Spring Bean 的创建全过程！

1.1 Spring测试环境搭建

Spring模块概览，绿色是模块，Spring中八大模块，黑色表示该模块包含的jar包（组件）。例如我们想要用IOC容器，也就是绿色的CoreContainer，我们需要导入Beans，Core,Context,SpEL（spring-expression）四个包。

Spring模块概览

• Test：测试相关
• Core Container：IOC容器
• AOP：面向切面编程
• Aspects：切面
• Instrumenttation：跟JDK关联，一般不用
• Messaging：消息服务，一般不用
• Data Access/Integration：数据访问与集成（JDBC访问，Transaction事务，ORM对象关系映射，OXM和XML映射一般不用，JMS为Java消息服务Java-message-service一般不用）
• Web：Web服务（WebSocket网络通信协议，Servlet, Web，Portlet一般不用）

最偷懒的方式，是直接导入Spring-Framework。但是可能导入不必要的包，导致项目打包后比较大

由于Spring-Content中的ApplicationContent是整个IOC的入口。我们导入Spring-context包即可

<!-- https://mvnrepository.com/artifact/org.springframework/spring-context -->
<dependency>
    <groupId>org.springframework</groupId>
    <artifactId>spring-context</artifactId>
    <version>5.2.3.RELEASE</version>
</dependency>

我们导入spring-content后，默认会导入该组件的依赖jar，spring-content底层的依赖可以看到，实际上我们是导入了Core Container模块：

  <dependencies>
    <dependency>
      <groupId>org.springframework</groupId>
      <artifactId>spring-aop</artifactId>
      <version>5.2.3.RELEASE</version>
      <scope>compile</scope>
    </dependency>
    <dependency>
      <groupId>org.springframework</groupId>
      <artifactId>spring-beans</artifactId>
      <version>5.2.3.RELEASE</version>
      <scope>compile</scope>
    </dependency>
    <dependency>
      <groupId>org.springframework</groupId>
      <artifactId>spring-core</artifactId>
      <version>5.2.3.RELEASE</version>
      <scope>compile</scope>
    </dependency>
    <dependency>
      <groupId>org.springframework</groupId>
      <artifactId>spring-expression</artifactId>
      <version>5.2.3.RELEASE</version>
      <scope>compile</scope>
    </dependency>
  </dependencies>

新建Spring配置文件spring.xml：

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd">

    <!--注册一个对象，spring回自动创建这个对象-->
    <!--
    一个bean标签就表示一个对象
    id:这个对象的唯一标识
    class:注册对象的完全限定名
    -->
    <bean id="hello" class="com.xiaodai.service.Hello">
        <!--使用property标签给对象的属性赋值
        name:表示属性的名称
        value：表示属性的值
        -->
        <property name="id" value="1"></property>
        <property name="name" value="zhangsan"></property>
        <property name="age" value="18"></property>
    </bean>
</beans>

编写测试类：

import com.xiaodai.service.Hello;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public class Test {

    public static void main(String[] args) {
        ApplicationContext applicationContext = new ClassPathXmlApplicationContext("spring.xml");
        Hello hello = applicationContext.getBean("hello", Hello.class);
        System.out.println(hello.getName());

    }

}

1.2 Debug容器创建过程

从测试类的new ClassPathXmlApplicationContext(“spring.xml”)开始debug，进入ClassPathXmlApplicationContext，可以看到:

 public ClassPathXmlApplicationContext(
   String[] configLocations, boolean refresh, @Nullable ApplicationContext parent)
   throws BeansException {

  super(parent);
  // 设置配置文件路径
  setConfigLocations(configLocations);
  if (refresh) {
          // 核心步骤
   refresh();
  }
 }

加载配置文件后，进入refresh()方法，该方法是容器初始化的核心步骤。该方法包含十三个方法：

@Override
public void refresh() throws BeansException, IllegalStateException {
  synchronized (this.startupShutdownMonitor) {
   // Prepare this context for refreshing.
   /** 
    * 准备刷新，做一些最基本的准备化工作
   **/
   prepareRefresh();

