一、過程（資源定位，Bean的載入，解析，以及註冊）

1. 第一個過程是Resource資源定位。這個Resouce指的是BeanDefinition的資源定位。這個過程就是容器找資料的過程，就像水桶裝水需要先找到水一樣

2. 第二個過程是BeanDefinition的載入過程。這個載入過程是把使用者定義好的Bean表示成Ioc容器內部的資料結構，而這個容器內部的資料結構就是BeanDefition

3. 第三個過程是向IOC容器註冊這些BeanDefinition的過程，這個過程就是將前面的BeanDefition儲存到HashMap中的過程

二、圖解

1.refresh()方法解讀

public
 
 void refresh() throws BeansException, IllegalStateException {
    synchronized (this.startupShutdownMonitor) {
        // Prepare this context for refreshing.
        // 獲取系統配置，環境
        prepareRefresh();

        // Tell the subclass to refresh the internal bean factory.
        // 2.建立BeanFactory，完成資源的定位，Bean的載入，解析，以及註冊
 

        ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

        // Prepare the bean factory for use in this context.
        // 給BeanFactory設定一些跟上下文有關的屬性（Aware 感知自身屬性），為BeanFactory配置容器特性，例如類載入器、事件處理器等
        prepareBeanFactory(beanFactory);

        try {
            // Allows post-processing of the bean factory in context subclasses.
 

            // 為容器的某些子類指定特殊的BeanPost事件處理器（註冊ServletContext,ServletConfig單例），
            postProcessBeanFactory(beanFactory);

            // Invoke factory processors registered as beans in the context.
            // 呼叫BeanFactory的後置處理器
            invokeBeanFactoryPostProcessors(beanFactory);

            // Register bean processors that intercept bean creation.
            // 註冊Bean的後置處理器
            registerBeanPostProcessors(beanFactory);

            // Initialize message source for this context.
            // 初始化國國際化資源
            initMessageSource();

            // Initialize event multicaster for this context.
            // 初始化容器事件傳播器(跟監聽器有關)
            initApplicationEventMulticaster();

            // Initialize other special beans in specific context subclasses.
            // 呼叫子類的某些特殊Bean初始化方法
            onRefresh();

            // Check for listener beans and register them.
            // 為事件傳播器註冊事件監聽器
            registerListeners();

            // Instantiate all remaining (non-lazy-init) singletons.
            // 初始化所有剩餘的單態Bean（所有非懶載入的單例Bean），初始化，以及依賴注入
            finishBeanFactoryInitialization(beanFactory);

            // Last step: publish corresponding event.
            // 初始化容器的生命週期事件處理器，併發布容器的生命週期事件
            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();
        }
    }
}

三、原始碼解讀

1. ClassPathXmlApplicationContext建構函式呼叫

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

        super(parent);
        setConfigLocations(configLocations);
        if (refresh) {
            // 1.spring容器啟動的入口
            refresh();
        }
    }

2. AbstractApplicationContext的refresh()函式啟動整個IoC容器對Bean定義的載入過程

    public void refresh() throws BeansException, IllegalStateException {
        synchronized (this.startupShutdownMonitor) {
            // Prepare this context for refreshing.
            // 獲取系統配置，環境
            prepareRefresh();

            // Tell the subclass to refresh the internal bean factory.
            // 2. 建立BeanFactory，完成資源的定位，Bean的載入，解析，以及註冊
            ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

            // Prepare the bean factory for use in this context.
            // 給BeanFactory設定一些跟上下文有關的屬性（Aware 感知自身屬性），為BeanFactory配置容器特性，例如類載入器、事件處理器等
            prepareBeanFactory(beanFactory);

            try {
                // Allows post-processing of the bean factory in context subclasses.
                // 為容器的某些子類指定特殊的BeanPost事件處理器（註冊ServletContext,ServletConfig單例），
                postProcessBeanFactory(beanFactory);

                // Invoke factory processors registered as beans in the context.
                // 呼叫BeanFactory的後置處理器
                invokeBeanFactoryPostProcessors(beanFactory);

                // Register bean processors that intercept bean creation.
                // 註冊Bean的後置處理器
                registerBeanPostProcessors(beanFactory);

                // Initialize message source for this context.
                // 初始化國國際化資源
                initMessageSource();

