code: protected void initZipOutputStream( ZipArchiveOutputStream zOut )
          throws ArchiverException, IOException
      {
          // If no webxml file is specified, it's an error.
          if ( expectWebXml && deploymentDescriptor == null && !isInUpdateMode() )
          {
              throw new ArchiverException( "webxml attribute is required (or pre-existing WEB-INF/web.xml if executing in update mode)" );
          }
          super.initZipOutputStream( zOut );
      }
docs: override of  parent; validates configuration
  before initializing the output stream.