Springboot源码学习-启动流程

程序入口

1、spring boot应用入口，启动类中调用SpringApplication.run方法，方法参数中可以传入启动类，也可以传入其他配置类。
示例所传入的配置类为启动类，也就是主类。

2、跟踪run方法，该方法返回的是一个ConfigurableApplicationContext对象，并且其内部创建了一个SpringApplication对象，然后调用了它的run方法。
ConfigurableApplicationContext：可配置的应用程序上下文，是一个接口，此处返回的是它的一个实现类对象。
可以将返回的这个context理解成Spring的IOC容器，我们可以通过其getBean方法获取到Bean。

IOC：控制反转，将对象的创建、管理和销毁等工作（即控制权）交给Spring容器，而不是开发者；开发者使用时只需要从容器中获取即可。
Bean：可以简单理解为交由Spring容器创建、组装和管理的Java对象。

3、看看SpringApplication的构造方法主要做了什么，注释版代码如下：

	public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
		this.resourceLoader = resourceLoader;
		Assert.notNull(primarySources, "PrimarySources must not be null");
		this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
		//通过判断classpath中是否能加载到指定的类，来推断当前web应用类型，最终返回为WebApplicationType.SERVLET。
		this.webApplicationType = WebApplicationType.deduceFromClasspath();
		//读取classpath下所有META-INF目录下的spring.factories文件，实例化其中配置的BootstrapRegistryInitializer对象。
		this.bootstrapRegistryInitializers = new ArrayList<>(
				getSpringFactoriesInstances(BootstrapRegistryInitializer.class));
		//同上，读取spring.factories文件，实例化其中配置的ApplicationContextInitializer对象。
		setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));
		//同上，读取spring.factories文件，实例化其中配置的ApplicationListener对象。
		setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
		//通过堆栈信息中main方法的调用信息，推断应用主类，比如示例工程的启动类Application。
		this.mainApplicationClass = deduceMainApplicationClass();
	}

具体分析：
1）deduceFromClasspath方法，通过判断classpath中是否能加载到指定的类，来推断当前web应用类型。
2）三个getSpringFactoriesInstances(xxx.class)方法，均扫描classpath下所有META-INF目录下的spring.factories文件（这些文件中包含了当前库需要告知spring的属性信息），然后实例化相关的对象。包括：

• BootstrapRegistryInitializer，引导注册表初始化器，此处结果为空，暂不关注；
• ApplicationContextInitializer，应用上下文，也就是Spring容器初始化器（不同的初始化器中封装了各自对Spring容器的初始化操作）；
• ApplicationListener，应用程序监听器。
  通过debug可以看到此次加载到的是下图中两个spring.factories文件中的ApplicationContextInitializer配置：
  
  4、SpringApplication对象实例化后，进入其run方法，核心操作包括：创建容器、准备容器、刷新容器。
  
  接下来重点分析这三个关于Spring容器的方法。

创建容器

调用createApplicationContext方法，创建一个ConfigurableApplicationContext接口实现类的对象作为Spring容器。
SpringApplication类中有一个接口类型的成员变量：applicationContextFactory

private ApplicationContextFactory applicationContextFactory = ApplicationContextFactory.DEFAULT;

ApplicationContextFactory是一个函数式接口，applicationContextFactory是该接口的默认实现（lambda表达式形式），createApplicationContext方法中，就是调用了applicationContextFactory的create方法，执行了这个默认的实现，返回了一个AnnotationConfigServletWebServerApplicationContext类型的实例，这个类是ConfigurableApplicationContext接口的实现类，也是我们容器的实际类型。

ApplicationContextFactory接口部分代码如下：

准备容器

prepareContext方法源码如下：

    private void prepareContext(DefaultBootstrapContext bootstrapContext, ConfigurableApplicationContext context,
                                ConfigurableEnvironment environment, SpringApplicationRunListeners listeners,
                                ApplicationArguments applicationArguments, Banner printedBanner) {
        context.setEnvironment(environment);
        postProcessApplicationContext(context);
        //使用之前设置的所有ApplicationContextInitializer，对容器进行初始化
        applyInitializers(context);
        listeners.contextPrepared(context);
        bootstrapContext.close(context);
        if (this.logStartupInfo) {
            logStartupInfo(context.getParent() == null);
            logStartupProfileInfo(context);
        }
        // Add boot specific singleton beans
        //获取该容器的beanFactory，并注册引导专用的单例bean
        ConfigurableListableBeanFactory beanFactory = context.getBeanFactory();
        beanFactory.registerSingleton("springApplicationArguments", applicationArguments);
        if (printedBanner != null) {
            beanFactory.registerSingleton("springBootBanner", printedBanner);
        }
        if (beanFactory instanceof AbstractAutowireCapableBeanFactory) {
            //默认关闭循环引用，即循环依赖
            ((AbstractAutowireCapableBeanFactory) beanFactory).setAllowCircularReferences(this.allowCircularReferences);
            if (beanFactory instanceof DefaultListableBeanFactory) {
                ((DefaultListableBeanFactory) beanFactory)
                        .setAllowBeanDefinitionOverriding(this.allowBeanDefinitionOverriding);
            }
        }
        if (this.lazyInitialization) {
            context.addBeanFactoryPostProcessor(new LazyInitializationBeanFactoryPostProcessor());
        }
        // Load the sources
        //加载来源，这里拿到的是示例代码主类"org.example.Application"
        Set<Object> sources = getAllSources();
        Assert.notEmpty(sources, "Sources must not be empty");
        //装载bean到容器中
        load(context, sources.toArray(new Object[0]));
        listeners.contextLoaded(context);
    }

其中有两个比较关键的地方：
1、ConfigurableListableBeanFactory beanFactory = context.getBeanFactory();
这里开始出现一个ConfigurableListableBeanFactory接口类型的变量，它可以通过context.getBeanFactory()方法获取到。
追踪源码，发现context中确实有一个该类型的成员变量，叫beanFactory，它是在context实例化过程中，由context父类的构造函数进行实例化的，实际类型为DefaultListableBeanFactory。源码如下：

