spring的compentScan注解掃描類機制

 1主要通過ConfigurationClassParser類的doProcessConfigurationClass(ConfigurationClass configClass, SourceClass sourceClass)方法來掃描被ComponentScan注解的類。

2 此方法在spring容器的切入點為ConfigurationClassPostProcessor類，此類實現了接口BeanFactoryPostProcessor、BeanDefinitionRegistryPostProcessor，所以在spring啟動時，會調用該類來插入配置掃描制定包下面的類，生成BeanDefinition。

3 真正處理掃描類為org.springframework.context.annotation.ClassPathBeanDefinitionScanner，該類的核心方法是：protected Set<BeanDefinitionHolder> doScan(String... basePackages)，

  該方法把掃描制定包到的BeanDefinition，然后注入到 BeanDefinitionRegistry中

	/**
	 * Perform a scan within the specified base packages,
	 * returning the registered bean definitions.
	 * <p>This method does <i>not</i> register an annotation config processor
	 * but rather leaves this up to the caller.
	 * @param basePackages the packages to check for annotated classes
	 * @return set of beans registered if any for tooling registration purposes (never {@code null})
	 */
	protected Set<BeanDefinitionHolder> doScan(String... basePackages) {
		Assert.notEmpty(basePackages, "At least one base package must be specified");
		Set<BeanDefinitionHolder> beanDefinitions = new LinkedHashSet<>();
		for (String basePackage : basePackages) {
			Set<BeanDefinition> candidates = findCandidateComponents(basePackage);
			for (BeanDefinition candidate : candidates) {
				ScopeMetadata scopeMetadata = this.scopeMetadataResolver.resolveScopeMetadata(candidate);
				candidate.setScope(scopeMetadata.getScopeName());
				String beanName = this.beanNameGenerator.generateBeanName(candidate, this.registry);
				if (candidate instanceof AbstractBeanDefinition) {
					postProcessBeanDefinition((AbstractBeanDefinition) candidate, beanName);
				}
				if (candidate instanceof AnnotatedBeanDefinition) {
					AnnotationConfigUtils.processCommonDefinitionAnnotations((AnnotatedBeanDefinition) candidate);
				}
				if (checkCandidate(beanName, candidate)) {
					BeanDefinitionHolder definitionHolder = new BeanDefinitionHolder(candidate, beanName);
					definitionHolder =
							AnnotationConfigUtils.applyScopedProxyMode(scopeMetadata, definitionHolder, this.registry);
					beanDefinitions.add(definitionHolder);
					registerBeanDefinition(definitionHolder, this.registry);
				}
			}
		}
		return beanDefinitions;
	}