Spring之IOC容器初始化

前言

在前面我们分析了最底层的IOC容器BeanFactory,接着简单分析了高级形态的容器ApplicationContext，在ApplicationContext 中我们知道一个核心方法 refresh，这里面就是IOC容器的初始化流程，在前面并没有直接去分析它，只是简单的分析了BeanDefinition的载入,解析注册，有了这些基础后，再来完整的分析IOC容器的启动流程。

refresh 源码分析

在 AbstractApplicationContext 中以及将refresh整个流程定义出来了，我们再来看refresh 源码。

@Override
public void refresh() throws BeansException, IllegalStateException {
    synchronized (this.startupShutdownMonitor) {
        // Prepare this context for refreshing.
        prepareRefresh();

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

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

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

            // Invoke factory processors registered as beans in the context.
            invokeBeanFactoryPostProcessors(beanFactory);

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

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

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

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

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

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

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

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

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

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

            // Propagate exception to caller.
            throw ex;
        }

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

下面我们针对refresh里面的执行流程一个一个的进行分析，来看它做了什么并且是如何实现的。

prepareRefresh()

protected void prepareRefresh() {
   // 记录启动时间，
   this.startupDate = System.currentTimeMillis();
   // 设置容器状态
   this.closed.set(false);
   this.active.set(true);

   if (logger.isInfoEnabled()) {
      logger.info("Refreshing " + this);
   }

   // 初始化一些属性设置(一个空方法，留给子类具体实现)
   initPropertySources();

   // 校验属性的合法性
   getEnvironment().validateRequiredProperties();

   //保存容器中一些早期的事件
   this.earlyApplicationEvents = new LinkedHashSet<ApplicationEvent>();
}

创建IOC容器

refresh方法中的下一行 obtainFreshBeanFactory()。

注意，这个方法很重要，这里将会初始化 BeanFactory、加载 Bean、注册 Bean 等等。

AbstractApplicationContext -> obtainFreshBeanFactory:

protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {
   // 关闭旧的 BeanFactory (如果有)，创建新的 BeanFactory，加载 Bean 定义、注册 Bean 等等
   refreshBeanFactory();

   ConfigurableListableBeanFactory beanFactory = getBeanFactory();
   if (logger.isDebugEnabled()) {
      logger.debug("Bean factory for " + getDisplayName() + ": " + beanFactory);
   }
   return beanFactory;
}

AbstractRefreshableApplicationContext->refreshBeanFactory:

@Override
protected final void refreshBeanFactory() throws BeansException {
  
   if (hasBeanFactory()) {
      destroyBeans();
      closeBeanFactory();
   }
   try {
      // 初始化一个 DefaultListableBeanFactory
      DefaultListableBeanFactory beanFactory = createBeanFactory();
      // 用于 BeanFactory 的序列化
      beanFactory.setSerializationId(getId());

      // 设置 BeanFactory 的两个配置属性：是否允许 Bean 覆盖、是否允许循环引用
      customizeBeanFactory(beanFactory);

      // 加载 Bean 到 BeanFactory 中
      loadBeanDefinitions(beanFactory);
      synchronized (this.beanFactoryMonitor) {
         this.beanFactory = beanFactory;
      }
   }
   catch (IOException ex) {
      throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
   }
}

这里主要看一下 customizeBeanFactory 方法：

protected void customizeBeanFactory(DefaultListableBeanFactory beanFactory) {
  if (this.allowBeanDefinitionOverriding != null) {
    // 是否允许 Bean 定义覆盖
    beanFactory.setAllowBeanDefinitionOverriding(this.allowBeanDefinitionOverriding);
  }
  if (this.allowCircularReferences != null) {
    // 是否允许 Bean 间的循环依赖
    beanFactory.setAllowCircularReferences(this.allowCircularReferences);
  }
}

prepareBeanFactory

BeanFactory预准备工作

protected void prepareBeanFactory(ConfigurableListableBeanFactory beanFactory) {
    //设置类加载器
    beanFactory.setBeanClassLoader(getClassLoader());
    //设置支持表达式的解析器
    beanFactory.setBeanExpressionResolver(new StandardBeanExpressionResolver(beanFactory.getBeanClassLoader()));
    beanFactory.addPropertyEditorRegistrar(new ResourceEditorRegistrar(this, getEnvironment()));

