestTemplateAuthInterceptorProvider
// your default implementation will be here
@Bean
public RestTemplate restTemplate(RestTemplateAuthInterceptorProvider provider) {
  RestTemplate restTemplate = new RestTemplate();
  List<ClientHttpRequestInterceptor> interceptors = restTemplate.getInterceptors();
  if (interceptors == null){
    interceptors = new ArrayList<>();
  }
  interceptors.addAll(provider.provideInterceptor()); // from argument 
  restTemplate.setInterceptors(interceptors);
  return restTemplate;
}

Edit: Another hacky approach is to manipulate already defined RestTemplate beans

@Component
@ConditionalOnBean(RestTemplate.class)
public class RestTemplateBeanCustomizer {

  private List<RestTemplate> restTemplateBeans;

  // This injects all restTemplate bean definitions to your bean as a list
  @Autowired
  public RestTemplateBeanCustomizer(List<RestTemplate> restTemplateBeans) {
    this.restTemplateBeans = restTemplateBeans;
  }

  @PostConstruct
  public void customizeRestTemplateBeans() {
      for (RestTemplate restTemplate : restTemplateBeans) {
        // Add your interceptors message handlers etc
        // restTemplate.set...
      }
  }
}