spring-boot源码解析之AnnotationConfigServletWebServerApplicationContext

主要功能

• 接受Configuration和Component标注的class，生成Bean
• package扫描

注意AnnotationConfigServletWebServerApplicationContext只能接受class注册，不能接受xml注册，xml注册是在SpringApplication那里面处理的，SpringApplication负责解析xml并手动调用registerBeanDefinition注册到AnnotationConfigServletWebServerApplicationContext

Factory

	/**
	 * {@link ApplicationContextFactory} registered in {@code spring.factories} to support
	 * {@link AnnotationConfigServletWebServerApplicationContext}.
	 */
	static class Factory implements ApplicationContextFactory {

		@Override
		public ConfigurableApplicationContext create(WebApplicationType webApplicationType) {
			return (webApplicationType != WebApplicationType.SERVLET) ? null
					: new AnnotationConfigServletWebServerApplicationContext();
		}

	}

Factory 会被写入spring.factory，这样AnnotationConfigServletWebServerApplicationContext就会生成。平时我们的web服务都是生成的这个。

Constructor

	/**
	 * Create a new {@link AnnotationConfigServletWebServerApplicationContext} that needs
	 * to be populated through {@link #register} calls and then manually
	 * {@linkplain #refresh refreshed}.
	 */
	public AnnotationConfigServletWebServerApplicationContext() {
		this.reader = new AnnotatedBeanDefinitionReader(this);
		this.scanner = new ClassPathBeanDefinitionScanner(this);
	}

在SpringApplication的分析那里我们已经知道SpringApplication只会注册一个BeanDefinition，那就是我们的mainClass。AnnotationConfigServletWebServerApplicationContext本身也是个BeanDefinitionRegistry，所以可以直接传给AnnotatedBeanDefinitionReader和ClassPathBeanDefinitionScanner。

Additional Component Candidates

	private final AnnotatedBeanDefinitionReader reader;

	private final ClassPathBeanDefinitionScanner scanner;

	private final Set<Class<?>> annotatedClasses = new LinkedHashSet<>();

	private String[] basePackages;
	@Override
	public final void register(Class<?>... annotatedClasses) {
		Assert.notEmpty(annotatedClasses, "At least one annotated class must be specified");
		this.annotatedClasses.addAll(Arrays.asList(annotatedClasses));
	}
		@Override
	public final void scan(String... basePackages) {
		Assert.notEmpty(basePackages, "At least one base package must be specified");
		this.basePackages = basePackages;
	}
	@Override
	protected void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) {
		super.postProcessBeanFactory(beanFactory);
		if (this.basePackages != null && this.basePackages.length > 0) {
			this.scanner.scan(this.basePackages);
		}
		if (!this.annotatedClasses.isEmpty()) {
			this.reader.register(ClassUtils.toClassArray(this.annotatedClasses));
		}
	}

可以看到，AnnotationConfigServletWebServerApplicationContext可以通过register() & scan()添加额外的需要扫描的class和package，但是，根据我们之前的分析， SpringApplication其实只是调用了一个org.springframework.context.support.GenericApplicationContext#registerBeanDefinition (GenericApplicationContext是AnnotationConfigServletWebServerApplicationContext的父父类)。所以annotatedClasses,和basePackages 其实都是空的。也就是没有被用到。但是一旦注册，就会在org.springframework.context.support.AbstractApplicationContext#refresh的postProcessBeanFactory阶段被调用到，随后被invokeBeanFactoryPostProcessors统一处理，比如cgiproxy等等。

Parent Class ServletWebServerApplicationContext

可以看到AnnotationConfigServletWebServerApplicationContext提供的额外的class注册和package扫描在实际使用过程中（SpringApplication）用到，所以我们继续查看它的父类。
主要功能：开启一个webServer并且注册Filter和Servlet。
1， 查找ServletWebServerFactory，并且初始化webserver，通常这个Factory就是ServletWebServerFactoryAutoConfiguration -》ServletWebServerFactoryConfiguration -》EmbeddedTomcat -》TomcatServletWebServerFactory
2， 创建mapping： Any Servlet named ‘dispatcherServlet’ will always be mapped to ‘/’. Filter beans will be mapped to all URLs (‘/*’).
3， 自定义配置：For more advanced configuration, the context can instead define beans that implement the ServletContextInitializer interface (most often ServletRegistrationBeans and/or FilterRegistrationBeans
4， 不建议单独使用：Although this context can be used directly, most developers should consider using the AnnotationConfigServletWebServerApplicationContext or XmlServletWebServerApplicationContext variants. 肯定也不会单独使用，还有Environment抽象这些功能呢，使用SpringApplication好一些。

