@Test
  void onlyOneHeaderMapAnnotationPermitted() throws Exception {
    try {
      parseAndValidateMetadata(HeaderMapInterface.class, "multipleHeaderMap", Map.class, Map.class);
      Fail.failBecauseExceptionWasNotThrown(IllegalStateException.class);
    } catch (final IllegalStateException ex) {
      assertThat(ex).hasMessage("HeaderMap annotation was present on multiple parameters.");
    }
  }
Body type is only for the body param.
core/src/test/java/feign/DefaultContractTest.java