Dubbo源码分析3之服务发布

dubbo-export-mulu.png

1.服务发布概述

Dubbo 服务导出过程始于 Spring 容器发布刷新事件[dubbo:service --> ServiceBean --> onApplicationEvent(ContextRefreshedEvent event)]，在接收到ContextRefreshedEvent 事件后执行服务导出逻辑。整个逻辑大致可分为三个部分：

第一部分是前置工作，主要用于检查参数，组装 URL；

第二部分是导出服务，包含导出服务到本地 (JVM)，和导出服务到远程两个过程；

第三部分是向注册中心注册服务，用于服务发现，包括注册到zk和订阅zk。

本文的重点实在整个发布流程，一些细节简单描述省略，比如配置检查，URL组装。

2.源码环境说明

基于dubbo2.6.4版本，使用官方的dubbo-demo项目，项目结构图如下：

dubbo-demo.png

修改注册中心为zookeeper

接口和实现类代码：

public interface DemoService {
    String sayHello(String name);
}

public class DemoServiceImpl implements DemoService {
    @Override
    public String sayHello(String name) {
        System.out.println("[" + new SimpleDateFormat("HH:mm:ss").format(new Date()) + "] Hello " + name + ", request from consumer: " + RpcContext.getContext().getRemoteAddress());
        return "Hello " + name + ", response from provider: " + RpcContext.getContext().getLocalAddress();
    }
}

3.源码分析

服务发布的入口方法是 ServiceBean 的 onApplicationEvent，如下：

代码块 ServiceBean #onApplicationEvent

  @Override
    public void onApplicationEvent(ContextRefreshedEvent event) {
        // 是否有延迟导出 && 是否已导出 && 是不是已被取消导出
        if (isDelay() && !isExported() && !isUnexported()) {
            if (logger.isInfoEnabled()) {
                logger.info("The service ready on spring started. service: " + getInterface());
            }
            export();
        }
    }

3.1 服务发布前置工作

3.1.1 概述

前置工作主要包含两个部分，分别是配置检查，以及 URL 装配。在导出服务之前，Dubbo 需要检查用户的配置是否合理，或者为用户补充缺省配置。配置检查完成后，接下来需要根据这些配置组装 URL。在 Dubbo 中，URL 的作用十分重要。Dubbo 使用 URL 作为配置载体，所有的拓展点都是通过 URL 获取配置。

代码块 ServiceConfig#doExport

protected synchronized void doExport() {
    if (unexported) {
        throw new IllegalStateException("Already unexported!");
    }
    if (exported) {
        return;
    }
    exported = true;
    // 检测 interfaceName 是否合法
    if (interfaceName == null || interfaceName.length() == 0) {
        throw new IllegalStateException("interface not allow null!");
    }
    // 检测 provider 是否为空，为空则新建一个，并通过系统变量为其初始化
    checkDefault();

    // 下面几个 if 语句用于检测 provider、application 等核心配置类对象是否为空，
    // 若为空，则尝试从其他配置类对象中获取相应的实例。
    if (provider != null) {
        if (application == null) {
            application = provider.getApplication();
        }
        if (module == null) {
            module = provider.getModule();
        }
        if (registries == null) {...}
        if (monitor == null) {...}
        if (protocols == null) {...}
    }
    if (module != null) {
        if (registries == null) {
            registries = module.getRegistries();
        }
        if (monitor == null) {...}
    }
    if (application != null) {
        if (registries == null) {
            registries = application.getRegistries();
        }
        if (monitor == null) {...}
    }

    // 检测 ref 是否为泛化服务类型
    if (ref instanceof GenericService) {
        // 设置 interfaceClass 为 GenericService.class
        interfaceClass = GenericService.class;
        if (StringUtils.isEmpty(generic)) {
            // 设置 generic = "true"
            generic = Boolean.TRUE.toString();
        }
        
    // ref 非 GenericService 类型
    } else {
        try {
            interfaceClass = Class.forName(interfaceName, true, Thread.currentThread()
                    .getContextClassLoader());
        } catch (ClassNotFoundException e) {
            throw new IllegalStateException(e.getMessage(), e);
        }
        // 对 interfaceClass，以及 <dubbo:method> 标签中的必要字段进行检查
        checkInterfaceAndMethods(interfaceClass, methods);
        // 对 ref 合法性进行检测
        checkRef();
        // 设置 generic = "false"
        generic = Boolean.FALSE.toString();
    }

    // local 和 stub 在功能应该是一致的，用于配置本地存根
    if (local != null) {
        if ("true".equals(local)) {
            local = interfaceName + "Local";
        }
        Class<?> localClass;
        try {
            // 获取本地存根类
            localClass = ClassHelper.forNameWithThreadContextClassLoader(local);
        } catch (ClassNotFoundException e) {
            throw new IllegalStateException(e.getMessage(), e);
        }
        // 检测本地存根类是否可赋值给接口类，若不可赋值则会抛出异常，提醒使用者本地存根类类型不合法
        if (!interfaceClass.isAssignableFrom(localClass)) {
            throw new IllegalStateException("The local implementation class " + localClass.getName() + " not implement interface " + interfaceName);
        }
    }

    if (stub != null) {
        // 此处的代码和上一个 if 分支的代码基本一致，这里省略
    }

    // 检测各种对象是否为空，为空则新建，或者抛出异常
    checkApplication();
    checkRegistry();
    checkProtocol();
    appendProperties(this);
    checkStubAndMock(interfaceClass);
    if (path == null || path.length() == 0) {
        path = interfaceName;
    }

    // 导出服务
    doExportUrls();

    // ProviderModel 表示服务提供者模型，此对象中存储了与服务提供者相关的信息。
    // 比如服务的配置信息，服务实例等。每个被导出的服务对应一个 ProviderModel。
    // ApplicationModel 持有所有的 ProviderModel。
    ProviderModel providerModel = new ProviderModel(getUniqueServiceName(), this, ref);
    ApplicationModel.initProviderModel(getUniqueServiceName(), providerModel);
}

3.1.2 对配置检查的逻辑进行简单的总结：

检测 <dubbo:service> 标签的 interface 属性合法性，不合法则抛出异常
检测 ProviderConfig、ApplicationConfig 等核心配置类对象是否为空，若为空，则尝试从其他配置类对象中获取相应的实例。
检测并处理泛化服务和普通服务类
检测本地存根配置，并进行相应的处理
对 ApplicationConfig、RegistryConfig 等配置类进行检测，为空则尝试创建，若无法创建则抛出异常

3.2 服务暴露

下面进入doExportUrls();方法：

    private void doExportUrls() {
         // 加载注册中心链接
        List<URL> registryURLs = loadRegistries(true);
        // 遍历 protocols，并在每个协议下导出服务
        for (ProtocolConfig protocolConfig : protocols) {
            doExportUrlsFor1Protocol(protocolConfig, registryURLs);
        }
    }

