@Override
protected <T> Invoker<T> doSelect(List<Invoker<T>> invokers, URL url, Invocation invocation) {
    String key = invokers.get(0).getUrl().getServiceKey() + "." + invocation.getMethodName();

    //活跃服务提供者集合
    ConcurrentMap<String, WeightedRoundRobin> map = methodWeightMap.get(key);
    if (map == null) {
        methodWeightMap.putIfAbsent(key, new ConcurrentHashMap<String, WeightedRoundRobin>());
        map = methodWeightMap.get(key);
    }
    int totalWeight = 0;
    long maxCurrent = Long.MIN_VALUE;
    long now = System.currentTimeMillis();
    Invoker<T> selectedInvoker = null;
    WeightedRoundRobin selectedWRR = null;

//找出活跃的权重最大的服务提供者为目标服务提供者
    for (Invoker<T> invoker : invokers) {
        String identifyString = invoker.getUrl().toIdentityString();
        WeightedRoundRobin weightedRoundRobin = map.get(identifyString);
        int weight = getWeight(invoker, invocation);
        if (weight < 0) {
            weight = 0;
        }
        if (weightedRoundRobin == null) {
            weightedRoundRobin = new WeightedRoundRobin();
            weightedRoundRobin.setWeight(weight);
            map.putIfAbsent(identifyString, weightedRoundRobin);
            weightedRoundRobin = map.get(identifyString);
        }
        if (weight != weightedRoundRobin.getWeight()) {
            //weight changed
            weightedRoundRobin.setWeight(weight);
        }
//每轮询一次，服务提供者的当前权限增长给定步长
        long cur = weightedRoundRobin.increaseCurrent();
        weightedRoundRobin.setLastUpdate(now);

//当前服务提供者权重大于最大权重，最大权重修改为当前服务提供者权重，目标服务提供者设置为当前服务提供者
        if (cur > maxCurrent) {
            maxCurrent = cur;
            selectedInvoker = invoker;
            selectedWRR = weightedRoundRobin;
        }

//用于重新目标服务提供者权重
        totalWeight += weight;
    }

//回收活跃的服务提供者
    if (!updateLock.get() && invokers.size() != map.size()) {
        if (updateLock.compareAndSet(false, true)) {
            try {
                // copy -> modify -> update reference
                ConcurrentMap<String, WeightedRoundRobin> newMap = new ConcurrentHashMap<String, WeightedRoundRobin>();
                newMap.putAll(map);
                Iterator<Entry<String, WeightedRoundRobin>> it = newMap.entrySet().iterator();
                while (it.hasNext()) {
                    Entry<String, WeightedRoundRobin> item = it.next();
                    if (now - item.getValue().getLastUpdate() > RECYCLE_PERIOD) {
                        it.remove();
                    }
                }
                methodWeightMap.put(key, newMap);
            } finally {
                updateLock.set(false);
            }
        }
    }

//目标服务提供者不为空，则将目标服务提供者权重设置为最低：current.addAndGet(-1 * total)
    if (selectedInvoker != null) {
        selectedWRR.sel(totalWeight);
        return selectedInvoker;
    }

//返回第一个为目标服务提供者
    // should not happen here
    return invokers.get(0);
}