Bean的加载原理

电玩女神 2022-04-02 08:50 210阅读 0赞

### 0. Bean 加载原理 ###

加载过程： 通过 `ResourceLoader`和其子类 `DefaultResourceLoader`完成资源文件位置定位，实现从类路径，文件系统，url等方式定位功能，完成定位后得到 `Resource`对象，再交给 `BeanDefinitionReader`，它再委托给 `BeanDefinitionParserDelegate`完成bean的解析并得到 `BeanDefinition`对象，然后通过 `registerBeanDefinition`方法进行注册，IOC容器内ibu维护了一个HashMap来保存该 `BeanDefinition`对象，Spring中的 `BeanDefinition`其实就是我们用的 `JavaBean`。

什么是BeanDefinition对象

BeanDefinition是一个接口，描述了一个bean实例，它具有属性值，构造函数参数值以及具体实现提供的更多信息。

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注重理解加载过程

在开始之前需要认真阅读和理解这个过程，有了这个过程，阅读源码难度就小了一半。

大多源码都进行了注释，有的是官方英文注释。中文是主线(本文也主要也是过一遍主线)，想要面面俱到需要自己再去摸索。

### 1. bean.xml ###

一个普通的bean配置文件，这里我要强调的是它里面的格式，因为解析标签的时候会用到。它有 `<beans>` `<bean>` `<import>` `<alias>`等标签，下文会对他们进行解析并翻译成BeanDefinition对象。

### 2. ResourceLoader.java ###

> 加载资源的策略接口(策略模式)。 DefaultResourceLoader is a standalone implementation that is usable outside an ApplicationContext, also used by ResourceEditor
> 
> An ApplicationContext is required to provide this functionality, plus extended ResourcePatternResolver support.

public interface ResourceLoader {   
    /** Pseudo URL prefix for loading from the class path: "classpath:". */
       String CLASSPATH_URL_PREFIX = ResourceUtils.CLASSPATH_URL_PREFIX;
           // 返回一个Resource 对象 (明确配置文件位置的对象)
       Resource getResource(String location);
           // 返回ResourceLoader的ClassLoader
       @Nullable
       ClassLoader getClassLoader();
    }

然后我们看看 `DefaultResourceLoader`对于 `getResource()`方法的实现。

public Resource getResource ( String location ) { 
    Assert . notNull (
    location , "Location must not be null" );
     for ( ProtocolResolver protocolResolver : this . protocolResolvers ) {
      Resource resource =protocolResolver . resolve ( location , this );
       if ( resource != null ){
        return resource ;
         }
          }
           // 如果location 以 / 开头
            if ( location . startsWith( "/" )) {
             return getResourceByPath ( location );
              } // 如果location以classpath: 开头
               else if ( location . startsWith ( CLASSPATH_URL_PREFIX )){
                return new ClassPathResource ( location . substring (CLASSPATH_URL_PREFIX . length ()), getClassLoader ());
                 } else {
                  try { 
                  //Try to parse the location as a URL... 
                  URL url = new URL ( location );
    return ( ResourceUtils . isFileURL ( url ) ? new FileUrlResource ( url ): new UrlResource ( url ));
     } catch ( MalformedURLException ex ) { 
     // No URL -> resolve as resource path. 
     return getResourceByPath ( location );}
     }
     }

可以看到，它判断了三种情况： `/` `classpath:` `url格式匹配`， 然后调用相对应的处理方法，我只分析 `classpath:`，因为这是最常用的。所以看一看 `ClassPathResource`实现：

public ClassPathResource(String path,
     @Nullable ClassLoader classLoader) {
           Assert.notNull(path, "Path must not be null");
           String pathToUse = StringUtils.cleanPath(path);
           if (pathToUse.startsWith("/")) {
               pathToUse = pathToUse.substring(1);
           }
           this.path = pathToUse;
           this.classLoader = (classLoader != null ? classLoader : ClassUtils.getDefaultClassLoader());
       }

看了上面的代码，意味着你配置静态资源文件路径的时候，不用纠结 `classpath:`后面用不用写 `/`,因为如果写了它会给你过滤掉。

那url如何定位的呢?

