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?

Ideally your projects should use the RestTemplateBuilder to create instances of a RestTEmplate this in turn will auto detect all pre-registered interceptors. The only thing your autp-config has to do is make a bean for the interceptor. Or define a RestTemplateCustomizer which adds the interceptor. However this only works if teams follow the practice of using the RestTemplateBuilder to create instances. — M. Deinum
– M. Deinum, Commented Dec 10, 2020 at 15:39

Ghokun · Accepted Answer · 2020-12-10 15:49:25Z

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... } } }

This would only allow them to define their custom interceptors, if I'm correct. My question is more about creating the RestTemplate itself. Let's say they want to add a custom messageHandler to it, but the starter already defined a bean without that. I could annotate it with @ConditionalOnMissingBean, but then if someone defines it, it wouldn't have the interceptor set.

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