2、load方法，装载bean到容器中。
1）该方法内部，首先调用createBeanDefinitionLoader方法，创建了一个BeanDefinitionLoader。该方法需要一个BeanDefinitionRegistry类型的入参。这里通过getBeanDefinitionRegistry方法强转context后传入参数。创建完loader后，又继续调用其load方法。

BeanDefinitionRegistry：bean定义信息注册表接口，提供BeanDefinition注册相关的操作。
BeanDefinition：描述bean信息的最小接口。
context所属的AnnotationConfigServletWebServerApplicationContext类，以及其成员beanFactory所属的DefaultListableBeanFactory类，均实现了BeanDefinitionRegistry接口。

2）继续跟进load方法，直到进入AnnotatedBeanDefinitionReader类的doRegisterBean方法。
这里主要进行了以下操作：

• 根据提供的配置类（这里是启动类，即主类），创建一个带有标准注解元数据（StandardAnnotationMetadata）的bean definition，这里是一个AnnotatedGenericBeanDefinition类的对象；
• 生成与bean definition对应的beanName；
• 处理@Lazy、@Primary、@DependsOn等通用注解，设置bean definition对应属性值；其中用到了AnnotationsScanner类的相关方法扫描注解；
• 调用BeanDefinitionReaderUtils.registerBeanDefinition方法，注册bean definition。

    private <T> void doRegisterBean(Class<T> beanClass, @Nullable String name,
                                    @Nullable Class<? extends Annotation>[] qualifiers, @Nullable Supplier<T> supplier,
                                    @Nullable BeanDefinitionCustomizer[] customizers) {
        //创建一个带有标准注解元数据（StandardAnnotationMetadata）的bean definition
		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());
		//生成beanName
		String beanName = (name != null ? name : this.beanNameGenerator.generateBeanName(abd, this.registry));
		
		//处理@Lazy、@Primary、@DependsOn等通用注解，设置bean definition相关属性值
        AnnotationConfigUtils.processCommonDefinitionAnnotations(abd);
        //...
        BeanDefinitionHolder definitionHolder = new BeanDefinitionHolder(abd, beanName);
        definitionHolder = AnnotationConfigUtils.applyScopedProxyMode(scopeMetadata, definitionHolder, this.registry);
        //注册bean definition
        BeanDefinitionReaderUtils.registerBeanDefinition(definitionHolder, this.registry);
    }

3）进入BeanDefinitionReaderUtils.registerBeanDefinition方法，最终调用的是registry.registerBeanDefinition，也就是DefaultListableBeanFactory类中的registerBeanDefinition方法。
注册bean definition的核心操作，就是在DefaultListableBeanFactory类的成员变量beanDefinitionMap和beanDefinitionNames中添加bean definition信息。

这里可以看到，除了刚注册的"application"这个bean definition，有5个bean definition已经存在了，它们是在application context的构造方法中创建reader的过程中注册的：

BeanDefinition的大致结构如下：

到这里，准备容器工作完成。主要就是从容器中获取beanFactory，并为主类构建一个beanDefinition，生成beanName，然后调用beanFactory的registerBeanDefinition(beanName, beanDefinition)，完成主类BeanDefinition的注册（将beanName、beanDefinition放入beanFactory中相关的集合里）。

刷新容器

跟踪refreshContext(context)方法，直到进入到AbstractApplicationContext类的refresh()方法中，部分源码如下：

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

			// Prepare this context for refreshing.
			//准备好要刷新的上下文，设置它的启动日期和活动标志，以及执行所有属性源的初始化。
			prepareRefresh();

			// Tell the subclass to refresh the internal bean factory.
			//获得新鲜的bean factory，这里是通知子类刷新内部的bean factory。
			//实际子类中实现的刷新操作是设置了bean factory的serializationId为配置类（即主类）的beanName。
			ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

			// Prepare the bean factory for use in this context.
			//配置bean factory的标准上下文特征，比如类加载器 class loader，以及一些后置处理器 post processor。
			prepareBeanFactory(beanFactory);

			try {
				// Allows post-processing of the bean factory in context subclasses.
				//允许context对bean factory 做一些处理。
				postProcessBeanFactory(beanFactory);

				StartupStep beanPostProcess = this.applicationStartup.start("spring.context.beans.post-process");
				// Invoke factory processors registered as beans in the context.
				//调用BeanFactoryPostProcessor，包括配置类解析，注册、修改bean definition等。
				invokeBeanFactoryPostProcessors(beanFactory);

				// Register bean processors that intercept bean creation.
				//实例化BeanPostProcessor对象，添加到bean factory中
				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.
				//启动web server，默认tomcat
				onRefresh();

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

				// Instantiate all remaining (non-lazy-init) singletons.
				//实例化剩余的非懒初始化的单例bean，完成bean factory的初始化
				finishBeanFactoryInitialization(beanFactory);

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

			catch (BeansException ex) {
				//...
			}

			finally {
				//...
			}
		}
	}

prepareBeanFactory(beanFactory)
配置bean factory的标准上下文特征，比如类加载器 class loader，以及一些后置处理器 post processor。
BeanPostProcessor：bean后置处理器接口。允许在bean初始化之前或之后对bean进行自定义修改，也就是postProcess操作。
通过debug可以看到配置结果：

invokeBeanFactoryPostProcessors(beanFactory)

调用BeanFactoryPostProcessor，包括配置类解析，注册、修改bean definition等。
BeanFactoryPostProcessor：bean factory后置处理器接口，允许对bean definition进行自定义修改。

1）进入PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors方法。
通过debug可以看到，容器中已经存在两个BeanFactoryPostProcessor：