                // Initialize event multicaster for this context.
                // 初始化容器事件傳播器(跟監聽器有關)
                initApplicationEventMulticaster();

                // Initialize other special beans in specific context subclasses.
                // 呼叫子類的某些特殊Bean初始化方法
                onRefresh();

                // Check for listener beans and register them.
                // 為事件傳播器註冊事件監聽器
                registerListeners();

                // Instantiate all remaining (non-lazy-init) singletons.
                // 初始化所有剩餘的單態Bean（所有非懶載入的單例Bean），初始化，以及依賴注入
                finishBeanFactoryInitialization(beanFactory);

                // Last step: publish corresponding event.
                // 初始化容器的生命週期事件處理器，併發布容器的生命週期事件
                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();
            }
        }
    }

3. AbstractRefreshableApplicationContext的refreshBeanFactory()方法

    protected final void refreshBeanFactory() throws BeansException {
        // 如果已經有容器，銷燬容器中的bean，關閉容器
        if (hasBeanFactory()) {
            destroyBeans();
            closeBeanFactory();
        }
        try {
            // 建立IoC容器
            DefaultListableBeanFactory beanFactory = createBeanFactory();
            beanFactory.setSerializationId(getId());
            // 對IoC容器進行定製化，如設定啟動引數，開啟註解的自動裝配等
            customizeBeanFactory(beanFactory);
            // 3. 呼叫載入Bean定義的方法，主要這裡又使用了一個委派模式，在當前類中只定義了抽象的loadBeanDefinitions方法，具體的實現呼叫子類容器
            loadBeanDefinitions(beanFactory);

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

4. AbstractXmlApplicationContext的loadBeanDefinitions()方法

    protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) throws BeansException, IOException {
        // Create a new XmlBeanDefinitionReader for the given BeanFactory.
        // 建立XmlBeanDefinitionReader，即建立Bean讀取器，並通過回撥設定到容器中去，容 器使用該讀取器讀取Bean定義資源
        XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(beanFactory);

        // Configure the bean definition reader with this context's
        // resource loading environment.
        beanDefinitionReader.setEnvironment(this.getEnvironment());
        beanDefinitionReader.setResourceLoader(this);
        beanDefinitionReader.setEntityResolver(new ResourceEntityResolver(this));

        // Allow a subclass to provide custom initialization of the reader,
        // then proceed with actually loading the bean definitions.
        // 當Bean讀取器讀取Bean定義的Xml資原始檔時，啟用Xml的校驗機制
        initBeanDefinitionReader(beanDefinitionReader);
        // 4.Bean讀取器真正實現載入的方法
        loadBeanDefinitions(beanDefinitionReader);
    }

    protected void loadBeanDefinitions(XmlBeanDefinitionReader reader) throws BeansException, IOException {
        // 獲取Bean定義資源的定位
        Resource[] configResources = getConfigResources();
        if (configResources != null) {
            reader.loadBeanDefinitions(configResources);
        }
        // 如果子類中獲取的Bean定義資源定位為空，則獲取FileSystemXmlApplicationContext構造方法中setConfigLocations方法設定的資源
        String[] configLocations = getConfigLocations();
        if (configLocations != null) {
            // 繼續呼叫真正的載入方法
            reader.loadBeanDefinitions(configLocations);
        }
    }

5. AbstractBeanDefinitionReader的loadBeanDefinitions()方法

    public int loadBeanDefinitions(String location, @Nullable Set<Resource> actualResources) throws BeanDefinitionStoreException {
        // 獲取在IoC容器初始化過程中設定的資源載入器
        ResourceLoader resourceLoader = getResourceLoader();
        if (resourceLoader == null) {
            throw new BeanDefinitionStoreException(
                    "Cannot import bean definitions from location [" + location + "]: no ResourceLoader available");
        }

