code: protected String extractClientCertificate(HttpServletRequest request)
      {
          String[] certs = extractClientCertificateChain(request);
  
          if (certs != null && certs.length > 0)
          {
              return certs[0];
          }
          else
          {
              return null;
          }
      }
docs: Utility method for extracting a single client certificate from the default
  certificate extractors. Calls extractClientCertificateChain and returns the
  first entry in the array, if any, null otherwise.
  
  @param request
  The incoming HTTP request to search for the client's certificate.
  
  @return
  The client's mutual TLS certificate.
  
  @since 2.8