postProcessBeanFactory

	protected void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) {
		beanFactory.addBeanPostProcessor(new WebApplicationContextServletContextAwareProcessor(this));
		beanFactory.ignoreDependencyInterface(ServletContextAware.class);
		registerWebApplicationScopes();
	}

这时候父类还没有扫描Beans，所以可以添加bean以及对beanFactory做配置。
WebApplicationContextServletContextAwareProcessor： 注入 ServletContext
registerWebApplicationScopes： Register web-specific scopes (“request”, “session”, “globalSession”) with the given BeanFactory

顺便说一下SpringBoot的命名，postProcessBeanFactory意思就是在BeanFactory创建初始化以后马上再process。其实叫做configureBeanFactory不是更好？

创建webserver

	@Override
	public final void refresh() throws BeansException, IllegalStateException {
		try {
			super.refresh();
		}
		catch (RuntimeException ex) {
			WebServer webServer = this.webServer;
			if (webServer != null) {
				webServer.stop();
			}
			throw ex;
		}
	}

	@Override
	protected void onRefresh() {
		super.onRefresh();
		try {
			createWebServer();
		}
		catch (Throwable ex) {
			throw new ApplicationContextException("Unable to start web server", ex);
		}
	}
	private void createWebServer() {
		WebServer webServer = this.webServer;
		ServletContext servletContext = getServletContext();
		if (webServer == null && servletContext == null) {
			StartupStep createWebServer = this.getApplicationStartup().start("spring.boot.webserver.create");
			ServletWebServerFactory factory = getWebServerFactory();
			createWebServer.tag("factory", factory.getClass().toString());
			this.webServer = factory.getWebServer(getSelfInitializer());
			createWebServer.end();
			getBeanFactory().registerSingleton("webServerGracefulShutdown",
					new WebServerGracefulShutdownLifecycle(this.webServer));
			getBeanFactory().registerSingleton("webServerStartStop",
					new WebServerStartStopLifecycle(this, this.webServer));
		}
		else if (servletContext != null) {
			try {
				getSelfInitializer().onStartup(servletContext);
			}
			catch (ServletException ex) {
				throw new ApplicationContextException("Cannot initialize servlet context", ex);
			}
		}
		initPropertySources();
	}
	protected void initPropertySources() {
		ConfigurableEnvironment env = getEnvironment();
		if (env instanceof ConfigurableWebEnvironment) {
			((ConfigurableWebEnvironment) env).initPropertySources(this.servletContext, null);
		}
	}

refresh 啥也没干交给org.springframework.context.support.AbstractApplicationContext#refresh， 转而在refresh的钩子函数onRefresh里面创建webserver，此时beanFactory以及扫描并初始化完成Bean了。
createWebServer逻辑也很简单，就是获取TomcatWebServerFactory，生成一个webserver，注意此时的WebServer还没有Run起来，直到注册的WebServerStartStopLifecycle获取到Start回调，也就是ApplicationContext完成的时候，才会Run起来

initPropertySources：通过之前对SpringApplication的分析知道env 就是ApplicationServletEnvironment， 追踪代码至：

	public static void initServletPropertySources(MutablePropertySources sources,
			@Nullable ServletContext servletContext, @Nullable ServletConfig servletConfig) {

		Assert.notNull(sources, "'propertySources' must not be null");
		String name = StandardServletEnvironment.SERVLET_CONTEXT_PROPERTY_SOURCE_NAME;
		if (servletContext != null && sources.get(name) instanceof StubPropertySource) {
			sources.replace(name, new ServletContextPropertySource(name, servletContext));
		}
		name = StandardServletEnvironment.SERVLET_CONFIG_PROPERTY_SOURCE_NAME;
		if (servletConfig != null && sources.get(name) instanceof StubPropertySource) {
			sources.replace(name, new ServletConfigPropertySource(name, servletConfig));
		}
	}

