github新增仓库 "dubbo-read"（点此查看），集合所有《Dubbo原理和源码解析》系列文章，后续将继续补充该系列，同时将针对Dubbo所做的功能扩展也进行分享。不定期更新，欢迎Follow。

一、框架设计

在官方《Dubbo 用户指南》架构部分，给出了服务调用的整体架构和流程：

Dubbo原理和源码解析之服务暴露

另外，在官方《Dubbo 开发指南》框架设计部分，给出了整体设计：

Dubbo原理和源码解析之服务暴露

以及暴露服务时序图：

Dubbo原理和源码解析之服务暴露

本文将根据以上几张图，分析服务暴露的实现原理，并进行详细的代码跟踪与解析。

二、原理和源码解析

2.1 标签解析

从文章《Dubbo原理和源码解析之标签解析》中我们知道，dubbo:service 标签会被解析成 ServiceBean。

ServiceBean 实现了 InitializingBean，在类加载完成之后会调用 afterPropertiesSet() 方法。在 afterPropertiesSet() 方法中，依次解析以下标签信息：

ServiceBean 还实现了 ApplicationListener，在 Spring 容器初始化的时候会调用 onApplicationEvent 方法。ServiceBean 重写了 onApplicationEvent 方法，实现了服务暴露的功能。

ServiceBean.java

public void onApplicationEvent(ApplicationEvent event) {
    if (ContextRefreshedEvent.class.getName().equals(event.getClass().getName())) {
        if (isDelay() && ! isExported() && ! isUnexported()) {
            if (logger.isInfoEnabled()) {
                logger.info("The service ready on spring started. service: " + getInterface());
            }
            export();
        }
    }
}

2.2 延迟暴露

ServiceBean 扩展了 ServiceConfig，调用 export() 方法时由 ServiceConfig 完成服务暴露的功能实现。

ServiceConfig.java

public synchronized void export() {
    if (provider != null) {
        if (export == null) {
            export = provider.getExport();
        }
        if (delay == null) {
            delay = provider.getDelay();
        }
    }
    if (export != null && ! export.booleanValue()) {
        return;
    }
    if (delay != null && delay > 0) {
        Thread thread = new Thread(new Runnable() {
            public void run() {
                try {
                    Thread.sleep(delay);
                } catch (Throwable e) {
                }
                doExport();
            }
        });
        thread.setDaemon(true);
        thread.setName("DelayExportServiceThread");
        thread.start();
    } else {
        doExport();
    }
}

由上面代码可知，如果设置了 delay 参数，Dubbo 的处理方式是启动一个守护线程在 sleep 指定时间后再 doExport。

2.3 参数检查

在 ServiceConfig 的 doExport() 方法中会进行参数检查和设置，包括：

泛化调用
本地实现
本地存根
本地伪装
配置（application、registry、protocol等）

ServiceConfig.java

protected synchronized void doExport() {
    if (unexported) {
        throw new IllegalStateException("Already unexported!");
    }
    if (exported) {
        return;
    }
    exported = true;
    if (interfaceName == null || interfaceName.length() == 0) {
        throw new IllegalStateException("<dubbo:service interface=\"\" /> interface not allow null!");
    }
    checkDefault();
    //省略
    if (ref instanceof GenericService) {
        interfaceClass = GenericService.class;
        generic = true;
    } else {
        try {
            interfaceClass = Class.forName(interfaceName, true, Thread.currentThread()
                    .getContextClassLoader());
        } catch (ClassNotFoundException e) {
            throw new IllegalStateException(e.getMessage(), e);
        }
        checkInterfaceAndMethods(interfaceClass, methods);
        checkRef();
        generic = false;
    }
    if(local !=null){
        if(local=="true"){
            local=interfaceName+"Local";
        }
        Class<?> localClass;
        try {
            localClass = ClassHelper.forNameWithThreadContextClassLoader(local);
        } catch (ClassNotFoundException e) {
            throw new IllegalStateException(e.getMessage(), e);
        }
        if(!interfaceClass.isAssignableFrom(localClass)){
            throw new IllegalStateException("The local implemention class " + localClass.getName() + " not implement interface " + interfaceName);
        }
    }
    if(stub !=null){
        if(stub=="true"){
            stub=interfaceName+"Stub";
        }
        Class<?> stubClass;
        try {
            stubClass = ClassHelper.forNameWithThreadContextClassLoader(stub);
        } catch (ClassNotFoundException e) {
            throw new IllegalStateException(e.getMessage(), e);
        }
        if(!interfaceClass.isAssignableFrom(stubClass)){
            throw new IllegalStateException("The stub implemention class " + stubClass.getName() + " not implement interface " + interfaceName);
        }
    }
    //此处省略：检查并设置相关参数
    doExportUrls();
}

