@GwtIncompatible
@ElementTypesAreNonnullByDefault
public abstract class FinalizablePhantomReference<T> extends PhantomReference<T>
    implements FinalizableReference {
  /**
   * Constructs a new finalizable phantom reference.
   *
   * @param referent to phantom reference
   * @param queue that should finalize the referent
   */
  protected FinalizablePhantomReference(@CheckForNull T referent, FinalizableReferenceQueue queue) {

 *   // You might also share this between several objects.
 *
 *   private static final Set<Reference<?>> references = Sets.newConcurrentHashSet();
 *   // This ensures that the FinalizablePhantomReference itself is not garbage-collected.
 *
 *   private final ServerSocket serverSocket;
 *
 *   private MyServer(...) {
 *     ...
 *     this.serverSocket = new ServerSocket(...);
 *     ...
 *   }
 *