    /**
     添加一个 BeanPostProcessor，这个 processor 比较简单：
     实现了 Aware 接口的 beans 在初始化的时候，这个 processor 负责回调，
     这个我们很常用，如我们会为了获取 ApplicationContext 而 implement ApplicationContextAware
     注意：它不仅仅回调 ApplicationContextAware，
     还会负责回调 EnvironmentAware、ResourceLoaderAware 等，看下源码就清楚了
    */
    beanFactory.addBeanPostProcessor(new ApplicationContextAwareProcessor(this));
    
    
    
    // 下面几行的意思就是，如果某个 bean 依赖于以下几个接口的实现类，在自动装配的时候忽略它们，
    // Spring 会通过其他方式来处理这些依赖。
    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.
    beanFactory.registerResolvableDependency(BeanFactory.class, beanFactory);
    beanFactory.registerResolvableDependency(ResourceLoader.class, this);
    beanFactory.registerResolvableDependency(ApplicationEventPublisher.class, this);
    beanFactory.registerResolvableDependency(ApplicationContext.class, this);

    // 这个 BeanPostProcessor 也很简单，在 bean 实例化后，如果是 ApplicationListener 的子类，
    // 那么将其添加到 listener 列表中，可以理解成：注册 事件监听器
    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());
    }
}

postProcessBeanFactory(beanFactory)

这个方法的作用主要是完成BeanFactory准备工作完成之后进行的后置处理工作，不要和BeanPostProcessor搞混了。

如果有 Bean 实现了BeanFactoryPostProcessor接口，那么在容器初始化以后，Spring 会负责调用里面的 postProcessBeanFactory 方法。

这里是提供给子类的扩展点，子类通过重写这个方法在BeanFactory创建并预准备完成以后做进一步的设置。

invokeBeanFactoryPostProcessors(beanFactory)

这个方法就是调用调用 BeanFactoryPostProcessor 各个实现类的 postProcessBeanFactory(factory) 方法

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()));
    }
}

我们主要来看PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(beanFactory, getBeanFactoryPostProcessors())这一句。

public static void invokeBeanFactoryPostProcessors(
    ConfigurableListableBeanFactory beanFactory, List<BeanFactoryPostProcessor> beanFactoryPostProcessors) {
    
    Set<String> processedBeans = new HashSet<>();
    //先判断当前的BeanFactory是不是BeanDefinitionRegistry(DefaultListableBeanFactory是BeanDefinitionRegistry)
    if (beanFactory instanceof BeanDefinitionRegistry) {
        BeanDefinitionRegistry registry = (BeanDefinitionRegistry) beanFactory;
        List<BeanFactoryPostProcessor> regularPostProcessors = new ArrayList<>();
        List<BeanDefinitionRegistryPostProcessor> registryProcessors = new ArrayList<>();
        //拿到所有的BeanFactoryPostProcessor在这进行遍历(一般第一次刷新容器不会走下面这个for循环，因为此时容器中还没有将BeanFactoryPostProcessor加入它对应的集合中，所以传入参数beanFactoryPostProcessors为空)
        for (BeanFactoryPostProcessor postProcessor : beanFactoryPostProcessors) {
            if (postProcessor instanceof BeanDefinitionRegistryPostProcessor) {
                BeanDefinitionRegistryPostProcessor registryProcessor =
                    (BeanDefinitionRegistryPostProcessor) postProcessor;
                registryProcessor.postProcessBeanDefinitionRegistry(registry);
                registryProcessors.add(registryProcessor);
            }
            else {
                regularPostProcessors.add(postProcessor);
            }
        }

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

        //首先，从容器中获取所有类型为BeanDefinitionRegistryPostProcessor的bean
        String[] postProcessorNames =           beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
        //然后先遍历执行实现PriorityOrdered接口的BeanDefinitionRegistryPostProcessor的接口方法
        for (String ppName : postProcessorNames) {
            if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
                currentRegistryProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
                processedBeans.add(ppName);
            }
        }
        sortPostProcessors(currentRegistryProcessors, beanFactory);
        registryProcessors.addAll(currentRegistryProcessors);
        invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry);
        currentRegistryProcessors.clear();

        //下来再执行实现了Ordered接口的BeanDefinitionRegistryPostProcessor的接口方法
        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);
        currentRegistryProcessors.clear();

        //最后执行没有实现任何接口的
        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);
            currentRegistryProcessors.clear();
        }

