『SpringBoot 源码分析』run() 方法执行流程：（3）刷新应用上下文-处理 @ComponentScan 注解

• 基于 2.2.9.RELEASE
• 问题：当方法进行了注释标记之后，springboot 又是怎么注入到容器中并创建类呢？

1. 首先创建测试主程序

package com.lagou;

@SpringBootApplication//标注在类上说明这个类是`SpringBoot`的主配置类
public class SpringBootMytestApplication{

	public static void main(String[] args) {
		SpringApplication.run(SpringBootMytestApplication.class, args);
	}
}

2. 创建测试 Controller

package com.lagou.controller;

@RestController
public class TestController {

	@RequestMapping("/test")
	public String test(){
		System.out.println("源码环境构建成功...");
		return "源码环境构建成功";
	}
}

处理 @ComponentScan 注解

1. 准备好上下文以及上下文环境之后，可以继续来看看 springboot 究竟是如何把主类所在的类路径下相关的类通过 @ComponentScan 注册到 IOC 容器中。首先先走到 refreshContext() 方法，然后调用 ApplicationContext 的 refresh()

public class SpringApplication {
	...
	public ConfigurableApplicationContext run(String... args) {
		...
		try {
			// 将运行时参数封装
			ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
			// 构造应用上下文环境
			ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments);
			// 处理需要忽略的 Bean
			configureIgnoreBeanInfo(environment);
			// 打印 banner
			Banner printedBanner = printBanner(environment);
			// 刷新应用上下文前的准备阶段
			context = createApplicationContext();
			// 实例化 SpringBootExceptionReporter.class，用来支持报告关于启动的错误
			exceptionReporters = getSpringFactoriesInstances(SpringBootExceptionReporter.class,
					new Class[] { ConfigurableApplicationContext.class }, context);
			// 刷新应用上下文前的准备阶段
			prepareContext(context, environment, listeners, applicationArguments, printedBanner);
			// 1. 刷新应用上下文
			refreshContext(context);
			...
		}
		catch (Throwable ex) {
			...
		}
		...
	}

	protected void refresh(ApplicationContext applicationContext) {
		Assert.isInstanceOf(AbstractApplicationContext.class, applicationContext);
		// 2. spring 的核心方法，刷新上下文
		((AbstractApplicationContext) applicationContext).refresh();
	}
}

2. 接着 ApplicationContext 调用父类的 refresh()

public class ServletWebServerApplicationContext extends GenericWebApplicationContext
		implements ConfigurableWebServerApplicationContext {
	...
	@Override
	public final void refresh() throws BeansException, IllegalStateException {
		try {
			// 1. 调用父类 refresh() 方法
			super.refresh();
		}
		catch (RuntimeException ex) {
			stopAndReleaseWebServer();
			throw ex;
		}
	}
}

3. 在 refresh() 中，首先对上下文进行预处理，设置一些必要的参数，然后获取 IOC 容器，并对 IOC 容器进行一些参数进行设置，然后调用工厂处理器注册相关的 bean 进入上下文

public abstract class AbstractApplicationContext extends DefaultResourceLoader
		implements ConfigurableApplicationContext {
		
	private final Object startupShutdownMonitor = new Object();
	...
	@Override
	public void refresh() throws BeansException, IllegalStateException {
		synchronized (this.startupShutdownMonitor) {
			// 1. 上下文设置一些必要的参数
			prepareRefresh();

			// 2. 获取 IOC 容器
			ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

			// 3. 对 IOC 容器进行一些参数设置
			prepareBeanFactory(beanFactory);

			try {
				// Allows post-processing of the bean factory in context subclasses.
				postProcessBeanFactory(beanFactory);

				// 4. 调用工厂处理器，注册 bean 到 IOC 容器
				invokeBeanFactoryPostProcessors(beanFactory);
				...
			} catch (BeansException ex) {
				...
			}
			...
		}
	}
	
	protected void invokeBeanFactoryPostProcessors(ConfigurableListableBeanFactory beanFactory) {
		// 5. 对 IOC 容器调用 beanFactoryPostProcessors 来处理  		
		PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(beanFactory, getBeanFactoryPostProcessors());

