SpringBoot SpringBootTest with RestClient.Builder that has base url + port already set.

I went scavenging the documentation couldn't find an annotation to configure RestClient.Builder although it does autowire it...

this is what I tried

@SpringBootTest(webEnvironment = SpringBootTest.WebEnvironment.RANDOM_PORT)
class AuthorizationServerTest {

  @Autowired
  RestClient.Builder restClientBuilder;

  @Test
  void noAuth() {
    var restClient = restClientBuilder
      .messageConverters(converters -> converters.addFirst(new OAuth2AccessTokenResponseHttpMessageConverter()))
      .build();

I expected it to be equivalent to

    var restClient = RestClient.builder()
      // .requestFactory(new HttpComponentsClientHttpRequestFactory()) not this because I'm doing this for tracing because of https://github.com/spring-projects/spring-framework/issues/32783 which might be better solved by Spring Boot
      .baseUrl("http://localhost:" + this.port)
      .messageConverters(converters -> converters)

org.springframework.web.client.ResourceAccessException: I/O error on GET request for "/authorize": Target host is not specified

Running with Spring Boot v3.2.5, Spring v6.1.6

Comment From: wilkinsona

There's no test-specific support for RestClient. What's being injected into the test class is the "main" RestClient.Builder that's auto-configured by RestClientAutoConfiguration. We can't change it in a test-specific way as it may break any consumers in main application code.

Support for a TestRestClient, which I'm guess may be what you're looking for, is being tracked by https://github.com/spring-projects/spring-framework/issues/31275. See also https://github.com/spring-projects/spring-boot/issues/40452 and https://github.com/spring-projects/spring-boot/issues/39848.