@Override
public void channelRead(ChannelHandlerContext ctx, Object msg) throws Exception {
    //以ctx.channel()为key，以NettyChannel为value，存储在ConcurrentHashMap中
    NettyChannel channel = NettyChannel.getOrAddChannel(ctx.channel(), url, handler);
    try {
        handler.received(channel, msg);
    } finally {
        NettyChannel.removeChannelIfDisconnected(ctx.channel());
    }
}

//直接在配置文件中配置url，实现直接通信（如果既配置了直连地址又配置了注册中心的地址，则自动忽略注册中心的地址）
if (url != null && url.length() > 0) { // user specified URL, could be peer-to-peer address, or register center's address.
    String[] us = Constants.SEMICOLON_SPLIT_PATTERN.split(url);
    if (us != null && us.length > 0) {
        for (String u : us) {
            URL url = URL.valueOf(u);
            if (url.getPath() == null || url.getPath().length() == 0) {
                url = url.setPath(interfaceName);
            }
            if (Constants.REGISTRY_PROTOCOL.equals(url.getProtocol())) {
                urls.add(url.addParameterAndEncoded(Constants.REFER_KEY, StringUtils.toQueryString(map)));
            } else {
                urls.add(ClusterUtils.mergeUrl(url, map));
            }
        }
    }
} else { // assemble URL from register center's configuration
    //获取配置注册中心的url（可以有多个注册中心的url）
    List<URL> us = loadRegistries(false);
    if (us != null && !us.isEmpty()) {
        for (URL u : us) {
            URL monitorUrl = loadMonitor(u);
            if (monitorUrl != null) {
                map.put(Constants.MONITOR_KEY, URL.encode(monitorUrl.toFullString()));
            }
            urls.add(u.addParameterAndEncoded(Constants.REFER_KEY, StringUtils.toQueryString(map)));
        }
    }
    //如果urls为空，则抛出异常
    if (urls.isEmpty()) {
        throw new IllegalStateException("No such any registry to reference " + interfaceName + " on the consumer " + NetUtils.getLocalHost() + " use dubbo version " + Version.getVersion() + ", please config <dubbo:registry address=\"...\" /> to your spring config.");
    }
}
//当urls的长度为一时，可能为服务的直连地址也可能为注册中心的地址
if (urls.size() == 1) {
    invoker = refprotocol.refer(interfaceClass, urls.get(0));
} else {//有多个直连地址，或者多个注册中心的地址
    List<Invoker<?>> invokers = new ArrayList<Invoker<?>>();
    URL registryURL = null;
    for (URL url : urls) {
        //获取每个url对应的invoker
        invokers.add(refprotocol.refer(interfaceClass, url));
        if (Constants.REGISTRY_PROTOCOL.equals(url.getProtocol())) {
            //获取最后一个注册中心的地址
            registryURL = url; // use last registry url
        }
    }
    if (registryURL != null) { // registry url is available
        // use AvailableCluster only when register's cluster is available
        //当有注册中心的地址时，第一层使用AvailableCluster集群策略，第二层使用默认的集群策略
        URL u = registryURL.addParameter(Constants.CLUSTER_KEY, AvailableCluster.NAME);
        invoker = cluster.join(new StaticDirectory(u, invokers));
    } else { // not a registry url
        //如果没有注册中心地址，则使用默认的集群策略
        invoker = cluster.join(new StaticDirectory(invokers));
    }
}

URL u = registryURL.addParameter(Constants.CLUSTER_KEY, AvailableCluster.NAME);
invoker = cluster.join(new StaticDirectory(u, invokers));

public class RandomLoadBalance extends AbstractLoadBalance {

    public static final String NAME = "random";

    @Override
    protected <T> Invoker<T> doSelect(List<Invoker<T>> invokers, URL url, Invocation invocation) {
        int length = invokers.size(); // Number of invokers
        int totalWeight = 0; // The sum of weights
        boolean sameWeight = true; // Every invoker has the same weight?
        for (int i = 0; i < length; i++) {
            //获取权重
            int weight = getWeight(invokers.get(i), invocation);
            //权重累积和
            totalWeight += weight; // Sum
        	//记录所有的invokers的weight是否是一样的
            if (sameWeight && i > 0
                    && weight != getWeight(invokers.get(i - 1), invocation)) {
                sameWeight = false;
            }
        }
        //如果不是所有的invoker权重都一样
        if (totalWeight > 0 && !sameWeight) {
            // If (not every invoker has the same weight & at least one invoker's weight>0), select randomly based on totalWeight.
            //获取随机数的范围是[0, totalWeight)
            int offset = ThreadLocalRandom.current().nextInt(totalWeight);
            // Return a invoker based on the random value.
            for (int i = 0; i < length; i++) {
                offset -= getWeight(invokers.get(i), invocation);
                if (offset < 0) {
                    //返回invoker
                    return invokers.get(i);
                }
            }
        }
        //如果所有的invoker都是一样的weight，则直接获取随机数，并返回
        //这里使用了ThreadLocalRandom做了优化
        // If all invokers have the same weight value or totalWeight=0, return evenly.
        return invokers.get(ThreadLocalRandom.current().nextInt(length));
    }

}