		// Detect a LoadTimeWeaver and prepare for weaving, if found in the meantime
		// (e.g. through an @Bean method registered by ConfigurationClassPostProcessor)
		if (beanFactory.getTempClassLoader() == null && beanFactory.containsBean(LOAD_TIME_WEAVER_BEAN_NAME)) {
			beanFactory.addBeanPostProcessor(new LoadTimeWeaverAwareProcessor(beanFactory));
			beanFactory.setTempClassLoader(new ContextTypeMatchClassLoader(beanFactory.getBeanClassLoader()));
		}
	}
}

4. 通过后置处理器注册的委派类 PostProcessorRegistrationDelegate 调用 invokeBeanFactoryPostProcessors() 的逻辑相对复杂一些，我们直接跳过，只关注问题本身，走到下面的一句话

final class PostProcessorRegistrationDelegate {
	...
	public static void invokeBeanFactoryPostProcessors(
			ConfigurableListableBeanFactory beanFactory, List<BeanFactoryPostProcessor> beanFactoryPostProcessors) {
		
		// Invoke BeanDefinitionRegistryPostProcessors first, if any.
		Set<String> processedBeans = new HashSet<>();
		
		if (beanFactory instanceof BeanDefinitionRegistry) {
			BeanDefinitionRegistry registry = (BeanDefinitionRegistry) beanFactory;
			List<BeanFactoryPostProcessor> regularPostProcessors = new ArrayList<>();
			List<BeanDefinitionRegistryPostProcessor> registryProcessors = new ArrayList<>();

			for (BeanFactoryPostProcessor postProcessor : beanFactoryPostProcessors) {
				if (postProcessor instanceof BeanDefinitionRegistryPostProcessor) {
					BeanDefinitionRegistryPostProcessor registryProcessor =
							(BeanDefinitionRegistryPostProcessor) postProcessor;
					registryProcessor.postProcessBeanDefinitionRegistry(registry);
					registryProcessors.add(registryProcessor);
				}
				else {
					regularPostProcessors.add(postProcessor);
				}
			}

			// Do not initialize FactoryBeans here: We need to leave all regular beans
			// uninitialized to let the bean factory post-processors apply to them!
			// Separate between BeanDefinitionRegistryPostProcessors that implement
			// PriorityOrdered, Ordered, and the rest.
			List<BeanDefinitionRegistryPostProcessor> currentRegistryProcessors = new ArrayList<>();

			// First, invoke the BeanDefinitionRegistryPostProcessors that implement PriorityOrdered.
			String[] postProcessorNames =
					beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
			for (String ppName : postProcessorNames) {
				if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
					currentRegistryProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
					processedBeans.add(ppName);
				}
			}
			sortPostProcessors(currentRegistryProcessors, beanFactory);
			registryProcessors.addAll(currentRegistryProcessors);
			// 1. 调用 bean 注册处理器，处理类的注册			
			invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry);
			...
		}
		...
	}

	private static void invokeBeanDefinitionRegistryPostProcessors(
			Collection<? extends BeanDefinitionRegistryPostProcessor> postProcessors, BeanDefinitionRegistry registry) {

		for (BeanDefinitionRegistryPostProcessor postProcessor : postProcessors) {
			// 2. 其中 postProcessors 只有一个类，即为	ConfigurationClassPostProcessor 		
			postProcessor.postProcessBeanDefinitionRegistry(registry);
		}
	}
}