这两个BeanFactoryPostProcessor是在容器准备阶段，执行applyInitializers(context)，应用各种初始化器的时候被添加进去的。

invokeBeanFactoryPostProcessors方法总体来说，就是先调用BeanDefinitionRegistryPostProcessor 注册 bean definition，其中最重要的是ConfigurationClassPostProcessor对配置类的解析（以启动类为入口）；然后调用BeanFactoryPostProcessor，对 bean definition做相关的处理。部分源码如下：

	public static void invokeBeanFactoryPostProcessors(
			ConfigurableListableBeanFactory beanFactory, List<BeanFactoryPostProcessor> beanFactoryPostProcessors) {

		// Invoke BeanDefinitionRegistryPostProcessors first, if any.
		Set<String> processedBeans = new HashSet<>();

		if (beanFactory instanceof BeanDefinitionRegistry) {
			BeanDefinitionRegistry registry = (BeanDefinitionRegistry) beanFactory;
			List<BeanFactoryPostProcessor> regularPostProcessors = new ArrayList<>();
			List<BeanDefinitionRegistryPostProcessor> registryProcessors = new ArrayList<>();
			
			//先处理一波现有的BeanDefinitionRegistryPostProcessor（BeanFactoryPostProcessor的子接口）
			for (BeanFactoryPostProcessor postProcessor : beanFactoryPostProcessors) {
				if (postProcessor instanceof BeanDefinitionRegistryPostProcessor) {
					BeanDefinitionRegistryPostProcessor registryProcessor =
							(BeanDefinitionRegistryPostProcessor) postProcessor;
					registryProcessor.postProcessBeanDefinitionRegistry(registry);
					registryProcessors.add(registryProcessor);
				}
				else {
					regularPostProcessors.add(postProcessor);
				}
			}

			List<BeanDefinitionRegistryPostProcessor> currentRegistryProcessors = new ArrayList<>();

			//获取bean factory中 BeanDefinitionRegistryPostProcessor 类型的bean definition；
			//这里最终找到的是 ConfigurationClassPostProcessor 配置类后处理器。
			String[] postProcessorNames =
					beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
			for (String ppName : postProcessorNames) {
				if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
					//优先处理实现了PriorityOrdered接口的post processor
					//获取ConfigurationClassPostProcessor实例
					currentRegistryProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
					processedBeans.add(ppName);
				}
			}
			sortPostProcessors(currentRegistryProcessors, beanFactory);
			registryProcessors.addAll(currentRegistryProcessors);
			//核心：调用ConfigurationClassPostProcessor的postProcessBeanDefinitionRegistry方法，处理配置bean。
			invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry, beanFactory.getApplicationStartup());
			currentRegistryProcessors.clear();

			// Next, invoke the BeanDefinitionRegistryPostProcessors that implement Ordered.
			//再次获取bean factory中 BeanDefinitionRegistryPostProcessor 类型的bean definition，这次处理实现Ordered接口的。
			postProcessorNames = beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
			for (String ppName : postProcessorNames) {
				if (!processedBeans.contains(ppName) && beanFactory.isTypeMatch(ppName, Ordered.class)) {
					currentRegistryProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
					processedBeans.add(ppName);
				}
			}
			sortPostProcessors(currentRegistryProcessors, beanFactory);
			registryProcessors.addAll(currentRegistryProcessors);
			invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry, beanFactory.getApplicationStartup());
			currentRegistryProcessors.clear();

			// Finally, invoke all other BeanDefinitionRegistryPostProcessors until no further ones appear.
			//最后循环处理完剩下的，以及可能再创建的BeanDefinitionRegistryPostProcessors
			boolean reiterate = true;
			while (reiterate) {
				reiterate = false;
				postProcessorNames = beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
				for (String ppName : postProcessorNames) {
					if (!processedBeans.contains(ppName)) {
						currentRegistryProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
						processedBeans.add(ppName);
						reiterate = true;
					}
				}
				sortPostProcessors(currentRegistryProcessors, beanFactory);
				registryProcessors.addAll(currentRegistryProcessors);
				invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry, beanFactory.getApplicationStartup());
				currentRegistryProcessors.clear();
			}

			// Now, invoke the postProcessBeanFactory callback of all processors handled so far.
			//对处理过的BeanFactoryPostProcessor，调用其postProcessBeanFactory方法。
			invokeBeanFactoryPostProcessors(registryProcessors, beanFactory);
			invokeBeanFactoryPostProcessors(regularPostProcessors, beanFactory);
		}

		else {
			// Invoke factory processors registered with the context instance.
			invokeBeanFactoryPostProcessors(beanFactoryPostProcessors, beanFactory);
		}

		// Do not initialize FactoryBeans here: We need to leave all regular beans
		// uninitialized to let the bean factory post-processors apply to them!
		String[] postProcessorNames =
				beanFactory.getBeanNamesForType(BeanFactoryPostProcessor.class, true, false);

		// Separate between BeanFactoryPostProcessors that implement PriorityOrdered,
		// Ordered, and the rest.
		//按照优先级处理剩下的BeanFactoryPostProcessor
		List<BeanFactoryPostProcessor> priorityOrderedPostProcessors = new ArrayList<>();
		List<String> orderedPostProcessorNames = new ArrayList<>();
		List<String> nonOrderedPostProcessorNames = new ArrayList<>();
		for (String ppName : postProcessorNames) {
			if (processedBeans.contains(ppName)) {
				// skip - already processed in first phase above
			}
			else if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
				priorityOrderedPostProcessors.add(beanFactory.getBean(ppName, BeanFactoryPostProcessor.class));
			}
			else if (beanFactory.isTypeMatch(ppName, Ordered.class)) {
				orderedPostProcessorNames.add(ppName);
			}
			else {
				nonOrderedPostProcessorNames.add(ppName);
			}
		}

		// First, invoke the BeanFactoryPostProcessors that implement PriorityOrdered.
		sortPostProcessors(priorityOrderedPostProcessors, beanFactory);
		invokeBeanFactoryPostProcessors(priorityOrderedPostProcessors, beanFactory);