   // Tell the subclass to refresh the internal bean factory.
   /** 
    * 获得一个刷新的bean容器，实质就是获取工厂。
    * 加载xml等配置文件，用该文件产生的BeanDefinition来创建一个工厂
   **/
   ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

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

   try {
    // Allows post-processing of the bean factory in context subclasses.
    // 后置增强，方便扩展
    postProcessBeanFactory(beanFactory);

    // Invoke factory processors registered as beans in the context.
    // 实例化并且执行BeanFactoryPostProcessors
    invokeBeanFactoryPostProcessors(beanFactory);

    // Register bean processors that intercept bean creation.
    // 实例化并且注册所有的BeanPostProcessor
    registerBeanPostProcessors(beanFactory);

    // Initialize message source for this context.
    // 国际化设置，一般用不到
    initMessageSource();

    // Initialize event multicaster for this context.
    // 初始化应用程序的多波器和广播器
    initApplicationEventMulticaster();

    // Initialize other special beans in specific context subclasses.
    // 空方法，预留给子类做扩展
    onRefresh();

    // Check for listener beans and register them.
    // 注册监听器
    registerListeners();

    // Instantiate all remaining (non-lazy-init) singletons.
    // 工作中常用，面试常问。实例化所有非懒加载的实例对象
    finishBeanFactoryInitialization(beanFactory);

    // Last step: publish corresponding event.
    // 完成刷新
    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();
   }
  }
}

1.3 AbstractApplicationContext的refresh()包含的13个方法分析

结合概览图一个一个方法分析：

方法1：prepareRefresh() => 准备工作

准备刷新，做一些最基本的准备化工作

 protected void prepareRefresh() {
  // Switch to active.
  // 设置开始时间
  this.startupDate = System.currentTimeMillis();
  // 关闭状态设置为false
  this.closed.set(false);
  // 活跃状态设置为true
  this.active.set(true);

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

  // Initialize any placeholder property sources in the context environment.
  // 初始化属性资源
  initPropertySources();

  // Validate that all properties marked as required are resolvable:
  // see ConfigurablePropertyResolver#setRequiredProperties
  // 获取环境信息，验证属性信息
  getEnvironment().validateRequiredProperties();

  // Store pre-refresh
  // 存储预刷新的一些应用信息的监听器
  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);
  }

  // Allow for the collection of early ApplicationEvents,
  // to be published once the multicaster is available...
  // 创建一些监听器事件的集合
  this.earlyApplicationEvents = new LinkedHashSet<>();
 }

总结：1.设置启动事件 2.设置关闭活跃的状态 3.获取环境对象并设置属性值 4.设置监听器以及需要发布事件的集合。

重要的点：

• 获取环境信息，验证属性信息，getEnvironment().validateRequiredProperties();
• 存储预刷新的一些应用信息的监听器，在Spring中是空实现，但是SpringBoot中，是有具体的值的

方法2：obtainFreshBeanFactory() => 获得一个刷新的bean容器

获得一个刷新的bean容器，实质就是获取工厂。创建容器对象DefaultListableBeanFactory;加载xml配置文件的属性到当前的工厂中，最重要的就是BeanDefinition

Bean工厂实例继承关系图

AbstractRefreshableApplicationContext:

// 只要进到这个方法，那么我们创建的一定是一个新的工厂
@Override
protected final void refreshBeanFactory() throws BeansException {
  if (hasBeanFactory()) {
      // 如果存在先销毁，后关闭
   destroyBeans();
   closeBeanFactory();
  }
  try {
      // 创建bean工厂，这里使用的就是DefaultListableBeanFactory。此时创建的工厂里面的属性值都是默认值
   DefaultListableBeanFactory beanFactory = createBeanFactory();
   // 序列化id
   beanFactory.setSerializationId(getId());
   // 设置一些属性值
   customizeBeanFactory(beanFactory);
   // 加载bean的定义属性值。该方法有很多重载，非常复杂，核心是do操作
   // 完成配置文件或者配置类文件的加载
   loadBeanDefinitions(beanFactory);
   synchronized (this.beanFactoryMonitor) {
    this.beanFactory = beanFactory;
   }
  }
  catch (IOException ex) {
   throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
  }
}