2.4 多协议、多注册中心

在检查完参数之后，开始暴露服务。Dubbo 支持多协议和多注册中心：

ServiceConfig.java

private void doExportUrls() {
    List<URL> registryURLs = loadRegistries(true);
    for (ProtocolConfig protocolConfig : protocols) {
        doExportUrlsFor1Protocol(protocolConfig, registryURLs);
    }
}

2.5 组装URL

针对每个协议、每个注册中心，开始组装 URL。

ServiceConfig.java

private void doExportUrlsFor1Protocol(ProtocolConfig protocolConfig, List<URL> registryURLs) {
    String name = protocolConfig.getName();
    if (name == null || name.length() == 0) {
        name = "dubbo";
    }

    //处理host

    //处理port

    Map<String, String> map = new HashMap<String, String>();
    //设置参数到map

    // 导出服务
    String contextPath = protocolConfig.getContextpath();
    if ((contextPath == null || contextPath.length() == 0) && provider != null) {
        contextPath = provider.getContextpath();
    }
    URL url = new URL(name, host, port, (contextPath == null || contextPath.length() == 0 ? "" : contextPath + "/") + path, map);

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

    //此处省略：服务暴露（详见 2.6 和 2.7）
    
    this.urls.add(url);
}

2.6 本地暴露

如果配置 scope=none，则不会进行服务暴露；如果没有配置 scope 或者 scope=local，则会进行本地暴露。

ServiceConfig.java

//public static final String LOCAL_PROTOCOL = "injvm";
//public static final String LOCALHOST = "127.0.0.1";

private void doExportUrlsFor1Protocol(ProtocolConfig protocolConfig, List<URL> registryURLs) {
    //......
    String scope = url.getParameter(Constants.SCOPE_KEY);
    //配置为none不暴露
    if (! Constants.SCOPE_NONE.toString().equalsIgnoreCase(scope)) {
        //配置不是remote的情况下做本地暴露 (配置为remote，则表示只暴露远程服务)
        if (!Constants.SCOPE_REMOTE.toString().equalsIgnoreCase(scope)) {
            exportLocal(url);
        }
        //......
    }
    //......
}

private void exportLocal(URL url) {
    if (!Constants.LOCAL_PROTOCOL.equalsIgnoreCase(url.getProtocol())) {
        URL local = URL.valueOf(url.toFullString())
                .setProtocol(Constants.LOCAL_PROTOCOL)
                .setHost(NetUtils.LOCALHOST)
                .setPort(0);
        Exporter<?> exporter = protocol.export(
                proxyFactory.getInvoker(ref, (Class) interfaceClass, local));
        exporters.add(exporter);
        logger.info("Export dubbo service " + interfaceClass.getName() +" to local registry");
    }
}

1. 暴露服务的时候，会通过代理创建 Invoker；

2. 本地暴露时使用 injvm 协议，injvm 协议是一个伪协议，它不开启端口，不能被远程调用，只在 JVM 内直接关联，但执行 Dubbo 的 Filter 链。

2.7 远程暴露

如果没有配置 scope 或者 scope=remote，则会进行远程暴露。

ServiceConfig.java

private void doExportUrlsFor1Protocol(ProtocolConfig protocolConfig, List<URL> registryURLs) {
    String scope = url.getParameter(Constants.SCOPE_KEY);
    //配置为none不暴露
    if (! Constants.SCOPE_NONE.toString().equalsIgnoreCase(scope)) {
        //......
        //如果配置不是local则暴露为远程服务.(配置为local，则表示只暴露远程服务)
        if (! Constants.SCOPE_LOCAL.toString().equalsIgnoreCase(scope) ){
            if (logger.isInfoEnabled()) {
                logger.info("Export dubbo service " + interfaceClass.getName() + " to url " + url);
            }
            if (registryURLs != null && registryURLs.size() > 0
                    && url.getParameter("register", true)) {
                for (URL registryURL : registryURLs) {
                    url = url.addParameterIfAbsent("dynamic", registryURL.getParameter("dynamic"));
                    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);
                    }
                    Invoker<?> invoker = proxyFactory.getInvoker(ref, (Class) interfaceClass, registryURL.addParameterAndEncoded(Constants.EXPORT_KEY, url.toFullString()));
    
                    Exporter<?> exporter = protocol.export(invoker);
                    exporters.add(exporter);
                }
            } else {
                Invoker<?> invoker = proxyFactory.getInvoker(ref, (Class) interfaceClass, url);
    
                Exporter<?> exporter = protocol.export(invoker);
                exporters.add(exporter);
            }
        }
    }
}