因为我们根本就没有配置额外的source，所以并不会起作用.

selfInitialize

	private void selfInitialize(ServletContext servletContext) throws ServletException {
		prepareWebApplicationContext(servletContext);
		registerApplicationScope(servletContext);
		WebApplicationContextUtils.registerEnvironmentBeans(getBeanFactory(), servletContext);
		for (ServletContextInitializer beans : getServletContextInitializerBeans()) {
			beans.onStartup(servletContext);
		}
	}
	protected void prepareWebApplicationContext(ServletContext servletContext) {
		Object rootContext = servletContext.getAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE);
		if (rootContext != null) {
			if (rootContext == this) {
				throw new IllegalStateException(
						"Cannot initialize context because there is already a root application context present - "
								+ "check whether you have multiple ServletContextInitializers!");
			}
			return;
		}
		servletContext.log("Initializing Spring embedded WebApplicationContext");
		try {
			servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, this);
			if (logger.isDebugEnabled()) {
				logger.debug("Published root WebApplicationContext as ServletContext attribute with name ["
						+ WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE + "]");
			}
			setServletContext(servletContext);
			if (logger.isInfoEnabled()) {
				long elapsedTime = System.currentTimeMillis() - getStartupDate();
				logger.info("Root WebApplicationContext: initialization completed in " + elapsedTime + " ms");
			}
		}
		catch (RuntimeException | Error ex) {
			logger.error("Context initialization failed", ex);
			servletContext.setAttribute(WebApplicationContext.ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE, ex);
			throw ex;
		}
	}

prepareWebApplicationContext: 其实就是把Tomcat生成的ServletContext设置到自己的servletContext里面，吧ApplicationContext设置到ServletContext的ROOT_WEB_APPLICATION_CONTEXT_ATTRIBUTE属性上
registerApplicationScope: 增加Application类型的scope
registerEnvironmentBeans：就是注册servletContext， servletConfig等bean，说明这些bean不能autowire，因为这是在其他bean都注册完成之后再注册的了，只能通过ServletContextAware接口获得。

getServletContextInitializerBeans： 获取所有ServletContextInitializer 的bean并且调用。

ServletContextInitializerBeans

在上一步有一个很重要的步骤，就是获取ServletContextInitializerBeans并且初始化。就是在这里获取了所有的Servlet（包括DispatchServlet）和Filter并且注册到servletContext上面。

org.springframework.boot.autoconfigure.web.servlet.DispatcherServletAutoConfiguration.DispatcherServletRegistrationConfiguration --> org.springframework.boot.autoconfigure.web.servlet.DispatcherServletRegistrationBean --> org.springframework.boot.web.servlet.DynamicRegistrationBean --> ServletContextInitializer

说明DispatcherServletRegistrationBean 就是一个ServletContextInitializer， 它注册了DispatchServlet到“/”这个mapping。

Ancestor Class GenericWebApplicationContext

在GenericApplicationContext的基础上增加web服务的一些通用功能。主要是增加了servletContext及其相关操作。还有themeSource（没用到），大部分method要么被ServletWebServerApplicationContext覆盖，要么被ServletWebServerApplicationContext调用。
被覆盖：postProcessBeanFactory，initPropertySources
被调用：onRefresh，创建themeSource

Ancestor Class GenericApplicationContext

GenericApplicationContext是第一个可以实例化的的继承AbstractApplicationContext的类。
主要是提供了默认的BeanFactory实现：DefaultListableBeanFactory
以及加载resource的实现：getResource。 GenericApplicationContext的resourceLoader总是null,所以总是会逐级调用到org.springframework.core.io.DefaultResourceLoader#getResource
GenericWebApplicationContext只有一个BeanFactory所以只能refresh一次。这好像不是个什么缺陷？

Ancestor Class AbstractApplicationContext

所有的具体ApplicationContext都是AbstractApplicationContext的子类，相当于所有ApplicationContext的模板父类。

• 实现了refresh的逻辑，子类可以实现onFresh等回调函数
• 自动检测并处理 BeanFactoryPostProcessors, BeanPostProcessors, and ApplicationListeners
• 实现SimpleApplicationEventMulticaster for事件发布
• getResourceByPath默认使用classpath

