Dubbo2.7源码分析-如何发布服务

Dubbo的服务发布逻辑是比较复杂的,我们还是以Dubbo自带的示例进行讲解,抽丝剥茧,拨云见日。

Provider配置如下:

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:dubbo="http://dubbo.apache.org/schema/dubbo"
    xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-4.3.xsd http://dubbo.apache.org/schema/dubbo http://dubbo.apache.org/schema/dubbo/dubbo.xsd">
 
    <!-- 提供方应用信息,用于计算依赖关系 -->
    <dubbo:application name="hello-world-app"  />
 
    <!-- 使用multicast广播注册中心暴露服务地址 -->
    <dubbo:registry address="multicast://224.5.6.7:1234" />
 
    <!-- 用dubbo协议在20880端口暴露服务 -->
    <dubbo:protocol name="dubbo" port="20880" />
 
    <!-- 声明需要暴露的服务接口 -->
    <dubbo:service interface="com.alibaba.dubbo.demo.DemoService" ref="demoService" />
 
    <!-- 和本地bean一样实现服务 -->
    <bean id="demoService" class="com.alibaba.dubbo.demo.provider.DemoServiceImpl" />
</beans>

ApplicationContext

ClassPathXmlApplicationContext父类AbstractApplicationContext的方法refresh()在实例化bean之后的最后一步finishRefresh()中,此方法作用是发布相应的事件。

    protected void finishRefresh() {
        //省略LifeCycleProcessor刷新代码

        // Publish the final event.
        publishEvent(new ContextRefreshedEvent(this));

        // 省略注册到 LiveBeansView MBean代码
    }

可以看到发布了一个ContextRefreshedEvent事件。

    protected void publishEvent(Object event, ResolvableType eventType) {
        //省略部分代码
      getApplicationEventMulticaster().multicastEvent(applicationEvent, eventType);
            //省略分部代码

首先获取ApplicationEvent事件广播对象,然后广播事件。

ApplicationEvent事件广播对象默认是SimpleApplicationEventMulticaster,这个对象是在AbstractApplicationContext的方法initApplicationEventMulticaster()初始化的,如果需要自定义,可以实现接口ApplicationEventMulticaster,并将bean的名字命名为applicationEventMulticaster

接下来看看SimpleApplicationEventMulticaster类的multicastEvent方法。

    @Override
    public void multicastEvent(final ApplicationEvent event, ResolvableType eventType) {
                //事件类型
        ResolvableType type = (eventType != null ? eventType : resolveDefaultEventType(event));
               
               //applicationListener
        for (final ApplicationListener<?> listener : getApplicationListeners(event, type)) {
                       //异常执行
            Executor executor = getTaskExecutor();
            if (executor != null) {
                executor.execute(new Runnable() {
                    @Override
                    public void run() {
                        invokeListener(listener, event);
                    }
                });
            }
            else {
                invokeListener(listener, event);
            }
        }
    }

可以看到此方法会调用applicationListener的方法,对于Dubbo而言,就是ServiceBean.

怎么样获取到ServiceBean的呢?

ServiceBean实现了好几个接口,其中有两个接口ApplicationContextAwareApplicationListener
,其中ApplicationContextAware使ServiceBean具有获取ApplicationContext的能力(了解bean的生命周期),而ApplicationListener使ServiceBean具有响应事件响应的能力。dubbo实现ApplicationContextAware的目的是通过反射把自己添加到ApplicationContext的ApplicationListener列表中,即使不实现ApplicationContextAware接口,spring也会将实现了ApplicationListener接口的bean添加到其listener列表中的,dubbo这样做估计是向后兼容。

接着看invokeListener(listener, event);方法

protected void invokeListener(ApplicationListener<?> listener, ApplicationEvent event) {
        ErrorHandler errorHandler = getErrorHandler();
        if (errorHandler != null) {
            try {
                doInvokeListener(listener, event);
            }
            catch (Throwable err) {
                errorHandler.handleError(err);
            }
        }
        else {
            doInvokeListener(listener, event);
        }
    }

    private void doInvokeListener(ApplicationListener listener, ApplicationEvent event) {
        try {
            listener.onApplicationEvent(event);
        }
        catch (ClassCastException ex) {
            //省略异常处理
            }
            else {
                throw ex;
            }
        }
    }

invokeListener方法内部调用了doInvokeListener方法,而doInvokeListener方法调用了listener(ServiceBean)的onApplicationEvent方法.

