return g;
        }

        List<MetadataGraphVertex> visited = new ArrayList<>(g.getVertices().size());
        visit(g.getEntry(), visited, g);

        List<MetadataGraphVertex> dropList = new ArrayList<>(g.getVertices().size());

        // collect drop list
        for (MetadataGraphVertex v : g.getVertices()) {
            if (!visited.contains(v)) {
                dropList.add(v);
            }
        }

   *
   * Visited Nodes = {}
   * EndpointPair [N1, N2] - return
   * EndpointPair [N1, N3] - return
   * Visited Nodes = {N1}
   * EndpointPair [N2, N1] - skip
   * EndpointPair [N2, N3] - return
   * Visited Nodes = {N1, N2}
   * EndpointPair [N3, N1] - skip
   * EndpointPair [N3, N2] - skip
   * Visited Nodes = {N1, N2, N3}
   * EndpointPair [N4, N4] - return
   * Visited Nodes = {N1, N2, N3, N4}

 *   }
 * }.visit(type);
 * }
 *
 * <p>One {@code Type} is visited at most once. The second time the same type is visited, it's
 * ignored by {@link #visit}. This avoids infinite recursion caused by recursive type bounds.
 *
 * <p>This class is not thread safe.
 *
 * @author Ben Yu
 */
abstract class TypeVisitor {

  private final Set<Type> visited = new HashSet<>();

  /**

        ClasspathContainer cpc;

        List<MetadataGraphVertex> visited;

        // -----------------------------------------------------------------------
        protected ClasspathGraphVisitor(MetadataGraph cleanGraph, ClasspathContainer cpc) {
            this.cpc = cpc;
            this.graph = cleanGraph;

            visited = new ArrayList<>(cleanGraph.getVertices().size());
        }

     *
     * @param sessionId The session ID.
     */
    void saveSession(String sessionId);

    /**
     * Checks if a URL has been visited.
     *
     * @param urlQueue The URL queue.
     * @return true if the URL has been visited, otherwise false.
     */
    boolean visited(QUEUE urlQueue);

    /**
     * Generates URL queues.
     *
     * @param previousSessionId The previous session ID.

    public void saveSession(final String sessionId) {
        // NOP
    }

    /*
     * (non-Javadoc)
     *
     * @see org.codelibs.fess.crawler.service.UrlQueueService#visited(UrlQueue)
     */
    @Override
    public boolean visited(final UrlQueueImpl<Long> urlQueue) {
        final Queue<UrlQueueImpl<Long>> urlQueueList = dataHelper.getUrlQueueList(urlQueue.getSessionId());
        synchronized (urlQueueList) {

        final CrawlerContext crawlerContext = CrawlingParameterUtil.getCrawlerContext();
        final UrlQueueService<UrlQueue<?>> urlQueueService = CrawlingParameterUtil.getUrlQueueService();
        if (!urlQueueService.visited(urlQueue)) {
            if (checkAccessCount(crawlerContext)) {
                final AccessResult<?> accessResult = createAccessResult(responseData, resultData);
                if (logger.isDebugEnabled()) {

        // TODO use cache
    }

    /**
     * Checks if a URL has been visited by looking in both the queue and access results.
     *
     * @param urlQueue The URL queue entry to check.
     * @return true if the URL has been visited, false otherwise.
     */
    @Override
    public boolean visited(final OpenSearchUrlQueue urlQueue) {
        final String url = urlQueue.getUrl();

 * class CrawlingService extends AbstractScheduledService {
 *   private Set<Uri> visited;
 *   private Queue<Uri> toCrawl;
 *   protected void startUp() throws Exception {
 *     toCrawl = readStartingUris();
 *   }
 *
 *   protected void runOneIteration() throws Exception {
 *     Uri uri = toCrawl.remove();
 *     Collection<Uri> newUris = crawl(uri);
 *     visited.add(uri);
 *     for (Uri newUri : newUris) {

      } finally {
        currentThread.setName(originalName);
      }
    }
  }

  public void fetch(HttpUrl url) throws IOException {
    // Skip hosts that we've visited many times.
    AtomicInteger hostnameCount = new AtomicInteger();
    AtomicInteger previous = hostnames.putIfAbsent(url.host(), hostnameCount);
    if (previous != null) hostnameCount = previous;