So I have two classes : A RestClient class and another MetadataService class which uses the restClient bean.

This is the RestClient class

public class RestClientConfig {
  //actual class is more complex than this as we are authenticating with kerberos and there's
  //code that generates kerberos token and injects it into the rest client builder 
  @Bean("customRestClient")
  public RestClient restClient(RestClient.Builder builder) {
    return builder.build();
  }
}
 

Below is the service class

public class MetadataService {
  @Autowired 
  @Qualifier("customRestClient")
  RestClient customRestClient;
  
  public void makeCall() {
     try {
         final String response = customRestClient.method(HTTPMethod.POST).uri("http://example.com")
                                      .headers(getHeaders()).body(//some body here).retrieve()
                                      .toEntity(String.class);
         log.info(response);
     }
  }
}

Now I want to write a test class for MetadataService.

Things I have tried :

1. Using @Mock annotation on the restClient bean but that lead to exceptions (more below)
2. Using @RestClientTest on the class albeit there’s a lot of confusion on how I should use this.

Here is my test class which I have written so far (what I have referenced in #1)

public class MetadataServiceTest {

@Mock
RestClient customRestClient;

@InjectMock
MetadataService metadataService;

@Test
public void testMakeCall() {

when(customRestClient.method(HTTPMethod.POST).uri("http://example.com")
     .headers(any()).body(any()).retrieve()
     .toEntity(String.class)).thenReturn(response); 

metadataService.makeCall();
}

}

This leads to exceptions like this one:

java.lang.NullPointerException: Cannot invoke “org.springframework.web.client.RestClient$RequestBodyUriSpec(String, Object[])” because the return value of “org.springframework.web.client.RestClient.method(org.springframework.http.HttpMethod)” is null