重要的内置bean（不存在就创建）：messageSource， lifecycleProcessor，applicationEventMulticaster

重要变量：

private final List<BeanFactoryPostProcessor> beanFactoryPostProcessors = new ArrayList<>();
	@Override
	public void addBeanFactoryPostProcessor(BeanFactoryPostProcessor postProcessor) {
		Assert.notNull(postProcessor, "BeanFactoryPostProcessor must not be null");
		this.beanFactoryPostProcessors.add(postProcessor);
	}

beanFactoryPostProcessors 并没有明显的的被调用，但是org.springframework.boot.SpringApplication#applyInitializers会读取spring.factories里面的ApplicationContextInitializer，比如

# Application Context Initializers
org.springframework.context.ApplicationContextInitializer=\
org.springframework.boot.context.ConfigurationWarningsApplicationContextInitializer,\
org.springframework.boot.context.ContextIdApplicationContextInitializer,\
org.springframework.boot.context.config.DelegatingApplicationContextInitializer,\
org.springframework.boot.rsocket.context.RSocketPortInfoApplicationContextInitializer,\
org.springframework.boot.web.context.ServerPortInfoApplicationContextInitializer

# Initializers
org.springframework.context.ApplicationContextInitializer=\
org.springframework.boot.autoconfigure.SharedMetadataReaderFactoryContextInitializer,\
org.springframework.boot.autoconfigure.logging.ConditionEvaluationReportLoggingListener

DelegatingApplicationContextInitializer: 他会读取环境变量中context.initializer.classes的内容，然后初始化并调用对应的initializer。
SharedMetadataReaderFactoryContextInitializer： 他会调用applicationContext.addBeanFactoryPostProcessor(postProcessor);
我们可以SpringApplication.addInitializers添加自己的的initializer，也就可以添加addBeanFactoryPostProcessor。

Refresh

	public void refresh() throws BeansException, IllegalStateException {
		synchronized (this.startupShutdownMonitor) {
			StartupStep contextRefresh = this.applicationStartup.start("spring.context.refresh");

			// Prepare this context for refreshing.
			prepareRefresh();

			// Tell the subclass to refresh the internal bean factory.
			ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

			// Prepare the bean factory for use in this context.
			prepareBeanFactory(beanFactory);

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

				StartupStep beanPostProcess = this.applicationStartup.start("spring.context.beans.post-process");
				// Invoke factory processors registered as beans in the context.
				invokeBeanFactoryPostProcessors(beanFactory);

				// Register bean processors that intercept bean creation.
				registerBeanPostProcessors(beanFactory);
				beanPostProcess.end();

				// Initialize message source for this context.
				initMessageSource();

				// Initialize event multicaster for this context.
				initApplicationEventMulticaster();

				// Initialize other special beans in specific context subclasses.
				onRefresh();

				// Check for listener beans and register them.
				registerListeners();

				// Instantiate all remaining (non-lazy-init) singletons.
				finishBeanFactoryInitialization(beanFactory);

				// Last step: publish corresponding event.
				finishRefresh();
			}

			catch (BeansException ex) {
				if (logger.isWarnEnabled()) {
					logger.warn("Exception encountered during context initialization - " +
							"cancelling refresh attempt: " + ex);
				}

				// Destroy already created singletons to avoid dangling resources.
				destroyBeans();

				// Reset 'active' flag.
				cancelRefresh(ex);

				// Propagate exception to caller.
				throw ex;
			}

			finally {
				// Reset common introspection caches in Spring's core, since we
				// might not ever need metadata for singleton beans anymore...
				resetCommonCaches();
				contextRefresh.end();
			}
		}
	}

prepareRefresh:只是设置isActive等几个变量，暂时看不出作用
obtainFreshBeanFactory： 得到DefaultListableBeanFactory，这是在GenericApplicationContext定义并且重载了getBeanFactory

prepareBeanFactory:

	protected void prepareBeanFactory(ConfigurableListableBeanFactory beanFactory) {
		// Tell the internal bean factory to use the context's class loader etc.
		beanFactory.setBeanClassLoader(getClassLoader());
		if (!shouldIgnoreSpel) {
			beanFactory.setBeanExpressionResolver(new StandardBeanExpressionResolver(beanFactory.getBeanClassLoader()));
		}
		beanFactory.addPropertyEditorRegistrar(new ResourceEditorRegistrar(this, getEnvironment()));

