IOC 源码--搞懂 Bean 加载过程

1. 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。

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。

<beans>

  <!-- this definition could be inside one beanRefFactory.xml file -->
  <bean id="a.qualified.name.of.some.sort"
      class="org.springframework.context.support.ClassPathXmlApplicationContext">
    <property name="configLocation" value="org/springframework/web/context/beans1.xml"/>
  </bean>

  <!-- while the following two could be inside another, also on the classpath,
    perhaps coming from another component jar -->
  <bean id="another.qualified.name"
      class="org.springframework.context.support.ClassPathXmlApplicationContext">
    <property name="configLocation" value="org/springframework/web/context/beans1.xml"/>
    <property name="parent" ref="a.qualified.name.of.some.sort"/>
  </bean>

  <alias name="another.qualified.name" alias="a.qualified.name.which.is.an.alias"/>

</beans>

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:

    @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Life Cycle:

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(DefaultListableBeanFactory beanFactory)方法中的loadBeanDefinitions(XmlBeanDefinitionReader reader)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:return builder.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。