context:component-scan配置和use-default-filters详解

最近在SpringMVC项目中对controller进行aop切面处理时，发现切面无法切入，原来是WebApplicationContext和ApplicationContext存在继承关系，加载先后顺序会影响aop代理注入。于是展开对context:component-scan的深入研究

<context:component-scan>标签

对于context标签，都是交由ContextNamespaceHandler处理

@Overridepublic 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());}

由源码可已看出，对component-scan的处理，是由ComponentScanBeanDefinitionParser类处理。

ComponentScanBeanDefinitionParser类通过configureScanner()方法，将配置文件中的信息封装成ClassPathBeanDefinitionScanner对象

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.setResourceLoader(parserContext.getReaderContext().getResourceLoader());scanner.setEnvironment(parserContext.getReaderContext().getEnvironment());scanner.setBeanDefinitionDefaults(parserContext.getDelegate().getBeanDefinitionDefaults());scanner.setAutowireCandidatePatterns(parserContext.getDelegate().getAutowireCandidatePatterns());if (element.hasAttribute(RESOURCE_PATTERN_ATTRIBUTE)) {scanner.setResourcePattern(element.getAttribute(RESOURCE_PATTERN_ATTRIBUTE));}try {parseBeanNameGenerator(element, scanner);}catch (Exception ex) {parserContext.getReaderContext().error(ex.getMessage(), parserContext.extractSource(element), ex.getCause());}try {parseScope(element, scanner);}catch (Exception ex) {parserContext.getReaderContext().error(ex.getMessage(), parserContext.extractSource(element), ex.getCause());}parseTypeFilters(element, scanner, parserContext);return scanner;}

对属性USE_DEFAULT_FILTERS_ATTRIBUTE(use-default-filters)判断，调用

new ClassPathBeanDefinitionScanner(readerContext.getRegistry(), useDefaultFilters);构造方法，然后进行相应的后续处理，返回ClassPathBeanDefinitionScanner对象

其中的parseTypeFilters(element, scanner, parserContext);方法 对exclude 和 include filter elements进行过滤 代码如下

protected void parseTypeFilters(Element element, ClassPathBeanDefinitionScanner scanner, ParserContext parserContext) {// Parse exclude and include filter elements.ClassLoader classLoader = scanner.getResourceLoader().getClassLoader();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 (Exception ex) {parserContext.getReaderContext().error(ex.getMessage(), parserContext.extractSource(element), ex.getCause());}}}}

可以看到，会根据<content:exclude-filter>和<content:include-filter>标签对 ClassPathBeanDefinitionScanner中的两个属性进行操作，这两个属性就是

private final List<TypeFilter> includeFilters = new LinkedList<TypeFilter>();private final List<TypeFilter> excludeFilters = new LinkedList<TypeFilter>();

ClassPathBeanDefinitionScanner继承自ClassPathScanningCandidateComponentProvider

/** * Create a ClassPathScanningCandidateComponentProvider with a {@link StandardEnvironment}. * @param useDefaultFilters whether to register the default filters for the * {@link Component @Component}, {@link Repository @Repository}, * {@link Service @Service}, and {@link Controller @Controller} * stereotype annotations * @see #registerDefaultFilters() */public ClassPathScanningCandidateComponentProvider(boolean useDefaultFilters) {this(useDefaultFilters, new StandardEnvironment());}

/** * Register the default filter for {@link Component @Component}. * <p>This will implicitly register all annotations that have the * {@link Component @Component} meta-annotation including the * {@link Repository @Repository}, {@link Service @Service}, and * {@link Controller @Controller} stereotype annotations. * <p>Also supports Java EE 6's {@link javax.annotation.ManagedBean} and * JSR-330's {@link javax.inject.Named} annotations, if available. * */@SuppressWarnings("unchecked")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.debug("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.debug("JSR-330 'javax.inject.Named' annotation found and supported for component scanning");}catch (ClassNotFoundException ex) {// JSR-330 API not available - simply skip.}}

默认use-default-filters为true，此时加载默认的filters；通过注释可以看到，默认是对@Component、@Repository、@Controller和 JavaEE6的ManagedBean和javax.inject.Named进行扫描管理

我们再看 isCandidateComponent(MetadataReader metadataReader);方法

/** * Determine whether the given class does not match any exclude filter * and does match at least one include filter. * @param metadataReader the ASM ClassReader for the class * @return whether the class qualifies as a candidate component */protected boolean isCandidateComponent(MetadataReader metadataReader) throws IOException {for (TypeFilter tf : this.excludeFilters) {if (tf.match(metadataReader, this.metadataReaderFactory)) {return false;}}for (TypeFilter tf : this.includeFilters) {if (tf.match(metadataReader, this.metadataReaderFactory)) {return isConditionMatch(metadataReader);}}return false;}

有注释可以看出，此方法用来决定一个是否不符合 exclude filter 和 符合include filter决定是否进行扫描。

总结：

<context:component-scan> 不配置 use-default-filters 或者配置 user-default-filters="true"

使用默认的filters 扫描包下的所有的注解

配置了 use-default-filters="false" 不使用默认的注解

无论时使用哪种都会经过   exclude-filter和include-filter的过滤