		// Next, invoke the BeanFactoryPostProcessors that implement Ordered.
		List<BeanFactoryPostProcessor> orderedPostProcessors = new ArrayList<>(orderedPostProcessorNames.size());
		for (String postProcessorName : orderedPostProcessorNames) {
			orderedPostProcessors.add(beanFactory.getBean(postProcessorName, BeanFactoryPostProcessor.class));
		}
		sortPostProcessors(orderedPostProcessors, beanFactory);
		invokeBeanFactoryPostProcessors(orderedPostProcessors, beanFactory);

		// Finally, invoke all other BeanFactoryPostProcessors.
		List<BeanFactoryPostProcessor> nonOrderedPostProcessors = new ArrayList<>(nonOrderedPostProcessorNames.size());
		for (String postProcessorName : nonOrderedPostProcessorNames) {
			nonOrderedPostProcessors.add(beanFactory.getBean(postProcessorName, BeanFactoryPostProcessor.class));
		}
		invokeBeanFactoryPostProcessors(nonOrderedPostProcessors, beanFactory);

		// Clear cached merged bean definitions since the post-processors might have
		// modified the original metadata, e.g. replacing placeholders in values...
		beanFactory.clearMetadataCache();
	}

BeanDefinitionRegistryPostProcessor：继承BeanFactoryPostProcessor接口，允许在BeanFactoryPostProcessor接口的方法回调之前，使用此接口来添加新的bean definition。

ConfigurationClassPostProcessor处理配置类的过程

	public void processConfigBeanDefinitions(BeanDefinitionRegistry registry) {
		List<BeanDefinitionHolder> configCandidates = new ArrayList<>();
		String[] candidateNames = registry.getBeanDefinitionNames();
		
		//遍历bean factory中的bean definition
		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);
				}
			}
			//检查是否是配置类，这里只有主类符合要求（@SpringBootApplication注解内包含@Configuration注解）
			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
		//构建配置类解析器
		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");
			//解析候选的BeanDefinitionHolder（BeanDefinitionHolder保存了beanDefinition和beanName）
			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();
		}
	}

ConfigurationClassUtils.checkConfigurationClassCandidate方法源码如下：

	public static boolean checkConfigurationClassCandidate(
			BeanDefinition beanDef, MetadataReaderFactory metadataReaderFactory) {

		String className = beanDef.getBeanClassName();
		if (className == null || beanDef.getFactoryMethodName() != null) {
			return false;
		}

		AnnotationMetadata metadata;
		if (beanDef instanceof AnnotatedBeanDefinition &&
				className.equals(((AnnotatedBeanDefinition) beanDef).getMetadata().getClassName())) {
			// Can reuse the pre-parsed metadata from the given BeanDefinition...
			//拿到注解元数据
			metadata = ((AnnotatedBeanDefinition) beanDef).getMetadata();
		}
		else if (beanDef instanceof AbstractBeanDefinition && ((AbstractBeanDefinition) beanDef).hasBeanClass()) {
			// Check already loaded Class if present...
			// since we possibly can't even load the class file for this Class.
			Class<?> beanClass = ((AbstractBeanDefinition) beanDef).getBeanClass();
			if (BeanFactoryPostProcessor.class.isAssignableFrom(beanClass) ||
					BeanPostProcessor.class.isAssignableFrom(beanClass) ||
					AopInfrastructureBean.class.isAssignableFrom(beanClass) ||
					EventListenerFactory.class.isAssignableFrom(beanClass)) {
				return false;
			}
			metadata = AnnotationMetadata.introspect(beanClass);
		}
		else {
			try {
				MetadataReader metadataReader = metadataReaderFactory.getMetadataReader(className);
				metadata = metadataReader.getAnnotationMetadata();
			}
			catch (IOException ex) {
				if (logger.isDebugEnabled()) {
					logger.debug("Could not find class file for introspecting configuration annotations: " +
							className, ex);
				}
				return false;
			}
		}

		//通过注解元数据获取@Configuration注解的属性，其中使用了AnnotationsScanner.scan方法进行注解扫描。
		Map<String, Object> config = metadata.getAnnotationAttributes(Configuration.class.getName());
		if (config != null && !Boolean.FALSE.equals(config.get("proxyBeanMethods"))) {
			beanDef.setAttribute(CONFIGURATION_CLASS_ATTRIBUTE, CONFIGURATION_CLASS_FULL);
		}
		else if (config != null || isConfigurationCandidate(metadata)) {
			beanDef.setAttribute(CONFIGURATION_CLASS_ATTRIBUTE, CONFIGURATION_CLASS_LITE);
		}
		else {
			return false;
		}

		// It's a full or lite configuration candidate... Let's determine the order value, if any.
		Integer order = getOrder(metadata);
		if (order != null) {
			beanDef.setAttribute(ORDER_ATTRIBUTE, order);
		}

		return true;
	}

沿着parser.parse(candidates)方法往下走，直到processConfigurationClass：

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

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

		// Recursively process the configuration class and its superclass hierarchy.
		SourceClass sourceClass = asSourceClass(configClass, filter);
		do {
			//处理配置类
			sourceClass = doProcessConfigurationClass(configClass, sourceClass, filter);
		}
		while (sourceClass != null);
		
		//处理完之后添加到配置类集合中
		this.configurationClasses.put(configClass, configClass);
	}

doProcessConfigurationClass方法中包含处理配置类的核心逻辑：

	@Nullable
	protected final SourceClass doProcessConfigurationClass(
			ConfigurationClass configClass, SourceClass sourceClass, Predicate<String> filter)
			throws IOException {
		
		//如果配置类注解元数据中包含@Component注解，则优先递归处理它的内部类。此处同样使用了AnnotationsScanner.scan方法进行注解扫描。
		if (configClass.getMetadata().isAnnotated(Component.class.getName())) {
			// Recursively process any member (nested) classes first
			processMemberClasses(configClass, sourceClass, filter);
		}

