code: @Override
    public boolean isModified()
    {
      long now = CurrentTime.currentTime();
      long lastCheckTime = _lastCheckTime.get();
      
      if (now <= lastCheckTime + _checkInterval) {
        return _isModified;
      }
  
      if (! _lastCheckTime.compareAndSet(lastCheckTime, now)) {
        return _isModified;
      }
  
      if (isModifiedImpl()) {
        _isModified = true;
        //_lastCheckTime.set(0);
      }
      else {
        _isModified = false;
      }
  
      return _isModified;
    }
docs: Returns true if the underlying resource has changed.
  
  XXX: restore isModified() with