Spring源码系列-第5章-容器刷新流程

第5章-容器刷新流程

流程图-容器刷新

容器创建

public class AnnotationMainTest {

   public static void main(String[] args) {

      ApplicationContext applicationContext =
            new AnnotationConfigApplicationContext(MainConfig.class);
   }

AnnotationConfigApplicationContext有参构造

public AnnotationConfigApplicationContext(Class<?>... componentClasses) {
   this();
   register(componentClasses);
   refresh(); //容器完整刷新（创建出所有组件，组织好所有功能）
}	

AnnotationConfigApplicationContext#this()无参构造

// 上面的this()就是调用的这里
public AnnotationConfigApplicationContext() {
    StartupStep createAnnotatedBeanDefReader = this.getApplicationStartup().start("spring.context.annotated-bean-reader.create");
    this.reader = new AnnotatedBeanDefinitionReader(this);
    createAnnotatedBeanDefReader.end();
    this.scanner = new ClassPathBeanDefinitionScanner(this);
}

AnnotatedBeanDefinitionReader有参构造

   public AnnotatedBeanDefinitionReader(BeanDefinitionRegistry registry) {
      this(registry, getOrCreateEnvironment(registry));
   }

public AnnotatedBeanDefinitionReader(BeanDefinitionRegistry registry, Environment environment) {
	Assert.notNull(registry, "BeanDefinitionRegistry must not be null");
	Assert.notNull(environment, "Environment must not be null");
	this.registry = registry;
	this.conditionEvaluator = new ConditionEvaluator(registry, environment, null);
	AnnotationConfigUtils.registerAnnotationConfigProcessors(this.registry);
}

AnnotationConfigUtils#registerAnnotationConfigProcessors()注册Spring内部几个核心组件

	/**
 * The bean name of the internally managed Configuration annotation processor.
 */
public static final String CONFIGURATION_ANNOTATION_PROCESSOR_BEAN_NAME =
		"org.springframework.context.annotation.internalConfigurationAnnotationProcessor";

/**
 * The bean name of the internally managed BeanNameGenerator for use when processing
 * {@link Configuration} classes. Set by {@link AnnotationConfigApplicationContext}
 * and {@code AnnotationConfigWebApplicationContext} during bootstrap in order to make
 * any custom name generation strategy available to the underlying
 * {@link ConfigurationClassPostProcessor}.
 * @since 3.1.1
 */
public static final String CONFIGURATION_BEAN_NAME_GENERATOR =
		"org.springframework.context.annotation.internalConfigurationBeanNameGenerator";

/**
 * The bean name of the internally managed Autowired annotation processor.
 */
public static final String AUTOWIRED_ANNOTATION_PROCESSOR_BEAN_NAME =
		"org.springframework.context.annotation.internalAutowiredAnnotationProcessor";

/**
 * The bean name of the internally managed Required annotation processor.
 * @deprecated as of 5.1, since no Required processor is registered by default anymore
 */
@Deprecated
public static final String REQUIRED_ANNOTATION_PROCESSOR_BEAN_NAME =
		"org.springframework.context.annotation.internalRequiredAnnotationProcessor";

/**
 * The bean name of the internally managed JSR-250 annotation processor.
 */
public static final String COMMON_ANNOTATION_PROCESSOR_BEAN_NAME =
		"org.springframework.context.annotation.internalCommonAnnotationProcessor";

/**
 * The bean name of the internally managed JPA annotation processor.
 */
public static final String PERSISTENCE_ANNOTATION_PROCESSOR_BEAN_NAME =
		"org.springframework.context.annotation.internalPersistenceAnnotationProcessor";

private static final String PERSISTENCE_ANNOTATION_PROCESSOR_CLASS_NAME =
		"org.springframework.orm.jpa.support.PersistenceAnnotationBeanPostProcessor";

/**
 * The bean name of the internally managed @EventListener annotation processor.
 */
public static final String EVENT_LISTENER_PROCESSOR_BEAN_NAME =
		"org.springframework.context.event.internalEventListenerProcessor";

/**
 * The bean name of the internally managed EventListenerFactory.
 */
public static final String EVENT_LISTENER_FACTORY_BEAN_NAME =
		"org.springframework.context.event.internalEventListenerFactory";


public static void registerAnnotationConfigProcessors(BeanDefinitionRegistry registry) {
      registerAnnotationConfigProcessors(registry, null);
   }



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());
		}
	}
	
       //下面是注册Spring底层的核心组件，主要是就是Spring自己的BeanPostproPostProcessor或者BeanFactoryPostProcessor
	Set<BeanDefinitionHolder> beanDefs = new LinkedHashSet<>(8);
	//注册底层的  配置文件处理器
	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));
	}
       //注册底层的自动装配处理器
	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));
	}
       //注册支持JSR-250的处理
	// 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;
}

上面的方法走完，我们可以看看到主要是下面4个后置处理器

RootBeanDefinition

public RootBeanDefinition(@Nullable Class<?> beanClass) {
   super();
   setBeanClass(beanClass);
}

AbstractBeanDefinition

protected AbstractBeanDefinition() {
   this(null, null);
}

ClassPathBeanDefinitionScanner

public ClassPathBeanDefinitionScanner(BeanDefinitionRegistry registry, boolean useDefaultFilters,
      Environment environment, @Nullable ResourceLoader resourceLoader) {

   Assert.notNull(registry, "BeanDefinitionRegistry must not be null");
   this.registry = registry;

   if (useDefaultFilters) {
      registerDefaultFilters();
   }
   setEnvironment(environment); //准备环境
   setResourceLoader(resourceLoader);//准备资源加载器
}

AnnotationConfigApplicationContext#register()

public void register(Class<?>... componentClasses) {
   //componentClasses就是咱们的配置类
   Assert.notEmpty(componentClasses, "At least one component class must be specified");
   StartupStep registerComponentClass = this.getApplicationStartup().start("spring.context.component-classes.register")
         .tag("classes", () -> Arrays.toString(componentClasses));
   this.reader.register(componentClasses);
   registerComponentClass.end();
}