方法3：prepareBeanFactory(beanFactory) => 准备（初始化）Bean工厂

为方法2拿到的工厂，设置某些具体的值

 protected void prepareBeanFactory(ConfigurableListableBeanFactory beanFactory) {
  // Tell the internal bean factory to use the context's class loader etc.
  // 为bean工厂设置类加载器
  beanFactory.setBeanClassLoader(getClassLoader());
  // 设置SPEL解析器
  beanFactory.setBeanExpressionResolver(new StandardBeanExpressionResolver(beanFactory.getBeanClassLoader()));
  beanFactory.addPropertyEditorRegistrar(new ResourceEditorRegistrar(this, getEnvironment()));

  // Configure the bean factory with context callbacks.
  // 添加一个BeanPostProcessor
  beanFactory.addBeanPostProcessor(new ApplicationContextAwareProcessor(this));
  // 忽略对应接口的实现
  beanFactory.ignoreDependencyInterface(EnvironmentAware.class);
  beanFactory.ignoreDependencyInterface(EmbeddedValueResolverAware.class);
  beanFactory.ignoreDependencyInterface(ResourceLoaderAware.class);
  beanFactory.ignoreDependencyInterface(ApplicationEventPublisherAware.class);
  beanFactory.ignoreDependencyInterface(MessageSourceAware.class);
  beanFactory.ignoreDependencyInterface(ApplicationContextAware.class);

  // BeanFactory interface not registered as resolvable type in a plain factory.
  // MessageSource registered (and found for autowiring) as a bean.
        // 注册一些依赖
  beanFactory.registerResolvableDependency(BeanFactory.class, beanFactory);
  beanFactory.registerResolvableDependency(ResourceLoader.class, this);
  beanFactory.registerResolvableDependency(ApplicationEventPublisher.class, this);
  beanFactory.registerResolvableDependency(ApplicationContext.class, this);

  // Register early post-processor for detecting inner beans as
  // ApplicationListeners添加一个BeanPostProcessor增强器
  ApplicationListeners.
  beanFactory.addBeanPostProcessor(new ApplicationListenerDetector(this));

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

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

方法4：postProcessBeanFactory(beanFactory) => 后置增强Bean(扩展实现)

空方法，方便扩展

方法5：invokeBeanFactoryPostProcessors(beanFactory) => 执行BFPP

实例化并且执行BeanFactoryPostProcessors

 /**
  * Instantiate and invoke all registered BeanFactoryPostProcessor beans,
  * respecting explicit order if given.
  * <p>Must be called before singleton instantiation.
  * 单例对象之前一定调用，因为单例bean创建后就只有一份
  */
 protected void invokeBeanFactoryPostProcessors(ConfigurableListableBeanFactory beanFactory) {
  PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(beanFactory, getBeanFactoryPostProcessors());

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

方法6：registerBeanPostProcessors(beanFactory) => 注册BPP

实例化并且注册所有的BeanPostProcessor。实例化Bean之前的准备工作

 /**
  * Instantiate and register all BeanPostProcessor beans,
  * respecting explicit order if given.
  * <p>Must be called before any instantiation of application beans.
  */
 protected void registerBeanPostProcessors(ConfigurableListableBeanFactory beanFactory) {
  PostProcessorRegistrationDelegate.registerBeanPostProcessors(beanFactory, this);
 }

方法7：initMessageSource() => 国际化设置

方法8：initApplicationEventMulticaster() => 初始化应用程序的多波器和广播器

也属于准备工作

方法9：onRefresh() => 预留给子类做扩展

空方法

方法10：registerListeners() => 注册监听器

也属于准备工作

 /**
  * Add beans that implement ApplicationListener as listeners.
  * Doesn't affect other listeners, which can be added without being beans.
  */
 protected void registerListeners() {
  // 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!
  String[] listenerBeanNames = getBeanNamesForType(ApplicationListener.class, true, false);
  for (String listenerBeanName : listenerBeanNames) {
   getApplicationEventMulticaster().addApplicationListenerBean(listenerBeanName);
  }

