/** {@inheritDoc} */
  @Override
  public void addAttributeListener(GridTaskSessionAttributeListener lsnr, boolean rewind) {
    A.notNull(lsnr, "lsnr");

    Map<Object, Object> attrs = null;

    List<GridTaskSessionAttributeListener> lsnrs;

    synchronized (mux) {
      lsnrs = new ArrayList<GridTaskSessionAttributeListener>(this.lsnrs.size());

      lsnrs.addAll(this.lsnrs);

      lsnrs.add(lsnr);

      lsnrs = Collections.unmodifiableList(lsnrs);

      this.lsnrs = lsnrs;

      if (rewind) attrs = new HashMap<Object, Object>(this.attrs);
    }

    if (rewind) {
      for (Map.Entry<Object, Object> entry : attrs.entrySet()) {
        for (GridTaskSessionAttributeListener l : lsnrs) {
          l.onAttributeSet(entry.getKey(), entry.getValue());
        }
      }
    }
  }
  /** {@inheritDoc} */
  @Override
  public boolean removeAttributeListener(GridTaskSessionAttributeListener lsnr) {
    A.notNull(lsnr, "lsnr");

    synchronized (mux) {
      List<GridTaskSessionAttributeListener> lsnrs =
          new ArrayList<GridTaskSessionAttributeListener>(this.lsnrs);

      boolean rmv = lsnrs.remove(lsnr);

      this.lsnrs = Collections.unmodifiableList(lsnrs);

      return rmv;
    }
  }