		// Process any @PropertySource annotations
		//处理@PropertySource注解，加载属性配置
		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
		//扫描并拿到@ComponentScan注解的属性。
		Set<AnnotationAttributes> componentScans = AnnotationConfigUtils.attributesForRepeatable(
				sourceClass.getMetadata(), ComponentScans.class, ComponentScan.class);
		if (!componentScans.isEmpty() &&
				!this.conditionEvaluator.shouldSkip(sourceClass.getMetadata(), ConfigurationPhase.REGISTER_BEAN)) {
			for (AnnotationAttributes componentScan : componentScans) {
				// The config class is annotated with @ComponentScan -> perform the scan immediately
				/*
				解析@ComponentScan注解属性，扫描目标package下的.class文件，拿到其注解元数据，然后根据排除和包含的规则（比如排除主类，
				包含@Component注解等）筛选出候选组件类，创建对应的BeanDefinition，将组装成的BeanDefinitionHolder放入返回集合，最后在
				bean工厂中注册BeanDefinition。
				*/
				Set<BeanDefinitionHolder> scannedBeanDefinitions =
						this.componentScanParser.parse(componentScan, sourceClass.getMetadata().getClassName());
				// Check the set of scanned definitions for any further config classes and parse recursively if needed
				//检查扫描到的组件类是否有进一步的配置注解，如果有则进行递归解析
				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());
					}
				}
			}
		}

		// Process any @Import annotations
		//处理@Import注解
		processImports(configClass, sourceClass, getImports(sourceClass), filter, true);

		// 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);
			}
		}

		// Process individual @Bean methods
		//检索@Bean注解的方法，添加到ConfigurationClass对象中。
		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;
	}

processImports(configClass, sourceClass, getImports(sourceClass), filter, true);
getImports(sourceClass)，递归查找。

	private void collectImports(SourceClass sourceClass, Set<SourceClass> imports, Set<SourceClass> visited)
			throws IOException {

		if (visited.add(sourceClass)) {
			//以主类为例，拿到注解信息后遍历，首层只有一个@SpringBootApplication
			for (SourceClass annotation : sourceClass.getAnnotations()) {
				//获取注解类名
				String annName = annotation.getMetadata().getClassName();
				//判断是否是Import，不是则继续往下一层找
				if (!annName.equals(Import.class.getName())) {
					//递归入口
					collectImports(annotation, imports, visited);
				}
			}
			//扫描当前sourceClass的@Import注解，拿到value属性，再把属性值组装成sourceClass，放进imports集合
			imports.addAll(sourceClass.getAnnotationAttributes(Import.class.getName(), "value"));
		}
	}

processImports(configClass, sourceClass, getImports(sourceClass), filter, true);

	private void processImports(ConfigurationClass configClass, SourceClass currentSourceClass,
			Collection<SourceClass> importCandidates, Predicate<String> exclusionFilter,
			boolean checkForCircularImports) {

		if (importCandidates.isEmpty()) {
			return;
		}

		if (checkForCircularImports && isChainedImportOnStack(configClass)) {
			this.problemReporter.error(new CircularImportProblem(configClass, this.importStack));
		}
		else {
			this.importStack.push(configClass);
			try {
				for (SourceClass candidate : importCandidates) {
					/*
					如果实现ImportSelector接口，实例化这个selector，调用getExclusionFilter拿到它的exclusionFilter（排除过滤器，lambda
					表达式形式），然后跟传进来的默认exclusionFilter组成or的关系，有一个成立即成立。这个排除规则在调用类似asSourceClass方
					法对配置类进行包装时生效。
					*/
					if (candidate.isAssignable(ImportSelector.class)) {
						// Candidate class is an ImportSelector -> delegate to it to determine imports
						Class<?> candidateClass = candidate.loadClass();
						ImportSelector selector = ParserStrategyUtils.instantiateClass(candidateClass, ImportSelector.class,
								this.environment, this.resourceLoader, this.registry);
						Predicate<String> selectorFilter = selector.getExclusionFilter();
						if (selectorFilter != null) {
							exclusionFilter = exclusionFilter.or(selectorFilter);
						}
						//如果实现DeferredImportSelector接口，则把这个selector先存起来，推迟处理
						if (selector instanceof DeferredImportSelector) {
							this.deferredImportSelectorHandler.handle(configClass, (DeferredImportSelector) selector);
						}
						else {
							String[] importClassNames = selector.selectImports(currentSourceClass.getMetadata());
							Collection<SourceClass> importSourceClasses = asSourceClasses(importClassNames, exclusionFilter);
							processImports(configClass, currentSourceClass, importSourceClasses, exclusionFilter, false);
						}
					}
					//判断是否实现ImportBeanDefinitionRegistrar接口，如果是的话，实例化这个registrar对象，绑定到当前配置类，后续会用它来注册bean definition
					else if (candidate.isAssignable(ImportBeanDefinitionRegistrar.class)) {
						// Candidate class is an ImportBeanDefinitionRegistrar ->
						// delegate to it to register additional bean definitions
						Class<?> candidateClass = candidate.loadClass();
						ImportBeanDefinitionRegistrar registrar =
								ParserStrategyUtils.instantiateClass(candidateClass, ImportBeanDefinitionRegistrar.class,
										this.environment, this.resourceLoader, this.registry);
						configClass.addImportBeanDefinitionRegistrar(registrar, currentSourceClass.getMetadata());
					}
					//既不是ImportSelector，也不是ImportBeanDefinitionRegistrar，按配置类处理。
					else {
						// Candidate class not an ImportSelector or ImportBeanDefinitionRegistrar ->
						// process it as an @Configuration class
						this.importStack.registerImport(
								currentSourceClass.getMetadata(), candidate.getMetadata().getClassName());
						processConfigurationClass(candidate.asConfigClass(configClass), exclusionFilter);
					}
				}
			}
			catch (BeanDefinitionStoreException ex) {
				throw ex;
			}
			catch (Throwable ex) {
				throw new BeanDefinitionStoreException(
						"Failed to process import candidates for configuration class [" +
						configClass.getMetadata().getClassName() + "]", ex);
			}
			finally {
				this.importStack.pop();
			}
		}
	}