AnnotatedBeanDefinitionReader#register()注册主配置类BeanDefinition

   public void register(Class<?>... componentClasses) {
      for (Class<?> componentClass : componentClasses) {
         registerBean(componentClass);
      }
   }

public void registerBean(Class<?> beanClass) {
	doRegisterBean(beanClass, null, null, null, null);
}


private <T> void doRegisterBean(Class<T> beanClass, @Nullable String name,
		@Nullable Class<? extends Annotation>[] qualifiers, @Nullable Supplier<T> supplier,
		@Nullable BeanDefinitionCustomizer[] customizers) {
	
       // 主配置类也被封装成一个Bean定义信息了
	AnnotatedGenericBeanDefinition abd = new AnnotatedGenericBeanDefinition(beanClass);
	if (this.conditionEvaluator.shouldSkip(abd.getMetadata())) {
		return;
	}

	abd.setInstanceSupplier(supplier);
	ScopeMetadata scopeMetadata = this.scopeMetadataResolver.resolveScopeMetadata(abd);
	abd.setScope(scopeMetadata.getScopeName());
	String beanName = (name != null ? name : this.beanNameGenerator.generateBeanName(abd, this.registry));
	//完善主配置类的BeanDefinition
	AnnotationConfigUtils.processCommonDefinitionAnnotations(abd);
	if (qualifiers != null) {
		for (Class<? extends Annotation> qualifier : qualifiers) {
			if (Primary.class == qualifier) {
				abd.setPrimary(true);
			}
			else if (Lazy.class == qualifier) {
				abd.setLazyInit(true);
			}
			else {
				abd.addQualifier(new AutowireCandidateQualifier(qualifier));
			}
		}
	}
	if (customizers != null) {
		for (BeanDefinitionCustomizer customizer : customizers) {
			customizer.customize(abd);
		}
	}

	BeanDefinitionHolder definitionHolder = new BeanDefinitionHolder(abd, beanName);
	definitionHolder = AnnotationConfigUtils.applyScopedProxyMode(scopeMetadata, definitionHolder, this.registry);
	BeanDefinitionReaderUtils.registerBeanDefinition(definitionHolder, this.registry);
}

AnnotationConfigUtils#processCommonDefinitionAnnotations()感知@Lazy，@Primary等注解

这里传进来的是主配置类，解析的就是主配置类的注解，其它的Bean这里还没有解析@Lazy，@Primary这些注解

public static void processCommonDefinitionAnnotations(AnnotatedBeanDefinition abd) {
   processCommonDefinitionAnnotations(abd, abd.getMetadata());
}
//解读所有Bean定义信息需要感知的注解
static void processCommonDefinitionAnnotations(AnnotatedBeanDefinition abd, AnnotatedTypeMetadata metadata) {
   AnnotationAttributes lazy = attributesFor(metadata, Lazy.class);
   if (lazy != null) {
      abd.setLazyInit(lazy.getBoolean("value"));
   }
   else if (abd.getMetadata() != metadata) {
      lazy = attributesFor(abd.getMetadata(), Lazy.class);
      if (lazy != null) {
         abd.setLazyInit(lazy.getBoolean("value"));
      }
   }

   if (metadata.isAnnotated(Primary.class.getName())) {
      abd.setPrimary(true);
   }
   AnnotationAttributes dependsOn = attributesFor(metadata, DependsOn.class);
   if (dependsOn != null) {
      abd.setDependsOn(dependsOn.getStringArray("value"));
   }

   AnnotationAttributes role = attributesFor(metadata, Role.class);
   if (role != null) {
      abd.setRole(role.getNumber("value").intValue());
   }
   AnnotationAttributes description = attributesFor(metadata, Description.class);
   if (description != null) {
      abd.setDescription(description.getString("value"));
   }
}

走完之后，注册中心肯定多了咱们的配置类

1. 接下来就是想办法让主配置类里的配置生效
2. 目前Spring就靠上面的4个后置处理器和一个主配置类白手起家了，开始我们后面最重要的refresh()容器刷新流程了

AbstractApplicationContext#refresh()开始容器刷新

@Override  //容器刷新的十二大步。模板模式
public void refresh() throws BeansException, IllegalStateException {
   synchronized (this.startupShutdownMonitor) {
       //容器启动过程状态的封装类
      StartupStep contextRefresh = this.applicationStartup.start("spring.context.refresh");

      //准备上下文环境 Prepare this context for refreshing.
      prepareRefresh();

      // Tell the subclass to refresh the internal bean factory.
      // 工厂创建：BeanFactory第一次开始创建的时候,有xml解析逻辑。注解版这一步什么都没做，直接返回this()环节早就new好的工厂
      ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

      //给容器中注册了环境信息作为单实例Bean方便后续自动装配；放了一些后置处理器处理（监听、xxAware功能） Prepare thebean factory for use in this context.
      prepareBeanFactory(beanFactory);

      try {
         //留给子类的模板方法，允许子类继续对工厂执行一些处理； Allows post-processing of the bean factory in context subclasses.
         postProcessBeanFactory(beanFactory);

         StartupStep beanPostProcess = this.applicationStartup.start("spring.context.beans.post-process");
         //【核心】工厂增强：执行所有的BeanFactory后置增强器；利用BeanFactory后置增强器对工厂进行修改或者增强,配置类会在这里进行解析。 Invoke factory processors registered as beans in the context.
         invokeBeanFactoryPostProcessors(beanFactory);

         //【核心】注册所有的Bean的后置处理器 Register bean processors that intercept bean creation.
         registerBeanPostProcessors(beanFactory);
         beanPostProcess.end();

         //初始化国际化功能 Initialize message source for this context.
         initMessageSource();