• 其中，配置类最关键的对象是 ConfigurationClassPostProcessor 后置处理器
  

5. 这时候就会交由 ConfigurationClassPostProcessor 来注册 bean 到 IOC 容器中

public class ConfigurationClassPostProcessor implements BeanDefinitionRegistryPostProcessor,
		PriorityOrdered, ResourceLoaderAware, BeanClassLoaderAware, EnvironmentAware {
	
	private final Set<Integer> registriesPostProcessed = new HashSet<>();
	...
	@Override
	public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) {
		int registryId = System.identityHashCode(registry);
		if (this.registriesPostProcessed.contains(registryId)) {
			throw new IllegalStateException(
					"postProcessBeanDefinitionRegistry already called on this post-processor against " + registry);
		}
		if (this.factoriesPostProcessed.contains(registryId)) {
			throw new IllegalStateException(
					"postProcessBeanFactory already called on this post-processor against " + registry);
		}
		this.registriesPostProcessed.add(registryId);
		// 1. 注册 bean 到 IOC 容器中
		processConfigBeanDefinitions(registry);
	}
}

6. 当调用 processConfigBeanDefinitions() 时，首先会创建 ConfigurationClassParser 用于解析 @Configuration

public class ConfigurationClassPostProcessor implements BeanDefinitionRegistryPostProcessor,
		PriorityOrdered, ResourceLoaderAware, BeanClassLoaderAware, EnvironmentAware {
	
	private MetadataReaderFactory metadataReaderFactory = new CachingMetadataReaderFactory();
	private ProblemReporter problemReporter = new FailFastProblemReporter();
	@Nullable
	private Environment environment;
	private ResourceLoader resourceLoader = new DefaultResourceLoader();
	private BeanNameGenerator componentScanBeanNameGenerator = AnnotationBeanNameGenerator.INSTANCE;
	...
	public void processConfigBeanDefinitions(BeanDefinitionRegistry registry) {
		List<BeanDefinitionHolder> configCandidates = new ArrayList<>();
		String[] candidateNames = registry.getBeanDefinitionNames();

		for (String beanName : candidateNames) {
			BeanDefinition beanDef = registry.getBeanDefinition(beanName);
			if (beanDef.getAttribute(ConfigurationClassUtils.CONFIGURATION_CLASS_ATTRIBUTE) != null) {
				if (logger.isDebugEnabled()) {
					logger.debug("Bean definition has already been processed as a configuration class: " + beanDef);
				}
			}
			// 把主类封装进 BeanDefinitionHolder 中
			else if (ConfigurationClassUtils.checkConfigurationClassCandidate(beanDef, this.metadataReaderFactory)) {
				configCandidates.add(new BeanDefinitionHolder(beanDef, beanName));
			}
		}

		// Return immediately if no @Configuration classes were found
		if (configCandidates.isEmpty()) {
			return;
		}

		// Sort by previously determined @Order value, if applicable
		configCandidates.sort((bd1, bd2) -> {
			int i1 = ConfigurationClassUtils.getOrder(bd1.getBeanDefinition());
			int i2 = ConfigurationClassUtils.getOrder(bd2.getBeanDefinition());
			return Integer.compare(i1, i2);
		});

		// Detect any custom bean name generation strategy supplied through the enclosing application context
		SingletonBeanRegistry sbr = null;
		if (registry instanceof SingletonBeanRegistry) {
			sbr = (SingletonBeanRegistry) registry;
			if (!this.localBeanNameGeneratorSet) {
				BeanNameGenerator generator = (BeanNameGenerator) sbr.getSingleton(
						AnnotationConfigUtils.CONFIGURATION_BEAN_NAME_GENERATOR);
				if (generator != null) {
					this.componentScanBeanNameGenerator = generator;
					this.importBeanNameGenerator = generator;
				}
			}
		}

		if (this.environment == null) {
			this.environment = new StandardEnvironment();
		}

		// 1. 创建解析器，用于解析 @Configuration 标注的类
		ConfigurationClassParser parser = new ConfigurationClassParser(
				this.metadataReaderFactory, this.problemReporter, this.environment,
				this.resourceLoader, this.componentScanBeanNameGenerator, registry);
		
		Set<BeanDefinitionHolder> candidates = new LinkedHashSet<>(configCandidates);
		Set<ConfigurationClass> alreadyParsed = new HashSet<>(configCandidates.size());
		
		do {
			// 2. 开始解析候选类
			parser.parse(candidates);
			...
		}
		while (!candidates.isEmpty());
		...
	}
}

