7.7 服务远程暴露 - 订阅与通知（TODO）

为了安全：服务启动的ip全部使用10.10.10.10

远程服务的暴露总体步骤：

• 将ref封装为invoker
• 将invoker转换为exporter
• 启动netty
• 注册服务到zookeeper
• 订阅与通知
• 返回新的exporter实例

在7.4 服务远程暴露 - 创建Exporter与启动netty服务端中，实现了前三步，在7.6 服务远程暴露 - 注册服务到zookeeper实现了第四步。本节实现第五步：订阅。总体代码如下：RegistryProtocol.export(final Invoker<T> originInvoker)

         // 订阅override数据
         // FIXME 提供者订阅时，会影响同一JVM即暴露服务，又引用同一服务的的场景，因为subscribed以服务名为缓存的key，导致订阅信息覆盖。
         final URL overrideSubscribeUrl = getSubscribedOverrideUrl(registedProviderUrl);
         final OverrideListener overrideSubscribeListener = new OverrideListener(overrideSubscribeUrl, originInvoker);
         overrideListeners.put(overrideSubscribeUrl, overrideSubscribeListener);
         registry.subscribe(overrideSubscribeUrl, overrideSubscribeListener);

说明：

• 第一句代码根据registedProviderUrl来获取overrideSubscribeUrl。
• 第二句代码创建overrideSubscribeListener
• 第三句代码将{ overrideSubscribeUrl : overrideSubscribeListener放入缓存 }
• 第四句代码实现真正的订阅与通知

一  获取overrideSubscribeUrl

final URL overrideSubscribeUrl = getSubscribedOverrideUrl(registedProviderUrl);

     /**
      * 1 将协议改为provider；
      * 2 添加参数：category=configurators和check=false；
      */
     private URL getSubscribedOverrideUrl(URL registedProviderUrl) {
         return registedProviderUrl.setProtocol(Constants.PROVIDER_PROTOCOL)
                                   .addParameters(Constants.CATEGORY_KEY, Constants.CONFIGURATORS_CATEGORY, Constants.CHECK_KEY, String.valueOf(false));
     }

开始时的registedProviderUrl如下：

• dubbo://10.10.10.10:20880/com.alibaba.dubbo.demo.DemoService?anyhost=true&application=demo-provider&dubbo=2.0.0&generic=false&interface=com.alibaba.dubbo.demo.DemoService&methods=sayHello&pid=5259&side=provider&timestamp=1507294508053

最终的overrideSubscribeUrl如下：

• provider://10.10.10.10:20880/com.alibaba.dubbo.demo.DemoService?anyhost=true&application=demo-provider&category=configurators&check=false&dubbo=2.0.0&generic=false&interface=com.alibaba.dubbo.demo.DemoService&methods=sayHello&pid=5259&side=provider&timestamp=1507294508053

二  创建overrideSubscribeListener

final OverrideListener overrideSubscribeListener = new OverrideListener(overrideSubscribeUrl, originInvoker);

overrideSubscribeListener是RegistryProtocol的内部类，来看一下声明和属性：

     private class OverrideListener implements NotifyListener {
         private final URL subscribeUrl;
         private final Invoker originInvoker;

         public OverrideListener(URL subscribeUrl, Invoker originalInvoker) {
             this.subscribeUrl = subscribeUrl;
             this.originInvoker = originalInvoker;
         }

这里创建出来的OverrideListener实例属性如下：

• subscribeUrl：provider://10.10.10.10:20880/com.alibaba.dubbo.demo.DemoService?anyhost=true&application=demo-provider&category=configurators&check=false&dubbo=2.0.0&generic=false&interface=com.alibaba.dubbo.demo.DemoService&methods=sayHello&pid=1818&side=provider&timestamp=1507366969962
• originInvoker：该实例还是在ServiceConfig.doExportUrlsFor1Protocol(ProtocolConfig protocolConfig, List<URL> registryURLs)创建出来的AbstractProxyInvoker实例（具体见7.4 服务远程暴露 - 创建Exporter与启动netty服务端）
  • proxy：DemoServiceImpl实例
  • type：Class<com.alibaba.dubbo.demo.DemoService>
  • url：registry://10.211.55.5:2181/com.alibaba.dubbo.registry.RegistryService?application=demo-provider&client=curator&dubbo=2.0.0&export=dubbo%3A%2F%2F10.10.10.10%3A20880%2Fcom.alibaba.dubbo.demo.DemoService%3Fanyhost%3Dtrue%26application%3Ddemo-provider%26dubbo%3D2.0.0%26generic%3Dfalse%26interface%3Dcom.alibaba.dubbo.demo.DemoService%26methods%3DsayHello%26pid%3D993%26side%3Dprovider%26timestamp%3D1507100322516&pid=993&registry=zookeeper&timestamp=1507100319830