ServiceBean

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

onApplicationEvent方法调用了export方法,export方法首先判断是否已经发布了服务,发布了则直接返回,没有发布则会判断是否需要延迟发布,如果需要延迟,则将发布服务做为一个任务添加到ScheduledThreadPoolExecutor线程池中,如果不延迟,则调用doExport方法立即发布服务。
doExport方法中会获取application/registries/monitor/module/protocols,并做一些检查和属性填充,然后调用doExportUrls();发布服务。doExportUrls()首先调用loadRegistries方法得到要注册的url,然后发布相关Protocol的服务。

简单叙述一下获取url的过程,url通过map组装参数和对应的值,参数有ApplicationConfigRegistryConfig对象的属性以及pathdubbotimestamppidprotocolregistry

本示例applicationConfig是:

registryURL
registryConfig是:

最终map组装结果是:url parameters

最后得到registryURL是:

registry://224.5.6.7:1234/org.apache.dubbo.registry.RegistryService?application=demo-provider&dubbo=2.0.2&pid=4892&qos.port=22222&registry=multicast&timestamp=1536112339884

然后调用doExportUrlsFor1Protocol方法发布服务,此方法开始部分是构造发布的服务URL,然后再发布url。

服务URL
URL包括以下几部分:服务端还是客户端标识Dubbo版本,时间戳,Pid,服务的方法名tokenApplicationConfig,MoudleConfig,ProviderConfig,ProtocolConfig,*MethodConfig对象的相关属性等。
例如本示例的url:

dubbo://192.168.124.1:20880/org.apache.dubbo.demo.DemoService?anyhost=true&application=demo-provider&bind.ip=192.168.124.1&bind.port=20880&dubbo=2.0.2&generic=false&interface=org.apache.dubbo.demo.DemoService&methods=sayHello&pid=8004&qos.port=22222&side=provider&timestamp=1536114090787

我们来着重看一下在构造URL过程中port的获取过程。

//protocolConfig是配置的<dubbo:protocol />生成的对象
//name是protocol的name,本示例为"dubbo"
//map保存了url的键值对
Integer port = this.findConfigedPorts(protocolConfig, name, map);

findConfigedPorts顾名思义是查找配置的port,从哪查呢,先从系统环境变量查,如果没找到,再查找名字为name的protocol协义。

    private Integer findConfigedPorts(ProtocolConfig protocolConfig, String name, Map<String, String> map) {
        Integer portToBind = null;

        // 从环境变量从查找绑定的port
        String port = getValueFromConfig(protocolConfig, Constants.DUBBO_PORT_TO_BIND);
        portToBind = parsePort(port);

        // 如果没有从环境变量从查到,则从名称为name的protocol查找
        if (portToBind == null) {
            portToBind = protocolConfig.getPort();
            if (provider != null && (portToBind == null || portToBind == 0)) {
                portToBind = provider.getPort();
            }
           //这一句是关键,示例中name值是"dubbo",所以会实例化DubboProtocol,得到默认的port:20880
            final int defaultPort = ExtensionLoader.getExtensionLoader(Protocol.class).getExtension(name).getDefaultPort();
            if (portToBind == null || portToBind == 0) {
                portToBind = defaultPort;
            }
            if (portToBind == null || portToBind <= 0) {
                portToBind = getRandomPort(name);
                if (portToBind == null || portToBind < 0) {
                    portToBind = getAvailablePort(defaultPort);
                    putRandomPort(name, portToBind);
                }
                logger.warn("Use random available port(" + portToBind + ") for protocol " + name);
            }
        }