        if (resourceLoader instanceof ResourcePatternResolver) {
            // Resource pattern matching available.
            try {
                // 將指定位置的Bean定義資原始檔解析為Spring IoC容器封裝的資源
                // 載入多個指定位置的Bean定義資原始檔
                Resource[] resources = ((ResourcePatternResolver) resourceLoader).getResources(location);
                // 5. 委派呼叫其子類XmlBeanDefinitionReader的方法，實現載入功能
                int loadCount = loadBeanDefinitions(resources);
                if (actualResources != null) {
                    for (Resource resource : resources) {
                        actualResources.add(resource);
                    }
                }
                if (logger.isDebugEnabled()) {
                    logger.debug("Loaded " + loadCount + " bean definitions from location pattern [" + location + "]");
                }
                return loadCount;
            }
            catch (IOException ex) {
                throw new BeanDefinitionStoreException(
                        "Could not resolve bean definition resource pattern [" + location + "]", ex);
            }
        }
        else {
            // Can only load single resources by absolute URL.
            Resource resource = resourceLoader.getResource(location);
            int loadCount = loadBeanDefinitions(resource);
            if (actualResources != null) {
                actualResources.add(resource);
            }
            if (logger.isDebugEnabled()) {
                logger.debug("Loaded " + loadCount + " bean definitions from location [" + location + "]");
            }
            return loadCount;
        }
    }

6. XmlBeanDefinitionReader的loadBeanDefinitions()

    public int loadBeanDefinitions(EncodedResource encodedResource) throws BeanDefinitionStoreException {
        Assert.notNull(encodedResource, "EncodedResource must not be null");
        if (logger.isInfoEnabled()) {
            logger.info("Loading XML bean definitions from " + encodedResource.getResource());
        }

        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 {
            // 將資原始檔轉為InputStream的IO流
            InputStream inputStream = encodedResource.getResource().getInputStream();
            try {
                // 從InputStream中得到XML的解析源 
                InputSource inputSource = new InputSource(inputStream);
                if (encodedResource.getEncoding() != null) {
                    inputSource.setEncoding(encodedResource.getEncoding());
                }
                // 6. 這裡是具體的讀取過程 
                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();
            }
        }
    }

7. 還是XmlBeanDefinitionReader的doLoadBeanDefinitions()

    protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource) throws BeanDefinitionStoreException {
        try {
            // 將XML檔案轉換為DOM物件，解析過程由documentLoader實現
            Document doc = doLoadDocument(inputSource, resource);
            // 7. 這裡是啟動對Bean定義解析的詳細過程，該解析過程會用到Spring的Bean配置規則
            return registerBeanDefinitions(doc, resource);
        }
        catch (BeanDefinitionStoreException ex) {
            throw ex;
        }
    }

8. 還是XmlBeanDefinitionReader的registerBeanDefinitions()方法

    public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
        // 得到BeanDefinitionDocumentReader來對xml格式的BeanDefinition解析
        BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();
        // 獲得容器中註冊的Bean數量
        int countBefore = getRegistry().getBeanDefinitionCount();
        // 8. 解析過程入口, 這裡使用了委派模式，BeanDefinitionDocumentReader只是個介面，具體的解析實現過程有實現類DefaultBeanDefinitionDocumentReader完成
        documentReader.registerBeanDefinitions(doc, createReaderContext(resource));
        // 統計解析的Bean數量
        return getRegistry().getBeanDefinitionCount() - countBefore;
    }       

9. BeanDefinitionDocumentReader介面通過registerBeanDefinitions()方法

    public void registerBeanDefinitions(Document doc, XmlReaderContext readerContext) {
        this.readerContext = readerContext;
        logger.debug("Loading bean definitions");
        // 獲取根節點
        Element root = doc.getDocumentElement();
        doRegisterBeanDefinitions(root);
    }

    protected void doRegisterBeanDefinitions(Element root) {
        // BeanDefinitionParserDelegate中定義了Spring Bean定義XML檔案的各種元素，委託
        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);
                if (!getReaderContext().getEnvironment().acceptsProfiles(specifiedProfiles)) {
                    if (logger.isInfoEnabled()) {
                        logger.info("Skipped XML bean definition file due to specified profiles [" + profileSpec +
                                "] not matching: " + getReaderContext().getResource());
                    }
                    return;
                }
            }
        }
        // 在解析Bean定義之前，進行自定義的解析，增強解析過程的可擴充套件性
        preProcessXml(root);
        // 9.從Document的根元素開始進行Bean定義的Document物件
        parseBeanDefinitions(root, this.delegate);
        // 在解析Bean定義之後，進行自定義的解析，增強解析過程的可擴充套件性
        postProcessXml(root);

