code: protected void addDirectory( ZipArchiver zipArchiver, File directory, String prefix, boolean includeEmptyFolders )
      {
          if ( directory != null && directory.exists() )
          {
              final DefaultFileSet fileSet = new DefaultFileSet();
              fileSet.setPrefix( endWithSlash( prefix ) );
              fileSet.setDirectory( directory );
              fileSet.setIncludingEmptyDirectories( includeEmptyFolders );
              zipArchiver.addFileSet( fileSet );
              getLog().debug( "Added files from " + directory );
          }
      }
docs: Adds a directory to a {@link JarArchiver} with a directory prefix.
  
  @param zipArchiver   ZipArchiver to use to archive the file.
  @param directory     The directory to add.
  @param prefix        An optional prefix for where in the Jar file the directory's contents should go.
  @param includeEmptyFolders   Whether to include an entry for empty folder in the archive.