code: @Nonnull
    public static DateTimeFormatter getDateTimeFormatter (@Nonnull @Nonempty final String sPattern,
                                                          @Nonnull final ResolverStyle eResolverStyle)
    {
      return getInstance ().getFromCache (new DateTimeFormatterPattern (sPattern, eResolverStyle));
    }
docs: Get the cached DateTimeFormatter using the provided resolver style.
  
  @param sPattern
  The pattern to retrieve. May neither be <code>null</code> nor empty.
  @param eResolverStyle
  The resolver style to be used. May not be <code>null</code>.
  @return The compiled DateTimeFormatter and never <code>null</code>.
  @throws IllegalArgumentException
  If the pattern is invalid