public static MqttConnectReturnCode connectReturnCodeForException(Throwable cause) {
	MqttConnectReturnCode code = MqttConnectReturnCode.CONNECTION_REFUSED_SERVER_UNAVAILABLE;
	if (cause instanceof MqttUnacceptableProtocolVersionException) {
		// 不支持的协议版本
		code = MqttConnectReturnCode.CONNECTION_REFUSED_UNACCEPTABLE_PROTOCOL_VERSION;
	} else if (cause instanceof MqttIdentifierRejectedException) {
		// 不合格的clientId
		code = MqttConnectReturnCode.CONNECTION_REFUSED_IDENTIFIER_REJECTED;
	} else {
		code = MqttConnectReturnCode.CONNECTION_REFUSED_SERVER_UNAVAILABLE;
	}
	return code;
}