最后，将创建出来的OverrideListener实例存储在RegistryProtocol的属性Map<URL, NotifyListener> overrideListeners中：

• key: （overrideSubscribeUrl，也就是subscribeUrl） provider://10.10.10.10:20880/com.alibaba.dubbo.demo.DemoService?anyhost=true&application=demo-provider&category=configurators&check=false&dubbo=2.0.0&generic=false&interface=com.alibaba.dubbo.demo.DemoService&methods=sayHello&pid=1818&side=provider&timestamp=1507366969962
• value:  上述的OverrideListener实例

三  真正的订阅

registry.subscribe(overrideSubscribeUrl, overrideSubscribeListener);

这里的registry是ZookeeperRegistry实例，subscribe(URL url, NotifyListener listener)方法在其父类FailbackRegistry中，如下：

     @Override
     public void subscribe(URL url, NotifyListener listener) {
         if (destroyed.get()){
             return;
         }
         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check=true，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);
         }
     }

步骤：

• 首先调用其父类AbstractRegistry的方法，将之前创建出来的overrideSubscribeListener实例加入到overrideSubscribeUrl所对应的监听器集合中；
• 然后从failedSubscribed／failedUnsubscribed中overrideSubscribeUrl所对应的监听器集合中删除overrideSubscribeListener实例；从failedNotified获取当前url的通知失败map Map<NotifyListener, List<URL>>，之后从中删除掉该NotifyListener实例以及其需要通知的所有的url。
• 之后使用具体的子类（这里是ZookeeperRegistry）向服务器端发送订阅请求
• 如果在订阅的过程中抛出了异常，那么尝试获取缓存url，如果有缓存url，则进行失败通知，之后“将失败的订阅请求记录到失败列表，定时重试”，如果没有缓存url，如果开启了启动时检测或者直接抛出的异常是SkipFailbackWrapperException，则直接抛出异常，不会“将失败的订阅请求记录到失败列表，定时重试”

将之前创建出来的overrideSubscribeListener实例加入到overrideSubscribeUrl所对应的监听器集合中

    private final ConcurrentMap<URL, Set<NotifyListener>> subscribed = new ConcurrentHashMap<URL, Set<NotifyListener>>();//已经订阅的<URL, Set<NotifyListener>>

      /**
      * 首先从ConcurrentMap<URL, Set<NotifyListener>> subscribed中获取key为url的集合Set<NotifyListener>，
      * 如果该集合存在，直接将当前的NotifyListener实例存入该集合，
      * 如果集合不存在，先创建，之后放入subscribed中，并将当前的NotifyListener实例存入刚刚创建的集合
      *
      * @param url      订阅条件，不允许为空，如：consumer://10.10.10.10/com.alibaba.foo.BarService?version=1.0.0&application=kylin
      * @param listener 变更事件监听器，不允许为空
      */
     public void subscribe(URL url, NotifyListener listener) {
         if (url == null) {
             throw new IllegalArgumentException("subscribe url == null");
         }
         if (listener == null) {
             throw new IllegalArgumentException("subscribe listener == null");
         }
         if (logger.isInfoEnabled()) {
             logger.info("Subscribe: " + url);
         }
         Set<NotifyListener> listeners = subscribed.get(url);
         if (listeners == null) {
             subscribed.putIfAbsent(url, new ConcurrentHashSet<NotifyListener>());
             listeners = subscribed.get(url);
         }
         listeners.add(listener);
     }

从失败集合移除overrideSubscribeListener实例

     /**
      * 1 从ConcurrentMap<URL, Set<NotifyListener>> failedSubscribed 中获取当前url的订阅失败列表Set<NotifyListener>，之后从中删除掉该NotifyListener实例；
      * 2 从ConcurrentMap<URL, Set<NotifyListener>> failedUnsubscribed 中获取当前url的反订阅失败列表Set<NotifyListener>，之后从中删除掉该NotifyListener实例；
      * 3 从ConcurrentMap<URL, Map<NotifyListener, List<URL>>> failedNotified 中获取当前url的通知失败map Map<NotifyListener, List<URL>>，之后从中删除掉该NotifyListener实例以及其需要通知的所有的url。
      *
      * @param url
      * @param listener
      */
     private void removeFailedSubscribed(URL url, NotifyListener listener) {
         Set<NotifyListener> listeners = failedSubscribed.get(url);
         if (listeners != null) {
             listeners.remove(listener);
         }
         listeners = failedUnsubscribed.get(url);
         if (listeners != null) {
             listeners.remove(listener);
         }
         Map<NotifyListener, List<URL>> notified = failedNotified.get(url);
         if (notified != null) {
             notified.remove(listener);
         }
     }