在服务暴露时，有两种情况：

不使用注册中心：直接暴露对应协议的服务，引用服务时只能通过直连方式引用
使用注册中心：暴露对应协议的服务后，会将服务节点注册到注册中心，引用服务时可以通过注册中心动态获取服务提供者列表，也可以通过直连方式引用

2.8 暴露服务

不使用注册中心时，直接调用对应协议（如 Dubbo 协议）的 export() 暴露服务。以 Dubbo 协议为例：

DubboProtocol.java

public <T> Exporter<T> export(Invoker<T> invoker) throws RpcException {
    URL url = invoker.getUrl();
    
    // export service.
    String key = serviceKey(url);
    DubboExporter<T> exporter = new DubboExporter<T>(invoker, key, exporterMap);
    exporterMap.put(key, exporter);
    
    //export an stub service for dispaching event
    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 ){
            if (logger.isWarnEnabled()){
                logger.warn(new IllegalStateException("consumer [" +url.getParameter(Constants.INTERFACE_KEY) +
                        "], has set stubproxy support event ,but no stub methods founded."));
            }
        } else {
            stubServiceMethodsMap.put(url.getServiceKey(), stubServiceMethods);
        }
    }

    openServer(url);
    
    return exporter;
}

调用 openServer() 方法创建并启动 Server：

DubboProtocol.java

private void openServer(URL url) {
    // find server.
    String key = url.getAddress();
    //client 也可以暴露一个只有server可以调用的服务。
    boolean isServer = url.getParameter(Constants.IS_SERVER_KEY,true);
    if (isServer) {
        ExchangeServer server = serverMap.get(key);
        if (server == null) {
            serverMap.put(key, createServer(url));
        } else {
            //server支持reset,配合override功能使用
            server.reset(url);
        }
    }
}

private ExchangeServer createServer(URL url) {
    //默认开启server关闭时发送readonly事件
    url = url.addParameterIfAbsent(Constants.CHANNEL_READONLYEVENT_SENT_KEY, Boolean.TRUE.toString());
    //默认开启heartbeat
    url = url.addParameterIfAbsent(Constants.HEARTBEAT_KEY, String.valueOf(Constants.DEFAULT_HEARTBEAT));
    String str = url.getParameter(Constants.SERVER_KEY, Constants.DEFAULT_REMOTING_SERVER);

    if (str != null && str.length() > 0 && ! ExtensionLoader.getExtensionLoader(Transporter.class).hasExtension(str))
        throw new RpcException("Unsupported server type: " + str + ", url: " + url);

    url = url.addParameter(Constants.CODEC_KEY, Version.isCompatibleVersion() ? COMPATIBLE_CODEC_NAME : DubboCodec.NAME);
    ExchangeServer server;
    try {
        server = Exchangers.bind(url, requestHandler);
    } catch (RemotingException e) {
        throw new RpcException("Fail to start server(url: " + url + ") " + e.getMessage(), e);
    }
    str = url.getParameter(Constants.CLIENT_KEY);
    if (str != null && str.length() > 0) {
        Set<String> supportedTypes = ExtensionLoader.getExtensionLoader(Transporter.class).getSupportedExtensions();
        if (!supportedTypes.contains(str)) {
            throw new RpcException("Unsupported client type: " + str);
        }
    }
    return server;
}

Exchanger （默认 HeaderExchanger）封装请求响应模式，同步转异步，以 Request、Response 为中心:

HeaderExchager.java

public ExchangeServer bind(URL url, ExchangeHandler handler) throws RemotingException {
    return new HeaderExchangeServer(Transporters.bind(url, new DecodeHandler(new HeaderExchangeHandler(handler))));
}

Transporters.java

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 {
        handler = new ChannelHandlerDispatcher(handlers);
    }
    return getTransporter().bind(url, handler);
}

底层传输默认使用 NettyTransporter，最终是创建 NettyServer：

NettyTransporter.java

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

NettyServer.java

public class NettyServer extends AbstractServer implements Server {
    public NettyServer(URL url, ChannelHandler handler) throws RemotingException{
        super(url, ChannelHandlers.wrap(handler, ExecutorUtil.setThreadName(url, SERVER_THREAD_POOL_NAME)));
    }
}

