Spring是怎么扩展解析xml接口的

public class BeanDefinitionParserDelegate {
	public static final String BEANS_NAMESPACE_URI = "http://www.springframework.org/schema/beans";
	public static final String MULTI_VALUE_ATTRIBUTE_DELIMITERS = ",; ";
	private final XmlReaderContext readerContext;
	private final DocumentDefaultsDefinition defaults = new DocumentDefaultsDefinition();
	/**
	*初始化 default lazy-init, autowire, dependency check settings,
	* init-method, destroy-method and merge settings等属性值
	* 并且填充到属性 DocumentDefaultsDefinition defaults;
	* 如果 当前Xml文件没有配置默认属性，则查父类delegate有没有设置属性；如果没有则 使用默认配置
	*/
	protected void populateDefaults(DocumentDefaultsDefinition defaults, DocumentDefaultsDefinition parentDefaults, Element root) {
		String lazyInit = root.getAttribute(DEFAULT_LAZY_INIT_ATTRIBUTE);
		if (DEFAULT_VALUE.equals(lazyInit)) {
			// Potentially inherited from outer <beans> sections, otherwise falling back to false.
			lazyInit = (parentDefaults != null ? parentDefaults.getLazyInit() : FALSE_VALUE);
		}
		defaults.setLazyInit(lazyInit);

		String merge = root.getAttribute(DEFAULT_MERGE_ATTRIBUTE);
		if (DEFAULT_VALUE.equals(merge)) {
			// Potentially inherited from outer <beans> sections, otherwise falling back to false.
			merge = (parentDefaults != null ? parentDefaults.getMerge() : FALSE_VALUE);
		}
		defaults.setMerge(merge);

		String autowire = root.getAttribute(DEFAULT_AUTOWIRE_ATTRIBUTE);
		if (DEFAULT_VALUE.equals(autowire)) {
			// Potentially inherited from outer <beans> sections, otherwise falling back to 'no'.
			autowire = (parentDefaults != null ? parentDefaults.getAutowire() : AUTOWIRE_NO_VALUE);
		}
		defaults.setAutowire(autowire);

		// Don't fall back to parentDefaults for dependency-check as it's no longer supported in
		// <beans> as of 3.0. Therefore, no nested <beans> would ever need to fall back to it.
		defaults.setDependencyCheck(root.getAttribute(DEFAULT_DEPENDENCY_CHECK_ATTRIBUTE));

		if (root.hasAttribute(DEFAULT_AUTOWIRE_CANDIDATES_ATTRIBUTE)) {
			defaults.setAutowireCandidates(root.getAttribute(DEFAULT_AUTOWIRE_CANDIDATES_ATTRIBUTE));
		}
		else if (parentDefaults != null) {
			defaults.setAutowireCandidates(parentDefaults.getAutowireCandidates());
		}

		if (root.hasAttribute(DEFAULT_INIT_METHOD_ATTRIBUTE)) {
			defaults.setInitMethod(root.getAttribute(DEFAULT_INIT_METHOD_ATTRIBUTE));
		}
		else if (parentDefaults != null) {
			defaults.setInitMethod(parentDefaults.getInitMethod());
		}

		if (root.hasAttribute(DEFAULT_DESTROY_METHOD_ATTRIBUTE)) {
			defaults.setDestroyMethod(root.getAttribute(DEFAULT_DESTROY_METHOD_ATTRIBUTE));
		}
		else if (parentDefaults != null) {
			defaults.setDestroyMethod(parentDefaults.getDestroyMethod());
		}

		defaults.setSource(this.readerContext.extractSource(root));
	}
	//解析自定义元素
public BeanDefinition parseCustomElement(Element ele, BeanDefinition containingBd) {
		//获取当前Element的命名空间
		String namespaceUri = getNamespaceURI(ele);
		//获取namespaceHandlerResolver然后根据命名空间得到具体的 NamespaceHandler处理类
		NamespaceHandler handler = this.readerContext.getNamespaceHandlerResolver().resolve(namespaceUri);
		if (handler == null) {
			error("Unable to locate Spring NamespaceHandler for XML schema namespace [" + namespaceUri + "]", ele);
			return null;
		}
		//具体的NamespaceHandler处理类去解析Element；返回 BeanDefinition 对象
		return handler.parse(ele, new ParserContext(this.readerContext, this, containingBd));
	}
	//返回defaults属性值
	public BeanDefinitionDefaults getBeanDefinitionDefaults() {
		BeanDefinitionDefaults bdd = new BeanDefinitionDefaults();
		bdd.setLazyInit("TRUE".equalsIgnoreCase(this.defaults.getLazyInit()));
		bdd.setDependencyCheck(this.getDependencyCheck(DEFAULT_VALUE));
		bdd.setAutowireMode(this.getAutowireMode(DEFAULT_VALUE));
		bdd.setInitMethodName(this.defaults.getInitMethod());
		bdd.setDestroyMethodName(this.defaults.getDestroyMethod());
		return bdd;
	}