ZookeeperRegistry.doSubscribe(final URL url, final NotifyListener listener)

 protected void doSubscribe(final URL url, final NotifyListener listener) {
         try {
             if (Constants.ANY_VALUE.equals(url.getServiceInterface())) {//这条分支先不说
                 String root = toRootPath();
                 ConcurrentMap<NotifyListener, ChildListener> listeners = zkListeners.get(url);
                 if (listeners == null) {
                     zkListeners.putIfAbsent(url, new ConcurrentHashMap<NotifyListener, ChildListener>());
                     listeners = zkListeners.get(url);
                 }
                 ChildListener zkListener = listeners.get(listener);
                 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);
                 }
                 zkClient.create(root, false);
                 List<String> services = zkClient.addChildListener(root, zkListener);
                 if (services != null && services.size() > 0) {
                     for (String service : services) {
                         service = URL.decode(service);
                         anyServices.add(service);
                         subscribe(url.setPath(service).addParameters(Constants.INTERFACE_KEY, service,
                                 Constants.CHECK_KEY, String.valueOf(false)), listener);
                     }
                 }
             } else {
                 /**
                  * ConcurrentMap<URL, ConcurrentMap<NotifyListener, ChildListener>> zkListeners
                  * 1 根据url获取ConcurrentMap<NotifyListener, ChildListener>，没有就创建
                  * 2 根据listener从ConcurrentMap<NotifyListener, ChildListener>获取ChildListener，没有就创建（创建的ChildListener用来监听子节点的变化）
                  * 3 创建path持久化节点
                  * 4 创建path子节点监听器
                  */
                 List<URL> urls = new ArrayList<URL>();
                 for (String path : toCategoriesPath(url)) {
                     ConcurrentMap<NotifyListener, ChildListener> listeners = zkListeners.get(url);
                     if (listeners == null) {
                         zkListeners.putIfAbsent(url, new ConcurrentHashMap<NotifyListener, ChildListener>());
                         listeners = zkListeners.get(url);
                     }
                     ChildListener zkListener = listeners.get(listener);
                     if (zkListener == null) {
                         listeners.putIfAbsent(listener, new ChildListener() {
                             //监听子节点列表的变化
                             public void childChanged(String parentPath, List<String> currentChilds) {
                                 ZookeeperRegistry.this.notify(url, listener, toUrlsWithEmpty(url, parentPath, currentChilds));
                             }
                         });
                         zkListener = listeners.get(listener);
                     }
                     zkClient.create(path, false);//创建持久化节点/dubbo/com.alibaba.dubbo.demo.DemoService/configurators
                     List<String> children = zkClient.addChildListener(path, zkListener);
                     if (children != null) {
                         urls.addAll(toUrlsWithEmpty(url, path, children));
                     }
                 }
                 notify(url, listener, urls);
             }
         } catch (Throwable e) {
             throw new RpcException("Failed to subscribe " + url + " to zookeeper " + getUrl() + ", cause: " + e.getMessage(), e);
         }
     }

说明：

• url（overrideSubscribeUrl）：provider://10.10.10.10:20880/com.alibaba.dubbo.demo.DemoService?anyhost=true&application=demo-provider&category=configurators&check=false&dubbo=2.0.0&generic=false&interface=com.alibaba.dubbo.demo.DemoService&methods=sayHello&pid=9544&side=provider&timestamp=1507643800076
• listener：之前创建出来的overrideSubscribeListener实例

步骤：

• 首先获取categorypath：实际上就是获取/dubbo/{servicename}/{url中的category参数，默认是providers，这里是final URL overrideSubscribeUrl = getSubscribedOverrideUrl(registedProviderUrl);这句代码中添加到overrideSubscribeUrl上的category=configurators}

     private String[] toCategoriesPath(URL url) {
         String[] categroies;
         if (Constants.ANY_VALUE.equals(url.getParameter(Constants.CATEGORY_KEY))) {
             categroies = new String[]{Constants.PROVIDERS_CATEGORY, Constants.CONSUMERS_CATEGORY,
                     Constants.ROUTERS_CATEGORY, Constants.CONFIGURATORS_CATEGORY};
         } else {
             categroies = url.getParameter(Constants.CATEGORY_KEY, new String[]{Constants.DEFAULT_CATEGORY});
         }
         String[] paths = new String[categroies.length];
         for (int i = 0; i < categroies.length; i++) {
             paths[i] = toServicePath(url) + Constants.PATH_SEPARATOR + categroies[i];
         }
         return paths; // /dubbo/com.alibaba.dubbo.demo.DemoService/configurators
     }