//----------------------------------------------------------------------------------
        //下面就和上面一样的模式来执行容器中的BeanFactoryPostProcessor接口实现类的接口方法
        // Now, invoke the postProcessBeanFactory callback of all processors handled so far.
        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.
    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<>();
    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<>();
    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();
}

registerBeanPostProcessors(beanFactory)

这个方法是用来注册BeanPostProcessor(Bean的后置处理器)，来拦截Bean的创建过程。

这里也就不展开了，和处理 BeanFactoryPostProcessors 类似按照优先级注册，只是这里注册BeanPostProcessors ，并没有执行其方法，具体的执行时机是在bean实例化的时候。

initMessageSource()

初始化MessageSource组件，这个主要是用来完成国际化、消息绑定、以及消息解析功能的。

initApplicationEventMulticaster()和registerListeners()

初始化事件派发器和注册事件监听器，这两个方法在之前的文章中有分析过，这里就不再做分析了。

finishBeanFactoryInitialization(beanFactory)

这个方法很重要，作用是来完成剩下的非懒加载的单实例Bean

protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
        // 首先，初始化名字为 conversionService 的 Bean。这里暂时不讲
        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));
        }

        //不管
        if (!beanFactory.hasEmbeddedValueResolver()) {
            beanFactory.addEmbeddedValueResolver(strVal -> getEnvironment().resolvePlaceholders(strVal));
        }

        // 这是 AspectJ 相关的内容，先不管
        String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
        for (String weaverAwareName : weaverAwareNames) {
            getBean(weaverAwareName);
        }
        // Stop using the temporary ClassLoader for type matching.
        beanFactory.setTempClassLoader(null);

        // 冻结 BeanDefinition，不再修改配置了
        beanFactory.freezeConfiguration();

        // Instantiate all remaining (non-lazy-init) singletons.
        // 开始初始化 单例bean
        beanFactory.preInstantiateSingletons();
}

DefaultListableBeanFactory-> preInstantiateSingletons:这个方法才是开始初始化单例bean

public void preInstantiateSingletons() throws BeansException {
        

        // this.beanDefinitionNames 保存了所有的 beanNames
        List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);

        // 触发所有的非懒加载的 singleton beans 的初始化操作
        for (String beanName : beanNames) {
            RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
            // 非抽象、非懒加载的 singletons。如果配置了 'abstract = true'，那是不需要初始化的
            if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
                // 处理 FactoryBean
                if (isFactoryBean(beanName)) {
                    // FactoryBean 的话，在 beanName 前面加上 ‘&’ 符号。再调用 getBean
                    Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
                    if (bean instanceof FactoryBean) {
                        final FactoryBean<?> factory = (FactoryBean<?>) bean;
                        boolean isEagerInit;
                        // 判断当前 FactoryBean 是否是 SmartFactoryBean 的实现，忽略
                        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 {
                    // 对于普通的 Bean，只要调用 getBean(beanName) 这个方法就可以进行初始化了
                    getBean(beanName);
                }
            }
        }

        // 到这里说明所有的非懒加载的 singleton beans 已经完成了初始化
        // 如果我们定义的 bean 是实现了 SmartInitializingSingleton 接口的，那么在这里得到回调，忽略
        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();
                }
            }
        }
}

getBean(beanName)

大多数bean都是通过这个方法来进行初始化的

AbstractBeanFactory->getBean:

@Override
public Object getBean(String name) throws BeansException {
    return doGetBean(name, null, null, false);
}

doGetBean：

protected <T> T doGetBean(final String name, final Class<T> requiredType,
            final Object[] args, boolean typeCheckOnly) throws BeansException {
        // 获取一个 标准的 beanName，处理两种情况：
        //一个是前面说的 FactoryBean(前面带 ‘&’)，
        //如果指定的是别名，将别名转换为规范的Bean名称 
        final String beanName = transformedBeanName(name);
        Object bean;

        // 检查下是不是已经存在了，如果已经创建了的单例bean,会放入Map 中
        Object sharedInstance = getSingleton(beanName);
        // 但是如果 args 不为空的时候，那么不管是否该bean已经存在都会重新创建
        if (sharedInstance != null && args == null) {
            
            // 下面这个方法：如果是普通 Bean 的话，直接返回 sharedInstance，
            // 如果是 FactoryBean 的话，返回它创建的那个实例对象，调用FactoryBean的getObject 方法
            //这里就不展开了
            bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
        }

        else {
            // 创建过了此 beanName 的 prototype 类型的 bean，那么抛异常，
            // 往往是因为陷入了循环引用
            if (isPrototypeCurrentlyInCreation(beanName)) {
                throw new BeanCurrentlyInCreationException(beanName);
            }

