dubbo服務端客戶端長連接心跳

當客戶端從註冊中心拿到服務端的ip地址之後，客戶端與服務端之間建立連接，那這條建立的連接如何去校驗狀態呢。

DubboProtocol類中可以看到，默認開啓heartbeat

    /**
     * 創建新連接.
     */
    private ExchangeClient initClient(URL url) {
        
        // client type setting.
        String str = url.getParameter(Constants.CLIENT_KEY, url.getParameter(Constants.SERVER_KEY, Constants.DEFAULT_REMOTING_CLIENT));

        String version = url.getParameter(Constants.DUBBO_VERSION_KEY);
        boolean compatible = (version != null && version.startsWith("1.0."));
        url = url.addParameter(Constants.CODEC_KEY, Version.isCompatibleVersion() && compatible ? COMPATIBLE_CODEC_NAME : DubboCodec.NAME);
        //默認開啓heartbeat
        url = url.addParameterIfAbsent(Constants.HEARTBEAT_KEY, String.valueOf(Constants.DEFAULT_HEARTBEAT));
        
        // BIO存在嚴重性能問題，暫時不允許使用
        if (str != null && str.length() > 0 && ! ExtensionLoader.getExtensionLoader(Transporter.class).hasExtension(str)) {
            throw new RpcException("Unsupported client type: " + str + "," +
                    " supported client type is " + StringUtils.join(ExtensionLoader.getExtensionLoader(Transporter.class).getSupportedExtensions(), " "));
        }
        
        ExchangeClient client ;
        try {
            //設置連接應該是lazy的 
            if (url.getParameter(Constants.LAZY_CONNECT_KEY, false)){
                client = new LazyConnectExchangeClient(url ,requestHandler);
            } else {
                client = Exchangers.connect(url ,requestHandler);
            }
        } catch (RemotingException e) {
            throw new RpcException("Fail to create remoting client for service(" + url
                    + "): " + e.getMessage(), e);
        }
        return client;
    }