• 然后就是获取并创建：ConcurrentMap<overrideSubscribeUrl, ConcurrentMap<overrideSubscribeListener实例, ChildListener>> zkListeners，这里创建出来的ChildListener实例中的childChanged(String parentPath, List<String> currentChilds)方法实际上就是最终当parentPath（实际上就是上边的categorypath）下的currentChilds发生变化时，执行的逻辑。
• 之后创建持久化节点：/dubbo/com.alibaba.dubbo.demo.DemoService/configurators
• 然后使用AbstractZookeeperClient<TargetChildListener>的addChildListener(String path, final ChildListener listener)方法为path下的子节点添加上边创建出来的内部类ChildListener实例
• 最后进行通知

AbstractZookeeperClient<TargetChildListener>.addChildListener(String path, final ChildListener listener)

     /**
      *  1 根据path从ConcurrentMap<String, ConcurrentMap<ChildListener, TargetChildListener>> childListeners获取ConcurrentMap<ChildListener, TargetChildListener>，没有就创建
      *  2 根据ChildListener获取TargetChildListener，没有就创建，TargetChildListener是真正的监听path的子节点变化的监听器
      *  createTargetChildListener(String path, final ChildListener listener)：创建一个真正的用来执行当path节点的子节点发生变化时的逻辑
      *  3 addTargetChildListener(path, targetListener)：将刚刚创建出来的子节点监听器订阅path的变化，这样之后，path的子节点发生了变化时，TargetChildListener才会执行相应的逻辑。
      *  而实际上TargetChildListener又会调用ChildListener的实现类的childChanged(String parentPath, List<String> currentChilds)方法，而该实现类，正好是ZookeeperRegistry中实现的匿名内部类，
      *  在该匿名内部类的childChanged(String parentPath, List<String> currentChilds)方法中，调用了ZookeeperRegistry.notify(URL url, NotifyListener listener, List<URL> urls)
      */
     public List<String> addChildListener(String path, final ChildListener listener) {
         ConcurrentMap<ChildListener, TargetChildListener> listeners = childListeners.get(path);
         if (listeners == null) {
             childListeners.putIfAbsent(path, new ConcurrentHashMap<ChildListener, TargetChildListener>());
             listeners = childListeners.get(path);
         }
         TargetChildListener targetListener = listeners.get(listener);
         if (targetListener == null) {
             listeners.putIfAbsent(listener, createTargetChildListener(path, listener));
             targetListener = listeners.get(listener);
         }
         return addTargetChildListener(path, targetListener);
     }

步骤：

• 首先是一顿获取和创建：ConcurrentMap<categorypath, ConcurrentMap<ZookeeperRegistry的内部类ChildListener实例, TargetChildListener>> childListeners，这里主要是创建TargetChildListener；
• 之后是真正的为path添加TargetChildListener实例。

CuratorZookeeperClient.createTargetChildListener(path, listener)

     public CuratorWatcher createTargetChildListener(String path, ChildListener listener) {
         return new CuratorWatcherImpl(listener);
     }

     private class CuratorWatcherImpl implements CuratorWatcher {

         private volatile ChildListener listener;

         public CuratorWatcherImpl(ChildListener listener) {
             this.listener = listener;
         }

         public void unwatch() {
             this.listener = null;
         }

         public void process(WatchedEvent event) throws Exception {
             if (listener != null) {
                 listener.childChanged(event.getPath(), client.getChildren().usingWatcher(this).forPath(event.getPath()));
             }
         }
     }

很简单，就是创建一个监听path子节点的watcher，当path下有子节点变化时，调用listener（即传入的ZookeeperRegistry的内部类ChildListener实例的childChanged(String parentPath, List<String> currentChilds)方法）。

CuratorZookeeperClient.addTargetChildListener(String path, CuratorWatcher targetChildListener)

     public List<String> addTargetChildListener(String path, CuratorWatcher listener) {
         try {
             return client.getChildren().usingWatcher(listener).forPath(path);
         } catch (NoNodeException e) {
             return null;
         } catch (Exception e) {
             throw new IllegalStateException(e.getMessage(), e);
         }
     }

从上边的分析我们可以看出，当path节点下的子节点发生变化的时候，会首先调用TargetChildListener的process(WatchedEvent event)方法，在该方法中又会调用ChildListener实例的childChanged(String parentPath, List<String> currentChilds)方法，那么我们来分析一下该方法：

                             //监听子节点列表的变化
                             public void childChanged(String parentPath, List<String> currentChilds) {
                                 ZookeeperRegistry.this.notify(url, listener, toUrlsWithEmpty(url, parentPath, currentChilds));
                             }

步骤：

• 首先获取子节点urls或者是一个consumer的empty协议的url

       /**
        * 过滤出providers中与consumer匹配的url集合
        */
       private List<URL> toUrlsWithoutEmpty(URL consumer, List<String> providers) {
           List<URL> urls = new ArrayList<URL>();
           if (providers != null && providers.size() > 0) {
               for (String provider : providers) {
                   provider = URL.decode(provider);
                   if (provider.contains("://")) {
                       URL url = URL.valueOf(provider);
                       if (UrlUtils.isMatch(consumer, url)) {
                           urls.add(url);
                       }
                   }
               }
           }
           return urls;
       }
  