            // 检查一下这个 BeanDefinition 在容器中是否存在
            BeanFactory parentBeanFactory = getParentBeanFactory();
            if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
                // 如果当前容器不存在这个 BeanDefinition，看看父容器中有没有
                String nameToLookup = originalBeanName(name);
                if (parentBeanFactory instanceof AbstractBeanFactory) {
                    return ((AbstractBeanFactory) parentBeanFactory).doGetBean(
                            nameToLookup, requiredType, args, typeCheckOnly);
                }
                else if (args != null) {
                    return (T) parentBeanFactory.getBean(nameToLookup, args);
                }
                else {
                    // No args -> delegate to standard getBean method.
                    return parentBeanFactory.getBean(nameToLookup, requiredType);
                }
            }

            if (!typeCheckOnly) {
                // typeCheckOnly 为 false，将当前 beanName 放入一个 alreadyCreated 的 Set 集合中，标记一下。
                markBeanAsCreated(beanName);
            }
            
            /*
             * 到这里的话，要准备创建 Bean 了，
             * 对于 singleton 的 Bean 来说，容器中还没创建过此 Bean；
             * 对于 prototype 的 Bean 来说，本来就是要创建一个新的 Bean。
             */

            try {
                //根据指定Bean名称获取其父级的Bean定义，主要解决Bean继承时子类  
                //合并父类公共属性问题
                final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
                checkMergedBeanDefinition(mbd, beanName, args);

                // 先初始化依赖的所有 Bean
                //检查是不是有循环依赖，这里的循环依赖和我们前面说的循环依赖又不一样
                //这里的依赖指的是 depends-on 中定义的依赖
                //depends-on用来表示一个Bean的实例化依靠另一个Bean先实例化。
                //如果在一个bean A上定义了depend-on B那么就表示：A 实例化前先实例化 B。
                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 + "'");
                        }
                        // 注册一下依赖关系
                        registerDependentBean(dep, beanName);
                        try {
                            //递归调用getBean方法，获取当前Bean的依赖Bean
                            getBean(dep);
                        }
                        catch (NoSuchBeanDefinitionException ex) {
                            throw new BeanCreationException(mbd.getResourceDescription(), beanName,
                                    "'" + beanName + "' depends on missing bean '" + dep + "'", ex);
                        }
                    }
                }

                 // 如果是 singleton scope 的，创建 singleton 的实例
                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;
                        }
                    });
                    bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
                }
                // 如果是 prototype scope 的，创建 prototype 的实例
                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 {
                
                    // 如果不是 singleton 和 prototype 的话，需要委托给相应的实现类来处理
                    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, () -> {
                            //回调beforePrototypeCreation方法
                            beforePrototypeCreation(beanName);
                            try {
                                return createBean(beanName, mbd, args);
                            }
                            finally {
                                //回调afterPrototypeCreation方法
                                afterPrototypeCreation(beanName);
                            }
                        });
                         //获取给定Bean的实例对象
                        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;
            }
        }

        //对创建的Bean实例对象进行类型检查
        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.isDebugEnabled()) {
                    logger.debug("Failed to convert bean '" + name + "' to required type '" +
                            ClassUtils.getQualifiedName(requiredType) + "'", ex);
                }
                throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
            }
        }
        return (T) bean;
}

上面，我们可以看见在创建实例时做了判断

• 如果Bean定义的单态模式(Singleton)，则容器在创建之前先从缓存中查找，以确保整个容器中只存在一个实例对象
• 如果Bean定义的是原型模式(Prototype)，则容器每次都会创建一个新的实例对象。
• 两者都不是，则根据Bean定义资源中配置的生命周期范围，选择实例化Bean的合适方法，这种在Web应用程序中 比较常用，如：request、session、application等生命周期。

通过上面的代码基本上理解大概逻辑是不成问题的，接下来肯定就是分析createBean 方法了。

AbstractAutowireCapableBeanFactory->createBean:

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

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

        // 确保 BeanDefinition 中的 Class 被加载
        Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
        if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
            mbdToUse = new RootBeanDefinition(mbd);
            mbdToUse.setBeanClass(resolvedClass);
        }