		// Configure the bean factory with context callbacks.
		beanFactory.addBeanPostProcessor(new ApplicationContextAwareProcessor(this));
		beanFactory.ignoreDependencyInterface(EnvironmentAware.class);
		beanFactory.ignoreDependencyInterface(EmbeddedValueResolverAware.class);
		beanFactory.ignoreDependencyInterface(ResourceLoaderAware.class);
		beanFactory.ignoreDependencyInterface(ApplicationEventPublisherAware.class);
		beanFactory.ignoreDependencyInterface(MessageSourceAware.class);
		beanFactory.ignoreDependencyInterface(ApplicationContextAware.class);
		beanFactory.ignoreDependencyInterface(ApplicationStartupAware.class);

		// BeanFactory interface not registered as resolvable type in a plain factory.
		// MessageSource registered (and found for autowiring) as a bean.
		beanFactory.registerResolvableDependency(BeanFactory.class, beanFactory);
		beanFactory.registerResolvableDependency(ResourceLoader.class, this);
		beanFactory.registerResolvableDependency(ApplicationEventPublisher.class, this);
		beanFactory.registerResolvableDependency(ApplicationContext.class, this);

		// Register early post-processor for detecting inner beans as ApplicationListeners.
		beanFactory.addBeanPostProcessor(new ApplicationListenerDetector(this));

		// Detect a LoadTimeWeaver and prepare for weaving, if found.
		if (!NativeDetector.inNativeImage() && beanFactory.containsBean(LOAD_TIME_WEAVER_BEAN_NAME)) {
			beanFactory.addBeanPostProcessor(new LoadTimeWeaverAwareProcessor(beanFactory));
			// Set a temporary ClassLoader for type matching.
			beanFactory.setTempClassLoader(new ContextTypeMatchClassLoader(beanFactory.getBeanClassLoader()));
		}

		// Register default environment beans.
		if (!beanFactory.containsLocalBean(ENVIRONMENT_BEAN_NAME)) {
			beanFactory.registerSingleton(ENVIRONMENT_BEAN_NAME, getEnvironment());
		}
		if (!beanFactory.containsLocalBean(SYSTEM_PROPERTIES_BEAN_NAME)) {
			beanFactory.registerSingleton(SYSTEM_PROPERTIES_BEAN_NAME, getEnvironment().getSystemProperties());
		}
		if (!beanFactory.containsLocalBean(SYSTEM_ENVIRONMENT_BEAN_NAME)) {
			beanFactory.registerSingleton(SYSTEM_ENVIRONMENT_BEAN_NAME, getEnvironment().getSystemEnvironment());
		}
		if (!beanFactory.containsLocalBean(APPLICATION_STARTUP_BEAN_NAME)) {
			beanFactory.registerSingleton(APPLICATION_STARTUP_BEAN_NAME, getApplicationStartup());
		}
	}

1, 配置众多aware接口
2， BeanFactory自动注入beanFactory， ResourceLoader，ApplicationEventPublisher，ApplicationContext其实都是注入当前的ApplicationContext，也就是AnnotationConfigServletWebServerApplicationContext实例（this）
3，注册ENVIRONMENT_BEAN_NAME，SYSTEM_PROPERTIES_BEAN_NAME， SYSTEM_ENVIRONMENT_BEAN_NAME， APPLICATION_STARTUP_BEAN_NAME四个bean
注意registerResolvableDependency和registerSingleton的区别，registerSingleton是会注入到beanFactory的，可以通过依赖查找获得，但是registerResolvableDependency不注入beanFactory，只能通过依赖注入获得。

postProcessBeanFactory：
空函数，留给子类实现，比如AnnotationConfigServletWebServerApplicationContext就实现了