       /**
        * 1 首先过滤出providers中与consumer匹配的providerUrl集合
        * 2 如果providerUrl集合不为空，直接返回这个集合
        * 3 如果为空，首先从path中获取category，然后将consumer的协议换成empty，添加参数category=configurators
        * @param consumer provider://10.10.10.10:20880/com.alibaba.dubbo.demo.DemoService?anyhost=true&application=demo-provider&category=configurators&check=false&dubbo=2.0.0&generic=false&interface=com.alibaba.dubbo.demo.DemoService&methods=sayHello&pid=9544&side=provider&timestamp=1507643800076
        * @param path /dubbo/com.alibaba.dubbo.demo.DemoService/configurators
        * @param providers
        */
       private List<URL> toUrlsWithEmpty(URL consumer, String path, List<String> providers) {
           List<URL> urls = toUrlsWithoutEmpty(consumer, providers);
           if (urls == null || urls.isEmpty()) {
               int i = path.lastIndexOf('/');
               String category = i < 0 ? path : path.substring(i + 1);//configurators
               URL empty = consumer.setProtocol(Constants.EMPTY_PROTOCOL).addParameter(Constants.CATEGORY_KEY, category);
               urls.add(empty);
           }
           return urls; // empty://10.10.10.10:20880/com.alibaba.dubbo.demo.DemoService?anyhost=true&application=demo-provider&category=configurators&check=false&dubbo=2.0.0&generic=false&interface=com.alibaba.dubbo.demo.DemoService&methods=sayHello&pid=1237&side=provider&timestamp=1507352638483
       }

• 之后调用ZookeeperRegistry的父类FailbackRegistry.notify(URL url, NotifyListener listener, List<URL> urls)

       @Override
       protected void notify(URL url, NotifyListener listener, List<URL> urls) {
           if (url == null) {
               throw new IllegalArgumentException("notify url == null");
           }
           if (listener == null) {
               throw new IllegalArgumentException("notify listener == null");
           }
           try {
               doNotify(url, listener, urls);
           } catch (Exception t) {
               // 将失败的通知请求记录到失败列表，定时重试
               Map<NotifyListener, List<URL>> listeners = failedNotified.get(url);
               if (listeners == null) {
                   failedNotified.putIfAbsent(url, new ConcurrentHashMap<NotifyListener, List<URL>>());
                   listeners = failedNotified.get(url);
               }
               listeners.put(listener, urls);
               logger.error("Failed to notify for subscribe " + url + ", waiting for retry, cause: " + t.getMessage(), t);
           }
       }
  
       protected void doNotify(URL url, NotifyListener listener, List<URL> urls) {
           super.notify(url, listener, urls);
       }

  说明：这里传入的

  • url（overrideSubscribeUrl）：provider://10.10.10.10:20880/com.alibaba.dubbo.demo.DemoService?anyhost=true&application=demo-provider&category=configurators&check=false&dubbo=2.0.0&generic=false&interface=com.alibaba.dubbo.demo.DemoService&methods=sayHello&pid=9544&side=provider&timestamp=1507643800076

  • listener：之前创建出来的overrideSubscribeListener实例
  • urls：[ empty://10.10.10.10:20880/com.alibaba.dubbo.demo.DemoService?anyhost=true&application=demo-provider&category=configurators&check=false&dubbo=2.0.0&generic=false&interface=com.alibaba.dubbo.demo.DemoService&methods=sayHello&pid=9544&side=provider&timestamp=1507643800076 ]
  • 这里首先执行父类的AbstractRegistry.notify(URL url, NotifyListener listener, List<URL> urls)，如果失败，则获取或创建ConcurrentMap<overrideSubscribeUrl, Map<overrideSubscribeListener实例, urls>> failedNotified，后续做重试

来看一下通知的最核心部分：

AbstractRegistry.notify(URL url, NotifyListener listener, List<URL> urls)

