docs: @Nonnull
    public Serializable serializeState ()
    {
      final ICommonsMap <String, Object> aState = new CommonsHashMap <> ();
      for (final Map.Entry <String, Object> entry : m_aAttributes.entrySet ())
      {
        final String sName = entry.getKey ();
        final Object aValue = entry.getValue ();
        if (aValue instanceof Serializable)
        {
          aState.put (sName, aValue);
        }
        else
        {
          // Not serializable... Servlet containers usually automatically
          // unbind the attribute in this case.
          if (aValue instanceof HttpSessionBindingListener)
          {
            ((HttpSessionBindingListener) aValue).valueUnbound (new HttpSessionBindingEvent (this, sName, aValue));
          }
        }
      }
      m_aAttributes.clear ();
      return aState;
    }