        // 准备方法覆写，这里又涉及到一个概念：MethodOverrides，
        // 它来自于 bean 定义中的 <replaced-method />
        // 没怎么了解
        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.
            //如果Bean配置了初始化前和初始化后的处理器，则试图返回一个代理对象
            //这个方法在之前的文章中也有解析
            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 {
            //创建 bean 
            Object beanInstance = doCreateBean(beanName, mbdToUse, args);
            return beanInstance;
        }
        catch (BeanCreationException | ImplicitlyAppearedSingletonException ex) {
            throw ex;
        }
        catch (Throwable ex) {
            throw new BeanCreationException(
                    mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", ex);
        }
}

doCreateBean

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

        // Instantiate the bean.
        BeanWrapper instanceWrapper = null;
        if (mbd.isSingleton()) {
            //移除BeanWrapper缓存
            instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
        }
        if (instanceWrapper == null) {
            //创建 BeanWrapper
            instanceWrapper = createBeanInstance(beanName, mbd, args);
        }
        //获得bean 实例
        final Object bean = instanceWrapper.getWrappedInstance();
        //获取实例化对象的类型
        Class<?> beanType = instanceWrapper.getWrappedClass();
        if (beanType != NullBean.class) {
            mbd.resolvedTargetType = beanType;
        }

        // Allow post-processors to modify the merged bean definition.
        //调用PostProcessor后置处理器
        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.
        // 下面代码是为了解决循环依赖的问题
        boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
                isSingletonCurrentlyInCreation(beanName));
        //提前曝光bean      
        if (earlySingletonExposure) {
            addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
        }

        // Initialize the bean instance.
        Object exposedObject = bean;
        try {
            //实例化后，需要进行属性的赋值
            populateBean(beanName, mbd, instanceWrapper);
            // 这里就是处理 bean 初始化完成后的各种回调，例如init-method 配置，BeanPostProcessor接口
            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) {
            //如果已经提交曝光了bean,那么就从缓存中获取bean
            Object earlySingletonReference = getSingleton(beanName, false);
            if (earlySingletonReference != null) {
                //根据名称获取的以注册的Bean和正在实例化的Bean是同一个
                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 {
            registerDisposableBeanIfNecessary(beanName, bean, mbd);
        }
        catch (BeanDefinitionValidationException ex) {
            throw new BeanCreationException(
                    mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
        }

        return exposedObject;
}

到这里，基本上简单的分析了 doCreateBean 方法，整个bean就已经初始化完成了，这里面有三个重点的方法（过程）

1、创建 Bean 实例(createBeanInstance) 方法，

2、依赖注入(populateBean) 方法，

3、一系列初始化或者回调(initializeBean)。

createBeanInstance：

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);

        // 校验 类的访问权限
        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)  {
            // 采用工厂方法实例化，配置 factory-method
            return instantiateUsingFactoryMethod(beanName, mbd, args);
        }

        // Shortcut when re-creating the same bean...
        // 如果不是第一次创建，比如第二次创建 prototype bean。
        // 这种情况下，我们可以从第一次创建知道，采用无参构造函数，还是构造函数依赖注入 来完成实例化
        // 这个可以通过代码来测试，多次通过getbean(name)来获取 prototype的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) {
               //配置了自动装配属性，使用容器的自动装配实例化  
               //容器的自动装配是根据参数类型匹配Bean的构造方法
                return autowireConstructor(beanName, mbd, null, null);
            }
            else {
                //使用默认的无参构造方法实例化 
                return instantiateBean(beanName, mbd);
            }
        }

        // 判断是否采用有参构造函数.
        Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
        if (ctors != null ||
                mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_CONSTRUCTOR ||
                mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args))  {
            //使用容器的自动装配特性，调用匹配的构造方法实例化 
            return autowireConstructor(beanName, mbd, ctors, args);
        }

        //使用默认的无参构造方法实例化
        return instantiateBean(beanName, mbd);
}

instantiateBean：

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>) () ->
                        getInstantiationStrategy().instantiate(mbd, beanName, parent),
                        getAccessControlContext());
            }
            else {
                 //实例化
                beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, parent);
            }
            // 包装一下
            BeanWrapper bw = new BeanWrapperImpl(beanInstance);
            //初始化BeanWrapper
            //会设置 conversionService，注册customEditors
            initBeanWrapper(bw);
            return bw;
        }
        catch (Throwable ex) {
            throw new BeanCreationException(
                    mbd.getResourceDescription(), beanName, "Instantiation of bean failed", ex);
        }
}

