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Dubbo源码分析-Dubbo是如何整合spring-framework的

这篇文章是Dubbo源码分析的开端,与其说这篇文章是Dubbo源码分析,不如是spring源码分析,因为大部分都是在分析spring如何解析xml配置文件的,为了与后面的Dubbo源码分析保持一致,姑且这样命名了。使用Dubbo框架开发分布式服务时,一般使用spring进行管理,在spring的配置文件中进行配置,例如服务提供者Provider端配置如下:

<?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" /></beans>

对于

(省略描述....)public interface NamespaceHandler { void init(); BeanDefinition parse(Element element, ParserContext parserContext); BeanDefinitionHolder decorate(Node source, BeanDefinitionHolder definition, ParserContext parserContext);}

可以看出其中有一个parse方法,它的作用就是用于解析Xml文档中的节点。这个接口有一个抽象的实现类NamespaceHandlerSupport,具体的标签解析器都继承这个抽象类,我们来看下有哪些:

可以看见我们熟悉的Aop还有Dubbo NamespaceHandler.那Spring是如何知道要使用Dubbo定义的handler来解析自定义的标签呢?他们的结合点就在一个配置文件,spring留了一个配置文件,只要我们配置了它,spring就可以找到。这个配置文件名字叫做spring.handlers,spring在解析xml文件时,会去加载spring.handlers配置文件,然后寻找能够解析自定义标签的handler。spring.handlers在Dubbo中是怎样的内容呢,我们一起来看下:路径在:dubbo-config/dubbon-config-spring/META-INF/spring.handlers

内容如下:

http://dubbo.apache.org/schema/dubbo=org.apache.dubbo.config.spring.schema.DubboNamespaceHandlerhttp://code.alibabatech.com/schema/dubbo=org.apache.dubbo.config.spring.schema.DubboNamespaceHandler

我们来看一下spring是如何找到这个文件并生成对应标签namespaceHandler的:ClassPathXmlApplicationContext context = new ClassPathXmlApplicationContext(new String[]{"application.xml"});我们使用spring框架,启动的时候都会写这么一句话,加载xml配置文件.当执行到AbstractXmlApplicationContext类的loadBeanDefinitions方法时,会创建一个XmlBeanDefinitionReader对象,读取读取并解析xml.

/** * Loads the bean definitions via an XmlBeanDefinitionReader. * @see org.springframework.beans.factory.xml.XmlBeanDefinitionReader * @see #initBeanDefinitionReader * @see #loadBeanDefinitions */ @Override protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) throws BeansException, IOException { // Create a new XmlBeanDefinitionReader for the given BeanFactory. XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(beanFactory); // Configure the bean definition reader with this context"s // resource loading environment. beanDefinitionReader.setEnvironment(this.getEnvironment()); beanDefinitionReader.setResourceLoader(this); beanDefinitionReader.setEntityResolver(new ResourceEntityResolver(this)); // Allow a subclass to provide custom initialization of the reader, // then proceed with actually loading the bean definitions. initBeanDefinitionReader(beanDefinitionReader); loadBeanDefinitions(beanDefinitionReader); }

最后一句话loadBeanDefinitions(beanDefinitionReader),是使用XmlBeanDefinitionReader将xml解析成BeanDefinition.接下来调用XmlBeanDefinitionReader的registerBeanDefinitions方法创建BeanDefinitionDocumentReader对象,这个对象才是真正解析XML的对象。

public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException { BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader(); int countBefore = getRegistry().getBeanDefinitionCount(); //创建ReaderContext,并调用documentReader的registerBeanDefinitions方法 documentReader.registerBeanDefinitions(doc, createReaderContext(resource)); return getRegistry().getBeanDefinitionCount() - countBefore; }

大家再来看下createReaderContext方法

/** * Create the {@link XmlReaderContext} to pass over to the document reader. */ public XmlReaderContext createReaderContext(Resource resource) { return new XmlReaderContext(resource, this.problemReporter, this.eventListener, this.sourceExtractor, this, getNamespaceHandlerResolver()); }