> 跟踪getResourceByPath(location)方法:

public Resource getResource(String location) {
           Assert.notNull(location, "Location must not be null");
           for (ProtocolResolver protocolResolver : this.protocolResolvers) {           Resource resource = protocolResolver.resolve(location, this);
               if (resource != null) {
                   return resource;
               }
           }
                  // 如果location 以 / 开头
           if (location.startsWith("/")) {
               return getResourceByPath(location);
           }
                   // 如果location 以classpath: 开头
           else if (location.startsWith(CLASSPATH_URL_PREFIX)) {
               return new ClassPathResource(location.substring(CLASSPATH_URL_PREFIX.length()), getClassLoader());
           }
           else {
               try {
                   // Try to parse the location as a URL...
                   URL url = new URL(location);
                   return (ResourceUtils.isFileURL(url) ? new FileUrlResource(url) : new UrlResource(url));
               }
               catch (MalformedURLException ex) {
                   // No URL -> resolve as resource path.
                   return getResourceByPath(location);
               }
           }
       }

@Override
       protected Resource getResourceByPath(String path) {
           if (path.startsWith("/")) {
               path = path.substring(1);
           }
           // 这里使用文件系统资源对象来定义bean文件
           return new FileSystemResource(path);
       }

好了，很明显...跑偏了，因为我们想要的是xml文件及路径的解析，不过还好，换汤不换药。下文中会涉及到。

触发bean加载

回到正题，我们在使用spring手动加载bean.xml的时候，用到：

ApplicationContext applicationContext = new ClassPathXmlApplicationContext("bean.xml");

那就从 `ClassPathXmlApplicationContext`类开始深入：

### 3. ClassPathXmlApplicationContext.java ###

> 这个类里面只有构造方法(多个)和一个getConfigResources()方法，构造方法最终都统一打到下面这个构造方法中(适配器模式):

public ClassPathXmlApplicationContext(
               String[] configLocations, boolean refresh, @Nullable ApplicationContext parent)
               throws BeansException {
       // 动态的确定用哪个加载器去加载 配置文件       1.super(parent);
       // 告诉读取器 配置文件在哪里， 定位加载配置文件       2.setConfigLocations(configLocations);
       // 刷新
           if (refresh) {
               // 在创建IOC容器前，如果容器已经存在，则需要把已有的容器摧毁和关闭，以保证refresh
               //之后使用的是新的IOC容器           3.refresh();
           }
       }

**注意:** 这个类非常关键，我认为它定义了一个 `xml`加载 `bean`的一个 `LifeCycle`:

1.  `super()` 方法完成类加载器的指定。
2.  `setConfigLocations(configLocations);`方法对配置文件进行定位和解析，拿到Resource对象。
3.  `refresh();`方法对标签进行解析拿到BeanDefition对象，在通过校验后将其注册到IOC容器。(主要研究该方法)

我标记的1. 2. 3. 对应后面的方法x, 方便阅读。

先深入了解下 `setConfigLocations(configLocations);`方法:

方法2. setConfigLocations(configLocations)

// 解析Bean定义资源文件的路径，处理多个资源文件字符串数组
       public void setConfigLocations(@Nullable String... locations) {
           if (locations != null) {
               Assert.noNullElements(locations, "Config locations must not be null");
               this.configLocations = new String[locations.length];
               for (int i = 0; i < locations.length; i++) {
                   // resolvePath 为同一个类中将字符串解析为路径的方法
                   this.configLocations[i] = resolvePath(locations[i]).trim();
               }
           }
           else {
               this.configLocations = null;
           }
       }

然后我们继续上面看 `ClassPathXmlApplicationContext`的 `refresh()`方法:

方法3. refresh()

public void refresh() throws BeansException, IllegalStateException {
           synchronized (this.startupShutdownMonitor) {
               // 为refresh 准备上下文
               prepareRefresh();
               // 通知子类去刷新 Bean工厂
               ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
               // 用该上下文来 准备bean工厂
               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();
               }
           }
       }

**注:**下面的方法全都是围绕 `refresh()`里深入阅读，该方法套的很深，下面的阅读可能会引起不适。

然后看看 `refresh()`方法中的 `obtainFreshBeanFactory()`方法:

方法3.1 obtainFreshBeanFactory()

// 调用--刷新bean工厂
       protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {
           // 委派模式：父类定义了refreshBeanFactory方法，具体实现调用子类容器
           refreshBeanFactory();
           return getBeanFactory();
       }

