十二、Spring之IOC容器初始化

Spring之IOC容器初始化

前言

在前面我們分析了最底層的IOC容器BeanFactory,接著簡單分析了高級形態的容器ApplicationContext，在ApplicationContext 中我們知道一個核心方法 refresh，這裡面就是IOC容器的初始化流程，在前面並沒有直接去分析它，只是簡單的分析了BeanDefinition的載入,解析註冊，有了這些基礎後，再來完整的分析IOC容器的啟動流程。

refresh 源碼分析

在 AbstractApplicationContext 中以及將refresh整個流程定義出來了，我們再來看refresh 源碼。

@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();
        }
    }
}

下麵我們針對refresh裡面的執行流程一個一個的進行分析，來看它做了什麼並且是如何實現的。

prepareRefresh()

protected void prepareRefresh() {
   // 記錄啟動時間，
   this.startupDate = System.currentTimeMillis();
   // 設置容器狀態
   this.closed.set(false);
   this.active.set(true);

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

   // 初始化一些屬性設置(一個空方法，留給子類具體實現)
   initPropertySources();

   // 校驗屬性的合法性
   getEnvironment().validateRequiredProperties();

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

創建IOC容器

refresh方法中的下一行 obtainFreshBeanFactory()。

註意，這個方法很重要，這裡將會初始化 BeanFactory、載入 Bean、註冊 Bean 等等。

AbstractApplicationContext -> obtainFreshBeanFactory:

protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {
   // 關閉舊的 BeanFactory (如果有)，創建新的 BeanFactory，載入 Bean 定義、註冊 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 {
      // 初始化一個 DefaultListableBeanFactory
      DefaultListableBeanFactory beanFactory = createBeanFactory();
      // 用於 BeanFactory 的序列化
      beanFactory.setSerializationId(getId());

      // 設置 BeanFactory 的兩個配置屬性：是否允許 Bean 覆蓋、是否允許迴圈引用
      customizeBeanFactory(beanFactory);

      // 載入 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);
   }
}

這裡主要看一下 customizeBeanFactory 方法：

protected void customizeBeanFactory(DefaultListableBeanFactory beanFactory) {
  if (this.allowBeanDefinitionOverriding != null) {
    // 是否允許 Bean 定義覆蓋
    beanFactory.setAllowBeanDefinitionOverriding(this.allowBeanDefinitionOverriding);
  }
  if (this.allowCircularReferences != null) {
    // 是否允許 Bean 間的迴圈依賴
    beanFactory.setAllowCircularReferences(this.allowCircularReferences);
  }
}

prepareBeanFactory

BeanFactory預準備工作

protected void prepareBeanFactory(ConfigurableListableBeanFactory beanFactory) {
    //設置類載入器
    beanFactory.setBeanClassLoader(getClassLoader());
    //設置支持表達式的解析器
    beanFactory.setBeanExpressionResolver(new StandardBeanExpressionResolver(beanFactory.getBeanClassLoader()));
    beanFactory.addPropertyEditorRegistrar(new ResourceEditorRegistrar(this, getEnvironment()));

    /**
     添加一個 BeanPostProcessor，這個 processor 比較簡單：
     實現了 Aware 介面的 beans 在初始化的時候，這個 processor 負責回調，
     這個我們很常用，如我們會為了獲取 ApplicationContext 而 implement ApplicationContextAware
     註意：它不僅僅回調 ApplicationContextAware，
     還會負責回調 EnvironmentAware、ResourceLoaderAware 等，看下源碼就清楚了
    */
    beanFactory.addBeanPostProcessor(new ApplicationContextAwareProcessor(this));
    
    
    
    // 下麵幾行的意思就是，如果某個 bean 依賴於以下幾個介面的實現類，在自動裝配的時候忽略它們，
    // Spring 會通過其他方式來處理這些依賴。
    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);

    // 這個 BeanPostProcessor 也很簡單，在 bean 實例化後，如果是 ApplicationListener 的子類，
    // 那麼將其添加到 listener 列表中，可以理解成：註冊 事件監聽器
    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)

這個方法的作用主要是完成BeanFactory準備工作完成之後進行的後置處理工作，不要和BeanPostProcessor搞混了。

如果有 Bean 實現了BeanFactoryPostProcessor介面，那麼在容器初始化以後，Spring 會負責調用裡面的 postProcessBeanFactory 方法。