        //保存port到map中,以便后面url使用
        map.put(Constants.BIND_PORT_KEY, String.valueOf(portToBind));

        // 从环境变量中查找注册的port,如果没有找到,则等于绑定的Port.
        String portToRegistryStr = getValueFromConfig(protocolConfig, Constants.DUBBO_PORT_TO_REGISTRY);
        Integer portToRegistry = parsePort(portToRegistryStr);
        if (portToRegistry == null) {
            portToRegistry = portToBind;
        }
        return portToRegistry;
    }

有人或许有疑问,ServiceConfig在实例化时,不是已经加载过Protocol了吗?为什么还要使用ExtensionLoader加载一次呢?

final int defaultPort =ExtensionLoader.getExtensionLoader(Protocol.class).getExtension(name).getDefaultPort();

答: ServiceConfig实例化时,加载的Protocol是自适应的Protocol,是动态生成的,类名是Protocol$Adaptive(见Dubbo源码分析-SPI的应用中有分析)。而这里获取Port时加载的也是Protocol类,但指名了具体加载的是哪个Protocol(本示例是名称为dubbo的Protocol,即DubboProtocol,此类默认的端口是20880)。

发布URL

发布本地服务

调用ServiceConfig类的exportLocal(URL url)发布本地服务。

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

本示例的本地服务 url是:

injvm://127.0.0.1/org.apache.dubbo.demo.DemoService?anyhost=true&application=demo-provider&bind.ip=192.168.124.1&bind.port=20880&dubbo=2.0.2&generic=false&interface=org.apache.dubbo.demo.DemoService&methods=sayHello&pid=3008&qos.port=22222&side=provider&timestamp=1536125473655

重点看这一句:

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

其中涉及到ProxyFactory和Protocol,下面分别来看一看。

ProxyFactory

proxyFactory也是通过SPI加载的自适应类对象,类名为ProxyFactory$Adaptive,我们来看一下其class文件反编译后的源码。

package org.apache.dubbo.rpc;

import org.apache.dubbo.common.URL;
import org.apache.dubbo.common.extension.ExtensionLoader;

public class ProxyFactory$Adaptive implements ProxyFactory {
    public ProxyFactory$Adaptive() {
    }

    public Invoker getInvoker(Object var1, Class var2, URL var3) throws RpcException {
        if (var3 == null) {
            throw new IllegalArgumentException("url == null");
        } else {
            String var5 = var3.getParameter("proxy", "javassist");
            if (var5 == null) {
                throw new IllegalStateException("Fail to get extension(org.apache.dubbo.rpc.ProxyFactory) name from url(" + var3.toString() + ") use keys([proxy])");
            } else {
                ProxyFactory var6 = (ProxyFactory)ExtensionLoader.getExtensionLoader(ProxyFactory.class).getExtension(var5);
                return var6.getInvoker(var1, var2, var3);
            }
        }
    }

    public Object getProxy(Invoker var1, boolean var2) throws RpcException {
        if (var1 == null) {
            throw new IllegalArgumentException("org.apache.dubbo.rpc.Invoker argument == null");
        } else if (var1.getUrl() == null) {
            throw new IllegalArgumentException("org.apache.dubbo.rpc.Invoker argument getUrl() == null");
        } else {
            URL var3 = var1.getUrl();
            String var4 = var3.getParameter("proxy", "javassist");
            if (var4 == null) {
                throw new IllegalStateException("Fail to get extension(org.apache.dubbo.rpc.ProxyFactory) name from url(" + var3.toString() + ") use keys([proxy])");
            } else {
                ProxyFactory var5 = (ProxyFactory)ExtensionLoader.getExtensionLoader(ProxyFactory.class).getExtension(var4);
                return var5.getProxy(var1, var2);
            }
        }
    }