	@Override
	protected void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) {
		super.postProcessBeanFactory(beanFactory);
		if (this.basePackages != null && this.basePackages.length > 0) {
			this.scanner.scan(this.basePackages);
		}
		if (!this.annotatedClasses.isEmpty()) {
			this.reader.register(ClassUtils.toClassArray(this.annotatedClasses));
		}
	}

invokeBeanFactoryPostProcessors:调用所有的BeanFactoryPostProcessor以及子类BeanDefinitionRegistryPostProcessor。
registerBeanPostProcessors：与invokeBeanFactoryPostProcessors唯一不同的是，registerBeanPostProcessors从beanFactory里面获取String[] postProcessorNames = beanFactory.getBeanNamesForType(BeanPostProcessor.class, true, false);并且交给beanFactory去处理，而不是在这里处理。
registerListeners： 从beanFactory里面获取String[] listenerBeanNames = getBeanNamesForType(ApplicationListener.class, true, false);

finishBeanFactoryInitialization： 缓存所有配置文件包括bean的定义，这时候就不能增加或者修改bean了，初始化所有的singleton实例

ConfigurationClassPostProcessor

在invokeBeanFactoryPostProcessors的时候会调用一个非常重要的processor: ConfigurationClassPostProcessor.他会读取我们的mainClass，扫描对应的package并且生成合适的bean。这就是@SpringApplication被处理的地方。

ConfigurationClassPostProcessor是何时被注册上去的

AnnotationConfigServletWebServerApplicationContext#constructor --> org.springframework.context.annotation.AnnotatedBeanDefinitionReader#AnnotatedBeanDefinitionReader(org.springframework.beans.factory.support.BeanDefinitionRegistry) -->
org.springframework.context.annotation.AnnotationConfigUtils#registerAnnotationConfigProcessors(org.springframework.beans.factory.support.BeanDefinitionRegistry)

	public static Set<BeanDefinitionHolder> registerAnnotationConfigProcessors(
			BeanDefinitionRegistry registry, @Nullable Object source) {

		DefaultListableBeanFactory beanFactory = unwrapDefaultListableBeanFactory(registry);
		if (beanFactory != null) {
			if (!(beanFactory.getDependencyComparator() instanceof AnnotationAwareOrderComparator)) {
				beanFactory.setDependencyComparator(AnnotationAwareOrderComparator.INSTANCE);
			}
			if (!(beanFactory.getAutowireCandidateResolver() instanceof ContextAnnotationAutowireCandidateResolver)) {
				beanFactory.setAutowireCandidateResolver(new ContextAnnotationAutowireCandidateResolver());
			}
		}

		Set<BeanDefinitionHolder> beanDefs = new LinkedHashSet<>(8);

		if (!registry.containsBeanDefinition(CONFIGURATION_ANNOTATION_PROCESSOR_BEAN_NAME)) {
			RootBeanDefinition def = new RootBeanDefinition(ConfigurationClassPostProcessor.class);
			def.setSource(source);
			beanDefs.add(registerPostProcessor(registry, def, CONFIGURATION_ANNOTATION_PROCESSOR_BEAN_NAME));
		}

		if (!registry.containsBeanDefinition(AUTOWIRED_ANNOTATION_PROCESSOR_BEAN_NAME)) {
			RootBeanDefinition def = new RootBeanDefinition(AutowiredAnnotationBeanPostProcessor.class);
			def.setSource(source);
			beanDefs.add(registerPostProcessor(registry, def, AUTOWIRED_ANNOTATION_PROCESSOR_BEAN_NAME));
		}

		// Check for JSR-250 support, and if present add the CommonAnnotationBeanPostProcessor.
		if (jsr250Present && !registry.containsBeanDefinition(COMMON_ANNOTATION_PROCESSOR_BEAN_NAME)) {
			RootBeanDefinition def = new RootBeanDefinition(CommonAnnotationBeanPostProcessor.class);
			def.setSource(source);
			beanDefs.add(registerPostProcessor(registry, def, COMMON_ANNOTATION_PROCESSOR_BEAN_NAME));
		}