当processConfigurationClass方法结束，也就是主配置类处理完毕，回到parser.parse方法中，我们自己编写的配置类已经都解析到了，但是我们依赖的第三方自动配置类还没有处理。这时用到刚才保存起来的 ImportSelector，导入自动配置类。

this.deferredImportSelectorHandler.process();

		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<>();
			}
		}

handler.processGroupImports();

		public void processGroupImports() {
			for (DeferredImportSelectorGrouping grouping : this.groupings.values()) {
				Predicate<String> exclusionFilter = grouping.getCandidateFilter();
				//此时这个AutoConfigurationImportSelector内部的AutoConfigurationGroup对象里的条目是空的，需要通过getImports()来获取。
				grouping.getImports().forEach(entry -> {
					ConfigurationClass configurationClass = this.configurationClasses.get(entry.getMetadata());
					try {
						//处理每个条目记录的ImportClass，也就是引入的自动配置类，可能会再进入processConfigurationClass方法进行解析。
						processImports(configurationClass, asSourceClass(configurationClass, exclusionFilter),
								Collections.singleton(asSourceClass(entry.getImportClassName(), exclusionFilter)),
								exclusionFilter, false);
					}
					catch (BeanDefinitionStoreException ex) {
						throw ex;
					}
					catch (Throwable ex) {
						throw new BeanDefinitionStoreException(
								"Failed to process import candidates for configuration class [" +
										configurationClass.getMetadata().getClassName() + "]", ex);
					}
				});
			}
		}

grouping.getImports()

		public Iterable<Group.Entry> getImports() {
			for (DeferredImportSelectorHolder deferredImport : this.deferredImports) {
				this.group.process(deferredImport.getConfigurationClass().getMetadata(),
						deferredImport.getImportSelector());
			}
			return this.group.selectImports();
		}

this.group.process(deferredImport.getConfigurationClass().getMetadata(),
deferredImport.getImportSelector());

		public void process(AnnotationMetadata annotationMetadata, DeferredImportSelector deferredImportSelector) {
			Assert.state(deferredImportSelector instanceof AutoConfigurationImportSelector,
					() -> String.format("Only %s implementations are supported, got %s",
							AutoConfigurationImportSelector.class.getSimpleName(),
							deferredImportSelector.getClass().getName()));
			//获取自动配置类条目
			AutoConfigurationEntry autoConfigurationEntry = ((AutoConfigurationImportSelector) deferredImportSelector)
					.getAutoConfigurationEntry(annotationMetadata);
			this.autoConfigurationEntries.add(autoConfigurationEntry);
			for (String importClassName : autoConfigurationEntry.getConfigurations()) {
				this.entries.putIfAbsent(importClassName, annotationMetadata);
			}
		}

((AutoConfigurationImportSelector) deferredImportSelector).getAutoConfigurationEntry(annotationMetadata)

	protected AutoConfigurationEntry getAutoConfigurationEntry(AnnotationMetadata annotationMetadata) {
		if (!isEnabled(annotationMetadata)) {
			return EMPTY_ENTRY;
		}
		AnnotationAttributes attributes = getAttributes(annotationMetadata);
		//读取META-INF/spring.factories文件（这里读了缓存）中EnableAutoConfiguration属性值，这里拿到135个配置类
		List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes);
		configurations = removeDuplicates(configurations);
		Set<String> exclusions = getExclusions(annotationMetadata, attributes);
		checkExcludedClasses(configurations, exclusions);
		configurations.removeAll(exclusions);
		/*
		getConfigurationClassFilter()读取spring.factories中的AutoConfigurationImportFilter属性值，拿到了3个过滤器：
		OnWebApplicationCondition，对应@ConditionalOnWebApplication注解；
		OnClassCondition，对应@ConditionalOnClass、@ConditionalOnMissingClass注解；
		OnBeanCondition，对应@ConditionalOnBean、@ConditionalOnSingleCandidate、@ConditionalOnMissingBean注解。
		然后对刚才的135个配置类进行过滤。
		调用filter方法，通过过滤器筛选出最终符合要求的自动配置类。比如OnClassCondition过滤器，就是用来判断某个类是否存在；他会检查配置类是否有@ConditionalOnClass(value=XXX)这种注解；如果它有这个注解，但是它所依赖的XXX类不存在，那么这个配置类就会被过滤掉。
		这里最终过滤之后剩下了32个自动配置类。
		*/
		configurations = getConfigurationClassFilter().filter(configurations);
		fireAutoConfigurationImportEvents(configurations, exclusions);
		return new AutoConfigurationEntry(configurations, exclusions);
	}

getImports方法获取到的Entry结构如下图：

随着引入的自动配置类逐个解析完毕，parser.parse方法结束，配置类解析完毕。接下来需要利用这些配置类进行bean definition的注册，
也就是ConfigurationClassPostProcessor#processConfigBeanDefinitions方法中的这一行：this.reader.loadBeanDefinitions(configClasses);
具体逻辑在下面这个方法中：

	private void loadBeanDefinitionsForConfigurationClass(
			ConfigurationClass configClass, TrackedConditionEvaluator trackedConditionEvaluator) {

		if (trackedConditionEvaluator.shouldSkip(configClass)) {
			String beanName = configClass.getBeanName();
			if (StringUtils.hasLength(beanName) && this.registry.containsBeanDefinition(beanName)) {
				this.registry.removeBeanDefinition(beanName);
			}
			this.importRegistry.removeImportingClass(configClass.getMetadata().getClassName());
			return;
		}

		//如果是被引入的，直接注册bean definition
		if (configClass.isImported()) {
			registerBeanDefinitionForImportedConfigurationClass(configClass);
		}
		//如果有@Bean方法，对方法进行解析（包括方法注解、方法名等），利用这些信息注册bean definition
		for (BeanMethod beanMethod : configClass.getBeanMethods()) {
			loadBeanDefinitionsForBeanMethod(beanMethod);
		}

