Wire Mock seems to offer a solid set of stubs and mocks for testing external web services.

@Rule
public WireMockRule wireMockRule = new WireMockRule(8089);


@Test
public void exactUrlOnly() {
    stubFor(get(urlEqualTo("/some/thing"))
            .willReturn(aResponse()
                .withHeader("Content-Type", "text/plain")
                .withBody("Hello world!")));

    assertThat(testClient.get("/some/thing").statusCode(), is(200));
    assertThat(testClient.get("/some/thing/else").statusCode(), is(404));
}

It can integrate with spock as well. Example found here.

Here is a working example:

@Component
public class QuotesClient {

  private final WebClient webClient;

  public QuotesClient(WebClient.Builder builder, @Value("${client.baseUrl}") String baseUrl) {
    this.webClient = builder.baseUrl(baseUrl).build();
  }

  public JsonNode getData() {
    return this.webClient
      .get()
      .retrieve()
      .bodyToMono(JsonNode.class)
      .block();
  }
}

Using the WebClient.Builder is optional.

The corresponding test can look like the following:

class QuotesClientTest {

  private QuotesClient quotesClient;
  private MockWebServer server;

  @BeforeEach
  public void setup() {
    this.server = new MockWebServer();
    this.quotesClient = new QuotesClient(WebClient.builder(), server.url("/").toString());
  }

  @Test
  public void test() {

    server.enqueue(new MockResponse()
      .setStatus("HTTP/1.1 200")
      .setBody("{\"bar\":\"barbar\",\"foo\":\"foofoo\"}")
      .addHeader("Content-Type", "application/json"));

    JsonNode data = quotesClient.getData();
    assertNotNull(data);

    System.out.println(data);

  }

}

If you are searching for a similar setup using WireMock, Spring Boot, and JUnit 5, take a look at the linked guide.