    public Object getProxy(Invoker var1) throws RpcException {
        if (var1 == null) {
            throw new IllegalArgumentException("org.apache.dubbo.rpc.Invoker argument == null");
        } else if (var1.getUrl() == null) {
            throw new IllegalArgumentException("org.apache.dubbo.rpc.Invoker argument getUrl() == null");
        } else {
            URL var2 = var1.getUrl();
            String var3 = var2.getParameter("proxy", "javassist");
            if (var3 == null) {
                throw new IllegalStateException("Fail to get extension(org.apache.dubbo.rpc.ProxyFactory) name from url(" + var2.toString() + ") use keys([proxy])");
            } else {
                ProxyFactory var4 = (ProxyFactory)ExtensionLoader.getExtensionLoader(ProxyFactory.class).getExtension(var3);
                return var4.getProxy(var1);
            }
        }
    }
}

其中有三个方法,两个获取代理,一个获取Invoker。我们来看其中的getInvoker方法,默认获取名称为javassist的ProxyFactory。
由于本地服务URL中没有proxy参数,所以会调用JavassistProxyFactory的getInvoker(T proxy, Class

type, URL url)
方法,返回AbstractProxyInvoker的匿名类对象,此对象代理了服务对象(本示例中为DemoServiceImpl对象)。

其实(ProxyFactory)ExtensionLoader.getExtensionLoader(ProxyFactory.class).getExtension(“javassist”);获取到的并不是JavassistProxyFactory对象,而是StubProxyFactoryWrapper对象,为什么呢?我们可以看下ExtensionLoader的getExtension(String name)方法

   public T getExtension(String name) {
       //检查name是否合法
       if (name == null || name.length() == 0)
           throw new IllegalArgumentException("Extension name == null");
      //如果name等于true,则加载SPI的默认插件
     if ("true".equals(name)) {
         return getDefaultExtension();
      }
      //从当前插件类的缓存实例对象中获取
      Holder<Object> holder = cachedInstances.get(name);
      if (holder == null) {
         cachedInstances.putIfAbsent(name, new Holder<Object>());
         holder = cachedInstances.get(name);
     }
     Object instance = holder.get();
     if (instance == null) {
       synchronized (holder) {
             instance = holder.get();
             if (instance == null) {
                 //创建插件实例
                 instance = createExtension(name);
                 holder.set(instance);
             }
         }
     }
     return (T) instance;
 }

   private T createExtension(String name) {
      //从文件目录中加载插件类
      Class<?> clazz = getExtensionClasses().get(name);
      if (clazz == null) {
         throw findException(name);
    }
   
    //从已加载的所有插件实例集合中获取
     try {
         T instance = (T) EXTENSION_INSTANCES.get(clazz);
         if (instance == null) {
             //实例化插件实例,并放入集合
             EXTENSION_INSTANCES.putIfAbsent(clazz, clazz.newInstance());
            instance = (T) EXTENSION_INSTANCES.get(clazz);
        }
         
        //注入属性
        injectExtension(instance);

        //插件的包裹类
        Set<Class<?>> wrapperClasses = cachedWrapperClasses;
        if (wrapperClasses != null && !wrapperClasses.isEmpty()) {
           for (Class<?> wrapperClass : wrapperClasses) {
                instance = injectExtension((T) wrapperClass.getConstructor(type).newInstance(instance));
            }
        }
        return instance;
    } catch (Throwable t) {
        throw new IllegalStateException("Extension instance(name: " + name + ", class: " +
                type + ")  could not be instantiated: " + t.getMessage(), t);
    }
 }

重点的地方就在于插件的包裹类,StubProxyFactoryWrapper就是JavassistProxyFactory的包裹类,为什么这么说呢,因为StubProxyFactoryWrapper有一个带ProxyFactory参数的构造函数而且实现了ProxyFactory接口,具体可以看Extension的loadExtensionClasses方法源码(装饰者模式)。

