Dubbo之服务发现

一、Dubbo简介
    Apache Dubbo (incubating) |ˈdʌbəʊ| is a high-performance, java based RPC framework open-sourced by Alibaba. As in many RPC systems, dubbo is based around the idea of defining a service, specifying the methods that can be called remotely with their parameters and return types. On the server side, the server implements this interface and runs a dubbo server to handle client calls. On the client side, the client has a stub that provides the same methods as the server.

二、服务发现

    Dubbo默认是Zookeeper注册服务中心。
        1. Dubbo通过ZookeeperRegistry类来注册和订阅服务，通过zookeeper事件监听服务变更，一旦服务变更，消费端收到通知，然后主动去zookeeper拉取该服务的提供者子目录，这些提供者子目录记录了注册该服务的URL信息。
        2. RegistryDirectory类封装服务提供者，映射成RegistryDirectory.methodInvokerMap的map类型的属性。
        3. 客户端端调用服务的方法时，会从RegistryDirectory.methodInvokerMap中取对应的方法的服务提供者URL信息进行RPC调用。

  java.lang.Thread.State: RUNNABLE
	  at com.alibaba.dubbo.registry.zookeeper.ZookeeperRegistry.doSubscribe(ZookeeperRegistry.java:175)
	  at com.alibaba.dubbo.registry.support.FailbackRegistry.subscribe(FailbackRegistry.java:212)
	  at com.alibaba.dubbo.registry.integration.RegistryDirectory.subscribe(RegistryDirectory.java:138)
	  at com.alibaba.dubbo.registry.integration.RegistryProtocol.doRefer(RegistryProtocol.java:277)
	  at com.alibaba.dubbo.registry.integration.RegistryProtocol.refer(RegistryProtocol.java:260)
	  at com.alibaba.dubbo.rpc.protocol.ProtocolFilterWrapper.refer(ProtocolFilterWrapper.java:60)
	  at com.alibaba.qos.protocol.QosProtocolWrapper.refer(QosProtocolWrapper.java:45)
	  at com.alibaba.dubbo.rpc.protocol.ProtocolListenerWrapper.refer(ProtocolListenerWrapper.java:67)
	  at com.alibaba.dubbo.rpc.Protocol$Adaptive.refer(Protocol$Adaptive.java:-1)
	  at com.alibaba.dubbo.config.ReferenceConfig.createProxy(ReferenceConfig.java:422)
	  at com.alibaba.dubbo.config.ReferenceConfig.init(ReferenceConfig.java:317)
	  at com.alibaba.dubbo.config.ReferenceConfig.get(ReferenceConfig.java:147)
	  - locked <0xaeb> (a com.alibaba.dubbo.config.spring.ReferenceBean)
	  at com.alibaba.dubbo.config.spring.ReferenceBean.getObject(ReferenceBean.java:130)
	  at org.springframework.beans.factory.support.FactoryBeanRegistrySupport.doGetObjectFromFactoryBean(FactoryBeanRegistrySupport.java:168)
	  at org.springframework.beans.factory.support.FactoryBeanRegistrySupport.getObjectFromFactoryBean(FactoryBeanRegistrySupport.java:103)
	  - locked <0xb51> (a java.util.concurrent.ConcurrentHashMap)
	  at org.springframework.beans.factory.support.AbstractBeanFactory.getObjectForBeanInstance(AbstractBeanFactory.java:1517)
	  at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:251)
	  at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:199)
	  at org.springframework.context.annotation.CommonAnnotationBeanPostProcessor.autowireResource(CommonAnnotationBeanPostProcessor.java:461)
	  at org.springframework.context.annotation.CommonAnnotationBeanPostProcessor.getResource(CommonAnnotationBeanPostProcessor.java:435)
	  at org.springframework.context.annotation.CommonAnnotationBeanPostProcessor$ResourceElement.getResourceToInject(CommonAnnotationBeanPostProcessor.java:559)
	  at org.springframework.beans.factory.annotation.InjectionMetadata$InjectedElement.inject(InjectionMetadata.java:169)
	  at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:88)
	  at org.springframework.context.annotation.CommonAnnotationBeanPostProcessor.postProcessPropertyValues(CommonAnnotationBeanPostProcessor.java:305)
	  at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1202)
	  at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:537)
	  at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:476)
	  at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:303)
	  at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:230)
	  at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:299)
	  at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:194)
	  at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:762)
	  at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:757)
	  at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:480)

• 21行：spring中用到了dubbo的服务，spring会调用AbstractBeanFactory.getBean()去获取这个bean
• 19行：dubbo服务在客户端都是以<dubbo:reference标签配置的，每一个配置都是一个ReferenceBean，ReferenceBean实现了FactoryBean，所以会最终调用FactoryBeanRegistrySupport.getObjectFromFactoryBean()


• 12-13行：如果这个dubbo服务类还没有被实例化，会先调用ReferenceConfig.init方法进行实例化，init方法中先合并application，module，consumer，reference中的配置，然后会调用ReferenceConfig.createProxy()生成一个dubbo服务代理类



• 11行：运用了扩展点，refprotocol.refer(interfaceClass, url)最终会调用RegistryProtocol.refer()组装了一个Invoker对象，最后通过proxyFactory.getProxy(invoker)生成一个代理（dubbo默认使用了javassist）


• 6行：registryFactory.getRegistry(url)，通过url中的协议创建相应的注册中心，我们这里是ZookeeperRegistry，ZookeeperRegistry是AbstractRegistry的子类，在AbstractRegistry的构造方法中，首先会加载本地的注册中心的对应dubbo服务，然后会定时把最新的dubbo服务对应关系刷新到本地




• 5行：zookeeper注册中心，每个服务下都有这四个配置[configurators, consumers, providers, routers]， registry.register()方法会先把Consumer注册到注册中心sonsumers目录下，directory.subscribe()会订阅和监听configurators，providers，routers目录



• 2行：遍历了configurators，providers，routers目录，监听和获取子目录，providers目录下就是可用的dubbo服务providers。新的providers的加入和下架都会通过这里的监听，回调Consumers。