搜云库技术团队
根据上一小节对于spring扩展schema的介绍，大概可以猜到dubbo中相关的内容是如何实现的。
再来回顾Dubbo实践（一）中定义的dubbo-provider.xml：
<?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:context="http://www.springframework.org/schema/context"
xmlns:dubbo="http://code.alibabatech.com/schema/dubbo"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans-4.3.xsd
http://www.springframework.org/schema/context
http://www.springframework.org/schema/context/spring-context-4.3.xsd
http://code.alibabatech.com/schema/dubbo
http://code.alibabatech.com/schema/dubbo/dubbo.xsd">
<!-- 自动检测并装配bean -->
<context:component-scan base-package="org.warehouse.component.dubbo.provider" />
<!-- 提供方应用信息，用于计算依赖关系 -->
<dubbo:application name="provider-of-hello-world-app" />
<!-- 使用zookeeper注册中心暴露服务地址 -->
<dubbo:registry protocol="zookeeper" address="192.168.1.103:2181" />
<!-- 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="org.warehouse.component.dubbo.facade.DemoFacade" ref="demoFacadeImpl" />
</beans>
对应的自定义schema文件，对应的handler配置，可以在dubbo-{version}.jar 中 META-INF目录下找到。
spring.handlers文件：
http\://code.alibabatech.com/schema/dubbo=com.alibaba.dubbo.config.spring.schema.DubboNamespaceHandler
可以看到，对应的handler是 DubboNamespaceHandler。对应的源码如下：
/*
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.alibaba.dubbo.config.spring.schema;
import com.alibaba.dubbo.common.Version;
import com.alibaba.dubbo.config.*;
import com.alibaba.dubbo.config.spring.ReferenceBean;
import com.alibaba.dubbo.config.spring.ServiceBean;
import org.springframework.beans.factory.xml.NamespaceHandlerSupport;
/**
* DubboNamespaceHandler
*
* @export
*/
public class DubboNamespaceHandler extends NamespaceHandlerSupport {
static {
Version.checkDuplicate(DubboNamespaceHandler.class);
}
public void init() {
registerBeanDefinitionParser("application", new DubboBeanDefinitionParser(ApplicationConfig.class, true));
registerBeanDefinitionParser("module", new DubboBeanDefinitionParser(ModuleConfig.class, true));
registerBeanDefinitionParser("registry", new DubboBeanDefinitionParser(RegistryConfig.class, true));
registerBeanDefinitionParser("monitor", new DubboBeanDefinitionParser(MonitorConfig.class, true));
registerBeanDefinitionParser("provider", new DubboBeanDefinitionParser(ProviderConfig.class, true));
registerBeanDefinitionParser("consumer", new DubboBeanDefinitionParser(ConsumerConfig.class, true));
registerBeanDefinitionParser("protocol", new DubboBeanDefinitionParser(ProtocolConfig.class, true));
registerBeanDefinitionParser("service", new DubboBeanDefinitionParser(ServiceBean.class, true));
registerBeanDefinitionParser("reference", new DubboBeanDefinitionParser(ReferenceBean.class, false));
registerBeanDefinitionParser("annotation", new AnnotationBeanDefinitionParser());
}
}
从这里也可以看到，支持的标签其实不多。所有的Parser都封装到了DubboBeanDefinitionParser中。对应的class，就是传入的beanClass。比如application的就是ApplicationConfig。module的就是ModuleConfig。经过Parser的转换，dubbo-provider.xml大概可以变成如下的样子：
<bean id="provider-of-hello-world-app" class="com.alibaba.dubbo.config.ApplicationConfig"/>
<bean id="registryConfig" class="com.alibaba.dubbo.config.RegistryConfig">
<property name="address" value="10.125.195.174:2181"/>
<property name="protocol" value="zookeeper"/>
</bean>
<bean id="dubbo" class="com.alibaba.dubbo.config.ProtocolConfig">
<property name="port" value="20880"/>
</bean>
<bean id="org.warehouse.component.dubbo.facade.DemoFacade" class="com.alibaba.dubbo.config.spring.ServiceBean">
<property name="interface" value="org.warehouse.component.dubbo.facade.DemoFacade"/>
<property name="ref" ref="demoFacadeImpl"/>
</bean>
Spring生成bean的过程主要是把这些属性都梳理清楚，生成对应的类。
ServiceBean暴露服务
Spring加载dubbo-provider.xml，通过DubboBeanDefinitionParser解析ServiceBean后会将xml定义的服务暴露到zookeeper。从代码分析ServiceBean中export过程（暴露服务）是在如下过程触发的：
public void onApplicationEvent(ContextRefreshedEvent event) {
if (isDelay() && !isExported() && !isUnexported()) {
if (logger.isInfoEnabled()) {
logger.info("The service ready on spring started. service: " + getInterface());
}
export();
}
}
onApplicationEvent方法定义在Spring的org.springframework.context.ApplicationListener接口中，该接口通过继承java.util.EventListener接口实现了观察者模式。
从debug的结果上看，onApplicationEvent是由org.springframework.context.support.AbstractApplicationContext中的方法触发：
/**
* Publish the given event to all listeners.
