### Exception Hierarchy

```
CrawlerSystemException (unchecked)  # Setup/config errors
CrawlingAccessException (checked)   # Runtime crawl errors
  ├─ MaxLengthExceededException
  └─ ChildUrlsException
ExtractException (checked)          # Extraction failures
  └─ UnsupportedExtractException
```

### Thread-Local Storage

```java
// Set (in CrawlerThread)

      } catch (Exception e) { // sneaky checked exception
        if (exception == null) {
          exception = e;
        } else {
          exception.addSuppressed(e);
        }
      }
    }
    if (exception != null) {
      // Normally this is a RuntimeException that doesn't need sneakyThrow.
      // But theoretically we could see sneaky checked exception
      sneakyThrow(exception);
    }
  }

      }

      @Override
      public void run() {
        try {
          delegate.run();
        } catch (Throwable t) {
          // Any Exception is either a RuntimeException or sneaky checked exception.

          /*
           * We fail this `ListenableFuture`, whose result is exposed to the user through the
           * `ListenableScheduledTask` we return from the `schedule*` methods.
           */

      return this;
    }

    private void add(@Nullable Object[] elements, int n) {
      ensureRoomFor(n);
      /*
       * The following call is not statically checked, since arraycopy accepts plain Object for its
       * parameters. If it were statically checked, the checker would still be OK with it, since
       * we're copying into a `contents` array whose type allows it to contain nulls. Still, it's

   * true when removing a connection from the pool; otherwise a racing caller might get it from the
   * pool when it shouldn't. Symmetrically, this must always be checked before returning a
   * connection from the pool.
   *
   * Once true this is always true. Guarded by this.
   */
  var noNewExchanges = false

  /**

    fun Response.hasVaryAll(): Boolean = "*" in headers.varyFields()

    /**
     * Returns the names of the request headers that need to be checked for equality when caching.
     */
    private fun Headers.varyFields(): Set<String> {
      var result: MutableSet<String>? = null
      for (i in 0 until size) {

    @Override
    public void rejectedExecution(Runnable r, ThreadPoolExecutor executor) {}
  }

  /**
   * A CyclicBarrier that uses timed await and fails with AssertionFailedErrors instead of throwing
   * checked exceptions.
   */
  public class CheckedBarrier extends CyclicBarrier {
    public CheckedBarrier(int parties) {
      super(parties);
    }

    @Override
    public int await() {

     * indicating that the URL should not be crawled. Otherwise, it returns {@code true}.
     *
     * @param paramMap the parameter map containing potential exclusion patterns
     * @param url the URL to be checked for crawlability
     * @return {@code true} if the URL is crawlable; {@code false} if it matches the exclusion pattern
     */
    protected boolean isUrlCrawlable(final DataStoreParams paramMap, final String url) {

     *          since casting to long is monotonic
     * k <= (long) Math.sqrt(x) <= k + 1
     *          since (long) Math.sqrt(k * k) == k, as checked exhaustively in
     *          {@link LongMathTest#testSqrtOfPerfectSquareAsDoubleIsPerfect}
     */
    long guess = (long) Math.sqrt((double) x);
    // Note: guess is always <= FLOOR_SQRT_MAX_LONG.

    assertThrows(UncheckedExecutionException.class, () -> cache.getUnchecked("bar"));
    assertThat(callCount.get()).isEqualTo(2);
  }

  /**
   * On a concurrent computation that throws a checked exception, all threads should get the
   * (wrapped) exception, with the loader called only once. The result should not be cached (a later
   * request should call the loader again).
   */