/**
   * collect saved Reflections resources from all urls that contains the given packagePrefix and
   * matches the given resourceNameFilter and de-serializes them using the default serializer {@link
   * XmlSerializer} or using the optionally supplied optionalSerializer
   *
   * <p>it is preferred to use a designated resource prefix (for example META-INF/reflections but
   * not just META-INF), so that relevant urls could be found much faster
   *
   * @param optionalSerializer - optionally supply one serializer instance. if not specified or
   *     null, {@link XmlSerializer} will be used
   */
  public static Reflections collect(
      final String packagePrefix,
      final Predicate<String> resourceNameFilter,
      @Nullable Serializer... optionalSerializer) {
    Serializer serializer =
        optionalSerializer != null && optionalSerializer.length == 1
            ? optionalSerializer[0]
            : new XmlSerializer();

    Collection<URL> urls = ClasspathHelper.forPackage(packagePrefix);
    if (urls.isEmpty()) return null;
    long start = System.currentTimeMillis();
    final Reflections reflections = new Reflections();
    Iterable<Vfs.File> files = Vfs.findFiles(urls, packagePrefix, resourceNameFilter);
    for (final Vfs.File file : files) {
      InputStream inputStream = null;
      try {
        inputStream = file.openInputStream();
        reflections.merge(serializer.read(inputStream));
      } catch (IOException e) {
        throw new ReflectionsException("could not merge " + file, e);
      } finally {
        close(inputStream);
      }
    }

    if (log != null) {
      Store store = reflections.getStore();
      int keys = 0;
      int values = 0;
      for (String index : store.keySet()) {
        keys += store.get(index).keySet().size();
        values += store.get(index).size();
      }

      log.info(
          format(
              "Reflections took %d ms to collect %d url%s, producing %d keys and %d values [%s]",
              System.currentTimeMillis() - start,
              urls.size(),
              urls.size() > 1 ? "s" : "",
              keys,
              values,
              Joiner.on(", ").join(urls)));
    }
    return reflections;
  }
  protected void scan() {
    if (configuration.getUrls() == null || configuration.getUrls().isEmpty()) {
      if (log != null) log.warn("given scan urls are empty. set urls in the configuration");
      return;
    }

    if (log != null && log.isDebugEnabled()) {
      log.debug("going to scan these urls:\n" + Joiner.on("\n").join(configuration.getUrls()));
    }

    long time = System.currentTimeMillis();
    int scannedUrls = 0;
    ExecutorService executorService = configuration.getExecutorService();
    List<Future<?>> futures = Lists.newArrayList();

    for (final URL url : configuration.getUrls()) {
      try {
        if (executorService != null) {
          futures.add(
              executorService.submit(
                  new Runnable() {
                    public void run() {
                      if (log != null && log.isDebugEnabled())
                        log.debug("[" + Thread.currentThread().toString() + "] scanning " + url);
                      scan(url);
                    }
                  }));
        } else {
          scan(url);
        }
        scannedUrls++;
      } catch (ReflectionsException e) {
        if (log != null && log.isWarnEnabled())
          log.warn("could not create Vfs.Dir from url. ignoring the exception and continuing", e);
      }
    }

    // todo use CompletionService
    if (executorService != null) {
      for (Future future : futures) {
        try {
          future.get();
        } catch (Exception e) {
          throw new RuntimeException(e);
        }
      }
    }

    time = System.currentTimeMillis() - time;

    // gracefully shutdown the parallel scanner executor service.
    if (executorService != null) {
      executorService.shutdown();
    }

    if (log != null) {
      int keys = 0;
      int values = 0;
      for (String index : store.keySet()) {
        keys += store.get(index).keySet().size();
        values += store.get(index).size();
      }

      log.info(
          format(
              "Reflections took %d ms to scan %d urls, producing %d keys and %d values %s",
              time,
              scannedUrls,
              keys,
              values,
              executorService != null && executorService instanceof ThreadPoolExecutor
                  ? format(
                      "[using %d cores]",
                      ((ThreadPoolExecutor) executorService).getMaximumPoolSize())
                  : ""));
    }
  }