		//通过导入的资源文件注册bean definition
		loadBeanDefinitionsFromImportedResources(configClass.getImportedResources());
		/*
		通过注册器注册bean definition，比如之前启动类@AutoConfigurationPackage注解中包含的
		@Import(AutoConfigurationPackages.Registrar.class)注解，引入的就是一个ImportBeanDefinitionRegistrar
		*/
		loadBeanDefinitionsFromRegistrars(configClass.getImportBeanDefinitionRegistrars());
	}

顺带看下刚才引入的AutoConfigurationPackages.Registrar：

	static class Registrar implements ImportBeanDefinitionRegistrar, DeterminableImports {

		@Override
		public void registerBeanDefinitions(AnnotationMetadata metadata, BeanDefinitionRegistry registry) {
			/*
			PackageImports构造方法中，解析当前配置类（这里是启动类）的@AutoConfigurationPackage注解属性，设置packageNames，如果没有值就用当前配置类所在的包名。
			register方法注册一个名为"org.springframework.boot.autoconfigure.AutoConfigurationPackages"的 bean definition，用来保存这些自动配置包路径名。
			*/
			register(registry, new PackageImports(metadata).getPackageNames().toArray(new String[0]));
		}

		@Override
		public Set<Object> determineImports(AnnotationMetadata metadata) {
			return Collections.singleton(new PackageImports(metadata));
		}

	}

到这里，ConfigurationClassPostProcessor的处理基本就结束了，后续还会处理一些其他的BeanDefinitionRegistryPostProcessor 和 BeanFactoryPostProcessor。

registerBeanPostProcessors(beanFactory)
这一步主要是从bean factory 中找到类型为BeanPostProcessor 的bean definition，然后进行实例化和初始化，得到BeanPostProcessor对象，添加到bean factory中。

onRefresh();

	@Override
	protected void onRefresh() {
		super.onRefresh();
		try {
			//启动web服务（默认tomcat）
			createWebServer();
		}
		catch (Throwable ex) {
			throw new ApplicationContextException("Unable to start web server", ex);
		}
	}

finishBeanFactoryInitialization(beanFactory);

初始化剩余的单例bean，完成bean factory的初始化。详情转到Spring boot启动流程-bean的实例化和初始化

doGetBean

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

		String beanName = transformedBeanName(name);
		Object beanInstance;

		// Eagerly check singleton cache for manually registered singletons.
		//getSingleton，先尝试从缓存中获取已经实例化的单例bean，这里也允许为正在创建中的bean提供一个早期访问的引用（解决循环依赖）。
		Object sharedInstance = getSingleton(beanName);
		if (sharedInstance != null && args == null) {
			if (logger.isTraceEnabled()) {
				if (isSingletonCurrentlyInCreation(beanName)) {
					logger.trace("Returning eagerly cached instance of singleton bean '" + beanName +
							"' that is not fully initialized yet - a consequence of a circular reference");
				}
				else {
					logger.trace("Returning cached instance of singleton bean '" + beanName + "'");
				}
			}
			beanInstance = getObjectForBeanInstance(sharedInstance, name, beanName, null);
		}

		else {
			// Fail if we're already creating this bean instance:
			// We're assumably within a circular reference.
			if (isPrototypeCurrentlyInCreation(beanName)) {
				throw new BeanCurrentlyInCreationException(beanName);
			}

			// Check if bean definition exists in this factory.
			//如果bean没有获取到，则在这里判断是否满足：父工厂存在且当前工厂中没有对应的bean definition；
			//如果满足，则在父工厂中查找；
			//如果不满足，则先确保当前bean所依赖的bean已经初始化，然后再创建当前bean实例。
			BeanFactory parentBeanFactory = getParentBeanFactory();
			if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
				// Not found -> check parent.
				String nameToLookup = originalBeanName(name);
				if (parentBeanFactory instanceof AbstractBeanFactory) {
					return ((AbstractBeanFactory) parentBeanFactory).doGetBean(
							nameToLookup, requiredType, args, typeCheckOnly);
				}
				else if (args != null) {
					// Delegation to parent with explicit args.
					return (T) parentBeanFactory.getBean(nameToLookup, args);
				}
				else if (requiredType != null) {
					// No args -> delegate to standard getBean method.
					return parentBeanFactory.getBean(nameToLookup, requiredType);
				}
				else {
					return (T) parentBeanFactory.getBean(nameToLookup);
				}
			}

			if (!typeCheckOnly) {
				markBeanAsCreated(beanName);
			}

			StartupStep beanCreation = this.applicationStartup.start("spring.beans.instantiate")
					.tag("beanName", name);
			try {
				if (requiredType != null) {
					beanCreation.tag("beanType", requiredType::toString);
				}
				//RootBeanDefinition，本质上是Spring BeanFactory运行时的统一bean definition视图
				//获取merged bean definition，所谓的merged，是检查当前bean definition是否为子bean definition，如果是的话需要与父bean definition进行合并。
				RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
				checkMergedBeanDefinition(mbd, beanName, args);

				// Guarantee initialization of beans that the current bean depends on.
				//确保当前bean所依赖的bean已经初始化完毕
				String[] dependsOn = mbd.getDependsOn();
				if (dependsOn != null) {
					for (String dep : dependsOn) {
						if (isDependent(beanName, dep)) {
							throw new BeanCreationException(mbd.getResourceDescription(), beanName,
									"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
						}
						registerDependentBean(dep, beanName);
						try {
							getBean(dep);
						}
						catch (NoSuchBeanDefinitionException ex) {
							throw new BeanCreationException(mbd.getResourceDescription(), beanName,
									"'" + beanName + "' depends on missing bean '" + dep + "'", ex);
						}
					}
				}