		// Check for JPA support, and if present add the PersistenceAnnotationBeanPostProcessor.
		if (jpaPresent && !registry.containsBeanDefinition(PERSISTENCE_ANNOTATION_PROCESSOR_BEAN_NAME)) {
			RootBeanDefinition def = new RootBeanDefinition();
			try {
				def.setBeanClass(ClassUtils.forName(PERSISTENCE_ANNOTATION_PROCESSOR_CLASS_NAME,
						AnnotationConfigUtils.class.getClassLoader()));
			}
			catch (ClassNotFoundException ex) {
				throw new IllegalStateException(
						"Cannot load optional framework class: " + PERSISTENCE_ANNOTATION_PROCESSOR_CLASS_NAME, ex);
			}
			def.setSource(source);
			beanDefs.add(registerPostProcessor(registry, def, PERSISTENCE_ANNOTATION_PROCESSOR_BEAN_NAME));
		}

		if (!registry.containsBeanDefinition(EVENT_LISTENER_PROCESSOR_BEAN_NAME)) {
			RootBeanDefinition def = new RootBeanDefinition(EventListenerMethodProcessor.class);
			def.setSource(source);
			beanDefs.add(registerPostProcessor(registry, def, EVENT_LISTENER_PROCESSOR_BEAN_NAME));
		}

		if (!registry.containsBeanDefinition(EVENT_LISTENER_FACTORY_BEAN_NAME)) {
			RootBeanDefinition def = new RootBeanDefinition(DefaultEventListenerFactory.class);
			def.setSource(source);
			beanDefs.add(registerPostProcessor(registry, def, EVENT_LISTENER_FACTORY_BEAN_NAME));
		}

		return beanDefs;
	}

CONFIGURATION_ANNOTATION_PROCESSOR_BEAN_NAME 就是 ‘org.springframework.context.annotation.internalRequiredAnnotationProcessor’
实例就是org.springframework.context.annotation.ConfigurationClassPostProcessor。

postProcessBeanDefinitionRegistry

	@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);

		processConfigBeanDefinitions(registry);
	}
	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);
				}
			}
			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();
		}

		// Parse each @Configuration class
		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 {
			StartupStep processConfig = this.applicationStartup.start("spring.context.config-classes.parse");
			parser.parse(candidates);
			parser.validate();

			Set<ConfigurationClass> configClasses = new LinkedHashSet<>(parser.getConfigurationClasses());
			configClasses.removeAll(alreadyParsed);

			// Read the model and create bean definitions based on its content
			if (this.reader == null) {
				this.reader = new ConfigurationClassBeanDefinitionReader(
						registry, this.sourceExtractor, this.resourceLoader, this.environment,
						this.importBeanNameGenerator, parser.getImportRegistry());
			}
			this.reader.loadBeanDefinitions(configClasses);
			alreadyParsed.addAll(configClasses);
			processConfig.tag("classCount", () -> String.valueOf(configClasses.size())).end();

			candidates.clear();
			if (registry.getBeanDefinitionCount() > candidateNames.length) {
				String[] newCandidateNames = registry.getBeanDefinitionNames();
				Set<String> oldCandidateNames = new HashSet<>(Arrays.asList(candidateNames));
				Set<String> alreadyParsedClasses = new HashSet<>();
				for (ConfigurationClass configurationClass : alreadyParsed) {
					alreadyParsedClasses.add(configurationClass.getMetadata().getClassName());
				}
				for (String candidateName : newCandidateNames) {
					if (!oldCandidateNames.contains(candidateName)) {
						BeanDefinition bd = registry.getBeanDefinition(candidateName);
						if (ConfigurationClassUtils.checkConfigurationClassCandidate(bd, this.metadataReaderFactory) &&
								!alreadyParsedClasses.contains(bd.getBeanClassName())) {
							candidates.add(new BeanDefinitionHolder(bd, candidateName));
						}
					}
				}
				candidateNames = newCandidateNames;
			}
		}
		while (!candidates.isEmpty());

		// Register the ImportRegistry as a bean in order to support ImportAware @Configuration classes
		if (sbr != null && !sbr.containsSingleton(IMPORT_REGISTRY_BEAN_NAME)) {
			sbr.registerSingleton(IMPORT_REGISTRY_BEAN_NAME, parser.getImportRegistry());
		}

		if (this.metadataReaderFactory instanceof CachingMetadataReaderFactory) {
			// Clear cache in externally provided MetadataReaderFactory; this is a no-op
			// for a shared cache since it'll be cleared by the ApplicationContext.
			((CachingMetadataReaderFactory) this.metadataReaderFactory).clearCache();
		}
	}

