Object nextLabel;
          synchronized (PerListenerQueue.this) {
            Preconditions.checkState(isThreadScheduled);
            nextToRun = waitQueue.poll();
            nextLabel = labelQueue.poll();
            if (nextToRun == null) {
              isThreadScheduled = false;
              stillRunning = false;
              break;
            }
          }

      while (true) {
        Future<T> f = futureQueue.poll();
        if (f == null) {
          if (ntasks > 0) {
            --ntasks;
            futures.add(submitAndAddQueueListener(executorService, it.next(), futureQueue));
            ++active;
          } else if (active == 0) {
            break;
          } else if (timed) {
            f = futureQueue.poll(timeoutNanos, TimeUnit.NANOSECONDS);

    assertEquals(0, queue.size());

    try {
      queue.element();
      fail();
    } catch (NoSuchElementException expected) {
    }

    assertNull(queue.peek());
    assertNull(queue.poll());
    try {
      queue.remove();
      fail();
    } catch (NoSuchElementException expected) {
    }
  }

  public void testRemainingCapacity_maxSize0() {

                // from execute().
                workerRunCount++;
                workerRunningState = RUNNING;
                hasSetRunning = true;
              }
            }
            task = queue.poll();
            if (task == null) {
              workerRunningState = IDLE;
              return;
            }
          }

    assertEquals(0, queue.size());

    try {
      queue.element();
      fail();
    } catch (NoSuchElementException expected) {
    }

    assertNull(queue.peek());
    assertNull(queue.poll());
    try {
      queue.remove();
      fail();
    } catch (NoSuchElementException expected) {
    }
  }

  public void testRemainingCapacity_maxSize0() {

                final Map<String, Object> localDataMap =
                        dataMap.entrySet().stream().collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue));
                String processingUrl = urlQueue.poll();
                processedUrls.add(processingUrl);
                if (deleteUrlList.contains(processingUrl)) {
                    deleteDocuments(); // delete before indexing
                }
                try {

    }
  }

  void checkEmpty(BlockingQueue<?> q) {
    try {
      assertTrue(q.isEmpty());
      assertEquals(0, q.size());
      assertNull(q.peek());
      assertNull(q.poll());
      assertNull(q.poll(0, MILLISECONDS));
      assertEquals("[]", q.toString());
      assertTrue(Arrays.equals(q.toArray(), new Object[0]));
      assertFalse(q.iterator().hasNext());
      try {
        q.element();

      }
    }

    @GuardedBy("this")
    void drainKeyReferenceQueue(ReferenceQueue<K> keyReferenceQueue) {
      Reference<? extends K> ref;
      int i = 0;
      while ((ref = keyReferenceQueue.poll()) != null) {
        @SuppressWarnings("unchecked")
        E entry = (E) ref;
        map.reclaimKey(entry);
        if (++i == DRAIN_MAX) {
          break;
        }
      }
    }

   */
  protected void startUp() throws Exception {}

  /**
   * Run the service. This method is invoked on the execution thread. Implementations must respond
   * to stop requests. You could poll for lifecycle changes in a work loop:
   *
   * <pre>
   *   public void run() {
   *     while ({@link #isRunning()}) {
   *       // perform a unit of work
   *     }
   *   }
   * </pre>
   *

        clearValueReferenceQueue();
      }
    }

    void clearKeyReferenceQueue() {
      while (keyReferenceQueue.poll() != null) {}
    }

    void clearValueReferenceQueue() {
      while (valueReferenceQueue.poll() != null) {}
    }

    // recency queue, shared by expiration and eviction

    /**