Protocol

protocol对象也是一个自适应插件类,类名为Protocol$Adaptive,在上一篇文章中已有讲解。这个类会根据url的协义取得对应转义的插件类,没有的话,默认为dubbo协义,本地服务url协义为injvm,所以会加载InjvmProtocol,但是在加载InjvmProtocol并实例化后,发现InjvmProtocol还有对应的包裹类即(其实是所有Protocol的包裹类):ProtocolFilterWrapper和ProtocolListenerWrapper。ProtocolFilterWrapper类的作用是添加一些过滤器,ProtocolListenerWrapper的作用是添加ExporterListener。InjvmProtocol的export方法仅仅创建一个InjvmExporter实例,没有开启服务。

发布远程服务

如果注册url不为空,调用proxyFactory得到服务对象的代理类,然后使用protocol发布服务。由于注册url的协义是registry,在使用ExtensionLoader.getExtensionLoader(org.apache.dubbo.rpc.Protocol.class).getExtension(“registry”);会加载RegistryProtocol类并实例化,而且会添加其包裹类:ProtocolFilterWrapper和ProtocolListenerWrapper。而在这两个包裹类的export方法的首行,都会对registry协义进行单独处理。

RegistryProtocol
        if (Constants.REGISTRY_PROTOCOL.equals(invoker.getUrl().getProtocol())) {
            return protocol.export(invoker);
        }

经过这两个包裹类后,最终会调用RegistryProtocol的export方法。

    public <T> Exporter<T> export(final Invoker<T> originInvoker) throws RpcException {
        //发布服务
       //originInvoker中包含了代理服务对象的代理类
        final ExporterChangeableWrapper<T> exporter = doLocalExport(originInvoker);

        //注册相关代码省略

        //订阅相关代码省略
    }

       private <T> ExporterChangeableWrapper<T> doLocalExport(final Invoker<T> originInvoker) {
       
         //key为发布的服务url
        String key = getCacheKey(originInvoker);
        //从map缓存中获取
        ExporterChangeableWrapper<T> exporter = (ExporterChangeableWrapper<T>) bounds.get(key);
      //double check
        if (exporter == null) {
            synchronized (bounds) {
                exporter = (ExporterChangeableWrapper<T>) bounds.get(key);
                if (exporter == null) {
                    final Invoker<?> invokerDelegete = new InvokerDelegete<T>(originInvoker, getProviderUrl(originInvoker));
                    exporter = new ExporterChangeableWrapper<T>((Exporter<T>) protocol.export(invokerDelegete), originInvoker);
                    bounds.put(key, exporter);
                }
            }
        }
        return exporter;
    }

最重要的是这一句:

exporter = new ExporterChangeableWrapper<T>((Exporter<T>) protocol.export(invokerDelegete), originInvoker);

其中protocol也是Protocol$Adaptive对象,而invokerDelegete的URL是服务的url.

本示例中为:

dubbo://192.168.124.1:20880/org.apache.dubbo.demo.DemoService?anyhost=true&application=demo-provider&bind.ip=192.168.124.1&bind.port=20880&dubbo=2.0.2&generic=false&interface=org.apache.dubbo.demo.DemoService&methods=sayHello&pid=8468&qos.port=22222&side=provider&timestamp=1536138127517

DubboProtocol

Protocol$Adaptive在解析URL的时得到dubbo,所以会加载DubboProtocol并实例化(DubboProtocol实际在前面获取默认接口时已经实例化并缓存起来了,此处取的是缓存的实例),并调用了DubboProtocol的export方法(与上面一样,在得到DubboProtocol实例后,仍然会在外面包裹一下)。

