/**
 * 解析自定義元素.
 * @param ele 元素.
 * @param containingBd Bean定義.
 * @return 解析Bean定義.
 */
@Nullable
public BeanDefinition parseCustomElement(Element ele, @Nullable BeanDefinition containingBd) {
    // 獲取元素命名空間.
    String namespaceUri = getNamespaceURI(ele);
    if (namespaceUri == null) {
        return null;
    }
    // 解析元素命名空間對應的NamespaceHandler.
    NamespaceHandler handler = this.readerContext.getNamespaceHandlerResolver().resolve(namespaceUri);
    if (handler == null) {
        error("Unable to locate Spring NamespaceHandler for XML schema namespace [" + namespaceUri + "]", ele);
        return null;
    }
    // 解析當前元素.
    return handler.parse(ele, new ParserContext(this.readerContext, this, containingBd));
}

/**
 * 從配置映射中查找所提供命名空間URI的NamespaceHandler.
 * @param namespaceUri 相關命名空間URI.
 * @return NamespaceHandler，若未找到，則為null.
 */
@Override
@Nullable
public NamespaceHandler resolve(String namespaceUri) {
    // 加載處理器映射.
    Map<String, Object> handlerMappings = getHandlerMappings();
    // 根據(jù)namespaceUri查找處理器類路徑.
    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);
            // 處理器需實現(xiàn)NamespaceHandler接口.
            if (!NamespaceHandler.class.isAssignableFrom(handlerClass)) {
                throw new FatalBeanException("Class [" + className + "] for namespace [" + namespaceUri +
                                             "] does not implement the [" + NamespaceHandler.class.getName() + "] interface");
            }
            NamespaceHandler namespaceHandler = (NamespaceHandler) BeanUtils.instantiateClass(handlerClass);
            // NamespaceHandler初始化，指定命名空間下各元素對應的解析器.
            namespaceHandler.init();
            handlerMappings.put(namespaceUri, namespaceHandler);
            return namespaceHandler;
        }
        catch (ClassNotFoundException ex) {
            throw new FatalBeanException("Could not find NamespaceHandler class [" + className +
                                         "] for namespace [" + namespaceUri + "]", ex);
        }
        catch (LinkageError err) {
            throw new FatalBeanException("Unresolvable class definition for NamespaceHandler class [" +
                                         className + "] for namespace [" + namespaceUri + "]", err);
        }
    }
}

/**
 * 延遲加載指定的NamespaceHandler映射.
 */
private Map<String, Object> getHandlerMappings() {
    Map<String, Object> handlerMappings = this.handlerMappings;
    if (handlerMappings == null) {
        synchronized (this) {
            handlerMappings = this.handlerMappings;
            if (handlerMappings == null) {
                try {
                    // 搜索META-INF/spring.handlers文件.
                    Properties mappings =
                        PropertiesLoaderUtils.loadAllProperties(this.handlerMappingsLocation, this.classLoader);
                    if (logger.isDebugEnabled()) {
                        logger.debug("Loaded NamespaceHandler mappings: " + mappings);
                    }
                    Map<String, Object> mappingsToUse = new ConcurrentHashMap<>(mappings.size());
                    // 將Properties數(shù)據(jù)轉(zhuǎn)換到Map中.
                    CollectionUtils.mergePropertiesIntoMap(mappings, mappingsToUse);
                    handlerMappings = mappingsToUse;
                    this.handlerMappings = handlerMappings;
                }
                catch (IOException ex) {
                    throw new IllegalStateException(
                        "Unable to load NamespaceHandler mappings from location [" + this.handlerMappingsLocation + "]", ex);
                }
            }
        }
    }
    return handlerMappings;
}

/**
 * 通過委托給為Element注冊的BeanDefinitionParser來解析提供的Element.
 */
@Override
@Nullable
public BeanDefinition parse(Element element, ParserContext parserContext) {
    // 使用提供元素的本地名稱查找BeanDefinitionParser.
    BeanDefinitionParser parser = findParserForElement(element, parserContext);
    // 調(diào)用BeanDefinitionParser實現(xiàn)進行解析.
    return (parser != null ? parser.parse(element, parserContext) : null);
}

/**
 * 使用提供的元素的本地名稱查找BeanDefinitionParser.
 */
@Nullable
private BeanDefinitionParser findParserForElement(Element element, ParserContext parserContext) {
    // 獲取提供元素的本地名稱.
    String localName = parserContext.getDelegate().getLocalName(element);
    // 根據(jù)本地名稱查找BeanDefinitionParser.
    BeanDefinitionParser parser = this.parsers.get(localName);
    if (parser == null) {
        parserContext.getReaderContext().fatal(
            "Cannot locate BeanDefinitionParser for element [" + localName + "]", element);
    }
    return parser;
}