* @param event the event to publish (may be an {@link ApplicationEvent}
* or a payload object to be turned into a {@link PayloadApplicationEvent})
* @param eventType the resolved event type, if known
* @since 4.2
*/
protected void publishEvent(Object event, @Nullable ResolvableType eventType) {
Assert.notNull(event, "Event must not be null");
if (logger.isTraceEnabled()) {
logger.trace("Publishing event in " + getDisplayName() + ": " + event);
}
// Decorate event as an ApplicationEvent if necessary
ApplicationEvent applicationEvent;
if (event instanceof ApplicationEvent) {
applicationEvent = (ApplicationEvent) event;
}
else {
applicationEvent = new PayloadApplicationEvent<>(this, event);
if (eventType == null) {
eventType = ((PayloadApplicationEvent) applicationEvent).getResolvableType();
}
}
// Multicast right now if possible - or lazily once the multicaster is initialized
if (this.earlyApplicationEvents != null) {
this.earlyApplicationEvents.add(applicationEvent);
}
else {
getApplicationEventMulticaster().multicastEvent(applicationEvent, eventType);
}
// Publish event via parent context as well...
if (this.parent != null) {
if (this.parent instanceof AbstractApplicationContext) {
((AbstractApplicationContext) this.parent).publishEvent(event, eventType);
}
else {
this.parent.publishEvent(event);
}
}
}
publishEvent方法是在org.springframework.context.support.AbstractApplicationContext的finishRefresh方法中调用：
/**
* Finish the refresh of this context, invoking the LifecycleProcessor's
* onRefresh() method and publishing the
* {@link org.springframework.context.event.ContextRefreshedEvent}.
*/
protected void finishRefresh() {
// Clear context-level resource caches (such as ASM metadata from scanning).
clearResourceCaches();
// Initialize lifecycle processor for this context.
initLifecycleProcessor();
// Propagate refresh to lifecycle processor first.
getLifecycleProcessor().onRefresh();
// Publish the final event.
publishEvent(new ContextRefreshedEvent(this));
// Participate in LiveBeansView MBean, if active.
LiveBeansView.registerApplicationContext(this);
}
finishRefresh方法在org.springframework.context.support.AbstractApplicationContext的refresh方法中调用：
@Override
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
// Prepare this context for refreshing.
prepareRefresh();
// Tell the subclass to refresh the internal bean factory.
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
// Prepare the bean factory for use in this context.
prepareBeanFactory(beanFactory);
try {
// Allows post-processing of the bean factory in context subclasses.
postProcessBeanFactory(beanFactory);
// Invoke factory processors registered as beans in the context.
invokeBeanFactoryPostProcessors(beanFactory);
// Register bean processors that intercept bean creation.
registerBeanPostProcessors(beanFactory);
// Initialize message source for this context.
initMessageSource();
// Initialize event multicaster for this context.
initApplicationEventMulticaster();
// Initialize other special beans in specific context subclasses.
onRefresh();
// Check for listener beans and register them.
registerListeners();
// Instantiate all remaining (non-lazy-init) singletons.
finishBeanFactoryInitialization(beanFactory);
// Last step: publish corresponding event.
finishRefresh();
}
catch (BeansException ex) {
if (logger.isWarnEnabled()) {
logger.warn("Exception encountered during context initialization - " +
"cancelling refresh attempt: " + ex);
}
// Destroy already created singletons to avoid dangling resources.
destroyBeans();
// Reset 'active' flag.
cancelRefresh(ex);
// Propagate exception to caller.
throw ex;
}
finally {
// Reset common introspection caches in Spring's core, since we
// might not ever need metadata for singleton beans anymore...
resetCommonCaches();
}
}
}
而refresh方法就有AbstractApplicationContext的具体子类调用了，这里就不再进行分析，感兴趣的读者可以自行debug跟踪。
