code: public static JWSHeader fromBase64String(String base64) throws IOException {
          String json = MessageSecurityHelper.base64UrltoString(base64);
          ObjectMapper mapper = new ObjectMapper();
          return mapper.readValue(json, JWSHeader.class);
      }
docs: Construct JWSHeader from base64url string.
  
  @param base64
  base64 url string.
  
  @return Constructed JWSHeader