AbstractServer.java

public abstract class AbstractServer extends AbstractEndpoint implements Server {
    public AbstractServer(URL url, ChannelHandler handler) throws RemotingException {
        super(url, handler);
        localAddress = getUrl().toInetSocketAddress();
        String host = url.getParameter(Constants.ANYHOST_KEY, false) 
                        || NetUtils.isInvalidLocalHost(getUrl().getHost()) 
                        ? NetUtils.ANYHOST : getUrl().getHost();
        bindAddress = new InetSocketAddress(host, getUrl().getPort());
        this.accepts = url.getParameter(Constants.ACCEPTS_KEY, Constants.DEFAULT_ACCEPTS);
        this.idleTimeout = url.getParameter(Constants.IDLE_TIMEOUT_KEY, Constants.DEFAULT_IDLE_TIMEOUT);
        try {
            doOpen();
            if (logger.isInfoEnabled()) {
                logger.info("Start " + getClass().getSimpleName() + " bind " + getBindAddress() + ", export " + getLocalAddress());
            }
        } catch (Throwable t) {
            throw new RemotingException(url.toInetSocketAddress(), null, "Failed to bind " + getClass().getSimpleName() 
                                        + " on " + getLocalAddress() + ", cause: " + t.getMessage(), t);
        }
        if (handler instanceof WrappedChannelHandler ){
            executor = ((WrappedChannelHandler)handler).getExecutor();
        }
    }
}

NettyServer.java

public class NettyServer extends AbstractServer implements Server {
    protected void doOpen() throws Throwable {
        NettyHelper.setNettyLoggerFactory();
        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));
        bootstrap = new ServerBootstrap(channelFactory);
        
        final NettyHandler nettyHandler = new NettyHandler(getUrl(), this);
        channels = nettyHandler.getChannels();
        // final Timer timer = new HashedWheelTimer(new NamedThreadFactory("NettyIdleTimer", true));
        bootstrap.setPipelineFactory(new ChannelPipelineFactory() {
            public ChannelPipeline getPipeline() {
                NettyCodecAdapter adapter = new NettyCodecAdapter(getCodec() ,getUrl(), NettyServer.this);
                ChannelPipeline pipeline = Channels.pipeline();
                /*int idleTimeout = getIdleTimeout();
                if (idleTimeout > 10000) {
                    pipeline.addLast("timer", new IdleStateHandler(timer, idleTimeout / 1000, 0, 0));
                }*/
                pipeline.addLast("decoder", adapter.getDecoder());
                pipeline.addLast("encoder", adapter.getEncoder());
                pipeline.addLast("handler", nettyHandler);
                return pipeline;
            }
        });
        // bind
        channel = bootstrap.bind(getBindAddress());
    }
}

2.9 服务注册

如果使用了注册中心，则在通过具体协议（如 Dubbo 协议）暴露服务之后（即在 2.8 基础之上）进入服务注册流程，将服务节点注册到注册中心。

RegistryProtocol.java

public <T> Exporter<T> export(final Invoker<T> originInvoker) throws RpcException {
    //export invoker
    final ExporterChangeableWrapper<T> exporter = doLocalExport(originInvoker);
    //registry provider
    final Registry registry = getRegistry(originInvoker);
    final URL registedProviderUrl = getRegistedProviderUrl(originInvoker);
    //向Zookeeper注册节点
    registry.register(registedProviderUrl);
    // 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);
    overrideListeners.put(overrideSubscribeUrl, overrideSubscribeListener);
    registry.subscribe(overrideSubscribeUrl, overrideSubscribeListener);
    //保证每次export都返回一个新的exporter实例
    return new Exporter<T>() {
        public Invoker<T> getInvoker() {
            return exporter.getInvoker();
        }
        public void unexport() {
            try {
                exporter.unexport();
            } catch (Throwable t) {
                logger.warn(t.getMessage(), t);
            }
            try {
                registry.unregister(registedProviderUrl);
            } catch (Throwable t) {
                logger.warn(t.getMessage(), t);
            }
            try {
                overrideListeners.remove(overrideSubscribeUrl);
                registry.unsubscribe(overrideSubscribeUrl, overrideSubscribeListener);
            } catch (Throwable t) {
                logger.warn(t.getMessage(), t);
            }
        }
    };
}

private Registry getRegistry(final Invoker<?> originInvoker){
    URL registryUrl = originInvoker.getUrl();
    if (Constants.REGISTRY_PROTOCOL.equals(registryUrl.getProtocol())) {
        String protocol = registryUrl.getParameter(Constants.REGISTRY_KEY, Constants.DEFAULT_DIRECTORY);
        registryUrl = registryUrl.setProtocol(protocol).removeParameter(Constants.REGISTRY_KEY);
    }
    return registryFactory.getRegistry(registryUrl);
}

getRegistry() 方法根据注册中心类型（默认 Zookeeper）获取注册中心客户端，由注册中心客户端实例来进行真正的服务注册。

注册中心客户端将节点注册到注册中心，同时订阅对应的 override 数据，实时监听服务的属性变动实现动态配置功能。

最终返回的 Exporter 实现了 unexport() 方法，这样在服务下线时清理相关资源。

至此，服务暴露流程结束。