public List<Channel> get(final String topic) {
        return TOPIC_CHANNEL_FACTORY.getOrDefault(topic, new CopyOnWriteArrayList<>());
    }
get Channels.
@param topic topic
@return Channels
shenyu-protocol/shenyu-protocol-mqtt/src/main/java/org/apache/shenyu/protocol/mqtt/repositories/SubscribeRepository.java