  // Publish early application events now that we finally have a multicaster...
  Set<ApplicationEvent> earlyEventsToProcess = this.earlyApplicationEvents;
  this.earlyApplicationEvents = null;
  if (earlyEventsToProcess != null) {
   for (ApplicationEvent earlyEvent : earlyEventsToProcess) {
    getApplicationEventMulticaster().multicastEvent(earlyEvent);
   }
  }
 }

方法11：finishBeanFactoryInitialization(beanFactory) => 实例化所有单例对象

面试常问，工作常用。过程比较复杂

 /**
  * Finish the initialization of this context's bean factory,
  * initializing all remaining singleton beans.
  */
 protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
  // Initialize conversion service for this context.
  /**
   * 把类型转化操作，设置到当前的beanFactory里面去
  **/
  if (beanFactory.containsBean(CONVERSION_SERVICE_BEAN_NAME) &&
    beanFactory.isTypeMatch(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class)) {
   beanFactory.setConversionService(
     beanFactory.getBean(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class));
  }

  // Register a default embedded value resolver if no bean post-processor
  // (such as a PropertyPlaceholderConfigurer bean) registered any before:
  // at this point, primarily for resolution in annotation attribute values.
  /**
   * 判断当前的beanFactory有没有内置的值处理器
  **/
  if (!beanFactory.hasEmbeddedValueResolver()) {
   beanFactory.addEmbeddedValueResolver(strVal -> getEnvironment().resolvePlaceholders(strVal));
  }

  // Initialize LoadTimeWeaverAware beans early to allow for registering their transformers early.
  /**
   * 织入Aware
  **/
  String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
  for (String weaverAwareName : weaverAwareNames) {
   getBean(weaverAwareName);
  }

  // Stop using the temporary ClassLoader for type matching.
  // 设置类加载器
  beanFactory.setTempClassLoader(null);

  // Allow for caching all bean definition metadata, not expecting further changes.
  /**
   * 冻结：某些bean不需要进行修改操作了，放入
  **/
  beanFactory.freezeConfiguration();

  // Instantiate all remaining (non-lazy-init) singletons.
  /**
   * 实例化所有非懒加载的实例对象(重要)
  **/
  beanFactory.preInstantiateSingletons();
 }

实例化所有非懒加载的实例对象方法：

 @Override
 public void preInstantiateSingletons() throws BeansException {
  if (logger.isTraceEnabled()) {
   logger.trace("Pre-instantiating singletons in " + this);
  }

  // Iterate over a copy to allow for init methods which in turn register new bean definitions.
  // While this may not be part of the regular factory bootstrap, it does otherwise work fine.
  /**
   * 拿到所有注册bean的名称
   **/
  List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);

  // Trigger initialization of all non-lazy singleton beans...
  // 循环去创建我们需要的单例对象
  for (String beanName : beanNames) {
      // 拿到bean的定义信息，就是我们在xml配置文件里面指定的一些属性
   RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
   // 是否是抽象的，是否是单例的，是否是懒加载的
   if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
       // 判断当前类是否实现了factoryBean接口。一般没实现，直接进入下面的getBean
    if (isFactoryBean(beanName)) {
     Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
     if (bean instanceof FactoryBean) {
      final FactoryBean<?> factory = (FactoryBean<?>) bean;
      boolean isEagerInit;
      if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
       isEagerInit = AccessController.doPrivileged((PrivilegedAction<Boolean>)
           ((SmartFactoryBean<?>) factory)::isEagerInit,
         getAccessControlContext());
      }
      else {
       isEagerInit = (factory instanceof SmartFactoryBean &&
         ((SmartFactoryBean<?>) factory).isEagerInit());
      }
      if (isEagerInit) {
       getBean(beanName);
      }
     }
    }
    else {
        // 通过beanName。拿到bean
     getBean(beanName);
    }
   }
  }

  // Trigger post-initialization callback for all applicable beans...
  for (String beanName : beanNames) {
   Object singletonInstance = getSingleton(beanName);
   if (singletonInstance instanceof SmartInitializingSingleton) {
    final SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
    if (System.getSecurityManager() != null) {
     AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
      smartSingleton.afterSingletonsInstantiated();
      return null;
     }, getAccessControlContext());
    }
    else {
     smartSingleton.afterSingletonsInstantiated();
    }
   }
  }
 }