     /**
      * 1 首先遍历List<URL> urls，将urls按照category进行分类，存储在Map<"categoryName", List<URL>> result中；
      * 2 之后遍历result：（每遍历一次，都是一个新的category）
      * （1）将Map<"categoryName", List<URL>>存储在ConcurrentMap<URL, Map<String, List<URL>>> notified的Map<String, List<URL>>中
      * （2）进行properties设置和文件保存
      * （3）调用传入放入listener的notify()方法。
      * @param url
      * @param listener
      * @param urls
      */
     protected void notify(URL url, NotifyListener listener, List<URL> urls) {
         if (url == null) {
             throw new IllegalArgumentException("notify url == null");
         }
         if (listener == null) {
             throw new IllegalArgumentException("notify listener == null");
         }
         if ((urls == null || urls.size() == 0)
                 && !Constants.ANY_VALUE.equals(url.getServiceInterface())) {
             logger.warn("Ignore empty notify urls for subscribe url " + url);
             return;
         }
         if (logger.isInfoEnabled()) {
             logger.info("Notify urls for subscribe url " + url + ", urls: " + urls);
         }
         /**
          * 遍历List<URL> urls，将urls按照category进行分类
          */
         Map<String, List<URL>> result = new HashMap<String, List<URL>>(); //{ "categoryName" : List<URL> }
         for (URL u : urls) {
             if (UrlUtils.isMatch(url, u)) {
                 String category = u.getParameter(Constants.CATEGORY_KEY, Constants.DEFAULT_CATEGORY);
                 List<URL> categoryList = result.get(category);
                 if (categoryList == null) {
                     categoryList = new ArrayList<URL>();
                     result.put(category, categoryList);
                 }
                 categoryList.add(u);
             }
         }
         if (result.size() == 0) {
             return;
         }
         Map<String, List<URL>> categoryNotified = notified.get(url);
         if (categoryNotified == null) {
             notified.putIfAbsent(url, new ConcurrentHashMap<String, List<URL>>());
             categoryNotified = notified.get(url);
         }
         for (Map.Entry<String, List<URL>> entry : result.entrySet()) {
             String category = entry.getKey();
             List<URL> categoryList = entry.getValue();
             categoryNotified.put(category, categoryList);//填充notified集合
             saveProperties(url);//该行代码为什么不写在循环体外边
             listener.notify(categoryList);
         }
     }

说明：这里传入的

• url（overrideSubscribeUrl）：provider://10.10.10.10:20880/com.alibaba.dubbo.demo.DemoService?anyhost=true&application=demo-provider&category=configurators&check=false&dubbo=2.0.0&generic=false&interface=com.alibaba.dubbo.demo.DemoService&methods=sayHello&pid=9544&side=provider&timestamp=1507643800076

• listener：之前创建出来的overrideSubscribeListener实例
• urls：[ empty://10.10.10.10:20880/com.alibaba.dubbo.demo.DemoService?anyhost=true&application=demo-provider&category=configurators&check=false&dubbo=2.0.0&generic=false&interface=com.alibaba.dubbo.demo.DemoService&methods=sayHello&pid=9544&side=provider&timestamp=1507643800076 ]

步骤：

• 首先遍历List<URL> urls，将urls按照category进行分类，存储在Map<"categoryName", List<URL>> result中；
• 然后获取或创建ConcurrentMap<overrideSubscribeUrl, Map<"categoryName", subList(urls)>> notified
• 最后遍历Map<"categoryName", List<URL>> result
  • 去填充notified集合
  • 保存传入的url到Properties properties（本地磁盘缓存中）
  • 调用传入的listener的notify方法（注意：这里调用的正是文章开头创建的overrideSubscribeListener实例的notify方法）

AbstractRegistry.saveProperties(URL url)

     /**
      * 1 按照url从将ConcurrentMap<URL, Map<String, List<URL>>> notified中将Map<String, List<URL>>拿出来，之后将所有category的list组成一串buf（以空格分隔）
      * 2 将< serviceKey<->buf >写入本地磁盘缓存中：Properties properties
      * 3 将AtomicLong lastCacheChanged加1
      * 4 之后根据syncSaveFile判断时同步保存properties到文件，还是异步保存properties到文件
      * @param url
      */
     private void saveProperties(URL url) {
         if (file == null) {
             return;
         }

         try {
             StringBuilder buf = new StringBuilder();
             Map<String, List<URL>> categoryNotified = notified.get(url);
             if (categoryNotified != null) {
                 for (List<URL> us : categoryNotified.values()) {
                     for (URL u : us) {
                         if (buf.length() > 0) {
                             buf.append(URL_SEPARATOR);
                         }
                         buf.append(u.toFullString());
                     }
                 }
             }
             properties.setProperty(url.getServiceKey(), buf.toString());
             long version = lastCacheChanged.incrementAndGet();
             if (syncSaveFile) {
                 doSaveProperties(version);
             } else {
                 registryCacheExecutor.execute(new SaveProperties(version));
             }
         } catch (Throwable t) {
             logger.warn(t.getMessage(), t);
         }
     }

说明：

• 入参：url：provider://10.10.10.10:20880/com.alibaba.dubbo.demo.DemoService?anyhost=true&application=demo-provider&category=configurators&check=false&dubbo=2.0.0&generic=false&interface=com.alibaba.dubbo.demo.DemoService&methods=sayHello&pid=5033&side=provider&timestamp=1507720343596
• properties：{ "com.alibaba.dubbo.demo.DemoService" -> "empty://10.10.10.10:20880/com.alibaba.dubbo.demo.DemoService?anyhost=true&application=demo-provider&category=configurators&check=false&dubbo=2.0.0&generic=false&interface=com.alibaba.dubbo.demo.DemoService&methods=sayHello&pid=5033&side=provider&timestamp=1507720343596" }
• 最后采用异步线程将properties中的内容写入到文件中

