1、Dubbo与Spring整合解析配置文件

1、Dubbo消费者调用服务提供者例子

本专栏分析的Dubbo源码是基于2.6.x版本

public class Consumer {

    public static void main(String[] args) {
        ClassPathXmlApplicationContext context = 
           new ClassPathXmlApplicationContext(new String[]{"META-INF/spring/dubbo-demo-consumer.xml"});
        context.start();
        DemoService demoService = (DemoService) context.getBean("demoService");

        while (true) {
            try {
                Thread.sleep(1000);
                String hello = demoService.sayHello("world");
                System.out.println(hello); 
            } catch (Throwable throwable) {
                throwable.printStackTrace();
            }
        }
    }
}

<?xml version="1.0" encoding="UTF-8"?>

<beans xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:dubbo="http://dubbo.apache.org/schema/dubbo"
       xmlns="http://www.springframework.org/schema/beans"
       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">

    <dubbo:application name="demo-consumer"/>
    <dubbo:registry address="zookeeper://127.0.0.1:2181"/>
    <dubbo:reference id="demoService" check="false" interface="com.alibaba.dubbo.demo.DemoService"/>
</beans>

public class Provider {

    public static void main(String[] args) throws Exception {
        System.setProperty("java.net.preferIPv4Stack", "true");
        ClassPathXmlApplicationContext context = 
          new ClassPathXmlApplicationContext(new String[]{"META-INF/spring/dubbo-demo-provider.xml"});
        context.start();
        System.in.read(); 
    }
}

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:dubbo="http://dubbo.apache.org/schema/dubbo"
       xmlns="http://www.springframework.org/schema/beans"
       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">

    <dubbo:application name="demo-provider"/>
    <dubbo:registry address="zookeeper://127.0.0.1:2181"/>
    <dubbo:protocol name="dubbo" port="20880"/>
    <bean id="demoService" class="com.alibaba.dubbo.demo.provider.DemoServiceImpl"/>
    <dubbo:service interface="com.alibaba.dubbo.demo.DemoService" ref="demoService"/>

</beans>

2、Spring解析dubbo配置文件

  先启动服务提供者，再启动消费者，发现控制台可以正常输出。下面分析一下Spring是如何解析dubbo的消费者和服务提供者的配置文件。Spring容器提供了IOC功能，可以替我们生成bean。通常，我们将bean的定义放在xml文件中，我们来分析一下Spring加载xml配置文件并生成bean过程。Spring提供的容器分为两种：BeanFactory和ApplicationContext。其中BeanFactory是懒加载，也就是延迟初始化，它在你调用getBean时才会初始化这个bean，而ApplicationContext是初始化容器时就会加载非延迟初始化的bean。先简单概况下Spring容器生成bean的过程，首先通过loadBeanDefinition过程将bean的信息封装成一个个BeanDefinition，然后再根据这些BeanDefinition创建bean。下面看Spring解析Dubbo的配置文件并生成bean的过程。

//1、new ClassPathXmlApplicationContext时Spring容器初始化,此时会先调用loadBeanDefinition方法去加载
//解析xml配置文件
context = 
     new ClassPathXmlApplicationContext(new String[]{"META-INF/spring/dubbo-demo-provider.xml"});

//2、加载配置文件最终会走到这里
protected void parseBeanDefinitions(Element root, BeanDefinitionParserDelegate delegate) {
        if (delegate.isDefaultNamespace(root)) {
            NodeList nl = root.getChildNodes();
            // 3、这里其实已经通过dom4j将xml文件解析成了Document,将xml中的一项一项配置解析成了一个个
            //Node去读取处理.
            for (int i = 0; i < nl.getLength(); i++) {
                Node node = nl.item(i);
                if (node instanceof Element) {
                    Element ele = (Element) node;
                    // 4、判断是否是Spring默认可以处理的Node.这里看下面截图,由于dubbo：application
                    //是dubbo中定义的,不属于Spring的命名空间管理
                    if (delegate.isDefaultNamespace(ele)) {
                        parseDefaultElement(ele, delegate);
                    }
                    else {
                        delegate.parseCustomElement(ele);
                    }
                }
            }
        }
}

public BeanDefinition parseCustomElement(Element ele, BeanDefinition containingBd) {
        //http://dubbo.apache.org/schema/dubbo
        String namespaceUri = getNamespaceURI(ele);
        //DubboNameSpaceHandler
        NamespaceHandler handler = 
                             this.readerContext.getNamespaceHandlerResolver().resolve(namespaceUri);
        if (handler == null) {
            return null;
        }
        return handler.parse(ele, new ParserContext(this.readerContext, this, containingBd));
}