代码块：ServiceConfig#doExportUrlsFor1Protocol

    private void doExportUrlsFor1Protocol(ProtocolConfig protocolConfig, List<URL> registryURLs) {
        /***
        代码有点长，省略组装url部分的代码
        配置检查完毕后，紧接着要做的事情是根据配置，以及其他一些信息组装 URL。
        URL 是 Dubbo 配置的载体，通过 URL 可让 Dubbo 的各种配置在各个模块之间传递。
        ***/
        //...
        String host = this.findConfigedHosts(protocolConfig, registryURLs, map);
        Integer port = this.findConfigedPorts(protocolConfig, name, map);
        URL url = new URL(name, host, port, (contextPath == null || contextPath.length() == 0 ? "" : contextPath + "/") + path, map);
        /***此处组装的url示例：
        dubbo://192.168.43.174:20880/com.alibaba.dubbo.demo.DemoService?anyhost=true&application=demo-provider&bind.ip=192.168.43.174&bind.port=20880&dubbo=2.0.2&generic=false&interface=com.alibaba.dubbo.demo.DemoService&methods=sayHello&pid=8564&qos.port=22222&side=provider&timestamp=1578456375449
        ***/

        if (ExtensionLoader.getExtensionLoader(ConfiguratorFactory.class)
                .hasExtension(url.getProtocol())) {
            url = ExtensionLoader.getExtensionLoader(ConfiguratorFactory.class)
                    .getExtension(url.getProtocol()).getConfigurator(url).configure(url);
        }

        //下面开始要进入暴露服务的代码了
        String scope = url.getParameter(Constants.SCOPE_KEY);
        // don't export when none is configured
        if (!Constants.SCOPE_NONE.toString().equalsIgnoreCase(scope)) {

            // export to local if the config is not remote (export to remote only when config is remote)
            if (!Constants.SCOPE_REMOTE.toString().equalsIgnoreCase(scope)) {
                //暴露服务到本地
                exportLocal(url);
            }
            // export to remote if the config is not local (export to local only when config is local)
            if (!Constants.SCOPE_LOCAL.toString().equalsIgnoreCase(scope)) {
                if (logger.isInfoEnabled()) {
                    logger.info("Export dubbo service " + interfaceClass.getName() + " to url " + url);
                }
                if (registryURLs != null && !registryURLs.isEmpty()) {
                    for (URL registryURL : registryURLs) {
                        url = url.addParameterIfAbsent(Constants.DYNAMIC_KEY, registryURL.getParameter(Constants.DYNAMIC_KEY));
                        URL monitorUrl = loadMonitor(registryURL);
                        if (monitorUrl != null) {
                            url = url.addParameterAndEncoded(Constants.MONITOR_KEY, monitorUrl.toFullString());
                        }
                        if (logger.isInfoEnabled()) {
                            logger.info("Register dubbo service " + interfaceClass.getName() + " url " + url + " to registry " + registryURL);
                        }

                        // For providers, this is used to enable custom proxy to generate invoker
                        String proxy = url.getParameter(Constants.PROXY_KEY);
                        if (StringUtils.isNotEmpty(proxy)) {
                            registryURL = registryURL.addParameter(Constants.PROXY_KEY, proxy);
                        }

                        Invoker<?> invoker = proxyFactory.getInvoker(ref, (Class) interfaceClass, registryURL.addParameterAndEncoded(Constants.EXPORT_KEY, url.toFullString()));
                        DelegateProviderMetaDataInvoker wrapperInvoker = new DelegateProviderMetaDataInvoker(invoker, this);
                    //暴露服务到远程
                        Exporter<?> exporter = protocol.export(wrapperInvoker);
                        exporters.add(exporter);
                    }
                } else {
                    Invoker<?> invoker = proxyFactory.getInvoker(ref, (Class) interfaceClass, url);
                    DelegateProviderMetaDataInvoker wrapperInvoker = new DelegateProviderMetaDataInvoker(invoker, this);

                    Exporter<?> exporter = protocol.export(wrapperInvoker);
                    exporters.add(exporter);
                }
            }
        }
        this.urls.add(url);
    }

上面代码根据 url 中的 scope 参数决定服务导出方式，分别如下：

scope = none，不导出服务，注意这里是none字符串
scope != remote，导出到本地
scope != local，导出到远程

我们示例中到这里socpe=null，所以会同时暴露服务到本地和远程

3.2.1 暴露服务到本地

接下来进入ServiceConfig#exportLocal(URL url)方法

private void exportLocal(URL url) {
    // 如果 URL 的协议头等于 injvm，说明已经导出到本地了，无需再次导出
    if (!Constants.LOCAL_PROTOCOL.equalsIgnoreCase(url.getProtocol())) {
        URL local = URL.valueOf(url.toFullString())
            .setProtocol(Constants.LOCAL_PROTOCOL)    // 设置协议头为 injvm
            .setHost(LOCALHOST)
            .setPort(0);
        ServiceClassHolder.getInstance().pushServiceClass(getServiceClass(ref));
        // 创建 Invoker，并导出服务，这里的 protocol 会在运行时调用 InjvmProtocol 的 export 方法
        Exporter<?> exporter = protocol.export(
            proxyFactory.getInvoker(ref, (Class) interfaceClass, local));
        exporters.add(exporter);
    }
}

到这里看出服务暴露的结果是生成了一个Exporter对象存起来，关联一个Invoker对象，这两个是什么呢？

介绍Invoker和Exporter

Invoker 是实体域，它是 Dubbo 的核心模型，其它模型都向它靠扰，或转换成它，它代表一个可执行体，可向它发起 invoke 调用，它有可能是一个本地的实现，也可能是一个远程的实现，也可能一个集群实现。

这是官方描述，看起来还是不清楚具体Invoker是做什么的，有什么用？

这样说，以开头的DemoService为例，中有一个sayHello(String s)方法，这个方法是给其他地方使用的，有可能是本地也可能是远程调用，通过对应的Invoker.invoke()方法就可以调用了。调用invoker的结果就是最终调用DemoService.sayHello()。

public interface Exporter<T> {
    Invoker<T> getInvoker();
    void unexport();
}

通过Exporter可以获取到Invoker，把缓存起来，后面需要调用的时候就可以获取inoker调用对应的本地或者远程方法了。先这么理解就可以了，Invoker具体如何来的就先不分析了

接下来继续看这段代码：

Exporter<?> exporter = protocol.export(
            proxyFactory.getInvoker(ref, (Class) interfaceClass, local));