         //初始化事件多播功能（事件派发） Initialize event multicaster for this context.
         initApplicationEventMulticaster();

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

         //注册监听器，从容器中获取所有的ApplicationListener； Check for listener beans and register them.
         registerListeners();

         // Instantiate all remaining (non-lazy-init) singletons.
         //【大核心】bean创建；完成 BeanFactory 初始化。（工厂里面所有的组件都好了）
         finishBeanFactoryInitialization(beanFactory);

         //发布事件 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()准备上下文环境

protected void prepareRefresh() {
   // Switch to active.
   this.startupDate = System.currentTimeMillis();
   this.closed.set(false);
   this.active.set(true);

   if (logger.isDebugEnabled()) {
      if (logger.isTraceEnabled()) {
         logger.trace("Refreshing " + this);
      }
      else {
         logger.debug("Refreshing " + getDisplayName());
      }
   }

   //其他子容器自行实现（比如：WebApplicationContext） Initialize any placeholder property sources in the context environment.
   initPropertySources();

   //准备环境变量信息 Validate that all properties marked as required are resolvable:
   // see ConfigurablePropertyResolver#setRequiredProperties
   getEnvironment().validateRequiredProperties();

   // 存储子容器早期运行的一些监听器； Store pre-refresh ApplicationListeners...
   if (this.earlyApplicationListeners == null) {
      this.earlyApplicationListeners = new LinkedHashSet<>(this.applicationListeners);
   }
   else {
      // Reset local application listeners to pre-refresh state.
      this.applicationListeners.clear();
      this.applicationListeners.addAll(this.earlyApplicationListeners);
   }

   //早期的一些事件存储到这里 Allow for the collection of early ApplicationEvents,
   // to be published once the multicaster is available...
   this.earlyApplicationEvents = new LinkedHashSet<>();
}

initPropertySources()

protected void initPropertySources() {
   // For subclasses: do nothing by default. 自行在此处加载一些自己感兴趣的信息。【WebApplicationContextUtils.initServletPropertySources】
    // web-ioc容器启动的时候一般在此加载当前应用的上下文信息（ApplicationContext）
}

prepareBeanFactory()给BeanFactory准备一些核心组件

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

      // Configure the bean factory with context callbacks.
      beanFactory.addBeanPostProcessor(new ApplicationContextAwareProcessor(this)); //准备一个处理Aware接口功能的后置处理器
      beanFactory.ignoreDependencyInterface(EnvironmentAware.class); //告诉Spring先别管这些接口
      beanFactory.ignoreDependencyInterface(EmbeddedValueResolverAware.class);
      beanFactory.ignoreDependencyInterface(ResourceLoaderAware.class);
      beanFactory.ignoreDependencyInterface(ApplicationEventPublisherAware.class);
      beanFactory.ignoreDependencyInterface(MessageSourceAware.class);
      beanFactory.ignoreDependencyInterface(ApplicationContextAware.class);
      beanFactory.ignoreDependencyInterface(ApplicationStartupAware.class);

      /*
      1.注册可以解析到的依赖，直接保存到容器的resolvableDependencies池里
      2.意思就是在我们自定义的类中，我们可以直接用@Autowired注解注入下面4个东西，并直接使用
      */
      beanFactory.registerResolvableDependency(BeanFactory.class, beanFactory);
      beanFactory.registerResolvableDependency(ResourceLoader.class, this);
      beanFactory.registerResolvableDependency(ApplicationEventPublisher.class, this);
      beanFactory.registerResolvableDependency(ApplicationContext.class, this);

      // 注册应用监听器的探测器，用来探测应用中有哪些监听器。这也是一个后置处理器
      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()));
      }

      //注册默认组件： 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());
      }
   }

String ENVIRONMENT_BEAN_NAME = "environment";

String SYSTEM_PROPERTIES_BEAN_NAME = "systemProperties";

String SYSTEM_ENVIRONMENT_BEAN_NAME = "systemEnvironment";

/**
 * Name of the {@link ApplicationStartup} bean in the factory.
 * @since 5.3
 */
String APPLICATION_STARTUP_BEAN_NAME = "applicationStartup";

postProcessBeanFactory()留给子类的模板方法

这里是一个空方法，主要是留给子类实现，比如Web里的

protected void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) {
}

invokeBeanFactoryPostProcessors()工厂增强【核心】

protected void invokeBeanFactoryPostProcessors(ConfigurableListableBeanFactory beanFactory) {
   PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(beanFactory, getBeanFactoryPostProcessors()); //执行所有的工厂增强器
   //上面的类叫：后置处理器的注册代理（门面模式-装饰模式）
   // Detect a LoadTimeWeaver and prepare for weaving, if found in the meantime
   // (e.g. through an @Bean method registered by ConfigurationClassPostProcessor)
   if (!NativeDetector.inNativeImage() && beanFactory.getTempClassLoader() == null && beanFactory.containsBean(LOAD_TIME_WEAVER_BEAN_NAME)) {
      beanFactory.addBeanPostProcessor(new LoadTimeWeaverAwareProcessor(beanFactory));
      beanFactory.setTempClassLoader(new ContextTypeMatchClassLoader(beanFactory.getBeanClassLoader()));
   }
}

这一步有个很关键的后置处理器

ConfigurationClassPostProcessor#postProcessBeanDefinitionRegistry()开始解析@Configuration标注的所有配置类相关信息，并生成Bean

AnnotationConfigApplicationContext#register()这里解析的是主配置类

   @Override //把配置类中所有bean的定义信息导入进来。
   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); //处理配置的BeanDefinition信息
   }



public void processConfigBeanDefinitions(BeanDefinitionRegistry registry) {
	List<BeanDefinitionHolder> configCandidates = new ArrayList<>();
	String[] candidateNames = registry.getBeanDefinitionNames(); //拿到工厂所有的bean定义信息

	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)); //将配置类加到候选集合里面，等待处理
		}
	}

