throws Throwable
  public boolean session_set_save_handler(Env env,                                          Callback open,                                          Callback close,                                          Callback read,                                          Callback write,                                          Callback directory,                                          Callback gc)    throws Throwable  {    SessionCallback cb = new SessionCallback(open,                                             close,                                             read,                                             write,                                             directory,                                             gc);    env.setSessionCallback(cb);    return true;  }