dubbo的总体架构如图所示：
服务注册对于服务提供方，它需要发布服务，而且由于应用系统的复杂性，服务的数量、类型也不断膨胀；对于服务消费方，它最关心如何获取到它所需要的服务，而面对复杂的应用系统，需要管理大量的服务调用。而且，对于服务提供方和服务消费方来说，他们还有可能兼具这两种角色，即既需要提供服务，有需要消费服务。通过将服务统一管理起来，可以有效地优化内部应用对服务发布/使用的流程和管理。服务注册中心可以通过特定协议来完成服务对外的统一。Dubbo提供的注册中心有如下几种类型可供选择：
服务首先暴露在服务端，然后调用Registry的register方法在注册中心（它是一个服务协调中心，dubbo以外的独立服务端，dubbo提供了客户端实现）注册服务，然后用户通过配置文件中配置的service的url去subscribe(订阅服务)，Registry接收到订阅消息后会往url对应的的List<NotifyListener>中塞入当前NotifyListener，反之从这个list中移除listener就是取消订阅。registry会调用据consumer的订阅情况调用notify方法推送服务列表给Consumer。这里我们以Zookeeper注册中心来说明：
ZookeeperRegistry.java的构造函数，创建Zookeeper客户端：
public ZookeeperRegistry(URL url, ZookeeperTransporter zookeeperTransporter) {super(url);//如果provider的url是“0.0.0.0”或者在参数中带anyHost=true则抛出异常注册地址不存在if (url.isAnyHost()) {throw new IllegalStateException("registry address == null");}//服务分组（默认“dubbo”）String group = url.getParameter(Constants.GROUP_KEY, DEFAULT_ROOT);//在group头补齐“/”if (! group.startsWith(Constants.PATH_SEPARATOR)) {group = Constants.PATH_SEPARATOR + group;}//服务分组根地址this.root = group;//创建Zookeeper客户端zkClient = zookeeperTransporter.connect(url);//添加状态监听器zkClient.addStateListener(new StateListener() {public void stateChanged(int state) {if (state == RECONNECTED) {try {recover();} catch (Exception e) {logger.error(e.getMessage(), e);}}}});}
ZookeeperRegistry的doRegister方法：
protected void doRegister(URL url) {try {//连接注册中心注册zkClient.create(toUrlPath(url), url.getParameter(Constants.DYNAMIC_KEY, true));} catch (Throwable e) {throw new RpcException("Failed to register " + url + " to zookeeper " + getUrl() + ", cause: " + e.getMessage(), e);}}
Provider初始化时会调用doRegister方法向注册中心发起注册。那么客户端又是怎么subscribe在注册中心订阅服务的呢？答案是服务消费者在初始化ConsumerConfig时会调用RegistryProtocol的refer方法进一步调用RegistryDirectory的subscribe方法最终调用ZookeeperRegistry的subscribe方法向注册中心订阅服务。
com.alibaba.dubbo.registry.support.FailBackRegistry的subscribe方法：
@Overridepublic void subscribe(URL url, NotifyListener listener) {super.subscribe(url, listener);removeFailedSubscribed(url, listener);try {// 向服务器端发送订阅请求doSubscribe(url, listener);} catch (Exception e) {Throwable t = e;List<URL> urls = getCacheUrls(url);if (urls != null && urls.size() > 0) {notify(url, listener, urls);logger.error("Failed to subscribe " + url + ", Using cached list: " + urls + " from cache file: " + getUrl().getParameter(Constants.FILE_KEY, System.getProperty("user.home") + "/dubbo-registry-" + url.getHost() + ".cache") + ", cause: " + t.getMessage(), t);} else {// 如果开启了启动时检测，则直接抛出异常boolean check = getUrl().getParameter(Constants.CHECK_KEY, true)&& url.getParameter(Constants.CHECK_KEY, true);boolean skipFailback = t instanceof SkipFailbackWrapperException;if (check || skipFailback) {if(skipFailback) {t = t.getCause();}throw new IllegalStateException("Failed to subscribe " + url + ", cause: " + t.getMessage(), t);} else {logger.error("Failed to subscribe " + url + ", waiting for retry, cause: " + t.getMessage(), t);}}// 将失败的订阅请求记录到失败列表，定时重试addFailedSubscribed(url, listener);}}
com.alibaba.dubbo.registry.zookeeper.Zookeeper的doSubscribe方法：
protected void doSubscribe(final URL url, final NotifyListener listener) {try {//如果provider的service的接口配置的是“*”if (Constants.ANY_VALUE.equals(url.getServiceInterface())) {//获取服务分组根路径String root = toRootPath();//获取服务的NotifyListenerConcurrentMap<NotifyListener, ChildListener> listeners = zkListeners.get(url);if (listeners == null) {//如果没有则创建一个zkListeners.putIfAbsent(url, new ConcurrentHashMap<NotifyListener, ChildListener>());listeners = zkListeners.get(url);}ChildListener zkListener = listeners.get(listener);//如果没有子监听器则创建一个if (zkListener == null) {listeners.putIfAbsent(listener, new ChildListener() {public void childChanged(String parentPath, List<String> currentChilds) {for (String child : currentChilds) {child = URL.decode(child);if (! anyServices.contains(child)) {anyServices.add(child);subscribe(url.setPath(child).addParameters(Constants.INTERFACE_KEY, child,Constants.CHECK_KEY, String.valueOf(false)), listener);}}}});zkListener = listeners.get(listener);}//向服务器订阅服务，注册中心会调用NotifyListener的notify函数返回服务列表zkClient.create(root, false);//获取服务地址列表List<String> services = zkClient.addChildListener(root, zkListener);if (services != null && services.size() > 0) {//如果存在服务for (String service : services) {service = URL.decode(service);anyServices.add(service);//如果serviceInterface是“*”则从分组根路径遍历service并订阅所有服务subscribe(url.setPath(service).addParameters(Constants.INTERFACE_KEY, service,Constants.CHECK_KEY, String.valueOf(false)), listener);}}} else {//如果serviceInterface不是“*”则创建Zookeeper客户端索取服务列表，并通知（notify）消费者（consumer）这些服务可以用了List<URL> urls = new ArrayList<URL>();//获取类似于http：//xxx.xxx.xxx.xxx/context/com.service.xxxService/consumer的地址for (String path : toCategoriesPath(url)) {//获取例如com.service.xxxService对应的NotifyListener mapConcurrentMap<NotifyListener, ChildListener> listeners = zkListeners.get(url);if (listeners == null) {zkListeners.putIfAbsent(url, new ConcurrentHashMap<NotifyListener, ChildListener>());listeners = zkListeners.get(url);}//获取ChildListenerChildListener zkListener = listeners.get(listener);if (zkListener == null) {listeners.putIfAbsent(listener, new ChildListener() {public void childChanged(String parentPath, List<String> currentChilds) {ZookeeperRegistry.this.notify(url, listener, toUrlsWithEmpty(url, parentPath, currentChilds));}});zkListener = listeners.get(listener);}//创建Zookeeper客户端zkClient.create(path, false);List<String> children = zkClient.addChildListener(path, zkListener);if (children != null) {urls.addAll(toUrlsWithEmpty(url, path, children));}}//提醒消费者notify(url, listener, urls);}} catch (Throwable e) {throw new RpcException("Failed to subscribe " + url + " to zookeeper " + getUrl() + ", cause: " + e.getMessage(), e);}} 至此，Dubbo的源码解析结束，以后还会对一些细节进行补充。特别在此建议看官使用apache的开源项目Zookeeper作注册中心，，来完成分布式服务的协调调用。
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