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

	// 对所有的配置类进行排序，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); //getBean--getSingleton，获取创建一个internalConfigurationBeanNameGenerator来用来生成配置类的名字
			if (generator != null) {
				this.componentScanBeanNameGenerator = generator;
				this.importBeanNameGenerator = generator;
			}
		}
	}

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

	//由ConfigurationClassParser解析每一个配置类 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); //解析配置类;所有需要扫描进来的组件的BeanDefinition信息都已经完成
		parser.validate();

		Set<ConfigurationClass> configClasses = new LinkedHashSet<>(parser.getConfigurationClasses());
		configClasses.removeAll(alreadyParsed);
		//每一个组件都可以当配置类，@Import之类的都能进行处理
		// 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();
	}
}

ConfigurationClassParser#parse()

   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();
   }


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



protected void processConfigurationClass(ConfigurationClass configClass, Predicate<String> filter) throws IOException {
	if (this.conditionEvaluator.shouldSkip(configClass.getMetadata(), ConfigurationPhase.PARSE_CONFIGURATION)) {
		return;
	}
	//Spring底层大量使用缓存来保证框架速度
	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 { //解析配置类里面的所有注解，
		sourceClass = doProcessConfigurationClass(configClass, sourceClass, filter);
	}
	while (sourceClass != null);
	//只要这个配置类解析过，就放在已经解析好的集合中防止重复解析
	this.configurationClasses.put(configClass, configClass);
}

ConfigurationClassParser#doProcessConfigurationClass()解析配置类中诸如@ComponentScan，@Import，@Bean等注解

private final Environment environment;

private final BeanDefinitionRegistry registry;

private final ComponentScanAnnotationParser componentScanParser;

private final Map<ConfigurationClass, ConfigurationClass> configurationClasses = new LinkedHashMap<>();

private final Map<String, ConfigurationClass> knownSuperclasses = new HashMap<>();

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");
		}
	}

	// Process any @ComponentScan annotations
	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) {
			//使用Scanner把ComponentScan指定的包下的所有组件都扫描进来 The config class is annotated with @ComponentScan -> perform the scan immediately
			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());
				}
			}
		}
	}

	//处理@Import注解的地方【AOP就是利用这个地方导入一个后置处理器的】 Process any @Import annotations
	processImports(configClass, sourceClass, getImports(sourceClass), filter, true);

	//处理@ImportResource  Process any @ImportResource annotations
	AnnotationAttributes importResource =
			AnnotationConfigUtils.attributesFor(sourceClass.getMetadata(), ImportResource.class);
	if (importResource != null) {
		String[] resources = importResource.getStringArray("locations");
		Class<? extends BeanDefinitionReader> readerClass = importResource.getClass("reader");
		for (String resource : resources) {
			String resolvedResource = this.environment.resolveRequiredPlaceholders(resource);
			configClass.addImportedResource(resolvedResource, readerClass);
		}
	}

	//处理@Bean Process individual @Bean methods
	Set<MethodMetadata> beanMethods = retrieveBeanMethodMetadata(sourceClass);
	for (MethodMetadata methodMetadata : beanMethods) {
		configClass.addBeanMethod(new BeanMethod(methodMetadata, configClass));
	}

	// Process default methods on interfaces
	processInterfaces(configClass, sourceClass);

	// Process superclass, if any
	if (sourceClass.getMetadata().hasSuperClass()) {
		String superclass = sourceClass.getMetadata().getSuperClassName();
		if (superclass != null && !superclass.startsWith("java") &&
				!this.knownSuperclasses.containsKey(superclass)) {
			this.knownSuperclasses.put(superclass, configClass);
			// Superclass found, return its annotation metadata and recurse
			return sourceClass.getSuperClass();
		}
	}

	// No superclass -> processing is complete
	return null;
}

ComponentScanAnnotationParser#parse()具体如何解析@ComponentScan注解。这里也是@Component注解生效的地方

public Set<BeanDefinitionHolder> parse(AnnotationAttributes componentScan, final String declaringClass) {
   ClassPathBeanDefinitionScanner scanner = new ClassPathBeanDefinitionScanner(this.registry,
componentScan.getBoolean("useDefaultFilters"), this.environment, this.resourceLoader);
   // ...省略不重要
   return scanner.doScan(StringUtils.toStringArray(basePackages)); //扫描器进行扫描
}

ClassPathBeanDefinitionScanner#doScan()

public class ClassPathBeanDefinitionScanner extends ClassPathScanningCandidateComponentProvider{  
    
    //...
    
	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); //找到候选组件（我们需要扫描进来的组件），这里是调用父类的方法
          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;
    }
    
    
    // ...
}

ClassPathScanningCandidateComponentProvider#findCandidateComponents()

   public Set<BeanDefinition> findCandidateComponents(String basePackage) {
      if (this.componentsIndex != null && indexSupportsIncludeFilters()) {
         return addCandidateComponentsFromIndex(this.componentsIndex, basePackage);
      }
      else {
         return scanCandidateComponents(basePackage); //扫描所有组件
      }
   }


private Set<BeanDefinition> scanCandidateComponents(String basePackage) {
	Set<BeanDefinition> candidates = new LinkedHashSet<>();
	try {
		String packageSearchPath = ResourcePatternResolver.CLASSPATH_ALL_URL_PREFIX +
				resolveBasePackage(basePackage) + '/' + this.resourcePattern;
		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 { //生成每一个资源的元数据信息
					MetadataReader metadataReader = getMetadataReaderFactory().getMetadataReader(resource);
					if (isCandidateComponent(metadataReader)) { //如果当前类在扫描范围
						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);
	}
	return candidates;
}

拿到所有类(资源),不管你有没有标@Component注解。然后挨个遍历每一个资源是不是候选的组件(根据前面准备的一些条件，在这里进行判断)