到这里，bean的实例就算是创建完成了。

populateBean

为bean的属性进行赋值

AbstractAutowireCapableBeanFactory->populateBean：

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

        // Give any InstantiationAwareBeanPostProcessors the opportunity to modify the
        // state of the bean before properties are set. This can be used, for example,
        // to support styles of field injection.
        boolean continueWithPropertyPopulation = true;

        if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
            for (BeanPostProcessor bp : getBeanPostProcessors()) {
                //InstantiationAwareBeanPostProcessor 在实例前和实例后进行回调处理
                if (bp instanceof InstantiationAwareBeanPostProcessor) {
                    InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
                    //在设置属性之前调用Bean的PostProcessor后置处理器 
                    if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
                        continueWithPropertyPopulation = false;
                        break;
                    }
                }
            }
        }

        if (!continueWithPropertyPopulation) {
            return;
        }
        // bean 实例的所有属性
        PropertyValues pvs = (mbd.hasPropertyValues() ? mbd.getPropertyValues() : null);

        if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME ||
                mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) {
            MutablePropertyValues newPvs = new MutablePropertyValues(pvs);

            // Add property values based on autowire by name if applicable.
            if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME) {
                // 通过名字找到所有属性值，如果是 bean 依赖，先初始化依赖的 bean。记录依赖关系
                autowireByName(beanName, mbd, bw, newPvs);
            }

            // Add property values based on autowire by type if applicable.
            if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) {
                // 通过类型装配
                autowireByType(beanName, mbd, bw, newPvs);
            }

            pvs = newPvs;
        }
        //检查容器是否持有用于处理单态模式Bean关闭时的后置处理器
        boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors();
        //Bean实例对象没有依赖(此依赖是depends-on)，即没有继承基类 
        boolean needsDepCheck = (mbd.getDependencyCheck() != RootBeanDefinition.DEPENDENCY_CHECK_NONE);

        if (hasInstAwareBpps || needsDepCheck) {
            if (pvs == null) {
                pvs = mbd.getPropertyValues();
            }
            PropertyDescriptor[] filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
            if (hasInstAwareBpps) {
                for (BeanPostProcessor bp : getBeanPostProcessors()) {
                    //处理特殊的BeanPostProcessor
                    
                    if (bp instanceof InstantiationAwareBeanPostProcessor) {
                        InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
                        //使用BeanPostProcessor处理器处理属性值  
                        pvs = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
                        if (pvs == null) {
                            return;
                        }
                    }
                }
            }
            if (needsDepCheck) {
                //为要设置的属性进行依赖检查
                checkDependencies(beanName, mbd, filteredPds, pvs);
            }
        }

        if (pvs != null) {
            // 设置 bean 实例的属性值
            applyPropertyValues(beanName, mbd, bw, pvs);
        }
}

initializeBean

protected Object initializeBean(final String beanName, final Object bean, @Nullable RootBeanDefinition mbd) {
        if (System.getSecurityManager() != null) {
            AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
                invokeAwareMethods(beanName, bean);
                return null;
            }, getAccessControlContext());
        }
        else {
            // 如果 bean 实现了 BeanNameAware、BeanClassLoaderAware 或 BeanFactoryAware 接口，回调
            invokeAwareMethods(beanName, bean);
        }

        Object wrappedBean = bean;
        if (mbd == null || !mbd.isSynthetic()) {
             // BeanPostProcessor 的 postProcessBeforeInitialization 回调
            wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
        }

        try {
            // 处理 bean 中定义的 init-method，
            // 或者如果 bean 实现了 InitializingBean 接口，调用 afterPropertiesSet() 方法
            invokeInitMethods(beanName, wrappedBean, mbd);
        }
        catch (Throwable ex) {
            throw new BeanCreationException(
                    (mbd != null ? mbd.getResourceDescription() : null),
                    beanName, "Invocation of init method failed", ex);
        }
        if (mbd == null || !mbd.isSynthetic()) {
            // BeanPostProcessor 的 postProcessAfterInitialization 回调
            wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
        }

        return wrappedBean;
}

finishRefresh()

完成刷新，初始化LifecycleProcessor，发布容器初始化完成的事件

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);
}

至此，SpringIOC容器的初始化过程就完成了，整个过程还是非常复杂的。本文着重分析了整个逻辑过程，希望对你对IOC初始化的理解能有所帮助。