然后看 `obtainFreshBeanFactory()`的 `refreshBeanFactory()`方法

方法3.1.1 refreshBeanFactory()

// 刷新bean工厂
       protected final void refreshBeanFactory() throws BeansException {
           // 如果存在容器，就先销毁并关闭
           if (hasBeanFactory()) {
               destroyBeans();
               closeBeanFactory();
           }
           try {
               // 创建IOC容器
               DefaultListableBeanFactory beanFactory = createBeanFactory();
               beanFactory.setSerializationId(getId());
               // 对容器进行初始化
               customizeBeanFactory(beanFactory);
               // 调用载入Bean定义的方法，(使用了委派模式)
               loadBeanDefinitions(beanFactory);
               synchronized (this.beanFactoryMonitor) {
                   this.beanFactory = beanFactory;
               }
           }
           catch (IOException ex) {
               throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
           }
       }

然后再跟进 `refreshBeanFactory()`的 `loadBeanDefinitions()`方法：

方法3.1.1.1 loadBeanDefinitions()

> 通过 XmlBeanDefinitionReader 加载 BeanDefinition

// 通过 XmlBeanDefinitionReader 加载 BeanDefinition
       @Override
       protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) throws BeansException, IOException {
           // Create a new XmlBeanDefinitionReader for the given BeanFactory.
           // 为beanFactory 创建一个新的 XmlBeanDefinitionReader
           XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(beanFactory);
           // Configure the bean definition reader with this context's
           // resource loading environment.
           beanDefinitionReader.setEnvironment(this.getEnvironment());
           // 为 Bean读取器设置Spring资源加载器 (因为祖父类是ResourceLoader的子类，所以也是ResourceLoader)
           beanDefinitionReader.setResourceLoader(this);
           //  为 Bean读取器设置SAX xml解析器DOM4J
           beanDefinitionReader.setEntityResolver(new ResourceEntityResolver(this));
           // Allow a subclass to provide custom initialization of the reader,
           // then proceed with actually loading the bean definitions.
           // 初始化 BeanDefinition读取器
           initBeanDefinitionReader(beanDefinitionReader);
           // 真正加载 bean定义
           loadBeanDefinitions(beanDefinitionReader);
       }

再跟进 `loadBeanDefinitions(DefaultListableBeanFactorybeanFactory)`方法中的 `loadBeanDefinitions(XmlBeanDefinitionReaderreader)`方法:

方法3.1.1.1.1 loadBeanDefinitions()

> XMLBean读取器加载BeanDefinition 资源

// XMLBean读取器加载Bean 定义资源
       protected void loadBeanDefinitions(XmlBeanDefinitionReader reader) throws BeansException, IOException {
           // 获取Bean定义资源的定位
           Resource[] configResources = getConfigResources();
           if (configResources != null) {
               // XMLBean读取器调用其父类 AbstractBeanDefinitionReader 读取定位的Bean定义资源
               reader.loadBeanDefinitions(configResources);
           }
           // 如果子类中获取的bean定义资源定位为空，
           // 则获取 FileSystemXmlApplicationContext构造方法中 setConfigLocations 方法设置的资源
           String[] configLocations = getConfigLocations();
           if (configLocations != null) {
               // XMLBean读取器调用其父类 AbstractBeanDefinitionReader 读取定位的Bean定义资源
               reader.loadBeanDefinitions(configLocations);
           }
       }

@Override
       public int loadBeanDefinitions(Resource... resources) throws BeanDefinitionStoreException {
           Assert.notNull(resources, "Resource array must not be null");
           int count = 0;
           //
           for (Resource resource : resources) {
               count += loadBeanDefinitions(resource);
           }
           return count;
       }

再跟下去 `loadBeanDefinitions()`: 这只是一个抽象方法，找到 `XmlBeanDefinitionReader`子类的实现：

@Override
       public int loadBeanDefinitions(Resource resource) throws BeanDefinitionStoreException {
           return loadBeanDefinitions(new EncodedResource(resource));
       }

再深入 `loadBeanDefinitions`:

> 通过明确的xml文件加载bean

// 通过明确的xml文件加载bean
       public int loadBeanDefinitions(EncodedResource encodedResource) throws BeanDefinitionStoreException {
           Assert.notNull(encodedResource, "EncodedResource must not be null");
           if (logger.isTraceEnabled()) {
               logger.trace("Loading XML bean definitions from " + encodedResource);
           }
           Set<EncodedResource> currentResources = this.resourcesCurrentlyBeingLoaded.get();
           if (currentResources == null) {
               currentResources = new HashSet<>(4);
               this.resourcesCurrentlyBeingLoaded.set(currentResources);
           }
           if (!currentResources.add(encodedResource)) {
               throw new BeanDefinitionStoreException(
                       "Detected cyclic loading of " + encodedResource + " - check your import definitions!");
           }       try {
               // 将资源文件转为InputStream的IO流
               InputStream inputStream = encodedResource.getResource().getInputStream();
               try {
                   // 从流中获取 xml解析资源
                   InputSource inputSource = new InputSource(inputStream);
                   if (encodedResource.getEncoding() != null) {
                       // 设置编码
                       inputSource.setEncoding(encodedResource.getEncoding());
                   }
                   // 具体的读取过程
                   return doLoadBeanDefinitions(inputSource, encodedResource.getResource());
               }
               finally {
                   inputStream.close();
               }
           }
           catch (IOException ex) {
               throw new BeanDefinitionStoreException(
                       "IOException parsing XML document from " + encodedResource.getResource(), ex);
           }
           finally {
               currentResources.remove(encodedResource);
               if (currentResources.isEmpty()) {
                   this.resourcesCurrentlyBeingLoaded.remove();
               }
           }
       }

再深入到 `doLoadBeanDefinitions()`:

> 真正开始加载 BeanDefinitions

protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource)
               throws BeanDefinitionStoreException {
           try {
               // 将xml 文件转换为DOM对象
               Document doc = doLoadDocument(inputSource, resource);
               // 对bean定义解析的过程，该过程会用到 Spring的bean配置规则
               int count = registerBeanDefinitions(doc, resource);
               if (logger.isDebugEnabled()) {
                   logger.debug("Loaded " + count + " bean definitions from " + resource);
               }
               return count;
           }
           ...  ...  ..
    }

`doLoadDocument()`方法将流进行解析，返回一个Document对象: `returnbuilder.parse(inputSource);`为了避免扰乱思路，这里的深入自己去完成。

还需要再深入到: `registerBeanDefinitions()`

> 注册 BeanDefinitions

public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
           BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();
           // 得到容器中注册的bean数量
           int countBefore = getRegistry().getBeanDefinitionCount();
           // 解析过程入口，这里使用了委派模式
           documentReader.registerBeanDefinitions(doc, createReaderContext(resource));
           // 统计解析的bean数量
           return getRegistry().getBeanDefinitionCount() - countBefore;
       }

再深入 `registerBeanDefinitions()`方法（该方法是委派模式的结果）:

@Override
       public void registerBeanDefinitions(Document doc, XmlReaderContext readerContext) {
           // 获得XML描述符
           this.readerContext = readerContext;
           doRegisterBeanDefinitions(doc.getDocumentElement());
       }

再深入 `doRegisterBeanDefinitions(doc.getDocumentElement());`：

> 真正开始注册 BeanDefinitions :

protected void doRegisterBeanDefinitions(Element root) {
           // Any nested <beans> elements will cause recursion in this method. In
           // order to propagate and preserve <beans> default-* attributes correctly,
           // keep track of the current (parent) delegate, which may be null. Create
           // the new (child) delegate with a reference to the parent for fallback purposes,
           // then ultimately reset this.delegate back to its original (parent) reference.
           // this behavior emulates a stack of delegates without actually necessitating one.
           BeanDefinitionParserDelegate parent = this.delegate;
           this.delegate = createDelegate(getReaderContext(), root, parent);
           if (this.delegate.isDefaultNamespace(root)) {
               String profileSpec = root.getAttribute(PROFILE_ATTRIBUTE);
               if (StringUtils.hasText(profileSpec)) {
                   String[] specifiedProfiles = StringUtils.tokenizeToStringArray(
                           profileSpec, BeanDefinitionParserDelegate.MULTI_VALUE_ATTRIBUTE_DELIMITERS);
                   // We cannot use Profiles.of(...) since profile expressions are not supported
                   // in XML config. See SPR-12458 for details.
                   if (!getReaderContext().getEnvironment().acceptsProfiles(specifiedProfiles)) {
                       if (logger.isDebugEnabled()) {
                           logger.debug("Skipped XML bean definition file due to specified profiles [" + profileSpec +                               "] not matching: " + getReaderContext().getResource());
                       }
                       return;
                   }
               }
           }
           // 在bean解析定义之前，进行自定义解析，看是否是用户自定义标签
           preProcessXml(root);
           // 开始进行解析bean定义的document对象
           parseBeanDefinitions(root, this.delegate);
           // 解析bean定义之后，进行自定义的解析，增加解析过程的可扩展性
           postProcessXml(root);
           this.delegate = parent;
       }