AbstractRegistry$SaveProperties

     private class SaveProperties implements Runnable {
         private long version;

         private SaveProperties(long version) {
             this.version = version;
         }

         public void run() {
             doSaveProperties(version);
         }
     }

AbstractRegistry.doSaveProperties(long version)

     /**
      * 1 先将文件中的内容读取到一个新的Properties newProperties中；
      * 2 之后将properties中的信息写入这个newProperties中；
      * 3 之后创建dubbo-registry-10.211.55.5.cache.lock文件；
      * 4 最后将这个newProperties中的内容写入到文件中
      */
     public void doSaveProperties(long version) {
         if (version < lastCacheChanged.get()) {
             return;
         }
         if (file == null) {
             return;
         }
         Properties newProperties = new Properties();
         // 保存之前先读取一遍，防止多个注册中心之间冲突
         InputStream in = null;
         try {
             if (file.exists()) {
                 in = new FileInputStream(file);
                 newProperties.load(in);
             }
         } catch (Throwable e) {
             logger.warn("Failed to load registry store file, cause: " + e.getMessage(), e);
         } finally {
             if (in != null) {
                 try {
                     in.close();
                 } catch (IOException e) {
                     logger.warn(e.getMessage(), e);
                 }
             }
         }
         // 保存
         try {
             newProperties.putAll(properties);
             File lockfile = new File(file.getAbsolutePath() + ".lock");
             if (!lockfile.exists()) {
                 lockfile.createNewFile();//创建lock文件
             }
             RandomAccessFile raf = new RandomAccessFile(lockfile, "rw");
             try {
                 FileChannel channel = raf.getChannel();
                 try {
                     FileLock lock = channel.tryLock();
                     if (lock == null) {
                         throw new IOException("Can not lock the registry cache file " + file.getAbsolutePath() + ", ignore and retry later, maybe multi java process use the file, please config: dubbo.registry.file=xxx.properties");
                     }
                     // 保存
                     try {
                         if (!file.exists()) {
                             file.createNewFile();
                         }
                         FileOutputStream outputFile = new FileOutputStream(file);
                         try {
                             newProperties.store(outputFile, "Dubbo Registry Cache");
                         } finally {
                             outputFile.close();
                         }
                     } finally {
                         lock.release();
                     }
                 } finally {
                     channel.close();
                 }
             } finally {
                 raf.close();
             }
         } catch (Throwable e) {
             if (version < lastCacheChanged.get()) {
                 return;
             } else {
                 registryCacheExecutor.execute(new SaveProperties(lastCacheChanged.incrementAndGet()));
             }
             logger.warn("Failed to save registry store file, cause: " + e.getMessage(), e);
         }
     }

步骤见注释。这里有一个version，实际上是一个CAS判断，我们在saveProperties(URL url)方法中执行了long version = lastCacheChanged.incrementAndGet();之后在doSaveProperties(long version)进行if (version < lastCacheChanged.get())判断，如果满足这个条件，说明当前线程在进行doSaveProperties(long version)时，已经有其他线程执行了saveProperties(URL url)，马上就要执行doSaveProperties(long version)，所以当前线程放弃操作，让后边的这个线程来做保存操作。

保存操作执行之后，会在文件夹/Users/jigangzhao/.dubbo下生成两个文件：

• dubbo-registry-10.211.55.5.cache
• dubbo-registry-10.211.55.5.cache.lock

前者的内容：

#Wed Oct 11 19:42:29 CST 2017
com.alibaba.dubbo.demo.DemoService=empty\://10.10.10.10\:20880/com.alibaba.dubbo.demo.DemoService?anyhost\=true&application\=demo-provider&category\=configurators&check\=false&dubbo\=2.0.0&generic\=false&interface\=com.alibaba.dubbo.demo.DemoService&methods\=sayHello&pid\=5165&side\=provider&timestamp\=1507722024953

最后就是OverrideListener.notify(List<URL> urls)

     /**
      * 重新export
      * 1.protocol中的exporter destroy问题
      * 1.要求registryprotocol返回的exporter可以正常destroy
      * 2.notify后不需要重新向注册中心注册
      * 3.export 方法传入的invoker最好能一直作为exporter的invoker.
      */
     private class OverrideListener implements NotifyListener {
         private final URL subscribeUrl;
         private final Invoker originInvoker;

         public OverrideListener(URL subscribeUrl, Invoker originalInvoker) {
             this.subscribeUrl = subscribeUrl;
             this.originInvoker = originalInvoker;
         }