        this.delegate = parent;
    }

10. DefaultBeanDefinitionDocumentReader的parseBeanDefinitions()方法

    protected void parseBeanDefinitions(Element root, BeanDefinitionParserDelegate delegate) {
        // Bean定義的Document物件使用了Spring預設的XML名稱空間
        if (delegate.isDefaultNamespace(root)) {
            // 獲取Bean定義的Document物件根元素的所有子節點
            NodeList nl = root.getChildNodes();
            for (int i = 0; i < nl.getLength(); i++) {
                Node node = nl.item(i);
                // 獲得Document節點是XML元素節點
                if (node instanceof Element) {
                    Element ele = (Element) node;
                    if (delegate.isDefaultNamespace(ele)) {
                        // 10. 使用Spring的Bean規則解析元素節點
                        parseDefaultElement(ele, delegate);
                    }
                    else {
                        // 沒有使用Spring預設的XML名稱空間，則使用使用者自定義的解析規則解析元素節點
                        delegate.parseCustomElement(ele);
                    }
                }
            }
        }
        else {
            delegate.parseCustomElement(root);
        }
    }   

    private void parseDefaultElement(Element ele, BeanDefinitionParserDelegate delegate) {
        // 如果元素節點是<Import>匯入元素，進行匯入解析
        if (delegate.nodeNameEquals(ele, IMPORT_ELEMENT)) {
            importBeanDefinitionResource(ele);
        }
        // 如果元素節點是<Alias>別名元素，進行別名解析
        else if (delegate.nodeNameEquals(ele, ALIAS_ELEMENT)) {
            processAliasRegistration(ele);
        }
        // 元素節點既不是匯入元素，也不是別名元素，即普通的<Bean>元素，按照Spring的Bean規則解析元素
        else if (delegate.nodeNameEquals(ele, BEAN_ELEMENT)) {
            processBeanDefinition(ele, delegate);
        }
        // Beans的解析
        else if (delegate.nodeNameEquals(ele, NESTED_BEANS_ELEMENT)) {
            // recurse
            doRegisterBeanDefinitions(ele);
        }
    }

11. DefaultBeanDefinitionDocumentReader的processBeanDefinition()方法

    protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
        // BeanDefinitionHolder是對BeanDefinition的封裝，即Bean定義的封裝類，包括（屬性beanDefinition, beanName, aliases）
        // 11. 對Document物件中<Bean>元素的解析由BeanDefinitionParserDelegate實現
        BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
        if (bdHolder != null) {
            bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);
            try {
                // Register the final decorated instance.
                // 向Spring IoC容器註冊解析得到的Bean定義，這是Bean定義向IoC容器註冊的入口
                BeanDefinitionReaderUtils.registerBeanDefinition(bdHolder, getReaderContext().getRegistry());
            }
            catch (BeanDefinitionStoreException ex) {
                getReaderContext().error("Failed to register bean definition with name '" +
                        bdHolder.getBeanName() + "'", ele, ex);
            }
            // Send registration event.
            // 在完成向Spring IoC容器註冊解析得到的Bean定義之後，傳送註冊事件
            getReaderContext().fireComponentRegistered(new BeanComponentDefinition(bdHolder));
        }
    }

12. BeanDefinitionParserDelegate的parseBeanDefinitionElement()方法，返回BeanDefinitionHolder

    public BeanDefinitionHolder parseBeanDefinitionElement(Element ele, @Nullable BeanDefinition containingBean) {
        // 獲取<Bean>元素中的id屬性值
        String id = ele.getAttribute(ID_ATTRIBUTE);
        // 獲取<Bean>元素中的name屬性值
        String nameAttr = ele.getAttribute(NAME_ATTRIBUTE);
        // 獲取<Bean>元素中的alias屬性值
        List<String> aliases = new ArrayList<>();
        if (StringUtils.hasLength(nameAttr)) {
            String[] nameArr = StringUtils.tokenizeToStringArray(nameAttr, MULTI_VALUE_ATTRIBUTE_DELIMITERS);
            aliases.addAll(Arrays.asList(nameArr));
        }