接下来看 `parseBeanDefinitions(root,this.delegate);`:

> document的根元素开始进行解析翻译成BeanDefinitions

// 从document的根元素开始进行解析翻译成BeanDefinitions
       protected void parseBeanDefinitions(Element root, BeanDefinitionParserDelegate delegate) {
           // bean定义的document对象使用了spring默认的xml命名空间
           if (delegate.isDefaultNamespace(root)) {
               // 获取bean定义的document对象根元素的所有字节点
               NodeList nl = root.getChildNodes();
               for (int i = 0; i < nl.getLength(); i++) {
                   Node node = nl.item(i);
                   // 获得document节点是xml元素节点
                   if (node instanceof Element) {
                       Element ele = (Element) node;
                       // bean定义的document的元素节点使用的是spring默认的xml命名空间
                       if (delegate.isDefaultNamespace(ele)) {
                           // 使用spring的bean规则解析元素 节点
                           parseDefaultElement(ele, delegate);
                       }
                       else {
                           // 没有使用spring默认的xml命名空间，则使用用户自定义的解析规则解析元素节点
                           delegate.parseCustomElement(ele);
                       }
                   }
               }
           }       else {
               delegate.parseCustomElement(root);
           }
       }
       private void parseDefaultElement(Element ele, BeanDefinitionParserDelegate delegate) {
           // 解析 <import> 标签元素，并进行导入解析
           if (delegate.nodeNameEquals(ele, IMPORT_ELEMENT)) {
               importBeanDefinitionResource(ele);
           }
           // alias
           else if (delegate.nodeNameEquals(ele, ALIAS_ELEMENT)) {
               processAliasRegistration(ele);
           }
           // bean
           else if (delegate.nodeNameEquals(ele, BEAN_ELEMENT)) {
               processBeanDefinition(ele, delegate);
           }
           // beans
           else if (delegate.nodeNameEquals(ele, NESTED_BEANS_ELEMENT)) {
               // recurse
               doRegisterBeanDefinitions(ele);
           }
       }

```importBeanDefinitionResource(ele);``processAliasRegistration(ele);``processBeanDefinition(ele,delegate);```这三个方法里分别展示了标签解析的详细过程。 这下看到了，它其实使用DOM4J来解析 `import` `bean` `alias`等标签，然后递归标签内部直到拿到所有属性并封装到BeanDefition对象中。比如说 `processBeanDefinition`方法:

> 给我一个element 解析成 BeanDefinition

// 给我一个element 解析成 BeanDefinition
       protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
           // 真正解析过程
           BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
           if (bdHolder != null) {
               bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);
               try {
                   // Register the final decorated instance.
                   // 注册： 将db注册到ioc，委托模式
                   BeanDefinitionReaderUtils.registerBeanDefinition(bdHolder, getReaderContext().getRegistry());
               }
               catch (BeanDefinitionStoreException ex) {
                   getReaderContext().error("Failed to register bean definition with name '" +                       bdHolder.getBeanName() + "'", ele, ex);
               }
               // Send registration event.
               getReaderContext().fireComponentRegistered(new BeanComponentDefinition(bdHolder));
           }
       }

继续深入 `registerBeanDefinition():`

> 注册BeanDefinitions 到 bean 工厂

// 注册BeanDefinitions 到 bean 工厂
       // definitionHolder ： bean定义，包含了 name和aliases
       // registry： 注册到的bean工厂
       public static void registerBeanDefinition(
               BeanDefinitionHolder definitionHolder, BeanDefinitionRegistry registry)
               throws BeanDefinitionStoreException {
           // Register bean definition under primary name.
           String beanName = definitionHolder.getBeanName();
           // 真正注册
           registry.registerBeanDefinition(beanName, definitionHolder.getBeanDefinition());
           // Register aliases for bean name, if any.
           String[] aliases = definitionHolder.getAliases();
           if (aliases != null) {
               for (String alias : aliases) {
                   registry.registerAlias(beanName, alias);
               }
           }
       }

再深入 `registry.registerBeanDefinition(beanName,definitionHolder.getBeanDefinition());`

> 注册BeanDefinitions 到IOC容器

**注意**:该方法所在类是接口，我们查看的是 `DefaultListableBeanFactory.java`所实现的该方法。

// 实现BeanDefinitionRegistry接口,注册BeanDefinitions