此处protocol为生产的动态代理类Protocol$Adaptive如下：

package com.alibaba.dubbo.rpc;

import com.alibaba.dubbo.common.extension.ExtensionLoader;

public class Protocol$Adpative implements com.alibaba.dubbo.rpc.Protocol {
    public void destroy() {
        throw new UnsupportedOperationException("method public abstract void com.alibaba.dubbo.rpc.Protocol.destroy() of interface com.alibaba.dubbo.rpc.Protocol is not adaptive method!");
    }

    public int getDefaultPort() {
        throw new UnsupportedOperationException("method public abstract int com.alibaba.dubbo.rpc.Protocol.getDefaultPort() of interface com.alibaba.dubbo.rpc.Protocol is not adaptive method!");
    }

    public com.alibaba.dubbo.rpc.Exporter export(com.alibaba.dubbo.rpc.Invoker arg0) throws com.alibaba.dubbo.rpc.RpcException {
        if (arg0 == null) throw new IllegalArgumentException("com.alibaba.dubbo.rpc.Invoker argument == null");
        if (arg0.getUrl() == null)
            throw new IllegalArgumentException("com.alibaba.dubbo.rpc.Invoker argument getUrl() == null");
        com.alibaba.dubbo.common.URL url = arg0.getUrl();
        String extName = (url.getProtocol() == null ? "dubbo" : url.getProtocol());
        //执行到此处的时候extName=Injvm
        
        if (extName == null)
            throw new IllegalStateException("Fail to get extension(com.alibaba.dubbo.rpc.Protocol) name from url(" + url.toString() + ") use keys([protocol])");
        com.alibaba.dubbo.rpc.Protocol extension = (com.alibaba.dubbo.rpc.Protocol) ExtensionLoader.getExtensionLoader(com.alibaba.dubbo.rpc.Protocol.class).getExtension(extName);
        return extension.export(arg0);
    }

    public com.alibaba.dubbo.rpc.Invoker refer(java.lang.Class arg0, com.alibaba.dubbo.common.URL arg1) throws com.alibaba.dubbo.rpc.RpcException {
        if (arg1 == null) throw new IllegalArgumentException("url == null");
        com.alibaba.dubbo.common.URL url = arg1;
        String extName = (url.getProtocol() == null ? "dubbo" : url.getProtocol());
        if (extName == null)
            throw new IllegalStateException("Fail to get extension(com.alibaba.dubbo.rpc.Protocol) name from url(" + url.toString() + ") use keys([protocol])");
        com.alibaba.dubbo.rpc.Protocol extension = (com.alibaba.dubbo.rpc.Protocol) ExtensionLoader.getExtensionLoader(com.alibaba.dubbo.rpc.Protocol.class).getExtension(extName);
        return extension.refer(arg0, arg1);
    }
}

protocol.export执行的时候先获取Protocol的扩展实例，在这里是InjvmProtocol，然后调用InjvmProtocol#export方法(如下)返回了一个InjvmExporter。

InjvmProtocol#export

  @Override
    public <T> Exporter<T> export(Invoker<T> invoker) throws RpcException {
        return new InjvmExporter<T>(invoker, invoker.getUrl().getServiceKey(), exporterMap);
    }

到这里服务本地暴露就分析完了。

3.2.2 暴露服务到远程

然后回到ServiceConfig#doExportUrlsFor1Protocol中的这行代码 Exporter<?> exporter = protocol.export(wrapperInvoker);

这里的wrapperInvoker信息如下：

interface com.alibaba.dubbo.demo.DemoService -> registry://127.0.0.1:2181/com.alibaba.dubbo.registry.RegistryService?application=demo-provider&dubbo=2.0.2&export=dubbo%3A%2F%2F192.168.43.174%3A20880%2Fcom.alibaba.dubbo.demo.DemoService%3Fanyhost%3Dtrue%26application%3Ddemo-provider%26bind.ip%3D192.168.43.174%26bind.port%3D20880%26dubbo%3D2.0.2%26generic%3Dfalse%26interface%3Dcom.alibaba.dubbo.demo.DemoService%26methods%3DsayHello%26pid%3D16888%26qos.port%3D22222%26side%3Dprovider%26timestamp%3D1578470846696&pid=16888&qos.port=22222&registry=zookeeper&timestamp=1578470846603

protocol.export在执行的时候会根据protocol扩展名获取具体的实现：

Protocol$Adaptive#export 方法中部分代码（这个类在上面以及贴过了）

 com.alibaba.dubbo.rpc.Protocol extension = (com.alibaba.dubbo.rpc.Protocol) ExtensionLoader.getExtensionLoader(com.alibaba.dubbo.rpc.Protocol.class).getExtension("registry");
//在这里extension就是RegistryProtocol了
extension.refer(arg0, arg1);

RegistryProtocol #export

public <T> Exporter<T> export(final Invoker<T> originInvoker) throws RpcException {
    // 导出服务
    final ExporterChangeableWrapper<T> exporter = doLocalExport(originInvoker);

    // 获取注册中心 URL，以 zookeeper 注册中心为例，得到的示例 URL 如下：
    // zookeeper://127.0.0.1:2181/com.alibaba.dubbo.registry.RegistryService?application=demo-provider&dubbo=2.0.2&export=dubbo%3A%2F%2F172.17.48.52%3A20880%2Fcom.alibaba.dubbo.demo.DemoService%3Fanyhost%3Dtrue%26application%3Ddemo-provider
    URL registryUrl = getRegistryUrl(originInvoker);

    // 根据 URL 加载 Registry 实现类，比如 ZookeeperRegistry
    final Registry registry = getRegistry(originInvoker);
    
    // 获取已注册的服务提供者 URL，比如：
    // dubbo://172.17.48.52:20880/com.alibaba.dubbo.demo.DemoService?anyhost=true&application=demo-provider&dubbo=2.0.2&generic=false&interface=com.alibaba.dubbo.demo.DemoService&methods=sayHello
    final URL registeredProviderUrl = getRegisteredProviderUrl(originInvoker);

    // 获取 register 参数
    boolean register = registeredProviderUrl.getParameter("register", true);

    // 向服务提供者与消费者注册表中注册服务提供者
    ProviderConsumerRegTable.registerProvider(originInvoker, registryUrl, registeredProviderUrl);

    // 根据 register 的值决定是否注册服务
    if (register) {
        // 向注册中心注册服务
        register(registryUrl, registeredProviderUrl);
        ProviderConsumerRegTable.getProviderWrapper(originInvoker).setReg(true);
    }

    // 获取订阅 URL，比如：
    // provider://172.17.48.52:20880/com.alibaba.dubbo.demo.DemoService?category=configurators&check=false&anyhost=true&application=demo-provider&dubbo=2.0.2&generic=false&interface=com.alibaba.dubbo.demo.DemoService&methods=sayHello
    final URL overrideSubscribeUrl = getSubscribedOverrideUrl(registeredProviderUrl);
    // 创建监听器
    final OverrideListener overrideSubscribeListener = new OverrideListener(overrideSubscribeUrl, originInvoker);
    overrideListeners.put(overrideSubscribeUrl, overrideSubscribeListener);
    // 向注册中心进行订阅 override 数据
    registry.subscribe(overrideSubscribeUrl, overrideSubscribeListener);
    // 创建并返回 DestroyableExporter
    return new DestroyableExporter<T>(exporter, originInvoker, overrideSubscribeUrl, registeredProviderUrl);
}