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

        // 服务名:本例中为org.apache.dubbo.demo.DemoService:20880
        String key = serviceKey(url);
       //exporter 控制服务打开与关闭
        DubboExporter<T> exporter = new DubboExporter<T>(invoker, key, exporterMap);
        exporterMap.put(key, exporter);

        //省略发布子服务的相关代码
       
       //打开服务
        openServer(url);
       //优化序列化处理
        optimizeSerialization(url);
        return exporter;
    }

经过层层探索,曲折迂回,终于到openServer了,进去看看。

    private void openServer(URL url) {
        // 服务ip:端口号
        String key = url.getAddress();
        
        boolean isServer = url.getParameter(Constants.IS_SERVER_KEY, true);
        if (isServer) {
            ExchangeServer server = serverMap.get(key);
            if (server == null) {
                synchronized (this) {
                    server = serverMap.get(key);
                    if (server == null) {
                        serverMap.put(key, createServer(url));
                    }
                }
            } else {
                // 服务支持重置
                server.reset(url);
            }
        }
    }

可以看到其中有一个重要方法createServer(url)。

    private ExchangeServer createServer(URL url) {
        // 当服务关闭时,默认启动发送只读事件
        url = url.addParameterIfAbsent(Constants.CHANNEL_READONLYEVENT_SENT_KEY, Boolean.TRUE.toString());
        // 默认启动心跳
        url = url.addParameterIfAbsent(Constants.HEARTBEAT_KEY, String.valueOf(Constants.DEFAULT_HEARTBEAT));

        //str默认为netty
        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);
        
        //添加编解码器
        url = url.addParameter(Constants.CODEC_KEY, DubboCodec.NAME);
        ExchangeServer server;
        try {
            //启动服务,并传入请求处理器
            server = Exchangers.bind(url, requestHandler);
        } catch (RemotingException e) {
            throw new RpcException("Fail to start server(url: " + url + ") " + e.getMessage(), e);
        }
       
         //判断客户端使用的是网络传输层框架是否支持服务端的网络传输层。
        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;
    }
Exchangers

进入Exchangers.bind方法一探究竟。

    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");
        }
        //如果编码码器没有,则添加参数exchange
        url = url.addParameterIfAbsent(Constants.CODEC_KEY, "exchange");
        return getExchanger(url).bind(url, handler);
    }

getExchanger(url)默认得到的是HeaderExchanger,可通过exchanger参数配置。
到HeaderExchanger中看看bind方法

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

看下Transporters的bind方法。

    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);
    }

通过getTransporter方法获取一个自适应的Transporter,类名为Transporter$Adaptive,我们来看一下其源码:

package org.apache.dubbo.remoting;

import org.apache.dubbo.common.URL;
import org.apache.dubbo.common.extension.ExtensionLoader;

public class Transporter$Adaptive implements Transporter {
    public Transporter$Adaptive() {
    }

    public Client connect(URL var1, ChannelHandler var2) throws RemotingException {
        if (var1 == null) {
            throw new IllegalArgumentException("url == null");
        } else {
            String var4 = var1.getParameter("client", var1.getParameter("transporter", "netty"));
            if (var4 == null) {
                throw new IllegalStateException("Fail to get extension(org.apache.dubbo.remoting.Transporter) name from url(" + var1.toString() + ") use keys([client, transporter])");
            } else {
                Transporter var5 = (Transporter)ExtensionLoader.getExtensionLoader(Transporter.class).getExtension(var4);
                return var5.connect(var1, var2);
            }
        }
    }

    public Server bind(URL var1, ChannelHandler var2) throws RemotingException {
        if (var1 == null) {
            throw new IllegalArgumentException("url == null");
        } else {
            String var4 = var1.getParameter("server", var1.getParameter("transporter", "netty"));
            if (var4 == null) {
                throw new IllegalStateException("Fail to get extension(org.apache.dubbo.remoting.Transporter) name from url(" + var1.toString() + ") use keys([server, transporter])");
            } else {
                Transporter var5 = (Transporter)ExtensionLoader.getExtensionLoader(Transporter.class).getExtension(var4);
                return var5.bind(var1, var2);
            }
        }
    }
}

