code: private void process(File file) throws WatchingException {
          getLog().info("Minifying CSS files from " + file.getName() + " using Clean CSS");
  
          File filtered = getFilteredVersion(file);
          if (filtered == null) {
              filtered = file;
          }
  
          File output = getMinifiedFile(file);
          if (output.exists()) {
              FileUtils.deleteQuietly(output);
          }
  
          if (!output.getParentFile().isDirectory()) {
              getLog().debug("Creating output directory for " + output.getAbsolutePath() + " : "
                      + output.getParentFile().mkdirs());
          }
  
          getLog().info("Minifying " + filtered.getAbsolutePath() + " to " + output.getAbsolutePath());
          try {
              List<String> arguments = new ArrayList<>();
              arguments.add("--output");
              arguments.add(output.getAbsolutePath());
              arguments.add("--source-map");
              if (cleanCssArguments != null) {
                  arguments.addAll(Splitter.on(' ').splitToList(cleanCssArguments));
              }
              // Input file:
              arguments.add(filtered.getAbsolutePath());
              int exit = cleancss.execute("cleancss", arguments.toArray(new String[arguments.size()]));
              getLog().debug("CSS minification execution exiting with " + exit + " status");
          } catch (MojoExecutionException e) {
              throw new WatchingException("Error during the minification of " + filtered.getName(), e);
          }
      }
docs: Minifies the CSS file using Clean CSS.
  
  @param file that we wish to minify.
  @throws WatchingException if errors occur during minification.