        @Override
       public void registerBeanDefinition(String beanName, BeanDefinition beanDefinition)
               throws BeanDefinitionStoreException {
           Assert.hasText(beanName, "Bean name must not be empty");
           Assert.notNull(beanDefinition, "BeanDefinition must not be null");
           // 校验是否是 AbstractBeanDefinition)
           if (beanDefinition instanceof AbstractBeanDefinition) {
               try {
                   // 标记 beanDefinition 生效
                   ((AbstractBeanDefinition) beanDefinition).validate();
               }
               catch (BeanDefinitionValidationException ex) {
                   throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
                           "Validation of bean definition failed", ex);
               }
           }
           // 判断beanDefinitionMap 里是否已经有这个bean
           BeanDefinition existingDefinition = this.beanDefinitionMap.get(beanName);
           //如果没有这个bean
           if (existingDefinition != null) {
               //如果不允许bd 覆盖已注册的bean， 就抛出异常
               if (!isAllowBeanDefinitionOverriding()) {
                   throw new BeanDefinitionOverrideException(beanName, beanDefinition, existingDefinition);
               }
               // 如果允许覆盖， 则同名的bean， 注册的覆盖先注册的
               else if (existingDefinition.getRole() < beanDefinition.getRole()) {
                   // e.g. was ROLE_APPLICATION, now overriding with ROLE_SUPPORT or ROLE_INFRASTRUCTURE
                   if (logger.isInfoEnabled()) {
                       logger.info("Overriding user-defined bean definition for bean '" + beanName +
                               "' with a framework-generated bean definition: replacing [" +                           existingDefinition + "] with [" + beanDefinition + "]");
                   }
               }           else if (!beanDefinition.equals(existingDefinition)) {
                   if (logger.isDebugEnabled()) {
                       logger.debug("Overriding bean definition for bean '" + beanName +                           "' with a different definition: replacing [" + existingDefinition +                           "] with [" + beanDefinition + "]");
                   }
               }           else {
                   if (logger.isTraceEnabled()) {
                       logger.trace("Overriding bean definition for bean '" + beanName +                           "' with an equivalent definition: replacing [" + existingDefinition +                           "] with [" + beanDefinition + "]");
                   }
               }
               // 注册到容器，beanDefinitionMap 就是个容器
               this.beanDefinitionMap.put(beanName, beanDefinition);
           }       else {
               if (hasBeanCreationStarted()) {
                   // Cannot modify startup-time collection elements anymore (for stable iteration)
                   synchronized (this.beanDefinitionMap) {
                       this.beanDefinitionMap.put(beanName, beanDefinition);
                       List<String> updatedDefinitions = new ArrayList<>
    (this.beanDefinitionNames.size() + 1);
                       updatedDefinitions.addAll(this.beanDefinitionNames);
                       updatedDefinitions.add(beanName);
                       this.beanDefinitionNames = updatedDefinitions;
                       if (this.manualSingletonNames.contains(beanName)) {
                           Set<String> updatedSingletons = new LinkedHashSet<>
    (this.manualSingletonNames);
                           updatedSingletons.remove(beanName);
                           this.manualSingletonNames = updatedSingletons;
                       }
                   }
               }           else {
                   // Still in startup registration phase
                   this.beanDefinitionMap.put(beanName, beanDefinition);
                   this.beanDefinitionNames.add(beanName);
                   this.manualSingletonNames.remove(beanName);
               }
               this.frozenBeanDefinitionNames = null;
           }
           if (existingDefinition != null || containsSingleton(beanName)) {           resetBeanDefinition(beanName);
           }
       }

这个方法中对所需要加载的bean进行校验，没有问题的话就 `put`到 `beanDefinitionMap`中， `beanDefinitionMap`其实就是IOC.这样我们的Bean就被加载到IOC容器中了。

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