orElseThrow

private static <T> HttpMessageWriter<T> findWriter(
    BodyInserter.Context context, ResolvableType elementType, @Nullable MediaType mediaType) {
  return context.messageWriters().stream()
      .filter(messageWriter -> messageWriter.canWrite(elementType, mediaType))
      .findFirst()
      .map(BodyInserters::<T>cast)
      .orElseThrow(() -> new IllegalStateException(
          "No HttpMessageWriter for \"" + mediaType + "\" and \"" + elementType + "\""));
}