最后我们看一下执行完之后的BeanDefinition信息

registerBeanPostProcessors()注册所有的Bean后置处理器

前面讲过

protected void registerBeanPostProcessors(ConfigurableListableBeanFactory beanFactory) {
   PostProcessorRegistrationDelegate.registerBeanPostProcessors(beanFactory, this);
}

PostProcessorRegistrationDelegate#registerBeanPostProcessors()

public static void registerBeanPostProcessors(
      ConfigurableListableBeanFactory beanFactory, AbstractApplicationContext applicationContext) {

   // WARNING: Although it may appear that the body of this method can be easily
   // refactored to avoid the use of multiple loops and multiple lists, the use
   // of multiple lists and multiple passes over the names of processors is
   // intentional. We must ensure that we honor the contracts for PriorityOrdered
   // and Ordered processors. Specifically, we must NOT cause processors to be
   // instantiated (via getBean() invocations) or registered in the ApplicationContext
   // in the wrong order.
   //
   // Before submitting a pull request (PR) to change this method, please review the
   // list of all declined PRs involving changes to PostProcessorRegistrationDelegate
   // to ensure that your proposal does not result in a breaking change:
   // https://github.com/spring-projects/spring-framework/issues?q=PostProcessorRegistrationDelegate+is%3Aclosed+label%3A%22status%3A+declined%22
   //获取到容器中所有的 BeanPostProcessor； Bean的后置处理器
   String[] postProcessorNames = beanFactory.getBeanNamesForType(BeanPostProcessor.class, true, false);

   // Register BeanPostProcessorChecker that logs an info message when
   // a bean is created during BeanPostProcessor instantiation, i.e. when
   // a bean is not eligible for getting processed by all BeanPostProcessors.
   int beanProcessorTargetCount = beanFactory.getBeanPostProcessorCount() + 1 + postProcessorNames.length;
   beanFactory.addBeanPostProcessor(new BeanPostProcessorChecker(beanFactory, beanProcessorTargetCount));

   // Separate between BeanPostProcessors that implement PriorityOrdered,
   // Ordered, and the rest.
   List<BeanPostProcessor> priorityOrderedPostProcessors = new ArrayList<>();
   List<BeanPostProcessor> internalPostProcessors = new ArrayList<>();
   List<String> orderedPostProcessorNames = new ArrayList<>();
   List<String> nonOrderedPostProcessorNames = new ArrayList<>();
   for (String ppName : postProcessorNames) {
      if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {  //获取所有实现了 PriorityOrdered 的 BeanPostProcessor
         BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
         priorityOrderedPostProcessors.add(pp);
         if (pp instanceof MergedBeanDefinitionPostProcessor) {
            internalPostProcessors.add(pp);
         }
      }
      else if (beanFactory.isTypeMatch(ppName, Ordered.class)) {
         orderedPostProcessorNames.add(ppName);
      }
      else {
         nonOrderedPostProcessorNames.add(ppName);
      }
   }

   //首先，注册所有的实现了 PriorityOrdered 的 BeanPostProcessor ；  First, register the BeanPostProcessors that implement PriorityOrdered.
   sortPostProcessors(priorityOrderedPostProcessors, beanFactory);
   registerBeanPostProcessors(beanFactory, priorityOrderedPostProcessors);

   //接下来，注册所有的实现了 Ordered 的 BeanPostProcessor  Next, register the BeanPostProcessors that implement Ordered.
   List<BeanPostProcessor> orderedPostProcessors = new ArrayList<>(orderedPostProcessorNames.size());
   for (String ppName : orderedPostProcessorNames) {
      BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
      orderedPostProcessors.add(pp);
      if (pp instanceof MergedBeanDefinitionPostProcessor) {
         internalPostProcessors.add(pp);
      }
   }
   sortPostProcessors(orderedPostProcessors, beanFactory);
   registerBeanPostProcessors(beanFactory, orderedPostProcessors);

   // 最后，注册所有普通的 BeanPostProcessor ；Now, register all regular BeanPostProcessors.
   List<BeanPostProcessor> nonOrderedPostProcessors = new ArrayList<>(nonOrderedPostProcessorNames.size());
   for (String ppName : nonOrderedPostProcessorNames) {
      BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class); //从容器中获取这个组件
      nonOrderedPostProcessors.add(pp);
      if (pp instanceof MergedBeanDefinitionPostProcessor) {
         internalPostProcessors.add(pp);
      }
   }
   registerBeanPostProcessors(beanFactory, nonOrderedPostProcessors);

   //最后，重新注册所有internal的BeanPostProcessors  Finally, re-register all internal BeanPostProcessors.
   sortPostProcessors(internalPostProcessors, beanFactory);
   registerBeanPostProcessors(beanFactory, internalPostProcessors);

   // 重新注册一下这个后置处理器 Re-register post-processor for detecting inner beans as ApplicationListeners,
   // 把他放到后置处理器的最后一位； moving it to the end of the processor chain (for picking up proxies etc).
   beanFactory.addBeanPostProcessor(new ApplicationListenerDetector(applicationContext));
}

initMessageSource()

初始化国际化功能

1. 看容器中是否有MessageSource的定义信息
2. 如果没有就注册一个默认的
3. 把国际化组件(MessageSource) 放到单例池中

initApplicationEventMulticaster()

初始化事件多播功能（事件派发）

1. 看容器中是否有applicationEventMulticaster的定义信息，按照id找
2. 如果没有就注册一个默认的
3. 把事件多播器组件(ApplicationEventMulticaster) 放到单例池中

onRefresh()模板方法留给子类继续增强处理

留给子类继续增强处理，模板模式。

registerListeners()

注册监听器，从容器中获取所有的ApplicationListener

/** 多播器和监听器是观察者模式（里面包含了所有的监听器）
 * Add beans that implement ApplicationListener as listeners.
 * Doesn't affect other listeners, which can be added without being beans.
 */
protected void registerListeners() {
   //把所有监听器保存到多播器的集合中 Register statically specified listeners first.
   for (ApplicationListener<?> listener : getApplicationListeners()) {
      getApplicationEventMulticaster().addApplicationListener(listener);
   }