上面代码看起来比较复杂，主要做如下一些操作：

调用 doLocalExport 导出服务
向注册中心注册服务
向注册中心进行订阅 override 数据
创建并返回 DestroyableExporter

下面先来分析 doLocalExport 方法的逻辑，如下：

private <T> ExporterChangeableWrapper<T> doLocalExport(final Invoker<T> originInvoker) {
    String key = getCacheKey(originInvoker);
    // 访问缓存
    ExporterChangeableWrapper<T> exporter = (ExporterChangeableWrapper<T>) bounds.get(key);
    if (exporter == null) {
        synchronized (bounds) {
            exporter = (ExporterChangeableWrapper<T>) bounds.get(key);
            if (exporter == null) {
                // 创建 Invoker 为委托类对象
                final Invoker<?> invokerDelegete = new InvokerDelegete<T>(originInvoker, getProviderUrl(originInvoker));
                // 调用 protocol 的 export 方法导出服务
                exporter = new ExporterChangeableWrapper<T>((Exporter<T>) protocol.export(invokerDelegete), originInvoker);              
                // 写缓存
                bounds.put(key, exporter);
            }
        }
    }
    return exporter;
}

接下来，我们把重点放在 Protocol 的 export 方法上。假设运行时协议为 dubbo，此处的 protocol 变量会在运行时加载 DubboProtocol，并调用 DubboProtocol 的 export 方法。所以，接下来我们目光转移到 DubboProtocol 的 export 方法上，相关分析如下：

public <T> Exporter<T> export(Invoker<T> invoker) throws RpcException {
    URL url = invoker.getUrl();

    // 获取服务标识，理解成服务坐标也行。由服务组名，服务名，服务版本号以及端口组成。比如：
    // demoGroup/com.alibaba.dubbo.demo.DemoService:1.0.1:20880
    String key = serviceKey(url);
    // 创建 DubboExporter
    DubboExporter<T> exporter = new DubboExporter<T>(invoker, key, exporterMap);
    // 将 <key, exporter> 键值对放入缓存中
    exporterMap.put(key, exporter);

    // 本地存根相关代码
    //本地存根是一个代理对象，一般用于在真正调用服务前做一些参数见检查之类的
    Boolean isStubSupportEvent = url.getParameter(Constants.STUB_EVENT_KEY, Constants.DEFAULT_STUB_EVENT);
    Boolean isCallbackservice = url.getParameter(Constants.IS_CALLBACK_SERVICE, false);
    if (isStubSupportEvent && !isCallbackservice) {
        String stubServiceMethods = url.getParameter(Constants.STUB_EVENT_METHODS_KEY);
        if (stubServiceMethods == null || stubServiceMethods.length() == 0) {
            // 省略日志打印代码
        } else {
            stubServiceMethodsMap.put(url.getServiceKey(), stubServiceMethods);
        }
    }

    // 启动服务器
    openServer(url);
    // 优化序列化
    optimizeSerialization(url);
    return exporter;
}

重点关注 DubboExporter 的创建以及 openServer 方法,下面分析 openServer 方法。

private void openServer(URL url) {
    // 获取 host:port，并将其作为服务器实例的 key，用于标识当前的服务器实例
    String key = url.getAddress();
    boolean isServer = url.getParameter(Constants.IS_SERVER_KEY, true);
    if (isServer) {
        // 访问缓存
        ExchangeServer server = serverMap.get(key);
        if (server == null) {
            // 创建服务器实例
            serverMap.put(key, createServer(url));
        } else {
            // 服务器已创建，则根据 url 中的配置重置服务器
            //在同一台机器上（单网卡），同一个端口上仅允许启动一个服务器实例。若某个端口上已有服务器实例，此时则调用 reset 方法重置服务器的一些配置。
            server.reset(url);
        }
    }
}

接下来分析服务器实例的创建过程，如下：

private ExchangeServer createServer(URL url) {
    url = url.addParameterIfAbsent(Constants.CHANNEL_READONLYEVENT_SENT_KEY,
    // 添加心跳检测配置到 url 中
    url = url.addParameterIfAbsent(Constants.HEARTBEAT_KEY, String.valueOf(Constants.DEFAULT_HEARTBEAT));
    // 获取 server 参数，默认为 netty
    String str = url.getParameter(Constants.SERVER_KEY, Constants.DEFAULT_REMOTING_SERVER);

    // 通过 SPI 检测是否存在 server 参数所代表的 Transporter 拓展，不存在则抛出异常
    if (str != null && str.length() > 0 && !ExtensionLoader.getExtensionLoader(Transporter.class).hasExtension(str))
        throw new RpcException("Unsupported server type: " + str + ", url: " + url);

    // 添加编码解码器参数
    url = url.addParameter(Constants.CODEC_KEY, DubboCodec.NAME);
    ExchangeServer server;
    try {
        // 创建 ExchangeServer
        server = Exchangers.bind(url, requestHandler);
    } catch (RemotingException e) {
        throw new RpcException("Fail to start server...");
    }
                                   
    // 获取 client 参数，可指定 netty，mina
    str = url.getParameter(Constants.CLIENT_KEY);
    if (str != null && str.length() > 0) {
        // 获取所有的 Transporter 实现类名称集合，比如 supportedTypes = [netty, mina]
        Set<String> supportedTypes = ExtensionLoader.getExtensionLoader(Transporter.class).getSupportedExtensions();
        // 检测当前 Dubbo 所支持的 Transporter 实现类名称列表中，
        // 是否包含 client 所表示的 Transporter，若不包含，则抛出异常
        if (!supportedTypes.contains(str)) {
            throw new RpcException("Unsupported client type...");
        }
    }
    return server;
}

继续看创建服务器的部分：

public static ExchangeServer bind(URL url, ExchangeHandler handler) throws RemotingException {
    if (url == null) {
        throw new IllegalArgumentException("url == null");
    }
    if (handler == null) {
        throw new IllegalArgumentException("handler == null");
    }
    url = url.addParameterIfAbsent(Constants.CODEC_KEY, "exchange");
    // 获取 Exchanger，默认为 HeaderExchanger。
    // 紧接着调用 HeaderExchanger 的 bind 方法创建 ExchangeServer 实例
    return getExchanger(url).bind(url, handler);
}

下面看一下 HeaderExchanger 的 bind 方法。

public ExchangeServer bind(URL url, ExchangeHandler handler) throws RemotingException {
    // 创建 HeaderExchangeServer 实例，该方法包含了多个逻辑，分别如下：
    //   1. new HeaderExchangeHandler(handler)
    //   2. new DecodeHandler(new HeaderExchangeHandler(handler))
    //   3. Transporters.bind(url, new DecodeHandler(new HeaderExchangeHandler(handler)))
    return new HeaderExchangeServer(Transporters.bind(url, new DecodeHandler(new HeaderExchangeHandler(handler))));
}