可以看到Transporter$Adaptive通过判断URL中是否有transporter参数,如果没有,就默认为netty。

示例中服务的URL为

dubbo://192.168.124.1:20880/org.apache.dubbo.demo.DemoService?anyhost=true&application=demo-provider&bind.ip=192.168.124.1&bind.port=20880&dubbo=2.0.2&generic=false&interface=org.apache.dubbo.demo.DemoService&methods=sayHello&pid=8004&qos.port=22222&side=provider&timestamp=1536114090787

其中没有transporter参数,所以就使用netty。然后dubbo就去查找netty对应的是哪个Transporter,结果找到是NettyTransporter。

package org.apache.dubbo.remoting.transport.netty4;

//省略导入部分

public class NettyTransporter implements Transporter {

    public static final String NAME = "netty";

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

    @Override
    public Client connect(URL url, ChannelHandler listener) throws RemotingException {
        return new NettyClient(url, listener);
    }

}

NettyTransporter很简单,只有两个方法,一个用于开启服务,一个用于连接服务。到这里已经明白了Dubbo是如何发布一个服务的。

我们再进一步看下NettyServer的构造函数

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

可以看出其调用父类的构造函数,并传入url和handler的包裹类。handler的包裹类有哪些呢,进去看一看。

    public static ChannelHandler wrap(ChannelHandler handler, URL url) {
        return ChannelHandlers.getInstance().wrapInternal(handler, url);
    }
    
     protected ChannelHandler wrapInternal(ChannelHandler handler, URL url) {
        return new MultiMessageHandler(new HeartbeatHandler(ExtensionLoader.getExtensionLoader(Dispatcher.class)
                .getAdaptiveExtension().dispatch(handler, url)));
    }

注意到有一个接口Dispatcher,其自适应插件类是AllDispatcher,AllDispatcher的dispatch方法返回AllChannelHandler实例(此实例会将所有请求做为任务放入线程池中处理),在此实例基础上又包裹了HeartbeatHandlerMultiMessageHandler
NettyServer会将MultiMessageHandler层层往上传到其父类AbstractPeer

我们来回忆一下正向流程

从ServiceConfig发布registryURL开始(见doExportUrlsFor1Protocol方法)
1.ServiceConfig生成服务实例的代理工厂类JavassistProxyFactory(ProxyFactory SPI默认代理工厂类)并包裹到DelegateProviderMetaDataInvoker(此类记录代理工厂类和服务信息ServiceBean(
标签对应的类))
2.由于registryURL的protocol协义是registry,所以会加载RegistryProtocol(Protocol类的外面都包裹了ProtocolFilterWrapper和ProtocolListenerWrapper,下面不再特殊说明),并传入上一步的invoker。
3.RegistryProtocol又找到DubboProtocol,也会带上Invoker(此时的Invoker包含上一次的Invoker并带有服务地址(dubbo://IP:端口/服务接口全称?参数=xxx))。

所以requestHandler又会调用正向传过来的Invoker,经过ProtocolFilterWrapper和ProtocolListenerWrapper,最终调用到服务实现类相应的方法。

最后以一张图总结:

标识为SPI的类,是可以动态加载的。图片看不清楚的话,请查看原图

再简单说下接收到请求后的处理流程:NettyServer接收到请求后,交给NettyServerHandler处理,NettyServerHandler转交给NettyServer的父类AbstractPeer处理,AbstractPeer又交给MultiMessageHandler处理,这样就开始了handler链的处理,handler的终点是HeaderExchangerHandler,HeaderExchangerHandler调用DubboProtocol传过来的成员变量requestHandler调用相应的服务类方法,然后得到结果,调用NettyServerHandler传过来的NettyChannel发送结果到Client。