private static BeanDefinition parse(Element element, ParserContext parserContext, 
                                                            Class<?> beanClass, boolean required) {
        RootBeanDefinition beanDefinition = new RootBeanDefinition();
        //class com.alibaba.dubbo.config.ApplicationConfig
        beanDefinition.setBeanClass(beanClass);
        beanDefinition.setLazyInit(false);
        //解析id属性
        String id = element.getAttribute("id");
        if ((id == null || id.length() == 0) && required) {
            String generatedBeanName = element.getAttribute("name");
            if (generatedBeanName == null || generatedBeanName.length() == 0) {
                if (ProtocolConfig.class.equals(beanClass)) {
                    generatedBeanName = "dubbo";
                } else {
                    generatedBeanName = element.getAttribute("interface");
                }
            }
            if (generatedBeanName == null || generatedBeanName.length() == 0) {
                generatedBeanName = beanClass.getName();
            }
            id = generatedBeanName;
            int counter = 2;
            while (parserContext.getRegistry().containsBeanDefinition(id)) {
                id = generatedBeanName + (counter++);
            }
        }
        if (id != null && id.length() > 0) {
            if (parserContext.getRegistry().containsBeanDefinition(id)) {
                throw new IllegalStateException("Duplicate spring bean id " + id);
            }
            //注册BeanDefinition
            parserContext.getRegistry().registerBeanDefinition(id, beanDefinition);
            //将id属性放入beanDefinition中,后续getBean创建bean时就是根据这些属性来创建bean,
            //这里创建的bean是ApplicationConfig
            beanDefinition.getPropertyValues().addPropertyValue("id", id);
        }

        //删去一些代码，reference是解析得到的value值，可见这里将属性和属性值都放入了BeanDefinition
        beanDefinition.getPropertyValues().addPropertyValue(property, reference);
        return beanDefinition;
}

到这里就解析完了，Spring将xml中的application节点解析成一个BeanDefinition，并注册到Registry中，Registry就是一个Map。下面分析Spring创建这个ApplicationConfig的过程。

3、Spring创建ApplicationConfig

context = 
       new ClassPathXmlApplicationContext(new String[]{"META-INF/spring/dubbo-demo-provider.xml"});

// Spring容器的初始化过程，new ClassPathXmlApplicationContext后会走到这里
public void refresh() throws BeansException, IllegalStateException {
        synchronized (this.startupShutdownMonitor) {
            prepareRefresh();
            // 这里面就会执行上面的分析过程，调用loadBeanDefinition解析BeanDefinition
            ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
            prepareBeanFactory(beanFactory);
            try {
                postProcessBeanFactory(beanFactory);
                invokeBeanFactoryPostProcessors(beanFactory);
                registerBeanPostProcessors(beanFactory);
                initMessageSource();
                initApplicationEventMulticaster();
                onRefresh();
                registerListeners();
                // Instantiate all remaining (non-lazy-init) singletons.可以看到Spring容器初始化
                //的后面会初始化非延迟加载的bean,这里会走到下图的preInstantiasteSingletons方法
                finishBeanFactoryInitialization(beanFactory);
                finishRefresh();
            }
        }
}

//Spring创建bean最终会走到这里
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, 
                                   final Object[] args) throws BeanCreationException {
        BeanWrapper instanceWrapper = null;
        if (mbd.isSingleton()) {
            instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
        }
        if (instanceWrapper == null) {
            //删除一些无用代码,这里会调用反射创建bean,创建完仅是一个空的bean，属性还没有赋值
            instanceWrapper = createBeanInstance(beanName, mbd, args);
        }
        final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);

        Object exposedObject = bean;
        try {
            //属性赋值,最终也是调用反射进行赋值
            populateBean(beanName, mbd, instanceWrapper);
            if (exposedObject != null) {
                exposedObject = initializeBean(beanName, exposedObject, mbd);
            }
        }
        return exposedObject;
}

protected void populateBean(String beanName, RootBeanDefinition mbd, BeanWrapper bw) {
        // 这里的pvs就是之前解析配置文件得到BeanDefinition时，给BeanDefinition注入进去的
        PropertyValues pvs = mbd.getPropertyValues();
        // 删除一些代码，最终这里会调用反射赋值，跳来跳去有点复杂
        applyPropertyValues(beanName, mbd, bw, pvs);
}

protected void addSingleton(String beanName, Object singletonObject) {
        //最终创建完bean以后会将它保存起来(猜测，Spring容器初始化以后，非懒加载的bean已经以如下方式
        //保存到Spring容器中了，后续通过@Autowired注解)来获取时就是从这里面获取，只是分析，还没有看源码)
        synchronized (this.singletonObjects) {
            this.singletonObjects.put(beanName, 
                                      (singletonObject != null ? singletonObject : NULL_OBJECT));
            this.singletonFactories.remove(beanName);
            this.earlySingletonObjects.remove(beanName);
            this.registeredSingletons.add(beanName);
        }
}