/** Notify the worker that new entry to delete appeared. */
  void signal() {
    lock.lock();

    try {
      force = true;

      cond.signalAll();
    } finally {
      lock.unlock();
    }
  }
  /** {@inheritDoc} */
  @Override
  protected void body() throws InterruptedException {
    if (log.isDebugEnabled()) log.debug("Delete worker started.");

    while (!cancelled) {
      lock.lock();

      try {
        if (!cancelled && !force) cond.await(FREQUENCY, TimeUnit.MILLISECONDS);

        force = false; // Reset force flag.
      } finally {
        lock.unlock();
      }

      if (!cancelled) delete();
    }
  }