根据响应时间计算权重，响应越长，权重越低，权重越低的服务器，被选择的可能性就越低

//定时统计权重
serverWeightTimer.schedule(new DynamicServerWeightTask(), 0,

        serverWeightTaskTimerInterval);

//权重数组（server存储的位置对应权重数组的位置）
//index处的权重为前面所有权重及index本身权重之和
private volatile List<Double> accumulatedWeights = new ArrayList<Double>();

//随机0-最大权重
double randomWeight = random.nextDouble() * maxTotalWeight;
// pick the server index based on the randomIndex
int n = 0;
for (Double d : currentWeights) {
    if (d >= randomWeight) { //选出落在这个权重位置的index
        serverIndex = n;
        break;
    } else {
        n++;
    }
}

server = allList.get(serverIndex);