Feign client

DemoApplication.java

@SpringCloudApplication
@EnableFeignClients
public class DemoApplication {
    public static void main(String[] args) {
        SpringApplication.run(DemoApplication.class, args);
    }

    @FeignClient(name = "httpbin", url = "httpbin.org")
    public interface HttpBinClient {
        @RequestMapping(path = "/post", method = POST, produces = APPLICATION_JSON_VALUE)
        public String echo(@HeaderMap MultiValueMap<String, String> headers);
    }
}

DemoApplicationTest

@RunWith(SpringRunner.class)
@SpringBootTest
public class DemoApplicationTest {
    @Autowired
    private DemoApplication.HttpBinClient client;
    private final ObjectReader objectReader = new ObjectMapper()
            .enable(ACCEPT_SINGLE_VALUE_AS_ARRAY)
            .readerFor(Request.class);

    @Test
    public void testEcho() throws IOException {
        MultiValueMap<String, String> headers = new LinkedMultiValueMap<>();
        headers.put(ACCEPT, singletonList(APPLICATION_JSON_VALUE));
        String lang = ENGLISH.toLanguageTag();
        headers.put(ACCEPT_LANGUAGE, singletonList(lang));
        String response = client.echo(headers);
        System.out.println(response);
        Request request = objectReader.readValue(response);

        assertThat(request.getUrl(), is("http://httpbin.org/post"));
        MultiValueMap<String, String> requestHeaders = request.getHeaders();
        assertThat(requestHeaders.keySet(), hasSize(greaterThanOrEqualTo(2)));
        assertThat(requestHeaders, hasEntry(is(ACCEPT), containsInAnyOrder(APPLICATION_JSON_VALUE)));
        assertThat(requestHeaders, hasEntry(is(ACCEPT_LANGUAGE), containsInAnyOrder(lang)));
    }

    @Data
    @JsonIgnoreProperties(ignoreUnknown = true)
    public static class Request {
        Map<String, String> args;
        @JsonDeserialize(as = LinkedMultiValueMap.class)
        MultiValueMap<String, String> headers;
        String url;
    }
}