code: protected String generateETagHeaderValue(byte[] bytes) {
          StringBuilder builder = new StringBuilder("\"0");
          DigestUtils.appendMd5DigestAsHex(bytes, builder);
          builder.append('"');
          return builder.toString();
      }
docs: Generate the ETag header value from the given response body byte array.
  
  <p>The default implementation generates an MD5 hash.
  
  @param bytes the response bdoy as byte array
  @return the ETag header value
  @see org.springframework.util.DigestUtils