HeaderExchanger 的 bind 方法包含的逻辑比较多，但目前我们仅需关心 Transporters 的 bind 方法逻辑即可。该方法的代码如下：

public static Server bind(URL url, ChannelHandler... handlers) throws RemotingException {
    if (url == null) {
        throw new IllegalArgumentException("url == null");
    }
    if (handlers == null || handlers.length == 0) {
        throw new IllegalArgumentException("handlers == null");
    }
    ChannelHandler handler;
    if (handlers.length == 1) {
        handler = handlers[0];
    } else {
        // 如果 handlers 元素数量大于1，则创建 ChannelHandler 分发器
        handler = new ChannelHandlerDispatcher(handlers);
    }
    // 获取自适应 Transporter 实例，并调用实例方法
    return getTransporter().bind(url, handler);
}

如上，getTransporter() 方法获取的 Transporter 是在运行时动态创建的，类名为 TransporterAdaptive，也就是自适应拓展类。TransporterAdaptive 会在运行时根据传入的 URL 参数决定加载什么类型的 Transporter，默认为 NettyTransporter。下面我们继续跟下去，这次分析的是 NettyTransporter 的 bind 方法。

public Server bind(URL url, ChannelHandler listener) throws RemotingException {
    // 创建 NettyServer
    return new NettyServer(url, listener);
}

public class NettyServer extends AbstractServer implements Server {
    public NettyServer(URL url, ChannelHandler handler) throws RemotingException {
        // 调用父类构造方法
        super(url, ChannelHandlers.wrap(handler, ExecutorUtil.setThreadName(url, SERVER_THREAD_POOL_NAME)));
    }
    //doOpen()..
    //doClose()..
    //...
}

public abstract class AbstractServer extends AbstractEndpoint implements Server {
    public AbstractServer(URL url, ChannelHandler handler) throws RemotingException {
        // 调用父类构造方法，这里就不用跟进去了，没什么复杂逻辑
        super(url, handler);
        localAddress = getUrl().toInetSocketAddress();

        // 获取 ip 和端口
        String bindIp = getUrl().getParameter(Constants.BIND_IP_KEY, getUrl().getHost());
        int bindPort = getUrl().getParameter(Constants.BIND_PORT_KEY, getUrl().getPort());
        if (url.getParameter(Constants.ANYHOST_KEY, false) || NetUtils.isInvalidLocalHost(bindIp)) {
            // 设置 ip 为 0.0.0.0
            bindIp = NetUtils.ANYHOST;
        }
        bindAddress = new InetSocketAddress(bindIp, bindPort);
        // 获取最大可接受连接数
        this.accepts = url.getParameter(Constants.ACCEPTS_KEY, Constants.DEFAULT_ACCEPTS);
        this.idleTimeout = url.getParameter(Constants.IDLE_TIMEOUT_KEY, Constants.DEFAULT_IDLE_TIMEOUT);
        try {
            // 调用模板方法 doOpen 启动服务器
            doOpen();
        } catch (Throwable t) {
            throw new RemotingException("Failed to bind ");
        }

        DataStore dataStore = ExtensionLoader.getExtensionLoader(DataStore.class).getDefaultExtension();
        executor = (ExecutorService) dataStore.get(Constants.EXECUTOR_SERVICE_COMPONENT_KEY, Integer.toString(url.getPort()));
    }
    
    protected abstract void doOpen() throws Throwable;

    protected abstract void doClose() throws Throwable;
}

我们重点关注 doOpen 抽象方法，该方法需要子类实现

NettyServer#doOpen

protected void doOpen() throws Throwable {
    NettyHelper.setNettyLoggerFactory();
    // 创建 boss 和 worker 线程池
    ExecutorService boss = Executors.newCachedThreadPool(new NamedThreadFactory("NettyServerBoss", true));
    ExecutorService worker = Executors.newCachedThreadPool(new NamedThreadFactory("NettyServerWorker", true));
    ChannelFactory channelFactory = new NioServerSocketChannelFactory(boss, worker, getUrl().getPositiveParameter(Constants.IO_THREADS_KEY, Constants.DEFAULT_IO_THREADS));
    
    // 创建 ServerBootstrap
    bootstrap = new ServerBootstrap(channelFactory);

    final NettyHandler nettyHandler = new NettyHandler(getUrl(), this);
    channels = nettyHandler.getChannels();
    bootstrap.setOption("child.tcpNoDelay", true);
    // 设置 PipelineFactory
    bootstrap.setPipelineFactory(new ChannelPipelineFactory() {
        @Override
        public ChannelPipeline getPipeline() {
            NettyCodecAdapter adapter = new NettyCodecAdapter(getCodec(), getUrl(), NettyServer.this);
            ChannelPipeline pipeline = Channels.pipeline();
            pipeline.addLast("decoder", adapter.getDecoder());
            pipeline.addLast("encoder", adapter.getEncoder());
            pipeline.addLast("handler", nettyHandler);
            return pipeline;
        }
    });
    // 绑定到指定的 ip 和端口上
    channel = bootstrap.bind(getBindAddress());
}

看到这段代码用过netty的同学应该很熟悉了，其启动netty服务端。到这里服务暴露到远程就分析完了。

上面涉及到protocol，exchange，transport这几个概念，回顾一下：

protocol 远程调用层：封装 RPC 调用，以 Invocation, Result 为中心，扩展接口为 Protocol, Invoker, Exporter
exchange 信息交换层：封装请求响应模式，同步转异步，以 Request, Response 为中心，扩展接口为 Exchanger, ExchangeChannel, ExchangeClient, ExchangeServer
transport 网络传输层：抽象 mina 和 netty 为统一接口，以 Message 为中心，扩展接口为 Channel, Transporter, Client, Server, Codec

3.3 服务注册

回到RegistryProtocol#export 方法上

public <T> Exporter<T> export(final Invoker<T> originInvoker) throws RpcException {
    // ${导出服务}
    // 省略其他代码
    boolean register = registeredProviderUrl.getParameter("register", true);
    if (register) {
        // 注册服务
        register(registryUrl, registeredProviderUrl);
        ProviderConsumerRegTable.getProviderWrapper(originInvoker).setReg(true);
    }
    
    final URL overrideSubscribeUrl = getSubscribedOverrideUrl(registeredProviderUrl);
    final OverrideListener overrideSubscribeListener = new OverrideListener(overrideSubscribeUrl, originInvoker);
    overrideListeners.put(overrideSubscribeUrl, overrideSubscribeListener);
    // 订阅 override 数据
    registry.subscribe(overrideSubscribeUrl, overrideSubscribeListener);

    // 省略部分代码
}

RegistryProtocol 的 export 方法包含了服务导出，注册，以及数据订阅等逻辑。其中服务导出逻辑上一节已经分析过了，本节将分析服务注册逻辑，相关代码如下：

public void register(URL registryUrl, URL registedProviderUrl) {
    // 获取 Registry
    Registry registry = registryFactory.getRegistry(registryUrl);
    // 注册服务
    registry.register(registedProviderUrl);
}