终于出现了NamespaceHandler,那就是getNamespaceHandlerResolver()方法,再来看下这个方法

public NamespaceHandlerResolver getNamespaceHandlerResolver() { if (this.namespaceHandlerResolver == null) { this.namespaceHandlerResolver = createDefaultNamespaceHandlerResolver(); } return this.namespaceHandlerResolver; } protected NamespaceHandlerResolver createDefaultNamespaceHandlerResolver() { return new DefaultNamespaceHandlerResolver(getResourceLoader().getClassLoader()); } public DefaultNamespaceHandlerResolver(ClassLoader classLoader) { this(classLoader, DEFAULT_HANDLER_MAPPINGS_LOCATION); }//最终会调用这个构造器实例化DefaultNamespaceHandlerResolver类,此时handlerMappingsLocation成员变量的值为META-INF/spring.handlers了 public DefaultNamespaceHandlerResolver(ClassLoader classLoader, String handlerMappingsLocation) { Assert.notNull(handlerMappingsLocation, "Handler mappings location must not be null"); this.classLoader = (classLoader != null ? classLoader : ClassUtils.getDefaultClassLoader()); this.handlerMappingsLocation = handlerMappingsLocation; } /** * The location to look for the mapping files. Can be present in multiple JAR files. */ public static final String DEFAULT_HANDLER_MAPPINGS_LOCATION = "META-INF/spring.handlers";//读取META-INF/spring.handlers配置文件的内容 private Map<String, Object> getHandlerMappings() { if (this.handlerMappings == null) { synchronized (this) { if (this.handlerMappings == null) { try { Properties mappings = PropertiesLoaderUtils.loadAllProperties(this.handlerMappingsLocation, this.classLoader); if (logger.isDebugEnabled()) { logger.debug("Loaded NamespaceHandler mappings: " + mappings); } Map<String, Object> handlerMappings = new ConcurrentHashMap<String, Object>(mappings.size()); CollectionUtils.mergePropertiesIntoMap(mappings, handlerMappings); this.handlerMappings = handlerMappings; } catch (IOException ex) { throw new IllegalStateException( "Unable to load NamespaceHandler mappings from location [" + this.handlerMappingsLocation + "]", ex); } } } } return this.handlerMappings; }

会创建一个默认的DefaultNamespaceHandlerResolver对象,其中有一个变量DEFAULT_HANDLER_MAPPINGS_LOCATION,其值是META-INF/spring.handlers,此时可以看到spring会去加载spring.hadlers中的内容。那加载完后,会在什么地方使用呢?我们来看一下DefaultBeanDefinitionDocumentReader类中的parseBeanDefinitions方法

protected void parseBeanDefinitions(Element root, BeanDefinitionParserDelegate delegate) { if (delegate.isDefaultNamespace(root)) { NodeList nl = root.getChildNodes(); for (int i = 0; i < nl.getLength(); i++) { Node node = nl.item(i); if (node instanceof Element) { Element ele = (Element) node; //判断标签是标准的标准还是自定义的,判断的依据就是标准的namespace是否是http://www.springframework.org/schema/beans //如果是,则是标准的标签,如果不是则不是标准的标签 if (delegate.isDefaultNamespace(ele)) { parseDefaultElement(ele, delegate); } else { delegate.parseCustomElement(ele); } } } } else { delegate.parseCustomElement(root); } }

接下来看一下BeanDefinitionParserDelegate类的parseCustomElement方法,BeanDefinitionParserDelegate对象是在DefaultBeanDefinitionDocumentReader的doRegisterBeanDefinitions方法中生成的,它的作用是用来解析标签的。

public BeanDefinition parseCustomElement(Element ele, BeanDefinition containingBd) { //根据标签得到标签的namespace //dubbo自定义的标签得到的namespace是http://dubbo.apache.org/schema/dubbo String namespaceUri = getNamespaceURI(ele); NamespaceHandler handler = this.readerContext.getNamespaceHandlerResolver().resolve(namespaceUri); if (handler == null) { error("Unable to locate Spring NamespaceHandler for XML schema namespace [" + namespaceUri + "]", ele); return null; } return handler.parse(ele, new ParserContext(this.readerContext, this, containingBd)); }

接下来看下DefaultBeanDefinitionDocumentReader的resolve方法

@Override public NamespaceHandler resolve(String namespaceUri) { //得到加载的内容 Map<String, Object> handlerMappings = getHandlerMappings(); //根据标签的namspace得到handler类名 //dubbo的namespace为http://dubbo.apache.org/schema/dubbo Object handlerOrClassName = handlerMappings.get(namespaceUri); if (handlerOrClassName == null) { return null; } else if (handlerOrClassName instanceof NamespaceHandler) { return (NamespaceHandler) handlerOrClassName; } else { String className = (String) handlerOrClassName; try { Class<?> handlerClass = ClassUtils.forName(className, this.classLoader); if (!NamespaceHandler.class.isAssignableFrom(handlerClass)) { throw new FatalBeanException("Class [" + className + "] for namespace [" + namespaceUri + "] does not implement the [" + NamespaceHandler.class.getName() + "] interface"); }//实例化namespaceHandler NamespaceHandler namespaceHandler = (NamespaceHandler) BeanUtils.instantiateClass(handlerClass); namespaceHandler.init(); handlerMappings.put(namespaceUri, namespaceHandler); return namespaceHandler; } catch (ClassNotFoundException ex) { throw new FatalBeanException("NamespaceHandler class [" + className + "] for namespace [" + namespaceUri + "] not found", ex); } catch (LinkageError err) { throw new FatalBeanException("Invalid NamespaceHandler class [" + className + "] for namespace [" + namespaceUri + "]: problem with handler class file or dependent class", err); } } }

得到DubboNamespaceHandler实例后,调用其init()方法

@Override public void init() { //将application、module等名称做为key注册到解析器Map中 //这些key名称正是dubbo自定义标签的localName,例如<dubbo:application />标签的LocalName是application 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()); }

可以看到所有的key(除了annotation)对应的标签解析器都是DubboBeanDefinitionParser实例对象,至于DubboBeanDefinitionParser是如何解析标签的,这里就不做分析了,如果想要了解,可以看看他的源码。

如果不做特别说明,此篇往后所有的Dubbo源码分析都是基于Dubbo2.7.0

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