重要方法：

• getMergedLocalBeanDefinition

   /**
    * Return a merged RootBeanDefinition, traversing the parent bean definition
    * if the specified bean corresponds to a child bean definition.
    * @param beanName the name of the bean to retrieve the merged definition for
    * @return a (potentially merged) RootBeanDefinition for the given bean
    * @throws NoSuchBeanDefinitionException if there is no bean with the given name
    * @throws BeanDefinitionStoreException in case of an invalid bean definition
    */
    // 返回一个合并好的RootBeanDefinition。整合子类和父类
   protected RootBeanDefinition getMergedLocalBeanDefinition(String beanName) throws BeansException {
    // Quick check on the concurrent map first, with minimal locking.
    RootBeanDefinition mbd = this.mergedBeanDefinitions.get(beanName);
    if (mbd != null && !mbd.stale) {
     return mbd;
    }
    return getMergedBeanDefinition(beanName, getBeanDefinition(beanName));
   }

• getBean() => doGetBean()

   /**
    * Return an instance, which may be shared or independent, of the specified bean.
    * @param name the name of the bean to retrieve
    * @param requiredType the required type of the bean to retrieve
    * @param args arguments to use when creating a bean instance using explicit arguments
    * (only applied when creating a new instance as opposed to retrieving an existing one)
    * @param typeCheckOnly whether the instance is obtained for a type check,
    * not for actual use
    * @return an instance of the bean
    * @throws BeansException if the bean could not be created
    */
   @SuppressWarnings("unchecked")
   protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
     @Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
  
          // 获取beanName
    final String beanName = transformedBeanName(name);
    Object bean;
  
    // Eagerly check singleton cache for manually registered singletons.
    // 单例检查，如果一级，二级，三级缓存中存在该Bean,直接获取到了
    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 + "'");
      }
     }
     bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
    }
  
    else {
     // Fail if we're already creating this bean instance:
     // We're assumably within a circular reference.
     // 如果是单例对象的话，尝试解决循环依赖问题
     if (isPrototypeCurrentlyInCreation(beanName)) {
      throw new BeanCurrentlyInCreationException(beanName);
     }
  
     // Check if bean definition exists in this factory.
     // 获取父类容器
     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);
      }
     }
  
              // 标志位。如果不是类型检查，表示要创建bean,此处在集合中做一个记录
     if (!typeCheckOnly) {
      markBeanAsCreated(beanName);
     }
  
     try {
         // 获取beanDefinition
      final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
      // 检测beanDefinition
      checkMergedBeanDefinition(mbd, beanName, args);
  
      // Guarantee initialization of beans that the current bean depends on.
      // 检查当前的bean是否有其他依赖的bean
      String[] dependsOn = mbd.getDependsOn();
      if (dependsOn != null) {
          // 如果有依赖的bean,我们要先递归解决其他依赖的bean
       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.
      // 是否是单例的
      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;
        }
       });
       bean = 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);
       }
       bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
      }
  
      else {
       String scopeName = mbd.getScope();
       final 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);
         }
        });
        bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
       }
       catch (IllegalStateException ex) {
        throw new BeanCreationException(beanName,
          "Scope '" + scopeName + "' is not active for the current thread; consider " +
          "defining a scoped proxy for this bean if you intend to refer to it from a singleton",
          ex);
       }
      }
     }
     catch (BeansException ex) {
      cleanupAfterBeanCreationFailure(beanName);
      throw ex;
     }
    }
  
    // Check if required type matches the type of the actual bean instance.
    if (requiredType != null && !requiredType.isInstance(bean)) {
     try {
      T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);
      if (convertedBean == null) {
       throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
      }
      return convertedBean;
     }
     catch (TypeMismatchException ex) {
      if (logger.isTraceEnabled()) {
       logger.trace("Failed to convert bean '" + name + "' to required type '" +
         ClassUtils.getQualifiedName(requiredType) + "'", ex);
      }
      throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
     }
    }
    return (T) bean;
   }