這裡是提供給子類的擴展點，子類通過重寫這個方法在BeanFactory創建並預準備完成以後做進一步的設置。

invokeBeanFactoryPostProcessors(beanFactory)

這個方法就是調用調用 BeanFactoryPostProcessor 各個實現類的 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()));
    }
}

我們主要來看PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(beanFactory, getBeanFactoryPostProcessors())這一句。

public static void invokeBeanFactoryPostProcessors(
    ConfigurableListableBeanFactory beanFactory, List<BeanFactoryPostProcessor> beanFactoryPostProcessors) {
    
    Set<String> processedBeans = new HashSet<>();
    //先判斷當前的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在這進行遍歷(一般第一次刷新容器不會走下麵這個for迴圈，因為此時容器中還沒有將BeanFactoryPostProcessor加入它對應的集合中，所以傳入參數beanFactoryPostProcessors為空)
        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，從容器中獲取所有類型為BeanDefinitionRegistryPostProcessor的bean
        String[] postProcessorNames =           beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
        //然後先遍歷執行實現PriorityOrdered介面的BeanDefinitionRegistryPostProcessor的介面方法
        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();

        //下來再執行實現了Ordered介面的BeanDefinitionRegistryPostProcessor的介面方法
        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();

        //最後執行沒有實現任何介面的
        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();
        }

//----------------------------------------------------------------------------------
        //下麵就和上面一樣的模式來執行容器中的BeanFactoryPostProcessor介面實現類的介面方法
        // 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)

這個方法是用來註冊BeanPostProcessor(Bean的後置處理器)，來攔截Bean的創建過程。

這裡也就不展開了，和處理 BeanFactoryPostProcessors 類似按照優先順序註冊，只是這裡註冊BeanPostProcessors ，並沒有執行其方法，具體的執行時機是在bean實例化的時候。

initMessageSource()

初始化MessageSource組件，這個主要是用來完成國際化、消息綁定、以及消息解析功能的。

initApplicationEventMulticaster()和registerListeners()

初始化事件派發器和註冊事件監聽器，這兩個方法在之前的文章中有分析過，這裡就不再做分析了。

finishBeanFactoryInitialization(beanFactory)

這個方法很重要，作用是來完成剩下的非懶載入的單實例Bean

protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
        // 首先，初始化名字為 conversionService 的 Bean。這裡暫時不講
        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));
        }

        // 這是 AspectJ 相關的內容，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);

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

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

DefaultListableBeanFactory-> preInstantiateSingletons:這個方法才是開始初始化單例bean

public void preInstantiateSingletons() throws BeansException {
        

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

        // 觸發所有的非懶載入的 singleton beans 的初始化操作
        for (String beanName : beanNames) {
            RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
            // 非抽象、非懶載入的 singletons。如果配置了 'abstract = true'，那是不需要初始化的
            if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
                // 處理 FactoryBean
                if (isFactoryBean(beanName)) {
                    // FactoryBean 的話，在 beanName 前面加上 ‘&’ 符號。再調用 getBean
                    Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
                    if (bean instanceof FactoryBean) {
                        final FactoryBean<?> factory = (FactoryBean<?>) bean;
                        boolean isEagerInit;
                        // 判斷當前 FactoryBean 是否是 SmartFactoryBean 的實現，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 {
                    // 對於普通的 Bean，只要調用 getBean(beanName) 這個方法就可以進行初始化了
                    getBean(beanName);
                }
            }
        }

        // 到這裡說明所有的非懶載入的 singleton beans 已經完成了初始化
        // 如果我們定義的 bean 是實現了 SmartInitializingSingleton 介面的，那麼在這裡得到回調，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)

大多數bean都是通過這個方法來進行初始化的

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 {
        // 獲取一個 標準的 beanName，處理兩種情況：
        //一個是前面說的 FactoryBean(前面帶 ‘&’)，
        //如果指定的是別名，將別名轉換為規範的Bean名稱 
        final String beanName = transformedBeanName(name);
        Object bean;

        // 檢查下是不是已經存在了，如果已經創建了的單例bean,會放入Map 中
        Object sharedInstance = getSingleton(beanName);
        // 但是如果 args 不為空的時候，那麼不管是否該bean已經存在都會重新創建
        if (sharedInstance != null && args == null) {
            
