在上一篇文章中分析了Spring是如何解析默认标签的，并封装为BeanDefinition注册到缓存中，这一篇就来看看对于像context这种自定义标签是如何解析的。同时我们常用的注解如：@Service、@Component、@Controller标注的类也是需要在xml中配置<context:component-scan>才能自动注入到IOC容器中，所以本篇也会重点分析注解解析原理。
正文 自定义标签解析原理在上一篇分析默认标签解析时看到过这个类DefaultBeanDefinitionDocumentReader的方法parseBeanDefinitions：
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; if (delegate.isDefaultNamespace(ele)) { //默认标签解析 parseDefaultElement(ele, delegate); } else { //自定义标签解析 delegate.parseCustomElement(ele); } } } } else { delegate.parseCustomElement(root); } }现在我们就来看看parseCustomElement这个方法，但在点进去之前不妨想想自定义标签解析应该怎么做。
public BeanDefinition parseCustomElement(Element ele, @Nullable BeanDefinition containingBd) { String namespaceUri = getNamespaceURI(ele); if (namespaceUri == null) { return null; } 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)); }可以看到和默认标签解析是一样的，只不过由decorate方法改为了parse方法，但具体是如何解析的呢？这里我就以component-scan标签的解析为例，看看注解是如何解析为BeanDefinition对象的。
注解解析原理进入到parse方法中，首先会进入NamespaceHandlerSupport类中：
public BeanDefinition parse(Element element, ParserContext parserContext) { BeanDefinitionParser parser = findParserForElement(element, parserContext); return (parser != null ? parser.parse(element, parserContext) : null); }首先通过findParserForElement方法去找到对应的解析器，然后委托给解析器ComponentScanBeanDefinitionParser解析。在往下看之前，我们先想一想，如果是我们自己要去实现这个注解解析过程会怎么做。是不是应该首先通过配置的basePackage属性，去扫描该路径下所有的class文件，然后判断class文件是否符合条件，即是否标注了@Service、@Component、@Controller等注解，如果有，则封装为BeanDefinition对象并注册到容器中去？下面就来验证我们的猜想：
public BeanDefinition parse(Element element, ParserContext parserContext) { String basePackage = element.getAttribute(BASE_PACKAGE_ATTRIBUTE); basePackage = parserContext.getReaderContext().getEnvironment().resolvePlaceholders(basePackage); String[] basePackages = StringUtils.tokenizeToStringArray(basePackage, ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS); // Actually scan for bean definitions and register them. // 创造ClassPathBeanDefinitionScanner对象，用来扫描basePackage包下符合条件（默认是@Component标注的类）的类， // 并创建BeanDefinition类注册到缓存中 ClassPathBeanDefinitionScanner scanner = configureScanner(parserContext, element); Set<BeanDefinitionHolder> beanDefinitions = scanner.doScan(basePackages); registerComponents(parserContext.getReaderContext(), beanDefinitions, element); return null; }可以看到流程和我们猜想的基本一致，首先创建了一个扫描器ClassPathBeanDefinitionScanner对象，然后通过这个扫描器去扫描classpath下的文件并注册，最后调用了registerComponents方法，这个方法的作用稍后来讲，我们先来看看扫描器是如何创建的：
protected ClassPathBeanDefinitionScanner configureScanner(ParserContext parserContext, Element element) { boolean useDefaultFilters = true; if (element.hasAttribute(USE_DEFAULT_FILTERS_ATTRIBUTE)) { useDefaultFilters = Boolean.valueOf(element.getAttribute(USE_DEFAULT_FILTERS_ATTRIBUTE)); } // Delegate bean definition registration to scanner class. ClassPathBeanDefinitionScanner scanner = createScanner(parserContext.getReaderContext(), useDefaultFilters); scanner.setBeanDefinitionDefaults(parserContext.getDelegate().getBeanDefinitionDefaults()); scanner.setAutowireCandidatePatterns(parserContext.getDelegate().getAutowireCandidatePatterns()); if (element.hasAttribute(RESOURCE_PATTERN_ATTRIBUTE)) { scanner.setResourcePattern(element.getAttribute(RESOURCE_PATTERN_ATTRIBUTE)); } ... parseTypeFilters(element, scanner, parserContext); return scanner; } public ClassPathBeanDefinitionScanner(BeanDefinitionRegistry registry, boolean useDefaultFilters, Environment environment, @Nullable ResourceLoader resourceLoader) { Assert.notNull(registry, "BeanDefinitionRegistry must not be null"); this.registry = registry; if (useDefaultFilters) { registerDefaultFilters(); } setEnvironment(environment); setResourceLoader(resourceLoader); } protected void registerDefaultFilters() { this.includeFilters.add(new AnnotationTypeFilter(Component.class)); ClassLoader cl = ClassPathScanningCandidateComponentProvider.class.getClassLoader(); try { this.includeFilters.add(new AnnotationTypeFilter( ((Class<? extends Annotation>) ClassUtils.forName("javax.annotation.ManagedBean", cl)), false)); logger.trace("JSR-250 'javax.annotation.ManagedBean' found and supported for component scanning"); } catch (ClassNotFoundException ex) { // JSR-250 1.1 API (as included in Java EE 6) not available - simply skip. } try { this.includeFilters.add(new AnnotationTypeFilter( ((Class<? extends Annotation>) ClassUtils.forName("javax.inject.Named", cl)), false)); logger.trace("JSR-330 'javax.inject.Named' annotation found and supported for component scanning"); } catch (ClassNotFoundException ex) { // JSR-330 API not available - simply skip. } } protected void parseTypeFilters(Element element, ClassPathBeanDefinitionScanner scanner, ParserContext parserContext) { // Parse exclude and include filter elements. ClassLoader classLoader = scanner.getResourceLoader().getClassLoader(); // 将component-scan的子标签include-filter和exclude-filter添加到scanner中 NodeList nodeList = element.getChildNodes(); for (int i = 0; i < nodeList.getLength(); i++) { Node node = nodeList.item(i); if (node.getNodeType() == Node.ELEMENT_NODE) { String localName = parserContext.getDelegate().getLocalName(node); try { if (INCLUDE_FILTER_ELEMENT.equals(localName)) { TypeFilter typeFilter = createTypeFilter((Element) node, classLoader, parserContext); scanner.addIncludeFilter(typeFilter); } else if (EXCLUDE_FILTER_ELEMENT.equals(localName)) { TypeFilter typeFilter = createTypeFilter((Element) node, classLoader, parserContext); scanner.addExcludeFilter(typeFilter); } } catch (ClassNotFoundException ex) { parserContext.getReaderContext().warning( "Ignoring non-present type filter class: " + ex, parserContext.extractSource(element)); } catch (Exception ex) { parserContext.getReaderContext().error( ex.getMessage(), parserContext.extractSource(element), ex.getCause()); } } } }