   // Do not initialize FactoryBeans here: We need to leave all regular beans
   // uninitialized to let post-processors apply to them! 获取ApplicationListener在ioc容器中注册的bean的名字
   String[] listenerBeanNames = getBeanNamesForType(ApplicationListener.class, true, false);
   for (String listenerBeanName : listenerBeanNames) {
      getApplicationEventMulticaster().addApplicationListenerBean(listenerBeanName); //获取所有的容器中的监听器，并保存他们的名字
   }

   //派发之前攒的一些早期事件 Publish early application events now that we finally have a multicaster...
   Set<ApplicationEvent> earlyEventsToProcess = this.earlyApplicationEvents;
   this.earlyApplicationEvents = null;
   if (!CollectionUtils.isEmpty(earlyEventsToProcess)) {
      for (ApplicationEvent earlyEvent : earlyEventsToProcess) {
         getApplicationEventMulticaster().multicastEvent(earlyEvent);
      }
   }
}

监听器也是一个重点，后面细讲

finishBeanFactoryInitialization()正式创建所有普通Bean

前面Bean的初始化流程讲的很仔细了

1. 主要就是遍历循环所有BeanDefinition里的BeanName，挨个创建Bean对象。注意BeanPostProcessor这些不算普通Bean，在registerBeanPostProcessors()，invokeBeanFactoryPostProcessors()已经创建完了，因为在普通Bean创建的过程中，后置处理器可能需要参与进来，所以需要比普通Bean提前创建好
2. 最后执行SmartInitializingSingleton.afterSingletonsInstantiated后初始化

finishRefresh()主要是发送容器刷新完成的事件

最后进行一些清理和事件发送等

protected void finishRefresh() {
   // Clear context-level resource caches (such as ASM metadata from scanning).
   clearResourceCaches();

   // Initialize lifecycle processor for this context.
   initLifecycleProcessor();

   //告诉LifecycleProcessor容器onRefresh Propagate refresh to lifecycle processor first.
   getLifecycleProcessor().onRefresh();

   //发布事件 Publish the final event.
   publishEvent(new ContextRefreshedEvent(this));

   //jconsole（暴露MBean端点信息） Participate in LiveBeansView MBean, if active.
   if (!NativeDetector.inNativeImage()) {
      LiveBeansView.registerApplicationContext(this);
   }
}

自此over

补充：循环引用解决方案

单开一节，讲一下之前一直忽略的这个。其实很简单的一个东西，被网上搞的很难一样。

测试类

A

@Component
public class A {
    private B b;

    public A() {
        System.out.println("A...构造....");
    }

    @Autowired
    public void setB(B b) {
        this.b = b;
    }
}

B

@Component
public class B {

   private A a;

   public B() {
      System.out.println("B...构造....");
   }

   @Autowired
   public void setA(A a) {
      this.a = a;
   }
}

这就是循环引用的场景，这种写法由于Spring内部获取Bean都是通过getBean方法来获取，就造成了下面的死循环。我们来看看Spring是怎么解决的。

核心代码

AbstractBeanFactory

 protected <T> T doGetBean(
       String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly)
       throws BeansException {

    String beanName = transformedBeanName(name); //转换Bean名字
    Object beanInstance;

    // 先检查单实例bean的缓存 Eagerly check singleton cache for manually registered singletons.
    // ☆☆☆ pos_1
    Object sharedInstance = getSingleton(beanName); //检查缓存中有没有，如果是第一次获取肯定是没有的     
    if (sharedInstance != null && args == null) {
       if (logger.isTraceEnabled()) {
          if (isSingletonCurrentlyInCreation(beanName)) {
	// ...
          }
          else {
             // ..
          }
       }
       beanInstance = getObjectForBeanInstance(sharedInstance, name, beanName, null);
    }

    else { //默认第一次获取组件都会进入else环节
// ...

          // 创建bean的实例；Create bean instance.
          if (mbd.isSingleton()) {
             // ☆☆☆ pos_2
             sharedInstance = getSingleton(beanName, () -> {
                try {
                   return createBean(beanName, mbd, args);  //创建bean对象的实例
                }
                catch (BeansException ex) {
                   destroySingleton(beanName);
                   throw ex;
                }
             }); //看当前bean是否是FactoryBean
             beanInstance = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
          }

// ...
 }

DefaultSingletonBeanRegistry

   public Object getSingleton(String beanName) {
      // ☆☆☆ pos_3
      return getSingleton(beanName, true);
   }

   /**
    * Return the (raw) singleton object registered under the given name.
    * <p>Checks already instantiated singletons and also allows for an early
    * reference to a currently created singleton (resolving a circular reference).
    * @param beanName the name of the bean to look for
    * @param allowEarlyReference whether early references should be created or not
    * @return the registered singleton object, or {@code null} if none found
    */
  //☆☆☆ pos_4
   @Nullable  //双检查锁
   protected Object getSingleton(String beanName, boolean allowEarlyReference) {
      //先检查单例缓存池，获取当前对象  Quick check for existing instance without full singleton lock
      Object singletonObject = this.singletonObjects.get(beanName); //一级缓存
      if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) { //如果当前bean正在创建过程中，而且缓存中没有则继续
         singletonObject = this.earlySingletonObjects.get(beanName); //二级
         if (singletonObject == null && allowEarlyReference) {
            synchronized (this.singletonObjects) {
               // Consistent creation of early reference within full singleton lock
               singletonObject = this.singletonObjects.get(beanName);
               if (singletonObject == null) {
                  singletonObject = this.earlySingletonObjects.get(beanName);
                  if (singletonObject == null) {
                     ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName); //三级
                     if (singletonFactory != null) {
                        singletonObject = singletonFactory.getObject();
                        this.earlySingletonObjects.put(beanName, singletonObject);
                        this.singletonFactories.remove(beanName);
                     }
                  }
               }
            }
         }
      }
      return singletonObject;
   }


