public static <FnT extends DoFn<?, ?>> DoFnSignature getSignature(Class<FnT> fn) {
    return signatureCache.computeIfAbsent(fn, DoFnSignatures::parseSignature);
  }
@Test
  public void testWrongTimestampType() throws Exception {
    thrown.expect(IllegalArgumentException.class);
    thrown.expectMessage("@Timestamp argument must have type org.joda.time.Instant");
    DoFnSignatures.getSignature(
        new DoFn<String, String>() {
          @ProcessElement
          public void process(@Timestamp String timestamp) {}
        }.getClass());
  }