3.3.1 创建注册中心

文章开头已经说了，本文使用的注册中心是 Zookeeper

AbstractRegistryFactory #getRegistry

public Registry getRegistry(URL url) {
    url = url.setPath(RegistryService.class.getName())
            .addParameter(Constants.INTERFACE_KEY, RegistryService.class.getName())
            .removeParameters(Constants.EXPORT_KEY, Constants.REFER_KEY);
    String key = url.toServiceString();
    LOCK.lock();
    try {
        // 访问缓存
        Registry registry = REGISTRIES.get(key);
        if (registry != null) {
            return registry;
        }  
        // 缓存未命中，创建 Registry 实例
        registry = createRegistry(url);
        if (registry == null) {
            throw new IllegalStateException("Can not create registry...");
        }
        // 写入缓存
        REGISTRIES.put(key, registry);
        return registry;
    } finally {
        LOCK.unlock();
    }
}

protected abstract Registry createRegistry(URL url);

如上，getRegistry 方法先访问缓存，缓存未命中则调用 createRegistry 创建 Registry，然后写入缓存。这里的 createRegistry 是一个模板方法，由具体的子类实现。

ZookeeperRegistryFactory #AbstractRegistryFactory

public class ZookeeperRegistryFactory extends AbstractRegistryFactory {

    // zookeeperTransporter 由 SPI 在运行时注入，类型为 ZookeeperTransporter$Adaptive
    private ZookeeperTransporter zookeeperTransporter;

    public void setZookeeperTransporter(ZookeeperTransporter zookeeperTransporter) {
        this.zookeeperTransporter = zookeeperTransporter;
    }

    @Override
    public Registry createRegistry(URL url) {
        // 创建 ZookeeperRegistry
        return new ZookeeperRegistry(url, zookeeperTransporter);
    }
}

public ZookeeperRegistry(URL url, ZookeeperTransporter zookeeperTransporter) {
    super(url);
    if (url.isAnyHost()) {
        throw new IllegalStateException("registry address == null");
    }
    
    // 获取组名，默认为 dubbo
    String group = url.getParameter(Constants.GROUP_KEY, DEFAULT_ROOT);
    if (!group.startsWith(Constants.PATH_SEPARATOR)) {
        // group = "/" + group
        group = Constants.PATH_SEPARATOR + group;
    }
    this.root = group;
    // 创建 Zookeeper 客户端，默认为 CuratorZookeeperTransporter
    //在2.5.x版本默认的是ZkclientZookeeperClient，
    //在2.6.4默认的CuratorZookeeperClient
    //在2.7.x版本已经移除Zkclient，若要使用需要自己扩展
    zkClient = zookeeperTransporter.connect(url);
    // 添加状态监听器
    zkClient.addStateListener(new StateListener() {
        @Override
        public void stateChanged(int state) {
            if (state == RECONNECTED) {
                try {
                    recover();
                } catch (Exception e) {
                    logger.error(e.getMessage(), e);
                }
            }
        }
    });
}

在上面的代码代码中，我们重点关注 ZookeeperTransporter 的 connect 方法调用，这个方法用于创建 Zookeeper 客户端。创建好 Zookeeper 客户端，意味着注册中心的创建过程就结束了。接下来，再来分析一下 Zookeeper 客户端的创建过程。

前面说过，这里的 zookeeperTransporter 类型为自适应拓展类，因此 connect 方法会在被调用时决定加载什么类型的 ZookeeperTransporter 拓展，默认为 CuratorZookeeperTransporter。下面我们到 CuratorZookeeperTransporter 中看一看。

public ZookeeperClient connect(URL url) {
    // 创建 CuratorZookeeperClient
    return new CuratorZookeeperClient(url);
}

public class CuratorZookeeperClient extends AbstractZookeeperClient<CuratorWatcher> {

    private final CuratorFramework client;
    
    public CuratorZookeeperClient(URL url) {
        super(url);
        try {
            // 创建 CuratorFramework 构造器
            CuratorFrameworkFactory.Builder builder = CuratorFrameworkFactory.builder()
                    .connectString(url.getBackupAddress())
                    .retryPolicy(new RetryNTimes(1, 1000))
                    .connectionTimeoutMs(5000);
            String authority = url.getAuthority();
            if (authority != null && authority.length() > 0) {
                builder = builder.authorization("digest", authority.getBytes());
            }
            // 构建 CuratorFramework 实例
            client = builder.build();
            // 添加监听器
            client.getConnectionStateListenable().addListener(new ConnectionStateListener() {
                @Override
                public void stateChanged(CuratorFramework client, ConnectionState state) {
                    if (state == ConnectionState.LOST) {
                        CuratorZookeeperClient.this.stateChanged(StateListener.DISCONNECTED);
                    } else if (state == ConnectionState.CONNECTED) {
                        CuratorZookeeperClient.this.stateChanged(StateListener.CONNECTED);
                    } else if (state == ConnectionState.RECONNECTED) {
                        CuratorZookeeperClient.this.stateChanged(StateListener.RECONNECTED);
                    }
                }
            });
            
            // 启动客户端
            client.start();
        } catch (Exception e) {
            throw new IllegalStateException(e.getMessage(), e);
        }
    }
}

再顺便看下ZkclientZookeeperClient

public class ZkclientZookeeperClient extends AbstractZookeeperClient<IZkChildListener> {

    private final ZkClientWrapper client;

    private volatile KeeperState state = KeeperState.SyncConnected;

    public ZkclientZookeeperClient(URL url) {
        super(url);
        client = new ZkClientWrapper(url.getBackupAddress(), 30000);
        client.addListener(new IZkStateListener() {
            @Override
            public void handleStateChanged(KeeperState state) throws Exception {
                ZkclientZookeeperClient.this.state = state;
                if (state == KeeperState.Disconnected) {
                    stateChanged(StateListener.DISCONNECTED);
                } else if (state == KeeperState.SyncConnected) {
                    stateChanged(StateListener.CONNECTED);
                }
            }

            @Override
            public void handleNewSession() throws Exception {
                stateChanged(StateListener.RECONNECTED);
            }
        });
        client.start();
    }

}

过程类似，都是创建客户端，然后增加一个监听器。

到这里注册中心实例创建好了，接下来要做的事情是向注册中心注册服务。

3.3.2 服务注册

以 Zookeeper 为例，所谓的服务注册，本质上是将服务配置数据写入到 Zookeeper 的某个路径的节点下。

Zookeeper 可视化客户端 ZooInspector 查看节点数据如下：

dubbo-zk.png

图中可以看到 com.alibaba.dubbo.demo.DemoService 这个服务对应的配置信息（存储在 URL 中）最终被注册到了 /dubbo/com.alibaba.dubbo.demo.DemoService/providers/ 节点下。