	/**
	 * Locate the {@link NamespaceHandler} for the supplied namespace URI
	 * from the configured mappings.
	 * @param namespaceUri the relevant namespace URI
	 * @return the located {@link NamespaceHandler}, or {@code null} if none found
	 */
	@Override
	public NamespaceHandler resolve(String namespaceUri) {
		Map<String, Object> handlerMappings = getHandlerMappings();
		Object handlerOrClassName = handlerMappings.get(namespaceUri);
		if (handlerOrClassName == null) {
			return null;
		}
		//如果已经把全类名路径转换成具体的实例了  就直接返回
		else if (handlerOrClassName instanceof NamespaceHandler) {
			return (NamespaceHandler) handlerOrClassName;
		}
		else {
			
			String className = (String) handlerOrClassName;
			try {
			   //根据全类名路径 来加载这个类
				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");
				}
				//加载完类之后，通过反射生成具体实例对象;例如：ContextNamespaceHandler
				NamespaceHandler namespaceHandler = (NamespaceHandler) BeanUtils.instantiateClass(handlerClass);
				//调用具体对象的init()方法，注册对应的解析类，例如annotation-config 对应的解析类是 AnnotationConfigBeanDefinitionParser
				namespaceHandler.init();
				//将实例对象覆盖 全类名路径
				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);
			}
		}
	}
     /**
	 * 第一次加载的时候 Map里面的对象 是String类型，全类名路径
	 */
	private Map<String, Object> getHandlerMappings() {
		if (this.handlerMappings == null) {
			synchronized (this) {
				if (this.handlerMappings == null) {
						//加载META-INF/spring.handlers的所有属性
						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;
	}

@Override
	public void init() {
		registerBeanDefinitionParser("property-placeholder", new PropertyPlaceholderBeanDefinitionParser());
		registerBeanDefinitionParser("property-override", new PropertyOverrideBeanDefinitionParser());
		registerBeanDefinitionParser("annotation-config", new AnnotationConfigBeanDefinitionParser());
		registerBeanDefinitionParser("component-scan", new ComponentScanBeanDefinitionParser());
		registerBeanDefinitionParser("load-time-weaver", new LoadTimeWeaverBeanDefinitionParser());
		registerBeanDefinitionParser("spring-configured", new SpringConfiguredBeanDefinitionParser());
		registerBeanDefinitionParser("mbean-export", new MBeanExportBeanDefinitionParser());
		registerBeanDefinitionParser("mbean-server", new MBeanServerBeanDefinitionParser());
	}

public abstract class NamespaceHandlerSupport implements NamespaceHandler {
		//将元素头 与对应的解析器对应起来
		protected final void registerBeanDefinitionParser(String elementName, BeanDefinitionParser parser) {
			this.parsers.put(elementName, parser);
		}
	/**
	*具体的NamespaceHandler处理类去解析Element；返回 BeanDefinition 对象
	* BeanDefinitionParserDelegate.parseCustomElement 中 在获取到了对应的handler后，会执行handler.parse(ele, new ParserContext(this.readerContext, this, containingBd))方法；
	* 例如：获取到的操作类是  ContextNamespaceHandler,调用parse的方法其实调用了NamespaceHandlerSupportde parse方法;也就是下面的 
	* 例如annotation-config得到的是AnnotationConfigBeanDefinitionParser实例， 再调用AnnotationConfigBeanDefinitionParser的parse方法；这样就实现了根据不同元素来解析成 Spring中的 定义Bean BeanDefinition了；
	*/
	@Override
	public BeanDefinition parse(Element element, ParserContext parserContext) {
		return findParserForElement(element, parserContext).parse(element, parserContext);
	}

	/**
	 * 根据元素节点的名称 获取对应的解析类实例；例如annotation-config得到的是AnnotationConfigBeanDefinitionParser实例，
	 */
	private BeanDefinitionParser findParserForElement(Element element, ParserContext parserContext) {
		String localName = parserContext.getDelegate().getLocalName(element);
		BeanDefinitionParser parser = this.parsers.get(localName);
		if (parser == null) {
			parserContext.getReaderContext().fatal(
					"Cannot locate BeanDefinitionParser for element [" + localName + "]", element);
		}
		return parser;
	}
	
	
}

public class ContextNamespaceHandler extends NamespaceHandlerSupport {
	@Override
	public void init() {
		registerBeanDefinitionParser("property-placeholder", new PropertyPlaceholderBeanDefinitionParser());
		registerBeanDefinitionParser("property-override", new PropertyOverrideBeanDefinitionParser());
		registerBeanDefinitionParser("annotation-config", new AnnotationConfigBeanDefinitionParser());
		registerBeanDefinitionParser("component-scan", new ComponentScanBeanDefinitionParser());
		registerBeanDefinitionParser("load-time-weaver", new LoadTimeWeaverBeanDefinitionParser());
		registerBeanDefinitionParser("spring-configured", new SpringConfiguredBeanDefinitionParser());
		registerBeanDefinitionParser("mbean-export", new MBeanExportBeanDefinitionParser());
		registerBeanDefinitionParser("mbean-server", new MBeanServerBeanDefinitionParser());
	}

}

	public ParserContext(XmlReaderContext readerContext, BeanDefinitionParserDelegate delegate,
			BeanDefinition containingBeanDefinition) {

		this.readerContext = readerContext;
		this.delegate = delegate;
		this.containingBeanDefinition = containingBeanDefinition;
	}