* string {@code "Foo<Bar>"}.
 *
 * <p>An {@code Escaper} instance is required to be stateless, and safe when used concurrently by
 * multiple threads.
 *
 * <p>Because, in general, escaping operates on the code points of a string and not on its
 * individual {@code char} values, it is not safe to assume that {@code escape(s)} is equivalent to

   * As such, this field must be cleared before every map mutation.
   *
   * Note about volatile: volatile doesn't make it safe to read from a mutable graph in one thread
   * while writing to it in another. All it does is help with _reading_ from multiple threads
   * concurrently. For more information, see AbstractNetworkTest.concurrentIteration.
   */
  private transient volatile @Nullable Entry<K, V> cacheEntry;

 * an {@code ArrayTable} concurrently and one of the threads modifies its value, there is no
 * guarantee that the new value will be fully visible to the other threads. To guarantee that
 * modifications are visible, synchronize access to the table. Unlike other {@code Table}
 * implementations, synchronization is unnecessary between a thread that writes to one cell and a

 *  New: Support [JPMS]. We replaced our `Automatic-Module-Name` metadata with proper
    `module-info.java` files.

 *  Fix: Recover gracefully when worker threads are interrupted. When we introduced fast fallback in
    OkHttp 5.0, we started using background threads while connecting. Sadly that code didn't handle
    interruptions well. This is now fixed.

 *  Upgrade: [Kotlin 2.2.20][kotlin_2_2_20].

     * Search criteria: target URLs.
     */
    public String urls;

    /**
     * Search criteria: user agent string.
     */
    public String userAgent;

    /**
     * Search criteria: number of threads.
     */
    public String numOfThread;

    /**
     * Search criteria: interval time.
     */
    public String intervalTime;

    /**
     * Search criteria: boost value.
     */

    // 1. Subscribers to events posted on different threads can be interleaved with each other
    //    freely. (A event on one thread, B event on another could yield any of
    //    [a1, a2, a3, b1, b2], [a1, b2, a2, a3, b2], [a1, b2, b3, a2, a3], etc.)
    // 2. It's possible for subscribers to actually be dispatched to in a different order than they
    //    were added to the queue. It's easily possible for one thread to take the head of the

 * acquired, permits need not be released.
 *
 * <p>{@code RateLimiter} is safe for concurrent use: It will restrict the total rate of calls from
 * all threads. Note, however, that it does not guarantee fairness.
 *
 * <p>Rate limiters are often used to restrict the rate at which some physical or logical resource

  ├─ ExecutionTimeoutException
  ├─ MimeTypeException
  ├─ MultipleCrawlingAccessException
  ├─ RobotsTxtException
  └─ SitemapsException
```

### Thread-Local Storage

Use `CrawlingParameterUtil` to set/get `CrawlerContext` and `UrlQueue` in worker threads. Always clear in finally block with `CrawlingParameterUtil.clearAll()`.

### Resource Cleanup Pattern

   * {@linkplain Thread#getContextClassLoader() context class loader}. In simple environments, the
   * context class loader will find resources from the class path. In environments where different
   * threads can have different class loaders, for example app servers, the context class loader
   * will typically have been set to an appropriate loader for the current thread.
   *

       * timeoutFuture can be set (and delegate cancelled) only once. (And "set only once" is
       * important for other reasons: run() can still be invoked concurrently in different threads,
       * even with the above null checks.)
       */
      timeoutFutureRef = null;
      if (delegate.isDone()) {
        timeoutFuture.setFuture(delegate);
      } else {
        try {