附一张dubbo注册到zookeper的节点层次说明图：

image

像注册中心注册的代码在RegistryProtocol#register(registryUrl, registeredProviderUrl)

public void register(URL registryUrl, URL registedProviderUrl) {
        Registry registry = registryFactory.getRegistry(registryUrl);
        registry.register(registedProviderUrl);
    }

FailbackRegistry#register(URL url)

public void register(URL url) {
    //需要注册的url:  dubbo://192.168.43.174:20880/com.alibaba.dubbo.demo.DemoService?anyhost=true&application=demo-provider&dubbo=2.0.2&generic=false&interface=com.alibaba.dubbo.demo.DemoService&methods=sayHello&pid=16024&side=provider&timestamp=1578478503772
    super.register(url);
    failedRegistered.remove(url);
    failedUnregistered.remove(url);
    try {
        // 模板方法，由子类实现
        doRegister(url);
    } catch (Exception e) {
        Throwable t = e;

        // 获取 check 参数，若 check = true 将会直接抛出异常
        boolean check = getUrl().getParameter(Constants.CHECK_KEY, true)
                && url.getParameter(Constants.CHECK_KEY, true)
                && !Constants.CONSUMER_PROTOCOL.equals(url.getProtocol());
        boolean skipFailback = t instanceof SkipFailbackWrapperException;
        if (check || skipFailback) {
            if (skipFailback) {
                t = t.getCause();
            }
            throw new IllegalStateException("Failed to register");
        } else {
            logger.error("Failed to register");
        }

        // 记录注册失败的链接
        failedRegistered.add(url);
    }
}

protected abstract void doRegister(URL url);

doRegister 方法是一个模板方法，因此我们到 FailbackRegistry 子类 ZookeeperRegistry 中进行分析。如下：

protected void doRegister(URL url) {
    try {
        // 通过 Zookeeper 客户端创建节点，节点路径由 toUrlPath 方法生成，路径格式如下:
        //   /${group}/${serviceInterface}/providers/${url}
        // 比如
        //   /dubbo/org.apache.dubbo.DemoService/providers/dubbo%3A%2F%2F127.0.0.1......
        zkClient.create(toUrlPath(url), url.getParameter(Constants.DYNAMIC_KEY, true));
    } catch (Throwable e) {
        throw new RpcException("Failed to register...");
    }
}

如上，ZookeeperRegistry 在 doRegister 中调用了 Zookeeper 客户端创建服务节点。节点路径由 toUrlPath 方法生成，该方法逻辑不难理解，就不分析了。接下来分析 create 方法，如下：

public void create(String path, boolean ephemeral) {
    //path:
  ///dubbo/com.alibaba.dubbo.demo.DemoService/providers/dubbo%3A%2F%2F192.168.43.174%3A20880%2Fcom.alibaba.dubbo.demo.DemoService%3Fanyhost%3Dtrue%26application%3Ddemo-provider%26dubbo%3D2.0.2%26generic%3Dfalse%26interface%3Dcom.alibaba.dubbo.demo.DemoService%26methods%3DsayHello%26pid%3D16024%26side%3Dprovider%26timestamp%3D1578478503772
    if (!ephemeral) {
        // 如果要创建的节点类型非临时节点，那么这里要检测节点是否存在
        if (checkExists(path)) {
            return;
        }
    }
    int i = path.lastIndexOf('/');
    if (i > 0) {
        // 递归创建上一级路径
        create(path.substring(0, i), false);
    }
    
    // 根据 ephemeral 的值创建临时或持久节点
    if (ephemeral) {
        createEphemeral(path);
    } else {
        createPersistent(path);
    }
}

经过这段代码会创建这些节点：

持久节点 /dubbo

持久节点 /com.alibaba.dubbo.demo.DemoService

持久节点 /providers

临时节点

/dubbo%3A%2F%2F192.168.43.174%3A20880%2Fcom.alibaba.dubbo.demo.DemoService%3Fanyhost%3Dtrue%26application%3Ddemo-provider%26dubbo%3D2.0.2%26generic%3Dfalse%26interface%3Dcom.alibaba.dubbo.demo.DemoService%26methods%3DsayHello%26pid%3D16024%26side%3Dprovider%26timestamp%3D1578478503772

对于上面的树型结构数据

3.3.2 订阅override 数据

又得回到RegistryProtocol#export方法，再贴一次

public <T> Exporter<T> export(final Invoker<T> originInvoker) throws RpcException {
    // 导出服务
    final ExporterChangeableWrapper<T> exporter = doLocalExport(originInvoker);

    // 获取注册中心 URL，以 zookeeper 注册中心为例，得到的示例 URL 如下：
    // zookeeper://127.0.0.1:2181/com.alibaba.dubbo.registry.RegistryService?application=demo-provider&dubbo=2.0.2&export=dubbo%3A%2F%2F172.17.48.52%3A20880%2Fcom.alibaba.dubbo.demo.DemoService%3Fanyhost%3Dtrue%26application%3Ddemo-provider
    URL registryUrl = getRegistryUrl(originInvoker);

    // 根据 URL 加载 Registry 实现类，比如 ZookeeperRegistry
    final Registry registry = getRegistry(originInvoker);
    
    // 获取已注册的服务提供者 URL，比如：
    // dubbo://172.17.48.52:20880/com.alibaba.dubbo.demo.DemoService?anyhost=true&application=demo-provider&dubbo=2.0.2&generic=false&interface=com.alibaba.dubbo.demo.DemoService&methods=sayHello
    final URL registeredProviderUrl = getRegisteredProviderUrl(originInvoker);

    // 获取 register 参数
    boolean register = registeredProviderUrl.getParameter("register", true);

    // 向服务提供者与消费者注册表中注册服务提供者
    ProviderConsumerRegTable.registerProvider(originInvoker, registryUrl, registeredProviderUrl);

    // 根据 register 的值决定是否注册服务
    if (register) {
        // 向注册中心注册服务
        register(registryUrl, registeredProviderUrl);
        ProviderConsumerRegTable.getProviderWrapper(originInvoker).setReg(true);
    }

    // 获取订阅 URL，比如：
    // provider://172.17.48.52:20880/com.alibaba.dubbo.demo.DemoService?category=configurators&check=false&anyhost=true&application=demo-provider&dubbo=2.0.2&generic=false&interface=com.alibaba.dubbo.demo.DemoService&methods=sayHello
    //表示订阅的是服务提供者provider://172.17.48.52:20880/com.alibaba.dubbo.demo.DemoService的configurators节点的信息
    final URL overrideSubscribeUrl = getSubscribedOverrideUrl(registeredProviderUrl);
    // 创建监听器
    final OverrideListener overrideSubscribeListener = new OverrideListener(overrideSubscribeUrl, originInvoker);
    overrideListeners.put(overrideSubscribeUrl, overrideSubscribeListener);
    // 向注册中心进行订阅 override 数据
    registry.subscribe(overrideSubscribeUrl, overrideSubscribeListener);
    // 创建并返回 DestroyableExporter
    return new DestroyableExporter<T>(exporter, originInvoker, overrideSubscribeUrl, registeredProviderUrl);
}

