private CopyOnWriteArraySet<BrokerChangeListener> getListenerList(String topic) {
        CopyOnWriteArraySet<BrokerChangeListener> set = this.brokerChangeListeners.get(topic);
        if (set == null) {
            set = new CopyOnWriteArraySet<ProducerZooKeeper.BrokerChangeListener>();
            CopyOnWriteArraySet<BrokerChangeListener> oldSet = this.brokerChangeListeners.putIfAbsent(topic, set);
            if (oldSet != null) {
                set = oldSet;
            }
        }
        return set;
    }
called when broker list changed.

@param topic
metamorphosis-client/src/main/java/com/taobao/metamorphosis/client/producer/ProducerZooKeeper.java