         /**
          * 目的：
          * 对原本注册了的providerUrl进行校验，如果url发生了变化，那么要重新export
          *
          * @param urls 已注册信息列表，总不为空，含义同{@link com.alibaba.dubbo.registry.RegistryService#lookup(URL)}的返回值。
          */
         public synchronized void notify(List<URL> urls) {
             logger.debug("original override urls: " + urls);
             List<URL> matchedUrls = getMatchedUrls(urls, subscribeUrl);
             logger.debug("subscribe url: " + subscribeUrl + ", override urls: " + matchedUrls);
             //没有匹配的
             if (matchedUrls.isEmpty()) {
                 return;
             }

             List<Configurator> configurators = RegistryDirectory.toConfigurators(matchedUrls);//这里是一个空列表

             final Invoker<?> invoker;
             if (originInvoker instanceof InvokerDelegete) {
                 invoker = ((InvokerDelegete<?>) originInvoker).getInvoker();
             } else {
                 invoker = originInvoker;
             }
             //最原始的invoker
             URL originUrl = RegistryProtocol.this.getProviderUrl(invoker);//dubbo://10.10.10.10:20880/com.alibaba.dubbo.demo.DemoService?anyhost=true&application=demo-provider&dubbo=2.0.0&generic=false&interface=com.alibaba.dubbo.demo.DemoService&methods=sayHello&pid=5279&side=provider&timestamp=1507723571451
             String key = getCacheKey(originInvoker);//dubbo://10.10.10.10:20880/com.alibaba.dubbo.demo.DemoService?anyhost=true&application=demo-provider&dubbo=2.0.0&generic=false&interface=com.alibaba.dubbo.demo.DemoService&methods=sayHello&pid=5279&side=provider&timestamp=1507723571451
             ExporterChangeableWrapper<?> exporter = bounds.get(key);//在doLocalExport方法中已经存放在这里了
             if (exporter == null) {
                 logger.warn(new IllegalStateException("error state, exporter should not be null"));
                 return;
             }
             //当前的，可能经过了多次merge
             URL currentUrl = exporter.getInvoker().getUrl();//dubbo://10.10.10.10:20880/com.alibaba.dubbo.demo.DemoService?anyhost=true&application=demo-provider&dubbo=2.0.0&generic=false&interface=com.alibaba.dubbo.demo.DemoService&methods=sayHello&pid=5279&side=provider&timestamp=1507723571451
             //与本次配置merge的
             URL newUrl = getConfigedInvokerUrl(configurators, originUrl);
             if (!currentUrl.equals(newUrl)) {
                 RegistryProtocol.this.doChangeLocalExport(originInvoker, newUrl);//重新将invoker暴露为exporter
                 logger.info("exported provider url changed, origin url: " + originUrl + ", old export url: " + currentUrl + ", new export url: " + newUrl);
             }
         }

         private List<URL> getMatchedUrls(List<URL> configuratorUrls, URL currentSubscribe) {
             List<URL> result = new ArrayList<URL>();
             for (URL url : configuratorUrls) {
                 URL overrideUrl = url;
                 // 兼容旧版本
                 if (url.getParameter(Constants.CATEGORY_KEY) == null
                         && Constants.OVERRIDE_PROTOCOL.equals(url.getProtocol())) {
                     overrideUrl = url.addParameter(Constants.CATEGORY_KEY, Constants.CONFIGURATORS_CATEGORY);
                 }

                 //检查是不是要应用到当前服务上
                 if (UrlUtils.isMatch(currentSubscribe, overrideUrl)) {
                     result.add(url);
                 }
             }
             return result;
         }

         //合并配置的url
         private URL getConfigedInvokerUrl(List<Configurator> configurators, URL url) {
             for (Configurator configurator : configurators) {
                 url = configurator.configure(url);
             }
             return url;
         }
     }

最后：总结一下：

当前的provider订阅了/dubbo/com.alibaba.dubbo.demo.DemoService/configurators，当其下的子节点发生变化时，如果其下的子节点的url或者当前的providerUrl发生了变化，需要重新暴露。

重新暴露：

     /**
      * 对修改了url的invoker重新export
      *
      * @param originInvoker
      * @param newInvokerUrl
      */
     @SuppressWarnings("unchecked")
     private <T> void doChangeLocalExport(final Invoker<T> originInvoker, URL newInvokerUrl) {
         String key = getCacheKey(originInvoker);
         final ExporterChangeableWrapper<T> exporter = (ExporterChangeableWrapper<T>) bounds.get(key);
         if (exporter == null) {
             logger.warn(new IllegalStateException("error state, exporter should not be null"));
         } else {
             final Invoker<T> invokerDelegete = new InvokerDelegete<T>(originInvoker, newInvokerUrl);
             exporter.setExporter(protocol.export(invokerDelegete));
         }
     }