* UnsignedBytesBenchmark. TODO(cpovirk): benchmark this
       */
      /*
       * A CopyOnWriteArraySet<WeakReference> is faster than a newSetFromMap of a MapMaker map with
       * weakKeys() and concurrencyLevel(1), even up to at least 12 cached exception types.
       */
      private static final Set<WeakReference<Class<? extends Exception>>> validClasses =

@GwtIncompatible
@ElementTypesAreNonnullByDefault
public class FinalizableReferenceQueue implements Closeable {
  /*
   * The Finalizer thread keeps a phantom reference to this object. When the client (for example, a
   * map built by MapMaker) no longer has a strong reference to this object, the garbage collector
   * will reclaim it and enqueue the phantom reference. The enqueued reference will trigger the
   * Finalizer to stop.
   *

@GwtIncompatible
@ElementTypesAreNonnullByDefault
public class FinalizableReferenceQueue implements Closeable {
  /*
   * The Finalizer thread keeps a phantom reference to this object. When the client (for example, a
   * map built by MapMaker) no longer has a strong reference to this object, the garbage collector
   * will reclaim it and enqueue the phantom reference. The enqueued reference will trigger the
   * Finalizer to stop.
   *