registry.getBeanDefinitionNames() 和registry.getBeanNamesForType(Object.class)返回结果不同的，只有调用过registry.registerBeanDefinition的才会在getBeanDefinitionNames()返回，通过registerSingleton的不会返回。
然后筛选出所有ConfigurationClass，在这里只有我们的mainClass demoApplication返回

parser.parse： 读出当前环境的所有Configuration class，会自动处理componentScan与及Import指令， 包括用户自定义的以及各种AutoConfiguration，注意现在还只是读出了CondifurationClass还没有读到各种Component class以及@Bean method生成的component

postProcessBeanFactory

	@Override
	public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) {
		int factoryId = System.identityHashCode(beanFactory);
		if (this.factoriesPostProcessed.contains(factoryId)) {
			throw new IllegalStateException(
					"postProcessBeanFactory already called on this post-processor against " + beanFactory);
		}
		this.factoriesPostProcessed.add(factoryId);
		if (!this.registriesPostProcessed.contains(factoryId)) {
			// BeanDefinitionRegistryPostProcessor hook apparently not supported...
			// Simply call processConfigurationClasses lazily at this point then.
			processConfigBeanDefinitions((BeanDefinitionRegistry) beanFactory);
		}

		enhanceConfigurationClasses(beanFactory);
		beanFactory.addBeanPostProcessor(new ImportAwareBeanPostProcessor(beanFactory));
	}

1, 给Configuration Class 加proxy，使所有的@Bean method返回同一个singleton
2，增加ImportAwareBeanPostProcessor， 使被引用的Consiguration可以获取引用着的信息。

DeferredImportSelectorHandler

在org.springframework.context.annotation.ConfigurationClassParser#parse(java.util.Set)

	public void parse(Set<BeanDefinitionHolder> configCandidates) {
		for (BeanDefinitionHolder holder : configCandidates) {
			BeanDefinition bd = holder.getBeanDefinition();
			try {
				if (bd instanceof AnnotatedBeanDefinition) {
					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();
	}

可以看到deferredImportSelectorHandler处理了importSelector

	private class DeferredImportSelectorHandler {

		@Nullable
		private List<DeferredImportSelectorHolder> deferredImportSelectors = new ArrayList<>();

		/**
		 * Handle the specified {@link DeferredImportSelector}. If deferred import
		 * selectors are being collected, this registers this instance to the list. If
		 * they are being processed, the {@link DeferredImportSelector} is also processed
		 * immediately according to its {@link DeferredImportSelector.Group}.
		 * @param configClass the source configuration class
		 * @param importSelector the selector to handle
		 */
		public void handle(ConfigurationClass configClass, DeferredImportSelector importSelector) {
			DeferredImportSelectorHolder holder = new DeferredImportSelectorHolder(configClass, importSelector);
			if (this.deferredImportSelectors == null) {
				DeferredImportSelectorGroupingHandler handler = new DeferredImportSelectorGroupingHandler();
				handler.register(holder);
				handler.processGroupImports();
			}
			else {
				this.deferredImportSelectors.add(holder);
			}
		}

		public void process() {
			List<DeferredImportSelectorHolder> deferredImports = this.deferredImportSelectors;
			this.deferredImportSelectors = null;
			try {
				if (deferredImports != null) {
					DeferredImportSelectorGroupingHandler handler = new DeferredImportSelectorGroupingHandler();
					deferredImports.sort(DEFERRED_IMPORT_COMPARATOR);
					deferredImports.forEach(handler::register);
					handler.processGroupImports();
				}
			}
			finally {
				this.deferredImportSelectors = new ArrayList<>();
			}
		}
	}

这就是EnableAutoConfiguration的原理

org.springframework.boot.autoconfigure.EnableAutoConfiguration --》
org.springframework.context.annotation.ConfigurationClassParser.DeferredImportSelectorHandler --》
org.springframework.boot.autoconfigure.AutoConfigurationImportSelector --》

		List<String> configurations = new ArrayList<>(
				SpringFactoriesLoader.loadFactoryNames(getSpringFactoriesLoaderFactoryClass(), getBeanClassLoader()));
	protected Class<?> getSpringFactoriesLoaderFactoryClass() {
		return EnableAutoConfiguration.class;
	}