        String beanName = id;
        // 如果<Bean>元素中沒有配置id屬性時，將別名中的第一個值賦值給beanName
        if (!StringUtils.hasText(beanName) && !aliases.isEmpty()) {
            beanName = aliases.remove(0);
            if (logger.isDebugEnabled()) {
                logger.debug("No XML 'id' specified - using '" + beanName +
                        "' as bean name and " + aliases + " as aliases");
            }
        }
        // 檢查<Bean>元素所配置的id或者name的唯一性，containingBean標識<Bean>元素中是否包含子<Bean>元素
        if (containingBean == null) {
            checkNameUniqueness(beanName, aliases, ele);
        }
        // 12. 詳細對<Bean>元素中配置的Bean定義進行解析的地方
        AbstractBeanDefinition beanDefinition = parseBeanDefinitionElement(ele, beanName, containingBean);

        if (beanDefinition != null) {
            if (!StringUtils.hasText(beanName)) {
                try {
                    if (containingBean != null) {
                        // 如果<Bean>元素中沒有配置id、別名或者name，且沒有包含子//<Bean>元素，為解析的Bean生成一個唯一beanName並註冊
                        beanName = BeanDefinitionReaderUtils.generateBeanName(
                                beanDefinition, this.readerContext.getRegistry(), true);
                    }
                    else {
                        beanName = this.readerContext.generateBeanName(beanDefinition);
                        // Register an alias for the plain bean class name, if still possible,
                        // if the generator returned the class name plus a suffix.
                        // This is expected for Spring 1.2/2.0 backwards compatibility.
                        String beanClassName = beanDefinition.getBeanClassName();
                        if (beanClassName != null &&
                                beanName.startsWith(beanClassName) && beanName.length() > beanClassName.length() &&
                                !this.readerContext.getRegistry().isBeanNameInUse(beanClassName)) {
                            aliases.add(beanClassName);
                        }
                    }
                    if (logger.isDebugEnabled()) {
                        logger.debug("Neither XML 'id' nor 'name' specified - " +
                                "using generated bean name [" + beanName + "]");
                    }
                }
                catch (Exception ex) {
                    error(ex.getMessage(), ele);
                    return null;
                }
            }
            String[] aliasesArray = StringUtils.toStringArray(aliases);
            // 返回解析完成的Bean
            return new BeanDefinitionHolder(beanDefinition, beanName, aliasesArray);
        }

        return null;
    }   

13. 繼續BeanDefinitionParserDelegate的parseBeanDefinitionElement()方法，返回AbstractBeanDefinition

    public AbstractBeanDefinition parseBeanDefinitionElement(
            Element ele, String beanName, @Nullable BeanDefinition containingBean) {
        // 記錄解析的<Bean>
        this.parseState.push(new BeanEntry(beanName));
        // 只是記錄配置的class名字，不做例項化，物件的例項化在依賴注入時完成
        String className = null;
        if (ele.hasAttribute(CLASS_ATTRIBUTE)) {
            className = ele.getAttribute(CLASS_ATTRIBUTE).trim();
        }
        String parent = null;
        // 如果<Bean>元素中配置了parent屬性，則獲取parent屬性的值
        if (ele.hasAttribute(PARENT_ATTRIBUTE)) {
            parent = ele.getAttribute(PARENT_ATTRIBUTE);
        }

        try {
            // 13. 根據<Bean>元素配置的class名稱和parent屬性值建立BeanDefinition為載入Bean定義資訊做準備
            AbstractBeanDefinition bd = createBeanDefinition(className, parent);

            // 對當前的<Bean>元素中配置的一些屬性進行解析和設定，如配置的單態(singleton)屬性等
            parseBeanDefinitionAttributes(ele, beanName, containingBean, bd);
            bd.setDescription(DomUtils.getChildElementValueByTagName(ele, DESCRIPTION_ELEMENT));
            // 對<Bean>元素的meta(元資訊)屬性解析
            parseMetaElements(ele, bd);
            parseLookupOverrideSubElements(ele, bd.getMethodOverrides());
            parseReplacedMethodSubElements(ele, bd.getMethodOverrides());
            // 解析<Bean>元素的構造方法設定
            parseConstructorArgElements(ele, bd);
            // 18.解析<Bean>元素的<property>設定
            parsePropertyElements(ele, bd);
            parseQualifierElements(ele, bd);

