Dubbo-服务注册与发现机制

服务引用过程中涉及到服务发现机制

　　RegistryProtocol#doRefer

private <T> Invoker<T> doRefer(Cluster cluster, Registry registry, Class<T> type, URL url) {
	// 创建 RegistryDirectory 实例, 基于注册中心动态发现服务提供者（服务提供者新增或减少）
	RegistryDirectory<T> directory = new RegistryDirectory<T>(type, url);
	// 设置注册中心和协议
	directory.setRegistry(registry);
	directory.setProtocol(protocol);
	// 获取服务消费者的配置属性
	Map<String, String> parameters = new HashMap<String, String>(directory.getConsumerUrl().getParameters());
	// 生成服务消费者URL, 如： consumer://192.168.56.1/com.alibaba.dubbo.demo.DemoService?application=demo-consumer&check=false&dubbo=2.0.0&interface=com.alibaba.dubbo.demo.DemoService&methods=sayHello&pid=9892&qos.port=33333&side=consumer&timestamp=1528380277185
	URL subscribeUrl = new URL(CONSUMER_PROTOCOL, parameters.remove(REGISTER_IP_KEY), 0, type.getName(), parameters);
	// 注册服务消费者，在 consumers 目录下新节点
	if (directory.isShouldRegister()) {
		directory.setRegisteredConsumerUrl(subscribeUrl);
		registry.register(directory.getRegisteredConsumerUrl());
	}
	directory.buildRouterChain(subscribeUrl);
	// 添加了category=consumers属性
	// consumer://192.168.56.1/com.alibaba.dubbo.demo.DemoService?application=demo-consumer&category=consumers&check=false&dubbo=2.0.0&interface=com.alibaba.dubbo.demo.DemoService&methods=sayHello&pid=9892&qos.port=33333&side=consumer&timestamp=1528380277185
	// 订阅 providers、configurators、routers 等节点数据，如Zookeeper的Node节点添加Watcher监听
	// 向注册中心订阅该URL，关注该服务下的providers,configurators,routers发生变化时通知RegistryDirectory，以便及时发现服务提供者、配置、路由规则的变化
	directory.subscribe(toSubscribeUrl(subscribeUrl));

	// 一个注册中心可能有多个服务提供者，因此这里需要将多个服务提供者合并为一个，利用集群策略返回集群Invoker
	Invoker<T> invoker = cluster.join(directory);
	List<RegistryProtocolListener> listeners = findRegistryProtocolListeners(url);
	if (CollectionUtils.isEmpty(listeners)) {
		return invoker;
	}

	RegistryInvokerWrapper<T> registryInvokerWrapper = new RegistryInvokerWrapper<>(directory, cluster, invoker);
	for (RegistryProtocolListener listener : listeners) {
		listener.onRefer(this, registryInvokerWrapper);
	}
	return registryInvokerWrapper;
}

　　RegistryDirectory构造器

public RegistryDirectory(Class<T> serviceType, URL url) {
	super(url);
	if (serviceType == null) {
		throw new IllegalArgumentException("service type is null.");
	}

	shouldRegister = !ANY_VALUE.equals(url.getServiceInterface()) && url.getParameter(REGISTER_KEY, true);
	shouldSimplified = url.getParameter(SIMPLIFIED_KEY, false);
	if (url.getServiceKey() == null || url.getServiceKey().length() == 0) {
		throw new IllegalArgumentException("registry serviceKey is null.");
	}
	this.serviceType = serviceType;
	// 获取注册中心URL的serviceKey：com.alibaba.dubbo.registry.RegistryService
	this.serviceKey = url.getServiceKey();
	// 获取注册中心URL消费提供者的所有配置参数:从url属性的refer
	this.queryMap = StringUtils.parseQueryString(url.getParameterAndDecoded(REFER_KEY));
	// 注册中心的URL，移除监控中心以及其他属性值，只保留消息消费者的配置属性
	this.overrideDirectoryUrl = this.directoryUrl = turnRegistryUrlToConsumerUrl(url);
	String group = directoryUrl.getParameter(GROUP_KEY, "");
	this.multiGroup = group != null && (ANY_VALUE.equals(group) || group.contains(","));
}