• getSingleton

   /**
    * Return the (raw) singleton object registered under the given name,
    * creating and registering a new one if none registered yet.
    * @param beanName the name of the bean
    * @param singletonFactory the ObjectFactory to lazily create the singleton
    * with, if necessary
    * @return the registered singleton object
    */
   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);
     if (singletonObject == null) {
      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 + "'");
      }
      beforeSingletonCreation(beanName);
      boolean newSingleton = false;
      boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
      if (recordSuppressedExceptions) {
       this.suppressedExceptions = new LinkedHashSet<>();
      }
      try {
          // 实际上就是调用了CreateBean
       singletonObject = singletonFactory.getObject();
       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.
       singletonObject = this.singletonObjects.get(beanName);
       if (singletonObject == null) {
        throw ex;
       }
      }
      catch (BeanCreationException ex) {
       if (recordSuppressedExceptions) {
        for (Exception suppressedException : this.suppressedExceptions) {
         ex.addRelatedCause(suppressedException);
        }
       }
       throw ex;
      }
      finally {
       if (recordSuppressedExceptions) {
        this.suppressedExceptions = null;
       }
       afterSingletonCreation(beanName);
      }
      if (newSingleton) {
       addSingleton(beanName, singletonObject);
      }
     }
     return singletonObject;
    }
   }

• doCreateBean => 通过上方法的singletonObject = singletonFactory.getObject();进入的

   /**
    * Actually create the specified bean. Pre-creation processing has already happened
    * at this point, e.g. checking {@code postProcessBeforeInstantiation} callbacks.
    * <p>Differentiates between default bean instantiation, use of a
    * factory method, and autowiring a constructor.
    * @param beanName the name of the bean
    * @param mbd the merged bean definition for the bean
    * @param args explicit arguments to use for constructor or factory method invocation
    * @return a new instance of the bean
    * @throws BeanCreationException if the bean could not be created
    * @see #instantiateBean
    * @see #instantiateUsingFactoryMethod
    * @see #autowireConstructor
    */
   protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
     throws BeanCreationException {
  
    // Instantiate the bean.
    BeanWrapper instanceWrapper = null;
    if (mbd.isSingleton()) {
     instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
    }
    if (instanceWrapper == null) {
        /**
         * 核心的创建实例化Bean的过程
        **/
     instanceWrapper = createBeanInstance(beanName, mbd, args);
    }
    final Object bean = instanceWrapper.getWrappedInstance();
    Class<?> beanType = instanceWrapper.getWrappedClass();
    if (beanType != NullBean.class) {
     mbd.resolvedTargetType = beanType;
    }
  
    // Allow post-processors to modify the merged bean definition.
    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
    // even when triggered by lifecycle interfaces like BeanFactoryAware.
    boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
      isSingletonCurrentlyInCreation(beanName));
      
    /**
     * 解决循环依赖。使用三级缓存
    **/
    if (earlySingletonExposure) {
     if (logger.isTraceEnabled()) {
      logger.trace("Eagerly caching bean '" + beanName +
        "' to allow for resolving potential circular references");
     }
     addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
    }
  
    // Initialize the bean instance.
    Object exposedObject = bean;
    try {
        /**
         * 填充属性，上文的实例化只是默认属性值。填充属性是初始化的第一步，第二步是执行init方法
        **/
     populateBean(beanName, mbd, instanceWrapper);
     /**
      * 执行init方法
     **/
     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 " +
          "'getBeanNamesOfType' with the 'allowEagerInit' flag turned off, for example.");
       }
      }
     }
    }
  
    // Register bean as disposable.
    try {
        /**
         * 需要销毁的时候，销毁的钩子函数
        **/
     registerDisposableBeanIfNecessary(beanName, bean, mbd);
    }
    catch (BeanDefinitionValidationException ex) {
     throw new BeanCreationException(
       mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
    }
  