            // 下麵這個方法：如果是普通 Bean 的話，直接返回 sharedInstance，
            // 如果是 FactoryBean 的話，返回它創建的那個實例對象，調用FactoryBean的getObject 方法
            //這裡就不展開了
            bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
        }

        else {
            // 創建過了此 beanName 的 prototype 類型的 bean，那麼拋異常，
            // 往往是因為陷入了迴圈引用
            if (isPrototypeCurrentlyInCreation(beanName)) {
                throw new BeanCurrentlyInCreationException(beanName);
            }

            // 檢查一下這個 BeanDefinition 在容器中是否存在
            BeanFactory parentBeanFactory = getParentBeanFactory();
            if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
                // 如果當前容器不存在這個 BeanDefinition，看看父容器中有沒有
                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 為 false，將當前 beanName 放入一個 alreadyCreated 的 Set 集合中，標記一下。
                markBeanAsCreated(beanName);
            }
            
            /*
             * 到這裡的話，要準備創建 Bean 了，
             * 對於 singleton 的 Bean 來說，容器中還沒創建過此 Bean；
             * 對於 prototype 的 Bean 來說，本來就是要創建一個新的 Bean。
             */

            try {
                //根據指定Bean名稱獲取其父級的Bean定義，主要解決Bean繼承時子類  
                //合併父類公共屬性問題
                final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
                checkMergedBeanDefinition(mbd, beanName, args);

                // 先初始化依賴的所有 Bean
                //檢查是不是有迴圈依賴，這裡的迴圈依賴和我們前面說的迴圈依賴又不一樣
                //這裡的依賴指的是 depends-on 中定義的依賴
                //depends-on用來表示一個Bean的實例化依靠另一個Bean先實例化。
                //如果在一個bean A上定義了depend-on B那麼就表示：A 實例化前先實例化 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 + "'");
                        }
                        // 註冊一下依賴關係
                        registerDependentBean(dep, beanName);
                        try {
                            //遞歸調用getBean方法，獲取當前Bean的依賴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，創建 singleton 的實例
                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，創建 prototype 的實例
                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 的話，需要委托給相應的實現類來處理
                    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, () -> {
                            //回調beforePrototypeCreation方法
                            beforePrototypeCreation(beanName);
                            try {
                                return createBean(beanName, mbd, args);
                            }
                            finally {
                                //回調afterPrototypeCreation方法
                                afterPrototypeCreation(beanName);
                            }
                        });
                         //獲取給定Bean的實例對象
                        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;
            }
        }

        //對創建的Bean實例對象進行類型檢查
        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，我們可以看見在創建實例時做了判斷

• 如果Bean定義的單態模式(Singleton)，則容器在創建之前先從緩存中查找，以確保整個容器中只存在一個實例對象
• 如果Bean定義的是原型模式(Prototype)，則容器每次都會創建一個新的實例對象。
• 兩者都不是，則根據Bean定義資源中配置的生命周期範圍，選擇實例化Bean的合適方法，這種在Web應用程式中 比較常用，如：request、session、application等生命周期。

通過上面的代碼基本上理解大概邏輯是不成問題的，接下來肯定就是分析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;

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

        // 準備方法覆寫，這裡又涉及到一個概念：MethodOverrides，
        // 它來自於 bean 定義中的 <replaced-method />
        // 沒怎麼瞭解
        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後的處理器，則試圖返回一個代理對象
            //這個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 {
            //創建 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緩存
            instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
        }
        if (instanceWrapper == null) {
            //創建 BeanWrapper
            instanceWrapper = createBeanInstance(beanName, mbd, args);
        }
        //獲得bean 實例
        final Object bean = instanceWrapper.getWrappedInstance();
        //獲取實例化對象的類型
        Class<?> beanType = instanceWrapper.getWrappedClass();
        if (beanType != NullBean.class) {
            mbd.resolvedTargetType = beanType;
        }

        // Allow post-processors to modify the merged bean definition.
        //調用PostProcessor後置處理器
        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.
        // 下麵代碼是為瞭解決迴圈依賴的問題
        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 {
            //實例化後，需要進行屬性的賦值
            populateBean(beanName, mbd, instanceWrapper);
            // 這裡就是處理 bean 初始化完成後的各種回調，例如init-method 配置，BeanPostProcessor介面
            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) {
            //如果已經提交曝光了bean,那麼就從緩存中獲取bean
            Object earlySingletonReference = getSingleton(beanName, false);
            if (earlySingletonReference != null) {
                //根據名稱獲取的以註冊的Bean和正在實例化的Bean是同一個
                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;
}

