概述

引用服务是指服务的消费者向注册中心查询提供者的信息或者自己配置提供者的信息，然后根据提供者的信息，将远程通信的逻辑封装到一个代理对象里，等待业务方进行调用。逻辑上分为两步：

1. url --> Invoker，使用Protocol#refer()
2. Invoker --> proxy（代理了远程通信等逻辑），使用ProxyFactory#getProxy()

ReferenceConfig

暴露服务的逻辑在ReferenceConfig#createProxy()，ReferenceConfig表示<dubbo:reference />

if (urls.size() == 1) {    invoker = refprotocol.refer(interfaceClass, urls.get(0));} else {    // 多个urls，循环产生Invoker，并最终用Cluster合并成一个Invoker，变成一个集群Invoker    List
    
     > invokers = new ArrayList
     
      >();    URL registryURL = null;    for (URL url : urls) {        invokers.add(refprotocol.refer(interfaceClass, url));        if (Constants.REGISTRY_PROTOCOL.equals(url.getProtocol())) {            registryURL = url; // use last registry url            // 如果有多个注册中心，用最后一个        }    }    if (registryURL != null) { // registry url is available        // use AvailableCluster only when register's cluster is available        // 对有注册中心的 Cluster 只用 AvailableCluster        URL u = registryURL.addParameter(Constants.CLUSTER_KEY, AvailableCluster.NAME);        invoker = cluster.join(new StaticDirectory(u, invokers));    } else { // not a registry url        invoker = cluster.join(new StaticDirectory(invokers));    }}
     
    

urls里可以是提供者url也可以是注册中心url

• 直接是提供者，解析出的url是比如：dubbo://service-host/com.foo.FooService?version=1.0.0，根据扩展自适配机制，使用DubboProtocol
• 注册中心，解析出的url是：registry://registry-host/com.alibaba.dubbo.registry.RegistryService?refer=URL.encode("consumer://consumer-host/com.foo.FooService?version=1.0.0")，根据扩展自适配机制，首先使用的RegistryProtocol，在refer()方法里向注册中心订阅提供者信息，订阅到信息之后，根据提供者的url是比如：dubbo://service-host/com.foo.FooService?version=1.0.0，使用DubboProtocol

RegistryProtocol

向注册中心订阅提供者信息，由注册中心推送提供者的信息。

/** * * @param cluster 自适应扩展实现 * @param registry 注册中心对象 * @param type 服务接口类型 * @param url 注册中心url * @param 
    
      * @return */private 
     
       Invoker
      
        doRefer(Cluster cluster, Registry registry, Class
       
         type, URL url) {    RegistryDirectory
        
          directory = new RegistryDirectory
         
          (type, url); directory.setRegistry(registry); directory.setProtocol(protocol); // all attributes of REFER_KEY Map
          
            parameters = new HashMap
           
            (directory.getUrl().getParameters()); // 订阅url URL subscribeUrl = new URL(Constants.CONSUMER_PROTOCOL, parameters.remove(Constants.REGISTER_IP_KEY), 0, type.getName(), parameters); // 向注册中心注册自己（服务消费者） if (!Constants.ANY_VALUE.equals(url.getServiceInterface()) && url.getParameter(Constants.REGISTER_KEY, true)) { registry.register(subscribeUrl.addParameters(Constants.CATEGORY_KEY, Constants.CONSUMERS_CATEGORY, Constants.CHECK_KEY, String.valueOf(false))); } // 向注册中心订阅信息，包含提供者、配置、路由 directory.subscribe(subscribeUrl.addParameter(Constants.CATEGORY_KEY, Constants.PROVIDERS_CATEGORY + "," + Constants.CONFIGURATORS_CATEGORY + "," + Constants.ROUTERS_CATEGORY)); Invoker invoker = cluster.join(directory); // 向本地注册表，注册消费者 ProviderConsumerRegTable.registerConsuemr(invoker, url, subscribeUrl, directory); return invoker;}
           
          
         
        
       
      
     
    

DubboProtocol

根据提供者的url，创建了到服务端的客户端连接，并且封装到了DubboInvoker里。

public 
    
      Invoker
     
       refer(Class
      
        serviceType, URL url) throws RpcException {    // create rpc invoker.    DubboInvoker
       
         invoker = new DubboInvoker
        
         (serviceType, url, getClients(url), invokers);    invokers.add(invoker);    return invoker;}/** * 连接远程服务提供者的客户端 * * @param url * @return */private ExchangeClient[] getClients(URL url) {    // whether to share connection    boolean service_share_connect = false;    int connections = url.getParameter(Constants.CONNECTIONS_KEY, 0);    // 配置连接数为0，就使用共享连接，    // if not configured, connection is shared, otherwise, one connection for one service    if (connections == 0) {        service_share_connect = true;        connections = 1;    }    ExchangeClient[] clients = new ExchangeClient[connections];    for (int i = 0; i < clients.length; i++) {        if (service_share_connect) {            clients[i] = getSharedClient(url);        } else {            clients[i] = initClient(url);        }    }    return clients;}/** * Get shared connection */private ExchangeClient getSharedClient(URL url) {    String key = url.getAddress();    ReferenceCountExchangeClient client = referenceClientMap.get(key);    if (client != null) {        if (!client.isClosed()) {            // 计数+1            client.incrementAndGetCount();            return client;        } else {            referenceClientMap.remove(key);        }    }    synchronized (key.intern()) {        ExchangeClient exchangeClient = initClient(url);        client = new ReferenceCountExchangeClient(exchangeClient, ghostClientMap);        referenceClientMap.put(key, client);        ghostClientMap.remove(key);        return client;    }}/** * Create new connection */private ExchangeClient initClient(URL url) {    // client type setting.    // 配置的Transporter的扩展实现，模式为netty    String str = url.getParameter(Constants.CLIENT_KEY, url.getParameter(Constants.SERVER_KEY, Constants.DEFAULT_REMOTING_CLIENT));    // 设置编码器默认为DubboCountCodec，Dubbo SPI 自适应机制会根据这个key找到真正的实现    url = url.addParameter(Constants.CODEC_KEY, DubboCodec.NAME);    // enable heartbeat by default    url = url.addParameterIfAbsent(Constants.HEARTBEAT_KEY, String.valueOf(Constants.DEFAULT_HEARTBEAT));    // BIO is not allowed since it has severe performance issue.    // 是否是支持的Transporter的扩展实现    if (str != null && str.length() > 0 && !ExtensionLoader.getExtensionLoader(Transporter.class).hasExtension(str)) {        throw new RpcException("Unsupported client type: " + str + "," +                " supported client type is " + StringUtils.join(ExtensionLoader.getExtensionLoader(Transporter.class).getSupportedExtensions(), " "));    }    ExchangeClient client;    try {        // connection should be lazy        if (url.getParameter(Constants.LAZY_CONNECT_KEY, false)) {            client = new LazyConnectExchangeClient(url, requestHandler);        } else {            client = Exchangers.connect(url, requestHandler);        }    } catch (RemotingException e) {        throw new RpcException("Fail to create remoting client for service(" + url + "): " + e.getMessage(), e);    }    return client;}
        
       
      
     
    

附录

参照：