public boolean offer(T o) {
      return standardOffer(o);
    }

    @Override
    public @Nullable T peek() {
      return standardPeek();
    }

    @Override
    public @Nullable T poll() {
      return standardPoll();
    }
  }

  @AndroidIncompatible // test-suite builders
  public static Test suite() {
    TestSuite suite = new TestSuite();

 *     .maximumSize(1000)
 *     .create();
 * }
 *
 * <p>As a {@link Queue} it functions exactly as a {@link PriorityQueue}: its head element -- the
 * implicit target of the methods {@link #peek()}, {@link #poll()} and {@link #remove()} -- is
 * defined as the <i>least</i> element in the queue according to the queue's comparator. But unlike
 * a regular priority queue, the methods {@link #peekLast}, {@link #pollLast} and {@link

  @CanIgnoreReturnValue
  @Override
  public @Nullable E poll() {
    Monitor monitor = this.monitor;
    if (monitor.enterIf(notEmpty)) {
      try {
        return extract();
      } finally {
        monitor.leave();
      }
    } else {
      return null;
    }
  }

  @CanIgnoreReturnValue
  @Override
  public @Nullable E poll(long timeout, TimeUnit unit) throws InterruptedException {

  /**
   * Starts the Finalizer thread. FinalizableReferenceQueue calls this method reflectively.
   *
   * @param finalizableReferenceClass FinalizableReference.class.
   * @param queue a reference queue that the thread will poll.
   * @param frqReference a phantom reference to the FinalizableReferenceQueue, which will be queued
   *     either when the FinalizableReferenceQueue is no longer referenced anywhere, or when its
   *     close() method is called.

  @Override
  public @Nullable E poll() {
    Monitor monitor = this.monitor;
    monitor.enter();
    try {
      return q.poll();
    } finally {
      monitor.leave();
    }
  }

  @CanIgnoreReturnValue // pushed down from class to method
  @Override
  public @Nullable E poll(long timeout, TimeUnit unit) throws InterruptedException {

  /**
   * Starts the Finalizer thread. FinalizableReferenceQueue calls this method reflectively.
   *
   * @param finalizableReferenceClass FinalizableReference.class.
   * @param queue a reference queue that the thread will poll.
   * @param frqReference a phantom reference to the FinalizableReferenceQueue, which will be queued
   *     either when the FinalizableReferenceQueue is no longer referenced anywhere, or when its
   *     close() method is called.

 *     .maximumSize(1000)
 *     .create();
 * }
 *
 * <p>As a {@link Queue} it functions exactly as a {@link PriorityQueue}: its head element -- the
 * implicit target of the methods {@link #peek()}, {@link #poll()} and {@link #remove()} -- is
 * defined as the <i>least</i> element in the queue according to the queue's comparator. But unlike
 * a regular priority queue, the methods {@link #peekLast}, {@link #pollLast} and {@link

- **CrawlerContext** (`CrawlerContext.java`): Execution context - `sessionId`, `status`, `accessCount`, `numOfThread`, `maxDepth`, `maxAccessCount`
- **CrawlerThread** (`CrawlerThread.java`): Worker thread - Poll URL → Validate → Execute → Process → Queue children

### HTTP Client Architecture

```
SwitchableHttpClient (extends FaultTolerantClient)
    ├── Hc5HttpClient (default) - Apache HttpComponents 5.x

        assertFalse(map.containsValue(valueOne));
        assertThat(map.get(keyOne)).isNull();
        assertEquals(1, map.size());
        assertThat(segment.getKeyReferenceQueueForTesting().poll()).isNull();
      }
    }
  }

  public void testDrainValueReferenceQueueOnWrite() {
    for (MapMaker maker : allWeakValueStrengthMakers()) {

        assertFalse(map.containsValue(valueOne));
        assertThat(map.get(keyOne)).isNull();
        assertEquals(1, map.size());
        assertThat(segment.getKeyReferenceQueueForTesting().poll()).isNull();
      }
    }
  }

  public void testDrainValueReferenceQueueOnWrite() {
    for (MapMaker maker : allWeakValueStrengthMakers()) {