• 其中，这里的 candidates 属性也就只有主类
  

7. 接下来会遍历 candidates 属性，解析主类的注解

class ConfigurationClassParser {

	private static final Predicate<String> DEFAULT_EXCLUSION_FILTER = className ->
			(className.startsWith("java.lang.annotation.") || className.startsWith("org.springframework.stereotype."));
	...
	public void parse(Set<BeanDefinitionHolder> configCandidates) {
		for (BeanDefinitionHolder holder : configCandidates) {
			BeanDefinition bd = holder.getBeanDefinition();
			try {
				if (bd instanceof AnnotatedBeanDefinition) {
					// 1. 解析主类配置，其中 getBeanName() 为 SpringBootMytestApplication
					parse(((AnnotatedBeanDefinition) bd).getMetadata(), holder.getBeanName());
				}
				else if (bd instanceof AbstractBeanDefinition && ((AbstractBeanDefinition) bd).hasBeanClass()) {
					parse(((AbstractBeanDefinition) bd).getBeanClass(), holder.getBeanName());
				}
				else {
					parse(bd.getBeanClassName(), holder.getBeanName());
				}
			}
			catch (BeanDefinitionStoreException ex) {
				throw ex;
			}
			catch (Throwable ex) {
				throw new BeanDefinitionStoreException(
						"Failed to parse configuration class [" + bd.getBeanClassName() + "]", ex);
			}
		}

		this.deferredImportSelectorHandler.process();
	}

	protected final void parse(AnnotationMetadata metadata, String beanName) throws IOException {
		processConfigurationClass(new ConfigurationClass(metadata, beanName), DEFAULT_EXCLUSION_FILTER);
	}
}

8. 再走进一层嵌套中，遇到 do 开头的方法，才是真正处理的时候

class ConfigurationClassParser {
	...
	protected void processConfigurationClass(ConfigurationClass configClass, Predicate<String> filter) throws IOException {
		if (this.conditionEvaluator.shouldSkip(configClass.getMetadata(), ConfigurationPhase.PARSE_CONFIGURATION)) {
			return;
		}

		ConfigurationClass existingClass = this.configurationClasses.get(configClass);
		if (existingClass != null) {
			if (configClass.isImported()) {
				if (existingClass.isImported()) {
					existingClass.mergeImportedBy(configClass);
				}
				// Otherwise ignore new imported config class; existing non-imported class overrides it.
				return;
			}
			else {
				// Explicit bean definition found, probably replacing an import.
				// Let's remove the old one and go with the new one.
				this.configurationClasses.remove(configClass);
				this.knownSuperclasses.values().removeIf(configClass::equals);
			}
		}

		// Recursively process the configuration class and its superclass hierarchy.
		SourceClass sourceClass = asSourceClass(configClass, filter);
		do {
			// 1. 真正处理 @Configuration 的方法，其中 sourceClass 为 com.lagou.SpringBootMytestApplication
			sourceClass = doProcessConfigurationClass(configClass, sourceClass, filter);
		}
		while (sourceClass != null);

		this.configurationClasses.put(configClass, configClass);
	}
}

9. 接下来会根据不同的注解，执行不同的处理方法，这里主要关注处理 @ComponentScan 注解的位置，首先会判断 sourceClass 是否标注了 @ComponentScan 注解，主类 @SpringBootApplication 是肯定包含这个注解。然后通过 ComponentScanAnnotationParser 来进行解析

class ConfigurationClassParser {

	private final ComponentScanAnnotationParser componentScanParser;
	private final ConditionEvaluator conditionEvaluator;
	private final Environment environment;
	...
	@Nullable
	protected final SourceClass doProcessConfigurationClass(
			ConfigurationClass configClass, SourceClass sourceClass, Predicate<String> filter)
			throws IOException {
			
		if (configClass.getMetadata().isAnnotated(Component.class.getName())) {
			// Recursively process any member (nested) classes first
			processMemberClasses(configClass, sourceClass, filter);
		}

