public String execute() {
        try {
            final long count = ComponentUtil.getThumbnailManager().purge(getExpiry());
            return "Deleted " + count + " thumbnail files.";
        } catch (final Exception e) {
            logger.error("Failed to purge thumbnails.", e);
            return e.getMessage();
        }
    }

    public long getExpiry() {
        return expiry;
    }

/src/main/webapp/WEB-INF/site/
/src/main/webapp/WEB-INF/plugin/*
!/src/main/webapp/WEB-INF/plugin/.keep
/src/main/webapp/WEB-INF/env/crawler/lib/
/src/main/webapp/WEB-INF/env/suggest/lib/
/src/main/webapp/WEB-INF/env/thumbnail/lib/
/src/main/webapp/WEB-INF/thumbnails/
/src/main/webapp/jar/
/dbflute_fess/extlib/*
/dbflute_fess/log/*.log
/dbflute_h2/log/*.log
/dbflute_mysql/log/*.log
/dbflute_oracle/log/*.log
/src/main/webapp/WEB-INF/conf/*.properties

            final int totalCount = process(options);
            if (totalCount != 0) {
                logger.info("Created {} thumbnail files.", totalCount);
            } else {
                logger.info("No new thumbnails found.");
            }
            exitCode = 0;
        } catch (final ContainerNotAvailableException e) {
            if (logger.isDebugEnabled()) {

        return getPath("WEB-INF/", "dict", names);
    }

    public static Path getThumbnailPath(final String... names) {
        return getPath("WEB-INF/", "thumbnails", names);
    }

    public static Path getSitePath(final String... names) {
        return getPath("WEB-INF/", "site", names);
    }

    public static Path getPluginPath(final String... names) {

        }

        final TimeoutTask timeoutTask = createTimeoutTask();
        try {
            executeThumbnailGenerator();
        } catch (final Exception e) {
            logger.warn("Failed to generate thumbnails.", e);
            resultBuf.append(e.getMessage()).append("\n");
        } finally {
            if (timeoutTask != null && !timeoutTask.isCanceled()) {
                timeoutTask.cancel();
            }
        }

        }

        if (logger.isDebugEnabled()) {
            logger.debug("Thumbnail Directory: {}", baseDir.getAbsolutePath());
        }

        thumbnailTaskQueue = new LinkedBlockingQueue<>(thumbnailTaskQueueSize);
        generating = !Constants.TRUE.equalsIgnoreCase(System.getProperty("fess.thumbnail.process"));
        thumbnailQueueThread = new Thread((Runnable) () -> {

    }

    @Override
    public boolean generate(final String thumbnailId, final File outputFile) {
        if (logger.isDebugEnabled()) {
            logger.debug("Generate Thumbnail: {}", thumbnailId);
        }

        if (outputFile.exists()) {
            if (logger.isDebugEnabled()) {
                logger.debug("The thumbnail file exists: {}", outputFile.getAbsolutePath());
            }
            return true;

  check_command convert
  check_command pdftoppm
  check_command unoconv
  tmp_pdf_file=/tmp/thumbnail.$$.pdf
  unoconv -e PageRange=1-1 -o ${tmp_pdf_file} -f pdf "${target_file}"
  if [[ ! -f ${tmp_pdf_file} ]] ; then
    echo "unoconv does not work."
    exit 1
  fi
  tmp_png_prefix=/tmp/thumbnail.png.$$
  pdftoppm -png -singlefile ${tmp_pdf_file} ${tmp_png_prefix}
  tmp_png_file="${tmp_png_prefix}.png"

                    }
                    break;
                default:
                    logger.error("Unknown thumbnail result: {} -> {}", thumbnailId, responseData.getUrl());
                    break;
                }
            } catch (final Throwable t) {

        final FessConfig fessConfig = ComponentUtil.getFessConfig();
        final String thumbnailId = DocumentUtil.getValue(docMap, fessConfig.getIndexFieldId(), String.class);
        final Tuple3<String, String, String> task = new Tuple3<>(getName(), thumbnailId, path);
        if (logger.isDebugEnabled()) {
            logger.debug("Create thumbnail task: {}", task);
        }
        return task;
    }