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深入探究 Spring 的扫描原理

news 来源：原创 2024/9/20 11:50:01

在 Spring 框架中，扫描机制是其核心功能之一，它负责在应用程序启动时自动发现和注册相关的组件。本文将深入探讨 Spring 的扫描原理，包括扫描过程以及相关源码的分析。

spring提供了两种扫描方式来进行扫描：1.我们可以通过AnnotationConfigApplicationContext的scan方法中进行设置固定的包名路径；2.我们可以通过@ComponentScan注解来进行配置要进行扫描的包路径。这两种方式底层都是进行执行的是ClassPathBeanDefinitionScanner类来进行扫描的操作。这两种方式的主要区别在于执行的时机不同，本文介绍@ComponentScan注解来进行配置的扫描方式

1.spring的扫描时机

spring 容器初始化的时候，会进行执行invokeBeanFactoryPostProcessors方法，内部会进行调用ConfigurationClassPostProcessor的postProcessBeanDefinitionRegistry方法的后完成了bean的扫描操作。

2.spring 扫描流程

通过我们对代码的分析，我们发现spring进行扫描的入口为org.springframework.context.annotation.ComponentScanAnnotationParser#parse。

public Set<BeanDefinitionHolder> parse(AnnotationAttributes componentScan, String declaringClass) {//扫描器的初始化操作	ClassPathBeanDefinitionScanner scanner = new ClassPathBeanDefinitionScanner(this.registry,componentScan.getBoolean("useDefaultFilters"), this.environment, this.resourceLoader);//获取名字生成器的策略 （针对的是bean的名字）Class<? extends BeanNameGenerator> generatorClass = componentScan.getClass("nameGenerator");boolean useInheritedGenerator = (BeanNameGenerator.class == generatorClass);scanner.setBeanNameGenerator(useInheritedGenerator ? this.beanNameGenerator :BeanUtils.instantiateClass(generatorClass));ScopedProxyMode scopedProxyMode = componentScan.getEnum("scopedProxy");if (scopedProxyMode != ScopedProxyMode.DEFAULT) {scanner.setScopedProxyMode(scopedProxyMode);}else {Class<? extends ScopeMetadataResolver> resolverClass = componentScan.getClass("scopeResolver");scanner.setScopeMetadataResolver(BeanUtils.instantiateClass(resolverClass));}//获取扫描的包路径scanner.setResourcePattern(componentScan.getString("resourcePattern"));//可以自己添加一些include的过滤器//mybatis 框架对此有一些自己的扩展for (AnnotationAttributes filter : componentScan.getAnnotationArray("includeFilters")) {for (TypeFilter typeFilter : typeFiltersFor(filter)) {scanner.addIncludeFilter(typeFilter);}}// 添加排除扫描的过滤器for (AnnotationAttributes filter : componentScan.getAnnotationArray("excludeFilters")) {for (TypeFilter typeFilter : typeFiltersFor(filter)) {scanner.addExcludeFilter(typeFilter);}}boolean lazyInit = componentScan.getBoolean("lazyInit");if (lazyInit) {scanner.getBeanDefinitionDefaults().setLazyInit(true);}// 扫描的包路径的配置Set<String> basePackages = new LinkedHashSet<>();String[] basePackagesArray = componentScan.getStringArray("basePackages");for (String pkg : basePackagesArray) {String[] tokenized = StringUtils.tokenizeToStringArray(this.environment.resolvePlaceholders(pkg),ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS);Collections.addAll(basePackages, tokenized);}for (Class<?> clazz : componentScan.getClassArray("basePackageClasses")) {basePackages.add(ClassUtils.getPackageName(clazz));}if (basePackages.isEmpty()) {basePackages.add(ClassUtils.getPackageName(declaringClass));}scanner.addExcludeFilter(new AbstractTypeHierarchyTraversingFilter(false, false) {@Overrideprotected boolean matchClassName(String className) {return declaringClass.equals(className);}});// 开始进行扫描return scanner.doScan(StringUtils.toStringArray(basePackages));}

2.1 扫描器的初始化操作

通过源码，我们可以看到spring的扫描器初始化的时候默认进行注册了三个include过滤器。

	public ClassPathBeanDefinitionScanner(BeanDefinitionRegistry registry, boolean useDefaultFilters,Environment environment, @Nullable ResourceLoader resourceLoader) {Assert.notNull(registry, "BeanDefinitionRegistry must not be null");this.registry = registry;// 针对@CompentScan注解这个值默认为trueif (useDefaultFilters) {registerDefaultFilters();}setEnvironment(environment);setResourceLoader(resourceLoader);}

进行构建ClassPathBeanDefinitionScanner类对象的时候会进行注册三个默认的include过滤器。从代码中我们可以看出这三种过滤器都是AnnotationTypeFilter类型。这三种过滤器分别是针对@Component，@ManagedBean，@Named三种注解进行扫描操作。这块有一个小问题为啥@Component注解和@ManagedBean，@Named的写法不一致呢，有大神知道的话，可以告诉我一下

@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.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.}}

2.2 根据包名进行解析

解析@CompentScan注解得到包名，然后根据包名扫描包下所有的文件，遍历这些文件，通过ASM字节码技术读取这些文件信息，封装成一个metadataReader对象。调用isCandidateComponent方法，进行判断是否排除，是否添加。如果通过include的filter 则进行实例化成ScannedGenericBeanDefinition对象。

private Set<BeanDefinition> scanCandidateComponents(String basePackage) {Set<BeanDefinition> candidates = new LinkedHashSet<>();try {//拼接扫描包的路径String packageSearchPath = ResourcePatternResolver.CLASSPATH_ALL_URL_PREFIX +resolveBasePackage(basePackage) + '/' + this.resourcePattern;//会将该包路径下的文件解析成一个Resource对象			
Resource[] resources = getResourcePatternResolver().getResources(packageSearchPath);boolean traceEnabled = logger.isTraceEnabled();boolean debugEnabled = logger.isDebugEnabled();for (Resource resource : resources) {if (traceEnabled) {logger.trace("Scanning " + resource);}try {//将resource对象 封装成metaDataReaderMetadataReader metadataReader = getMetadataReaderFactory().getMetadataReader(resource);// includeFilter和excludeFilter的双重过滤if (isCandidateComponent(metadataReader)) {//将metadataReader对象封装成一个ScannedGenericBeanDefinition 对象ScannedGenericBeanDefinition sbd = new ScannedGenericBeanDefinition(metadataReader);sbd.setSource(resource);// 进行判断ScannedGenericBeanDefinition 对象是否是接口// 是否是抽象类，是否加了LookUp注解等if (isCandidateComponent(sbd)) {if (debugEnabled) {logger.debug("Identified candidate component class: " + resource);}candidates.add(sbd);}else {if (debugEnabled) {logger.debug("Ignored because not a concrete top-level class: " + resource);}}}else {if (traceEnabled) {logger.trace("Ignored because not matching any filter: " + resource);}}}catch (FileNotFoundException ex) {if (traceEnabled) {logger.trace("Ignored non-readable " + resource + ": " + ex.getMessage());}}catch (Throwable ex) {throw new BeanDefinitionStoreException("Failed to read candidate component class: " + resource, ex);}}}catch (IOException ex) {throw new BeanDefinitionStoreException("I/O failure during classpath scanning", ex);}return candidates;}