到這裡，基本上簡單的分析了 doCreateBean 方法，整個bean就已經初始化完成了，這裡面有三個重點的方法（過程）

1、創建 Bean 實例(createBeanInstance) 方法，

2、依賴註入(populateBean) 方法，

3、一系列初始化或者回調(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);

        // 校驗 類的訪問許可權
        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)  {
            // 採用工廠方法實例化，配置 factory-method
            return instantiateUsingFactoryMethod(beanName, mbd, args);
        }

        // Shortcut when re-creating the same bean...
        // 如果不是第一次創建，比如第二次創建 prototype bean。
        // 這種情況下，我們可以從第一次創建知道，採用無參構造函數，還是構造函數依賴註入 來完成實例化
        // 這個可以通過代碼來測試，多次通過getbean(name)來獲取 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) {
               //配置了自動裝配屬性，使用容器的自動裝配實例化  
               //容器的自動裝配是根據參數類型匹配Bean的構造方法
                return autowireConstructor(beanName, mbd, null, null);
            }
            else {
                //使用預設的無參構造方法實例化 
                return instantiateBean(beanName, mbd);
            }
        }

        // 判斷是否採用有參構造函數.
        Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
        if (ctors != null ||
                mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_CONSTRUCTOR ||
                mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args))  {
            //使用容器的自動裝配特性，調用匹配的構造方法實例化 
            return autowireConstructor(beanName, mbd, ctors, args);
        }

        //使用預設的無參構造方法實例化
        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 {
                 //實例化
                beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, parent);
            }
            // 包裝一下
            BeanWrapper bw = new BeanWrapperImpl(beanInstance);
            //初始化BeanWrapper
            //會設置 conversionService，註冊customEditors
            initBeanWrapper(bw);
            return bw;
        }
        catch (Throwable ex) {
            throw new BeanCreationException(
                    mbd.getResourceDescription(), beanName, "Instantiation of bean failed", ex);
        }
}

到這裡，bean的實例就算是創建完成了。

populateBean

為bean的屬性進行賦值

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 在實例前和實例後進行回調處理
                if (bp instanceof InstantiationAwareBeanPostProcessor) {
                    InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
                    //在設置屬性之前調用Bean的PostProcessor後置處理器 
                    if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
                        continueWithPropertyPopulation = false;
                        break;
                    }
                }
            }
        }

        if (!continueWithPropertyPopulation) {
            return;
        }
        // bean 實例的所有屬性
        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) {
                // 通過名字找到所有屬性值，如果是 bean 依賴，先初始化依賴的 bean。記錄依賴關係
                autowireByName(beanName, mbd, bw, newPvs);
            }

            // Add property values based on autowire by type if applicable.
            if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) {
                // 通過類型裝配
                autowireByType(beanName, mbd, bw, newPvs);
            }

            pvs = newPvs;
        }
        //檢查容器是否持有用於處理單態模式Bean關閉時的後置處理器
        boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors();
        //Bean實例對象沒有依賴(此依賴是depends-on)，即沒有繼承基類 
        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()) {
                    //處理特殊的BeanPostProcessor
                    
                    if (bp instanceof InstantiationAwareBeanPostProcessor) {
                        InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
                        //使用BeanPostProcessor處理器處理屬性值  
                        pvs = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
                        if (pvs == null) {
                            return;
                        }
                    }
                }
            }
            if (needsDepCheck) {
                //為要設置的屬性進行依賴檢查
                checkDependencies(beanName, mbd, filteredPds, pvs);
            }
        }

        if (pvs != null) {
            // 設置 bean 實例的屬性值
            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 實現了 BeanNameAware、BeanClassLoaderAware 或 BeanFactoryAware 介面，回調
            invokeAwareMethods(beanName, bean);
        }

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

        try {
            // 處理 bean 中定義的 init-method，
            // 或者如果 bean 實現了 InitializingBean 介面，調用 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 回調
            wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
        }

        return wrappedBean;
}

finishRefresh()

完成刷新，初始化LifecycleProcessor，發佈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容器的初始化過程就完成了，整個過程還是非常複雜的。本文著重分析了整個邏輯過程，希望對你對IOC初始化的理解能有所幫助。