    return exposedObject;
   }

  面试题：为甚么循环依赖的解决要使用三级缓存？关注公众号Java技术栈回复面试获取 Spring 系列面试题及答案。

• createBeanInstance => 核心的创建和实例化bean的过程，由doCreateBean调用

大量的反射出现在该方法中，用来创建对象

 /**
  * Create a new instance for the specified bean, using an appropriate instantiation strategy:
  * factory method, constructor autowiring, or simple instantiation.
  * @param beanName the name of the bean
  * @param mbd the bean definition for the bean
  * @param args explicit arguments to use for constructor or factory method invocation
  * @return a BeanWrapper for the new instance
  * @see #obtainFromSupplier
  * @see #instantiateUsingFactoryMethod
  * @see #autowireConstructor
  * @see #instantiateBean
  */
 protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {
  // Make sure bean class is actually resolved at this point.
  Class<?> beanClass = resolveBeanClass(mbd, beanName);

  if (beanClass != null && !Modifier.isPublic(beanClass.getModifiers()) && !mbd.isNonPublicAccessAllowed()) {
   throw new BeanCreationException(mbd.getResourceDescription(), beanName,
     "Bean class isn't public, and non-public access not allowed: " + beanClass.getName());
  }

  Supplier<?> instanceSupplier = mbd.getInstanceSupplier();
  if (instanceSupplier != null) {
   return obtainFromSupplier(instanceSupplier, beanName);
  }

  if (mbd.getFactoryMethodName() != null) {
   return instantiateUsingFactoryMethod(beanName, mbd, args);
  }

  // Shortcut when re-creating the same bean...
  boolean resolved = false;
  boolean autowireNecessary = false;
  if (args == null) {
   synchronized (mbd.constructorArgumentLock) {
    if (mbd.resolvedConstructorOrFactoryMethod != null) {
     resolved = true;
     autowireNecessary = mbd.constructorArgumentsResolved;
    }
   }
  }
  if (resolved) {
   if (autowireNecessary) {
    return autowireConstructor(beanName, mbd, null, null);
   }
   else {
    return instantiateBean(beanName, mbd);
   }
  }

  // Candidate constructors for autowiring?
  // 构造器
  Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
  if (ctors != null || mbd.getResolvedAutowireMode() == AUTOWIRE_CONSTRUCTOR ||
    mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
   return autowireConstructor(beanName, mbd, ctors, args);
  }

  // Preferred constructors for default construction?
  ctors = mbd.getPreferredConstructors();
  if (ctors != null) {
   return autowireConstructor(beanName, mbd, ctors, null);
  }

  // No special handling: simply use no-arg constructor.
  /**
   * 默认无参构造
   **/
  return instantiateBean(beanName, mbd);
 }
instantiateBean(beanName, mbd) => 默认无参构造
 /**
  * Instantiate the given bean using its default constructor.
  * @param beanName the name of the bean
  * @param mbd the bean definition for the bean
  * @return a BeanWrapper for the new instance
  */
 protected BeanWrapper instantiateBean(final String beanName, final RootBeanDefinition mbd) {
  try {
   Object beanInstance;
   final BeanFactory parent = this;
   if (System.getSecurityManager() != null) {
    beanInstance = AccessController.doPrivileged((PrivilegedAction<Object>) () ->
            // 实例化只会分配内存空间，设置默认值
      getInstantiationStrategy().instantiate(mbd, beanName, parent),
      getAccessControlContext());
   }
   else {
    beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, parent);
   }
   BeanWrapper bw = new BeanWrapperImpl(beanInstance);
   initBeanWrapper(bw);
   return bw;
  }
  catch (Throwable ex) {
   throw new BeanCreationException(
     mbd.getResourceDescription(), beanName, "Instantiation of bean failed", ex);
  }
 }

• instantiate

   @Override
   public Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner) {
    // Don't override the class with CGLIB if no overrides.
    if (!bd.hasMethodOverrides()) {
     Constructor<?> constructorToUse;
     synchronized (bd.constructorArgumentLock) {
      constructorToUse = (Constructor<?>) bd.resolvedConstructorOrFactoryMethod;
      if (constructorToUse == null) {
       final Class<?> clazz = bd.getBeanClass();
       if (clazz.isInterface()) {
        throw new BeanInstantiationException(clazz, "Specified class is an interface");
       }
       try {
        if (System.getSecurityManager() != null) {
         constructorToUse = AccessController.doPrivileged(
           (PrivilegedExceptionAction<Constructor<?>>) clazz::getDeclaredConstructor);
        }
        else {
         constructorToUse = clazz.getDeclaredConstructor();
        }
        bd.resolvedConstructorOrFactoryMethod = constructorToUse;
       }
       catch (Throwable ex) {
        throw new BeanInstantiationException(clazz, "No default constructor found", ex);
       }
      }
     }
     return BeanUtils.instantiateClass(constructorToUse);
    }
    else {
     // Must generate CGLIB subclass.
     return instantiateWithMethodInjection(bd, beanName, owner);
    }
   }