		// Process any @PropertySource annotations
		for (AnnotationAttributes propertySource : AnnotationConfigUtils.attributesForRepeatable(
				sourceClass.getMetadata(), PropertySources.class,
				org.springframework.context.annotation.PropertySource.class)) {
			if (this.environment instanceof ConfigurableEnvironment) {
				processPropertySource(propertySource);
			}
			else {
				logger.info("Ignoring @PropertySource annotation on [" + sourceClass.getMetadata().getClassName() +
						"]. Reason: Environment must implement ConfigurableEnvironment");
			}
		}

		// 1. 扫描主类 @ComponentScan 注解里面的配置属性
		Set<AnnotationAttributes> componentScans = AnnotationConfigUtils.attributesForRepeatable(
				sourceClass.getMetadata(), ComponentScans.class, ComponentScan.class);
		if (!componentScans.isEmpty() &&
				!this.conditionEvaluator.shouldSkip(sourceClass.getMetadata(), ConfigurationPhase.REGISTER_BEAN)) {
			for (AnnotationAttributes componentScan : componentScans) {
				// 2. 解析 @ComponentScan 的注解的属性，然后把扫描到的相关类转化为 BeanDefinition
				Set<BeanDefinitionHolder> scannedBeanDefinitions =
						this.componentScanParser.parse(componentScan, sourceClass.getMetadata().getClassName());
				// Check the set of scanned definitions for any further config classes and parse recursively if needed
				for (BeanDefinitionHolder holder : scannedBeanDefinitions) {
					BeanDefinition bdCand = holder.getBeanDefinition().getOriginatingBeanDefinition();
					if (bdCand == null) {
						bdCand = holder.getBeanDefinition();
					}
					if (ConfigurationClassUtils.checkConfigurationClassCandidate(bdCand, this.metadataReaderFactory)) {
						parse(bdCand.getBeanClassName(), holder.getBeanName());
					}
				}
			}
		}
		...
	}
}

10. parse() 主体逻辑就是首先创建 ClassPathBeanDefinitionScanner 扫描器，然后判断主类的 @ComponentScan 注解是否标注了 basePackages 属性，没有的话就把主类所在的包放进去，然后扫描所有的 @Component 类，封装成 BeanDefinition

• 重点：所以这里就是为什么主类所在的包，以及其子包的类都能扫出来

class ComponentScanAnnotationParser {
	
	private final BeanNameGenerator beanNameGenerator;
	private final ResourceLoader resourceLoader;
	private final Environment environment;
	private final BeanDefinitionRegistry registry;
	...
	// declaringClass 为 com.lagou.SpringBootMytestApplication
	public Set<BeanDefinitionHolder> parse(AnnotationAttributes componentScan, final String declaringClass) {
		// 1. 创建 BeanDefinition 扫描器
		ClassPathBeanDefinitionScanner scanner = new ClassPathBeanDefinitionScanner(this.registry,
				componentScan.getBoolean("useDefaultFilters"), this.environment, this.resourceLoader);

		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));
		}

		scanner.setResourcePattern(componentScan.getString("resourcePattern"));

		for (AnnotationAttributes filter : componentScan.getAnnotationArray("includeFilters")) {
			for (TypeFilter typeFilter : typeFiltersFor(filter)) {
				scanner.addIncludeFilter(typeFilter);
			}
		}
		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);
		}

		Set<String> basePackages = new LinkedHashSet<>();
		// 2. 尝试查看注解有没添加 basePackages 属性
		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()) {
			// 3. 没有就获取类所在的包名，这里是 com.lagou			
			basePackages.add(ClassUtils.getPackageName(declaringClass));
		}

		scanner.addExcludeFilter(new AbstractTypeHierarchyTraversingFilter(false, false) {
			@Override
			protected boolean matchClassName(String className) {
				return declaringClass.equals(className);
			}
		});
		// 4. 扫描对应的包，把所有相关的 @Component 的类封装成 BeanDefinition 返回
		return scanner.doScan(StringUtils.toStringArray(basePackages));
	}
}

