import java.lang.ref.PhantomReference;
import java.lang.ref.ReferenceQueue;

public class TestIsEnqueued {

  private static final long ENQUEUE_CHECK_DELAY = 500; // 1/2 sec check period
  private static final long ENQUEUE_TIMEOUT = 30000;   // 30 sec timeout

  private static final ReferenceQueue<Object> q = new ReferenceQueue<>();
  private static Object o = new Object();
  private static final
  PhantomReference<Object> r = new PhantomReference<>(o, q);

  public static void main(String[] args) throws Throwable {

    // delete reference to o, and verify r gets notified.
    o = null;
    System.gc();
    // wait for enqueue, as it might not be instantaneous.
    for (long i = 0; i < ENQUEUE_TIMEOUT; i += ENQUEUE_CHECK_DELAY) {
      Thread.sleep(ENQUEUE_CHECK_DELAY);
      if (r.isEnqueued()) break;
    }
    if (!r.isEnqueued()) {
      throw new RuntimeException("reference not notified");
    }

    // now remove r from q, and test r's isEnqueued() state.
    Object qr = q.remove(ENQUEUE_TIMEOUT);
    if (qr == null) {
      throw new RuntimeException("empty queue");
    } else if (qr != r) {
      throw new RuntimeException("unexpected queue entry");
    } else if (!r.isEnqueued()) {
      throw new RuntimeException(
        "TEST FAILED: reference transitioned to not enqueued");
    }
  }
}