• BeanUtils.instantiateClass => 通过构造器反射创建bean

   /**
    * Convenience method to instantiate a class using the given constructor.
    * <p>Note that this method tries to set the constructor accessible if given a
    * non-accessible (that is, non-public) constructor, and supports Kotlin classes
    * with optional parameters and default values.
    * @param ctor the constructor to instantiate
    * @param args the constructor arguments to apply (use {@code null} for an unspecified
    * parameter, Kotlin optional parameters and Java primitive types are supported)
    * @return the new instance
    * @throws BeanInstantiationException if the bean cannot be instantiated
    * @see Constructor#newInstance
    */
   public static <T> T instantiateClass(Constructor<T> ctor, Object... args) throws BeanInstantiationException {
    Assert.notNull(ctor, "Constructor must not be null");
    try {
     ReflectionUtils.makeAccessible(ctor);
     if (KotlinDetector.isKotlinReflectPresent() && KotlinDetector.isKotlinType(ctor.getDeclaringClass())) {
      return KotlinDelegate.instantiateClass(ctor, args);
     }
     else {
      Class<?>[] parameterTypes = ctor.getParameterTypes();
      Assert.isTrue(args.length <= parameterTypes.length, "Can't specify more arguments than constructor parameters");
      Object[] argsWithDefaultValues = new Object[args.length];
      for (int i = 0 ; i < args.length; i++) {
       if (args[i] == null) {
        Class<?> parameterType = parameterTypes[i];
        argsWithDefaultValues[i] = (parameterType.isPrimitive() ? DEFAULT_TYPE_VALUES.get(parameterType) : null);
       }
       else {
        argsWithDefaultValues[i] = args[i];
       }
      }
      return ctor.newInstance(argsWithDefaultValues);
     }
    }
    catch (InstantiationException ex) {
     throw new BeanInstantiationException(ctor, "Is it an abstract class?", ex);
    }
    catch (IllegalAccessException ex) {
     throw new BeanInstantiationException(ctor, "Is the constructor accessible?", ex);
    }
    catch (IllegalArgumentException ex) {
     throw new BeanInstantiationException(ctor, "Illegal arguments for constructor", ex);
    }
    catch (InvocationTargetException ex) {
     throw new BeanInstantiationException(ctor, "Constructor threw exception", ex.getTargetException());
    }
   }

方法12：finishRefresh() => 完成刷新

 /**
  * Finish the refresh of this context, invoking the LifecycleProcessor's
  * onRefresh() method and publishing the
  * {@link org.springframework.context.event.ContextRefreshedEvent}.
  */
 protected void finishRefresh() {
  // Clear context-level resource caches (such as ASM metadata from scanning).
  // 
  clearResourceCaches();

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

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

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

  // Participate in LiveBeansView MBean, if active.
  LiveBeansView.registerApplicationContext(this);
 }

方法13：resetCommonCaches() => 缓存重置

 /**
  * Reset Spring's common reflection metadata caches, in particular the
  * {@link ReflectionUtils}, {@link AnnotationUtils}, {@link ResolvableType}
  * and {@link CachedIntrospectionResults} caches.
  * @since 4.2
  * @see ReflectionUtils#clearCache()
  * @see AnnotationUtils#clearCache()
  * @see ResolvableType#clearCache()
  * @see CachedIntrospectionResults#clearClassLoader(ClassLoader)
  */
 protected void resetCommonCaches() {
  ReflectionUtils.clearCache();
  AnnotationUtils.clearCache();
  ResolvableType.clearCache();
  CachedIntrospectionResults.clearClassLoader(getClassLoader());
 }

最后回顾整个流程概览图：