// ☆☆☆ pos_5
public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
	Assert.notNull(beanName, "Bean name must not be null");
	synchronized (this.singletonObjects) {
		Object singletonObject = this.singletonObjects.get(beanName);//再检查一次单例池
		if (singletonObject == null) { //单实例池子里面没有当前对象（说明没有创建完成）
			if (this.singletonsCurrentlyInDestruction) {
				throw new BeanCreationNotAllowedException(beanName,
						"Singleton bean creation not allowed while singletons of this factory are in destruction " +
						"(Do not request a bean from a BeanFactory in a destroy method implementation!)");
			}
			if (logger.isDebugEnabled()) {
				logger.debug("Creating shared instance of singleton bean '" + beanName + "'");
			}
                // ☆☆☆ pos_6
			beforeSingletonCreation(beanName); //单实例创建之前，这个就是加到
			boolean newSingleton = false;
			boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
			if (recordSuppressedExceptions) {
				this.suppressedExceptions = new LinkedHashSet<>();
			}
			try {
				singletonObject = singletonFactory.getObject();//会调用lamda表达式的内容，真正创建对象
				newSingleton = true;
			}
			catch (IllegalStateException ex) {
				// Has the singleton object implicitly appeared in the meantime ->
				// if yes, proceed with it since the exception indicates that state.
                   // ☆☆☆ pos_7
				singletonObject = this.singletonObjects.get(beanName);
				if (singletonObject == null) {
					throw ex;
				}
			}
			catch (BeanCreationException ex) {
				// ...
			}
			finally {
				if (recordSuppressedExceptions) {
					this.suppressedExceptions = null;
				}
				afterSingletonCreation(beanName);
			}
			if (newSingleton) {
				addSingleton(beanName, singletonObject);
			}
		}
		return singletonObject;
	}
}

// 加到singletonsCurrentlyInCreation
protected void beforeSingletonCreation(String beanName) {
	if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.add(beanName)) {
		throw new BeanCurrentlyInCreationException(beanName);
	}
}

AbstractAutowireCapableBeanFactory

 protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
       throws BeanCreationException {

    // Instantiate the bean.
    BeanWrapper instanceWrapper = null;
    if (mbd.isSingleton()) { //是否单例的
       instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
    }
    if (instanceWrapper == null) {
       //创建Bean的实例,默认使用无参构造器创建的对象，组件的原始对象就创建了 ☆☆☆ pos_8
       instanceWrapper = createBeanInstance(beanName, mbd, args);
    }
    Object bean = instanceWrapper.getWrappedInstance();
    Class<?> beanType = instanceWrapper.getWrappedClass();
    if (beanType != NullBean.class) {
       mbd.resolvedTargetType = beanType;
    }

// ...

    // 提前暴露单实例。专门来解决循环引用问题；Eagerly cache singletons to be able to resolve circular references
    // even when triggered by lifecycle interfaces like BeanFactoryAware.  
    // ☆☆☆ pos_9
    boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
          isSingletonCurrentlyInCreation(beanName));
    if (earlySingletonExposure) {
       if (logger.isTraceEnabled()) {
          logger.trace("Eagerly caching bean '" + beanName +
                "' to allow for resolving potential circular references");
       }
       addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean)); //三级缓存中的Bean也会被后置处理来增强，
    }

    // Initialize the bean instance.
    Object exposedObject = bean;
    try {
       // ☆☆☆ pos_10
       populateBean(beanName, mbd, instanceWrapper); //给创建好的对象每个属性进行赋值，@Autowired发生在这里
       exposedObject = initializeBean(beanName, exposedObject, mbd);//初始化bean
    }
    catch (Throwable ex) {
       if (ex instanceof BeanCreationException && beanName.equals(((BeanCreationException) ex).getBeanName())) {
          throw (BeanCreationException) ex;
       }
       else {
          throw new BeanCreationException(
                mbd.getResourceDescription(), beanName, "Initialization of bean failed", ex);
       }
    }
    //早期单实例暴露
    if (earlySingletonExposure) {
       Object earlySingletonReference = getSingleton(beanName, false); //检查早期缓存中是否存在这个组件
       if (earlySingletonReference != null) {
          if (exposedObject == bean) {
             exposedObject = earlySingletonReference;
          }
          else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
             String[] dependentBeans = getDependentBeans(beanName);
             Set<String> actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
             for (String dependentBean : dependentBeans) {
                if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
                   actualDependentBeans.add(dependentBean);
                }
             }
             if (!actualDependentBeans.isEmpty()) {
                throw new BeanCurrentlyInCreationException(beanName,
                      "Bean with name '" + beanName + "' has been injected into other beans [" +
                      StringUtils.collectionToCommaDelimitedString(actualDependentBeans) +
                      "] in its raw version as part of a circular reference, but has eventually been " +
                      "wrapped. This means that said other beans do not use the final version of the " +
                      "bean. This is often the result of over-eager type matching - consider using " +
                      "'getBeanNamesForType' with the 'allowEagerInit' flag turned off, for example.");
             }
          }
       }
    }

    // Register bean as disposable.
    try {
       registerDisposableBeanIfNecessary(beanName, bean, mbd);
    }
    catch (BeanDefinitionValidationException ex) {
       throw new BeanCreationException(
             mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
    }

    return exposedObject;
 }

流程

1. pos_1位置先进入pos_3位置的getSingleton(beanName, true)，查看缓存中有没有A组件

2. 然后走到pos_2调用pos_5的getSingleton()开始创建A的流程
3. 在pos_5的getSingleton()中走到pos_6的beforeSingletonCreation()，就变成下面这样