            bd.setResource(this.readerContext.getResource());
            bd.setSource(extractSource(ele));

            return bd;
        }
        catch (ClassNotFoundException ex) {
            error("Bean class [" + className + "] not found", ele, ex);
        }
        catch (NoClassDefFoundError err) {
            error("Class that bean class [" + className + "] depends on not found", ele, err);
        }
        catch (Throwable ex) {
            error("Unexpected failure during bean definition parsing", ele, ex);
        }
        finally {
            this.parseState.pop();
        }

        return null;
    }

14. BeanDefinitionReaderUtils的createBeanDefinition()方法最終生成BeanDefinition

    protected AbstractBeanDefinition createBeanDefinition(@Nullable String className, @Nullable String parentName)throws ClassNotFoundException {

        return BeanDefinitionReaderUtils.createBeanDefinition(parentName, className, this.readerContext.getBeanClassLoader());
    }   

    public static AbstractBeanDefinition createBeanDefinition(@Nullable String parentName, @Nullable String className, @Nullable ClassLoader classLoader) throws ClassNotFoundException {
        // 14.最終生成BeanDefinition物件的地方
        GenericBeanDefinition bd = new GenericBeanDefinition();
        bd.setParentName(parentName);
        if (className != null) {
            if (classLoader != null) {
                bd.setBeanClass(ClassUtils.forName(className, classLoader));
            }
            else {
                bd.setBeanClassName(className);
            }
        }
        return bd;
    }

15. 經過對Spring Bean定義資原始檔轉換的Document物件中的元素層層解析，Spring IoC現在已經將XML形式定義的Bean定義資原始檔轉換為Spring IoC所識別的資料結構——BeanDefinition，它是Bean定義資原始檔中配置的POJO物件在Spring IoC容器中的對映（回到11部分）

    protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
        // BeanDefinitionHolder是對BeanDefinition的封裝，即Bean定義的封裝類，包括（屬性beanDefinition, beanName, aliases）
        // 11. 對Document物件中<Bean>元素的解析由BeanDefinitionParserDelegate實現
        BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
        if (bdHolder != null) {
            bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);
            try {
                // Register the final decorated instance.
                // 15. 向Spring IoC容器註冊解析得到的Bean定義，這是Bean定義向IoC容器註冊的入口
                BeanDefinitionReaderUtils.registerBeanDefinition(bdHolder, getReaderContext().getRegistry());
            }
            catch (BeanDefinitionStoreException ex) {
                getReaderContext().error("Failed to register bean definition with name '" +
                        bdHolder.getBeanName() + "'", ele, ex);
            }
            // Send registration event.
            // 在完成向Spring IoC容器註冊解析得到的Bean定義之後，傳送註冊事件
            getReaderContext().fireComponentRegistered(new BeanComponentDefinition(bdHolder));
        }
    }

16. 在IOC容器中BeanDefinition儲存的只是一些靜態資訊，接下來需要向容器註冊Bean定義資訊才能全部完成IOC容器的初始化過程

    public static void registerBeanDefinition(BeanDefinitionHolder definitionHolder, BeanDefinitionRegistry registry) throws BeanDefinitionStoreException {

        // Register bean definition under primary name.
        // 獲取解析的BeanDefinition的名稱
        String beanName = definitionHolder.getBeanName();
        // 16. 向IoC容器註冊BeanDefinition
        registry.registerBeanDefinition(beanName, definitionHolder.getBeanDefinition());

        // Register aliases for bean name, if any.
        // 如果解析的BeanDefinition有別名，向容器為其註冊別名
        String[] aliases = definitionHolder.getAliases();
        if (aliases != null) {
            for (String alias : aliases) {
                registry.registerAlias(beanName, alias);
            }
        }
    }

17. 真正完成註冊功能的是DefaultListableBeanFactory，DefaultListableBeanFactory中使用一個HashMap的集合物件存放IoC容器中註冊解析的BeanDefinition

    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");
        // 校驗解析的BeanDefiniton
        if (beanDefinition instanceof AbstractBeanDefinition) {
            try {
                ((AbstractBeanDefinition) beanDefinition).validate();
            }
            catch (BeanDefinitionValidationException ex) {
                throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
                        "Validation of bean definition failed", ex);
            }
        }

