smack PacketListener的總結

    用smack作一個xmpp客戶端彷佛是一件很簡單的事情。可是前幾天發現的一個bug，仍是讓我思考了不少。

    用smack創建一個xmpp connection。而後在connection 上註冊了一個PacketListener。

    可是程序裏面在一個for循環裏面去new PakcetListener，並把listener註冊到connection上面。這樣致使的結果就是

    smack客戶端收到許多相同的IQ response。

    若是稍微看一下smack的源代碼就明白問題出在哪裏。

    smack connection class：

/**
     * Registers a packet listener with this connection. A packet filter determines
     * which packets will be delivered to the listener. If the same packet listener
     * is added again with a different filter, only the new filter will be used.
     * 
     * @param packetListener the packet listener to notify of new received packets.
     * @param packetFilter   the packet filter to use.
     */
    public void addPacketListener(PacketListener packetListener, PacketFilter packetFilter) {
        if (packetListener == null) {
            throw new NullPointerException("Packet listener is null.");
        }
        ListenerWrapper wrapper = new ListenerWrapper(packetListener, packetFilter);
        recvListeners.put(packetListener, wrapper);
    }

    

/**
     * List of PacketListeners that will be notified when a new packet was received.
     */
    protected final Map<PacketListener, ListenerWrapper> recvListeners =
            new ConcurrentHashMap<PacketListener, ListenerWrapper>();

看完上面這兩段代碼，就知道了。若是重複註冊listener。其實就意味着一個packet會被notify屢次。

程序就回出現一些奇怪的現象。