• 其中，componentScan 包含的相关属性如下
  

11. doScan() 先通过 basePackages 属性，找到所有候选 Components，然后把它们注册到 IOC 容器中并返回

public class ClassPathBeanDefinitionScanner extends ClassPathScanningCandidateComponentProvider {
	
	private final BeanDefinitionRegistry registry;
	private BeanNameGenerator beanNameGenerator = AnnotationBeanNameGenerator.INSTANCE;
	private ScopeMetadataResolver scopeMetadataResolver = new AnnotationScopeMetadataResolver();
	...
	protected Set<BeanDefinitionHolder> doScan(String... basePackages) {
		// 1. basePackages 为 com.lagou
		Assert.notEmpty(basePackages, "At least one base package must be specified");
		Set<BeanDefinitionHolder> beanDefinitions = new LinkedHashSet<>();
		for (String basePackage : basePackages) {
			// 2. 根据 basePackage 找到所有候选的 Components
			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);
					// 3. 将候选 Components 注册进 IOC 容器
					registerBeanDefinition(definitionHolder, this.registry);
				}
			}
		}
		// 4. 返回 beanDefinitions
		return beanDefinitions;
	}
}

• 其中，findCandidateComponents() 会调用嵌套方法 scanCandidateComponents()。它会把 basePackage 属性拼接起来，形成 classpath*:com/lagou/**/*.class 路径，扫描所有的类，并返回

public class ClassPathScanningCandidateComponentProvider implements EnvironmentCapable, ResourceLoaderAware {
	
	static final String DEFAULT_RESOURCE_PATTERN = "**/*.class";
	private String resourcePattern = DEFAULT_RESOURCE_PATTERN;
	...

	private Set<BeanDefinition> scanCandidateComponents(String basePackage) {
		Set<BeanDefinition> candidates = new LinkedHashSet<>();
		try {
			// 1. 拼接的路径为 classpath*:com/lagou/**/*.class
			String packageSearchPath = ResourcePatternResolver.CLASSPATH_ALL_URL_PREFIX +
					resolveBasePackage(basePackage) + '/' + this.resourcePattern;
			// 2. 得到已经扫描出来的源
			Resource[] resources = getResourcePatternResolver().getResources(packageSearchPath);
			boolean traceEnabled = logger.isTraceEnabled();
			boolean debugEnabled = logger.isDebugEnabled();
			for (Resource resource : resources) {
				if (traceEnabled) {
					logger.trace("Scanning " + resource);
				}
				if (resource.isReadable()) {
					try {
						MetadataReader metadataReader = getMetadataReaderFactory().getMetadataReader(resource);
						// 先判断 resource 是不是候选的 component
						if (isCandidateComponent(metadataReader)) {
							//  封装成通用 BeanDefinition
							ScannedGenericBeanDefinition sbd = new ScannedGenericBeanDefinition(metadataReader);
							sbd.setSource(resource);
							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 (Throwable ex) {
						throw new BeanDefinitionStoreException(
								"Failed to read candidate component class: " + resource, ex);
					}
				}
				else {
					if (traceEnabled) {
						logger.trace("Ignored because not readable: " + resource);
					}
				}
			}
		}
		catch (IOException ex) {
			throw new BeanDefinitionStoreException("I/O failure during classpath scanning", ex);
		}
		// 3. 返回所有扫描出来的候选 components
		return candidates;
}

• （续）
  • 扫描出来的 resources 属性如下
    
• （续）
  • 最后的候选 components 属性（只有 com.lagou.config.MyDataSourceConfig 与 com.lagou.controller.TestController）如下
    
• 其中，registerBeanDefinition() 会调用 BeanDefinitionReaderUtils 来注册扫描出来的类

public class ClassPathBeanDefinitionScanner extends ClassPathScanningCandidateComponentProvider {
	...
	protected void registerBeanDefinition(BeanDefinitionHolder definitionHolder, BeanDefinitionRegistry registry) {
		// 1. 注册扫描出来的类。这里先注册 com.lagou.config.MyDataSourceConfig，后注册 com.lagou.controller.TestController
		BeanDefinitionReaderUtils.registerBeanDefinition(definitionHolder, registry);
	}
}