				// Create bean instance.（创建bean实例）
				if (mbd.isSingleton()) {
					sharedInstance = getSingleton(beanName, () -> {
						try {
							return createBean(beanName, mbd, args);
						}
						catch (BeansException ex) {
							// Explicitly remove instance from singleton cache: It might have been put there
							// eagerly by the creation process, to allow for circular reference resolution.
							// Also remove any beans that received a temporary reference to the bean.
							destroySingleton(beanName);
							throw ex;
						}
					});
					beanInstance = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
				}

				else if (mbd.isPrototype()) {
					// It's a prototype -> create a new instance.
					Object prototypeInstance = null;
					try {
						beforePrototypeCreation(beanName);
						prototypeInstance = createBean(beanName, mbd, args);
					}
					finally {
						afterPrototypeCreation(beanName);
					}
					beanInstance = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
				}

				else {
					String scopeName = mbd.getScope();
					if (!StringUtils.hasLength(scopeName)) {
						throw new IllegalStateException("No scope name defined for bean '" + beanName + "'");
					}
					Scope scope = this.scopes.get(scopeName);
					if (scope == null) {
						throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
					}
					try {
						Object scopedInstance = scope.get(beanName, () -> {
							beforePrototypeCreation(beanName);
							try {
								return createBean(beanName, mbd, args);
							}
							finally {
								afterPrototypeCreation(beanName);
							}
						});
						beanInstance = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
					}
					catch (IllegalStateException ex) {
						throw new ScopeNotActiveException(beanName, scopeName, ex);
					}
				}
			}
			catch (BeansException ex) {
				beanCreation.tag("exception", ex.getClass().toString());
				beanCreation.tag("message", String.valueOf(ex.getMessage()));
				cleanupAfterBeanCreationFailure(beanName);
				throw ex;
			}
			finally {
				beanCreation.end();
			}
		}

		return adaptBeanInstance(name, beanInstance, requiredType);
	}

Object sharedInstance = getSingleton(beanName);

	protected Object getSingleton(String beanName, boolean allowEarlyReference) {
		// Quick check for existing instance without full singleton lock
		Object singletonObject = this.singletonObjects.get(beanName);
		if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
			singletonObject = this.earlySingletonObjects.get(beanName);
			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);
							if (singletonFactory != null) {
								singletonObject = singletonFactory.getObject();
								this.earlySingletonObjects.put(beanName, singletonObject);
								this.singletonFactories.remove(beanName);
							}
						}
					}
				}
			}
		}
		return singletonObject;
	}

getSingleton重载方法 -> createBean

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

		if (logger.isTraceEnabled()) {
			logger.trace("Creating instance of bean '" + beanName + "'");
		}
		RootBeanDefinition mbdToUse = mbd;

		// Make sure bean class is actually resolved at this point, and
		// clone the bean definition in case of a dynamically resolved Class
		// which cannot be stored in the shared merged bean definition.
		Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
		if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
			mbdToUse = new RootBeanDefinition(mbd);
			mbdToUse.setBeanClass(resolvedClass);
		}

		// Prepare method overrides.
		try {
			mbdToUse.prepareMethodOverrides();
		}
		catch (BeanDefinitionValidationException ex) {
			throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),
					beanName, "Validation of method overrides failed", ex);
		}

		try {
			// Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
			//实例化之前，给BeanPostProcessors一个机会来返回一个目标bean实例的代理，需要实现InstantiationAwareBeanPostProcessor接口。
			Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
			if (bean != null) {
				return bean;
			}
		}
		catch (Throwable ex) {
			throw new BeanCreationException(mbdToUse.getResourceDescription(), beanName,
					"BeanPostProcessor before instantiation of bean failed", ex);
		}

		try {
			Object beanInstance = doCreateBean(beanName, mbdToUse, args);
			if (logger.isTraceEnabled()) {
				logger.trace("Finished creating instance of bean '" + beanName + "'");
			}
			return beanInstance;
		}
		catch (BeanCreationException | ImplicitlyAppearedSingletonException ex) {
			// A previously detected exception with proper bean creation context already,
			// or illegal singleton state to be communicated up to DefaultSingletonBeanRegistry.
			throw ex;
		}
		catch (Throwable ex) {
			throw new BeanCreationException(
					mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", ex);
		}
	}

createBean -> doCreateBean：

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

		// Instantiate the bean.
		BeanWrapper instanceWrapper = null;
		if (mbd.isSingleton()) {
			instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
		}
		if (instanceWrapper == null) {
			//实例化bean
			instanceWrapper = createBeanInstance(beanName, mbd, args);
		}
		Object bean = instanceWrapper.getWrappedInstance();
		Class<?> beanType = instanceWrapper.getWrappedClass();
		if (beanType != NullBean.class) {
			mbd.resolvedTargetType = beanType;
		}

		// Allow post-processors to modify the merged bean definition.
		//调用BeanPostProcessor，解析@Autowire，@Resource等注解？
		synchronized (mbd.postProcessingLock) {
			if (!mbd.postProcessed) {
				try {
					applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
				}
				catch (Throwable ex) {
					throw new BeanCreationException(mbd.getResourceDescription(), beanName,
							"Post-processing of merged bean definition failed", ex);
				}
				mbd.postProcessed = true;
			}
		}

		// Eagerly cache singletons to be able to resolve circular references（提前缓存单例以解决循环引用问题）
		// even when triggered by lifecycle interfaces like BeanFactoryAware.
		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");
			}
			//获取对bean进行早期访问的引用，添加到bean factory的singletonFactories变量中。
			addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
		}

		// Initialize the bean instance.（初始化bean）
		Object exposedObject = bean;
		try {
			populateBean(beanName, mbd, instanceWrapper);
			exposedObject = initializeBean(beanName, exposedObject, mbd);
		}
		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);
			}
		}

		if (earlySingletonExposure) {
			Object earlySingletonReference = getSingleton(beanName, false);
			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;
	}

doCreateBean -> createBeanInstance

createBeanInstance -> instantiateBean：

populateBean：

instantiateBean

instantiate

instantiateClass

populateBean

initializeBean

adaptBeanInstance