@Test
  void matrixParametersAreSupported() throws Exception {
    TestInterface api = new TestInterfaceBuilder().target("http://localhost:" + server.getPort());

    server.enqueue(new MockResponse());

    List<String> owners = new ArrayList<>();
    owners.add("Mark");
    owners.add("Jeff");
    owners.add("Susan");
    api.matrixParameters(owners);
    assertThat(server.takeRequest()).hasPath("/owners;owners=Mark;owners=Jeff;owners=Susan");
  }
when you must parse a 2xx status to determine if the operation succeeded or not.
core/src/test/java/feign/FeignTest.java