• （续）
  • BeanDefinitionReaderUtils 又会通过 BeanDefinitionRegistry （其实就是我们一开始创建的上下文转换强转过来的）调用 registerBeanDefinition() 方法进行注册

public abstract class BeanDefinitionReaderUtils {
	...
	public static void registerBeanDefinition(
			BeanDefinitionHolder definitionHolder, BeanDefinitionRegistry registry)
			throws BeanDefinitionStoreException {

		// Register bean definition under primary name.
		String beanName = definitionHolder.getBeanName();
		// 1. 通过 BeanDefinitionRegistry 注册扫描出来的类，这里是先注册 com.lagou.config.MyDataSourceConfig，后注册 com.lagou.controller.TestController
		registry.registerBeanDefinition(beanName, definitionHolder.getBeanDefinition());

		// Register aliases for bean name, if any.
		String[] aliases = definitionHolder.getAliases();
		if (aliases != null) {
			for (String alias : aliases) {
				registry.registerAlias(beanName, alias);
			}
		}
	}
}

• （续）
  • 最后会把这些扫描出来的类，放到 beanDefinitionMap 属性中

public class DefaultListableBeanFactory extends AbstractAutowireCapableBeanFactory
		implements ConfigurableListableBeanFactory, BeanDefinitionRegistry, Serializable {
	
	/** List of bean definition names, in registration order. */
	private volatile List<String> beanDefinitionNames = new ArrayList<>(256);
	/** Map of bean definition objects, keyed by bean name. */
	private final Map<String, BeanDefinition> beanDefinitionMap = new ConcurrentHashMap<>(256);
	...
	@Override
	public void registerBeanDefinition(String beanName, BeanDefinition beanDefinition)
			throws BeanDefinitionStoreException {

		Assert.hasText(beanName, "Bean name must not be empty");
		Assert.notNull(beanDefinition, "BeanDefinition must not be null");

		if (beanDefinition instanceof AbstractBeanDefinition) {
			try {
				((AbstractBeanDefinition) beanDefinition).validate();
			}
			catch (BeanDefinitionValidationException ex) {
				throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
						"Validation of bean definition failed", ex);
			}
		}
		
		BeanDefinition existingDefinition = this.beanDefinitionMap.get(beanName);
		if (existingDefinition != null) {
			if (!isAllowBeanDefinitionOverriding()) {
				throw new BeanDefinitionOverrideException(beanName, beanDefinition, existingDefinition);
			}
			else if (existingDefinition.getRole() < beanDefinition.getRole()) {
				// e.g. was ROLE_APPLICATION, now overriding with ROLE_SUPPORT or ROLE_INFRASTRUCTURE
				if (logger.isInfoEnabled()) {
					logger.info("Overriding user-defined bean definition for bean '" + beanName +
							"' with a framework-generated bean definition: replacing [" +
							existingDefinition + "] with [" + beanDefinition + "]");
				}
			}
			else if (!beanDefinition.equals(existingDefinition)) {
				if (logger.isDebugEnabled()) {
					logger.debug("Overriding bean definition for bean '" + beanName +
							"' with a different definition: replacing [" + existingDefinition +
							"] with [" + beanDefinition + "]");
				}
			}
			else {
				if (logger.isTraceEnabled()) {
					logger.trace("Overriding bean definition for bean '" + beanName +
							"' with an equivalent definition: replacing [" + existingDefinition +
							"] with [" + beanDefinition + "]");
				}
			}
			this.beanDefinitionMap.put(beanName, beanDefinition);
		}
		else {
			if (hasBeanCreationStarted()) {
				// Cannot modify startup-time collection elements anymore (for stable iteration)
				synchronized (this.beanDefinitionMap) {
					// 1. 把对应的 beanDefinition 注册到 beanDefinitionMap 中
					this.beanDefinitionMap.put(beanName, beanDefinition);
					List<String> updatedDefinitions = new ArrayList<>(this.beanDefinitionNames.size() + 1);
					updatedDefinitions.addAll(this.beanDefinitionNames);
					updatedDefinitions.add(beanName);
					this.beanDefinitionNames = updatedDefinitions;
					removeManualSingletonName(beanName);
				}
			}
			else {
				// Still in startup registration phase
				this.beanDefinitionMap.put(beanName, beanDefinition);
				this.beanDefinitionNames.add(beanName);
				removeManualSingletonName(beanName);
			}
			this.frozenBeanDefinitionNames = null;
		}