4. 接着pos_7的会调用pos_2的lamda表达式里的createbean()，里面再调用doCreateBean()。前面讲过不多说，最终调用A的无参构造(pos_8)，创建完之后发现A的B属性是null。

   

   5.在pos_9处的addSingletonFactory()来准备解决循环引用

protected void addSingletonFactory(String beanName, ObjectFactory<?> singletonFactory) {
   Assert.notNull(singletonFactory, "Singleton factory must not be null");
   synchronized (this.singletonObjects) {
      if (!this.singletonObjects.containsKey(beanName)) { //当前对象如果没有在单例池中
         this.singletonFactories.put(beanName, singletonFactory);
         this.earlySingletonObjects.remove(beanName);
         this.registeredSingletons.add(beanName);
      }
   }
}

6. 接着在pos_10处的populateBean()开始给属性赋值，这一步就是要把B b自动装配进来。主要是

protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {
   if (bw == null) {
      if (mbd.hasPropertyValues()) {
         throw new BeanCreationException(
               mbd.getResourceDescription(), beanName, "Cannot apply property values to null instance");
      }
      else {
         // Skip property population phase for null instance.
         return;
      }
   }

   // @Autowired赋值也在这里(但是没做事)。可以中断初始化行为； 在属性赋值之前，后置处理器可以提前准备些东西 Give any InstantiationAwareBeanPostProcessors the opportunity to modify the

   if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
      for (InstantiationAwareBeanPostProcessor bp : getBeanPostProcessorCache().instantiationAware) {
         if (!bp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
            return;
         }
      }
   } //以上的后置处理器可以中断以下的行为

   PropertyValues pvs = (mbd.hasPropertyValues() ? mbd.getPropertyValues() : null);

   int resolvedAutowireMode = mbd.getResolvedAutowireMode();
   if (resolvedAutowireMode == AUTOWIRE_BY_NAME || resolvedAutowireMode == AUTOWIRE_BY_TYPE) {
      // ...
   }

   boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors();
   boolean needsDepCheck = (mbd.getDependencyCheck() != AbstractBeanDefinition.DEPENDENCY_CHECK_NONE);

   PropertyDescriptor[] filteredPds = null;
   if (hasInstAwareBpps) {
      if (pvs == null) {
         pvs = mbd.getPropertyValues(); //xml中property标签指定的
      } //使用后置处理器处理属性
      for (InstantiationAwareBeanPostProcessor bp : getBeanPostProcessorCache().instantiationAware) {
         PropertyValues pvsToUse = bp.postProcessProperties(pvs, bw.getWrappedInstance(), beanName);
         if (pvsToUse == null) {
            if (filteredPds == null) {
               filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
            }
            pvsToUse = bp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
            if (pvsToUse == null) {
               return;
            }
         }
         pvs = pvsToUse; //封装了当前bean的所有属性名和值，可以由后置处理器处理得到
      }
   }
   // ...

   if (pvs != null) { //把以前处理好的PropertyValues给bean里面设置一下。主要是上面步骤没有给bean里面设置的属性
      applyPropertyValues(beanName, mbd, bw, pvs); //xml版的所有配置会来到这里给属性赋值
   }
}

通过前面讲过的AutowiredAnnotationBeanPostProcessor来注入B，最后发现要调用setB方法给B赋值

7. 继续走，发现要想获得B还是要调用getBean

public Object resolveCandidate(String beanName, Class<?> requiredType, BeanFactory beanFactory)
      throws BeansException {

   return beanFactory.getBean(beanName); //所有自动注入的属性都是beanFactory.getBean(beanName);的结果
}

然后图就是这样子

8. B也是走这一套

9. B为了获取A，还要再走一次getBean()流程，最终还是走到

//☆☆☆ pos_4
 @Nullable  //双检查锁
 protected Object getSingleton(String beanName, boolean allowEarlyReference) {
    //先检查单例缓存池，获取当前对象  Quick check for existing instance without full singleton lock
    Object singletonObject = this.singletonObjects.get(beanName); //一级缓存
    if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) { //如果当前bean正在创建过程中，而且缓存中没有则继续
       singletonObject = this.earlySingletonObjects.get(beanName); //二级
       if (singletonObject == null && allowEarlyReference) {
          synchronized (this.singletonObjects) {
             // Consistent creation of early reference within full singleton lock
             singletonObject = this.singletonObjects.get(beanName);
             if (singletonObject == null) {
                singletonObject = this.earlySingletonObjects.get(beanName);
                if (singletonObject == null) {
                   ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName); //三级
                   if (singletonFactory != null) {
                      singletonObject = singletonFactory.getObject();// 用那个lamda表达式拿对象
                      this.earlySingletonObjects.put(beanName, singletonObject);
                      this.singletonFactories.remove(beanName);
                   }
                }
             }
          }
       }
    }
    return singletonObject;
 }

11. 得到A之后，B赋值结束。B进入初始化

    populateBean(beanName, mbd, instanceWrapper); //给创建好的对象每个属性进行赋值，@Autowired发生在这里
    exposedObject = initializeBean(beanName, exposedObject, mbd);//初始化bean

12. B初始化完之后，回到getSingleton，把自己放到单例池里

protected void addSingleton(String beanName, Object singletonObject) {
   synchronized (this.singletonObjects) {
      this.singletonObjects.put(beanName, singletonObject);
      this.singletonFactories.remove(beanName);
      this.earlySingletonObjects.remove(beanName);
      this.registeredSingletons.add(beanName);
   }
}

13. B全部结束之后回到A的流程，A赋值工作结束了，然后就开始A的初始化。初始化的过程中

protected void addSingleton(String beanName, Object singletonObject) {
   synchronized (this.singletonObjects) {
      this.singletonObjects.put(beanName, singletonObject); //加到一级
      this.singletonFactories.remove(beanName); //移除三级
      this.earlySingletonObjects.remove(beanName); //移除二级
      this.registeredSingletons.add(beanName);
   }
}

全部的流程图