关注： registry.subscribe(overrideSubscribeUrl, overrideSubscribeListener);

FailbackRegistry#subscribe

public void subscribe(URL url, NotifyListener listener) {
    //url示例
    //provider://192.168.43.174:20880/com.alibaba.dubbo.demo.DemoService?anyhost=true&application=demo-provider&category=configurators&check=false&dubbo=2.0.2&generic=false&interface=com.alibaba.dubbo.demo.DemoService&methods=sayHello&pid=17976&side=provider&timestamp=1578479464018
        super.subscribe(url, listener);
        removeFailedSubscribed(url, listener);
        try {
            // Sending a subscription request to the server side
            doSubscribe(url, listener);
        } catch (Exception e) {
            Throwable t = e;

            List<URL> urls = getCacheUrls(url);
            if (urls != null && !urls.isEmpty()) {
                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 {
                // If the startup detection is opened, the Exception is thrown directly.
                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);
                }
            }

            // Record a failed registration request to a failed list, retry regularly
            addFailedSubscribed(url, listener);
        }
    }

关注doSubscribe(url, listener);方法

ZookeeperRegistry#doSubscribe(url, listener)

protected void doSubscribe(final URL url, final NotifyListener listener) {
        try {
            if (Constants.ANY_VALUE.equals(url.getServiceInterface())) {
                    //省略...
            } else {
                List<URL> urls = new ArrayList<URL>();
                for (String path : toCategoriesPath(url)) {
                    //toCategoriesPath(url)解析出要订阅的节点路径
                    //path:/dubbo/com.alibaba.dubbo.demo.DemoService/configurators
                    
                    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) {
                        //添加监听器，如果有变化调用notify(url, listener, urls)
                        listeners.putIfAbsent(listener, new ChildListener() {
                            @Override
                            public void childChanged(String parentPath, List<String> currentChilds) {
                                ZookeeperRegistry.this.notify(url, listener, toUrlsWithEmpty(url, parentPath, currentChilds));
                            }
                        });
                        zkListener = listeners.get(listener);
                    }
                    //创建持久节点 /dubbo/com.alibaba.dubbo.demo.DemoService/configurators
                    zkClient.create(path, false);
                    List<String> children = zkClient.addChildListener(path, zkListener);
                    if (children != null) {
                       //toUrlsWithEmpty(url, path, children) 这个方法url的协议头由provider替换为了empty       
                       //获得provider中，和consumer匹配的url数组
                        //若不存在则创建 empty://的url返回，可以处理类似服务提供者为空的情况
                        urls.addAll(toUrlsWithEmpty(url, path, children));
                         //此时url为：empty://192.168.43.174:20880/com.alibaba.dubbo.demo.DemoService?anyhost=true&application=demo-provider&category=configurators&check=false&dubbo=2.0.2&generic=false&interface=com.alibaba.dubbo.demo.DemoService&methods=sayHello&pid=1384&side=provider&timestamp=1578532572533
                    }
                }
                notify(url, listener, urls);
            }
        } catch (Throwable e) {
            throw new RpcException("Failed to subscribe " + url + " to zookeeper " + getUrl() + ", cause: " + e.getMessage(), e);
        }
    }

notify(url, listener, urls)方法会调用AbstractRegistry# notify(URL url, NotifyListener listener, List<URL> urls)方法，如下：

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.isEmpty())
                && !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);
        }
        Map<String, List<URL>> result = new HashMap<String, 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);
            /***
            表示服务提供者存入本地缓存文件key=com.alibaba.dubbo.demo.DemoService
            value=
            provider://192.168.43.174:20880/com.alibaba.dubbo.demo.DemoService?anyhost=true&application=demo-provider&category=configurators&check=false&dubbo=2.0.2&generic=false&interface=com.alibaba.dubbo.demo.DemoService&methods=sayHello&pid=16140&side=provider&timestamp=1578536147384
           ***/
            saveProperties(url);
            //调用RegistryProtocol中的OverrideListener#notify(List<URL> urls)方法
            listener.notify(categoryList);
        }
    }

OverrideListener#notify(List<URL> urls)方法

@Override
        public synchronized void notify(List<URL> urls) {
            //urls这里只有一条数据empty://192.168.43.174:20880/com.alibaba.dubbo.demo.DemoService?anyhost=true&application=demo-provider&category=configurators&check=false&dubbo=2.0.2&generic=false&interface=com.alibaba.dubbo.demo.DemoService&methods=sayHello&pid=16140&side=provider&timestamp=1578536147384
            logger.debug("original override urls: " + urls);
            //subscribeUrl
            //provider://192.168.43.174:20880/com.alibaba.dubbo.demo.DemoService?anyhost=true&application=demo-provider&category=configurators&check=false&dubbo=2.0.2&generic=false&interface=com.alibaba.dubbo.demo.DemoService&methods=sayHello&pid=16140&side=provider&timestamp=1578536147384
            //获取匹配的url
            List<URL> matchedUrls = getMatchedUrls(urls, subscribeUrl);
            logger.debug("subscribe url: " + subscribeUrl + ", override urls: " + matchedUrls);
            // No matching results
            if (matchedUrls.isEmpty()) {
                return;
            }
//提取出变化的配置
            List<Configurator> configurators = RegistryDirectory.toConfigurators(matchedUrls);

            final Invoker<?> invoker;
            if (originInvoker instanceof InvokerDelegete) {
                invoker = ((InvokerDelegete<?>) originInvoker).getInvoker();
            } else {
                invoker = originInvoker;
            }
            //The origin invoker
            URL originUrl = RegistryProtocol.this.getProviderUrl(invoker);
            String key = getCacheKey(originInvoker);
            ExporterChangeableWrapper<?> exporter = bounds.get(key);
            if (exporter == null) {
                logger.warn(new IllegalStateException("error state, exporter should not be null"));
                return;
            }
            //The current, may have been merged many times
            URL currentUrl = exporter.getInvoker().getUrl();
            //Merged with this configuration
            //根据变化的配置信息组装新的url
            URL newUrl = getConfigedInvokerUrl(configurators, originUrl);
            if (!currentUrl.equals(newUrl)) {
                //如果新的url和原来的不一样，则重新导出服务
                RegistryProtocol.this.doChangeLocalExport(originInvoker, newUrl);
                logger.info("exported provider url changed, origin url: " + originUrl + ", old export url: " + currentUrl + ", new export url: " + newUrl);
            }
        }

到这里订阅override数据的部分也分析完了

3.3.4 小结

到这里服务注册的过程分析完了，分为两个部分：先创建注册中心实例，之后再通过注册中心实例注册服务，然后订阅配置信息变化。

4.总结

服务发布整个流程讲完了，总结下主要由以下一个步骤：

前置工作：检查参数组装URl
暴露服务到本地
暴露服务到远程
启动netty暴露服务
创建连接zk注册中心
服务注册到zk
到zk订阅override数据

原文地址