		if (existingDefinition != null || containsSingleton(beanName)) {
			resetBeanDefinition(beanName);
		}
		else if (isConfigurationFrozen()) {
			clearByTypeCache();
		}
	}
}

12. 返回完 ComponentScans 注解扫描的类之后，会判断候选类是不是有 @Configuration 标记，有的话，会继续深度调用 processConfigurationClass()，继续扫描所有候选类，注册进 beanDefinitionMap 中

class ConfigurationClassParser {

	private final ComponentScanAnnotationParser componentScanParser;
	private final ConditionEvaluator conditionEvaluator;
	private final Environment environment;
	...
	@Nullable
	protected final SourceClass doProcessConfigurationClass(
			ConfigurationClass configClass, SourceClass sourceClass, Predicate<String> filter)
			throws IOException {
			
		if (configClass.getMetadata().isAnnotated(Component.class.getName())) {
			// Recursively process any member (nested) classes first
			processMemberClasses(configClass, sourceClass, filter);
		}

		// Process any @PropertySource annotations
		for (AnnotationAttributes propertySource : AnnotationConfigUtils.attributesForRepeatable(
				sourceClass.getMetadata(), PropertySources.class,
				org.springframework.context.annotation.PropertySource.class)) {
			if (this.environment instanceof ConfigurableEnvironment) {
				processPropertySource(propertySource);
			}
			else {
				logger.info("Ignoring @PropertySource annotation on [" + sourceClass.getMetadata().getClassName() +
						"]. Reason: Environment must implement ConfigurableEnvironment");
			}
		}

		// 扫描处理所有 @ComponentScan 注解
		Set<AnnotationAttributes> componentScans = AnnotationConfigUtils.attributesForRepeatable(
				sourceClass.getMetadata(), ComponentScans.class, ComponentScan.class);
		if (!componentScans.isEmpty() &&
				!this.conditionEvaluator.shouldSkip(sourceClass.getMetadata(), ConfigurationPhase.REGISTER_BEAN)) {
			for (AnnotationAttributes componentScan : componentScans) {
				// 解析 @ComponentScan 的注解，然后把相关对象
				Set<BeanDefinitionHolder> scannedBeanDefinitions =
						this.componentScanParser.parse(componentScan, sourceClass.getMetadata().getClassName());
				// Check the set of scanned definitions for any further config classes and parse recursively if needed
				for (BeanDefinitionHolder holder : scannedBeanDefinitions) {
					BeanDefinition bdCand = holder.getBeanDefinition().getOriginatingBeanDefinition();
					if (bdCand == null) {
						bdCand = holder.getBeanDefinition();
					}
					// 1. 判断扫描的类是不是用  @Configuration 标记
					if (ConfigurationClassUtils.checkConfigurationClassCandidate(bdCand, this.metadataReaderFactory)) {
						// 2. 是的话继续解析该配置类 
						parse(bdCand.getBeanClassName(), holder.getBeanName());
					}
				}
			}
		}
		...
	}

	protected final void parse(@Nullable String className, String beanName) throws IOException {
		Assert.notNull(className, "No bean class name for configuration class bean definition");
		MetadataReader reader = this.metadataReaderFactory.getMetadataReader(className);
		// 3. 形成递归调用解析
		processConfigurationClass(new ConfigurationClass(reader, beanName), DEFAULT_EXCLUSION_FILTER);
	}
}

13. 直到这里我们就能够明白如何把主类所在的类路径下相关的类通过 @ComponentScan 注册到 IOC 容器中，像相关的 @Import、@ImportResource 注册，也是在 processConfigurationClass() 这个方法里解析，下节再解析
14. 总结