        BeanDefinition oldBeanDefinition;

        oldBeanDefinition = this.beanDefinitionMap.get(beanName);
        // 檢查是否有同名的BeanDefinition已經在IoC容器中註冊，如果已經註冊，並且不允許覆蓋已註冊的Bean，則丟擲註冊失敗異常
        if (oldBeanDefinition != null) {
            if (!isAllowBeanDefinitionOverriding()) {
                throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
                        "Cannot register bean definition [" + beanDefinition + "] for bean '" + beanName +
                        "': There is already [" + oldBeanDefinition + "] bound.");
            }
            else if (oldBeanDefinition.getRole() < beanDefinition.getRole()) {
                // e.g. was ROLE_APPLICATION, now overriding with ROLE_SUPPORT or ROLE_INFRASTRUCTURE
                if (this.logger.isWarnEnabled()) {
                    this.logger.warn("Overriding user-defined bean definition for bean '" + beanName +
                            "' with a framework-generated bean definition: replacing [" +
                            oldBeanDefinition + "] with [" + beanDefinition + "]");
                }
            }
            else if (!beanDefinition.equals(oldBeanDefinition)) {
                if (this.logger.isInfoEnabled()) {
                    this.logger.info("Overriding bean definition for bean '" + beanName +
                            "' with a different definition: replacing [" + oldBeanDefinition +
                            "] with [" + beanDefinition + "]");
                }
            }
            else {
                if (this.logger.isDebugEnabled()) {
                    this.logger.debug("Overriding bean definition for bean '" + beanName +
                            "' with an equivalent definition: replacing [" + oldBeanDefinition +
                            "] with [" + beanDefinition + "]");
                }
            }
            // 如果允許覆蓋，則同名的Bean，後註冊的覆蓋先註冊的
            this.beanDefinitionMap.put(beanName, beanDefinition);

        }
        else {
            if (hasBeanCreationStarted()) {
                // Cannot modify startup-time collection elements anymore (for stable iteration)
                // 註冊的過程中需要執行緒同步，以保證資料的一致性
                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
                // 16. IoC容器中沒有已經註冊同名的Bean，按正常註冊流程註冊
                this.beanDefinitionMap.put(beanName, beanDefinition);
                this.beanDefinitionNames.add(beanName);
                this.manualSingletonNames.remove(beanName);
            }
            this.frozenBeanDefinitionNames = null;
        }

        if (oldBeanDefinition != null || containsSingleton(beanName)) {
            // 重置所有已經註冊過的BeanDefinition的快取
            resetBeanDefinition(beanName);
        }
    }

18. 初始化完成了，後續的分支走一走（回到13， 對的解析）

    public void parsePropertyElements(Element beanEle, BeanDefinition bd) {
        // 獲取<Bean>元素中所有的子元素
        NodeList nl = beanEle.getChildNodes();
        for (int i = 0; i < nl.getLength(); i++) {
            Node node = nl.item(i);
            // 如果子元素是<property>子元素，則呼叫解析<property>子元素方法解析
            if (isCandidateElement(node) && nodeNameEquals(node, PROPERTY_ELEMENT)) {
                // 解析<property>元素
                parsePropertyElement((Element) node, bd);
            }
        }
    }

    public void parsePropertyElement(Element ele, BeanDefinition bd) {
        // 獲取<property>元素的名字
        String propertyName = ele.getAttribute(NAME_ATTRIBUTE);
        if (!StringUtils.hasLength(propertyName)) {
            error("Tag 'property' must have a 'name' attribute", ele);
            return;
        }
        this.parseState.push(new PropertyEntry(propertyName));
        try {
            // 如果一個Bean中已經有同名的property存在，則不進行解析，直接返回。 即如果在同一個Bean中配置同名的property，則只有第一個起作用
            if (bd.getPropertyValues().contains(propertyName)) {
                error("Multiple 'property' definitions for property '" + propertyName + "'", ele);
                return;
            }
            // 18. 解析獲取property的值
            Object val = parsePropertyValue(ele, bd, propertyName);
            // 根據property的名字和值建立property例項
            PropertyValue pv = new PropertyValue(propertyName, val);
            // 解析<property>元素中的屬性
            parseMetaElements(ele, pv);
            pv.setSource(extractSource(ele));
            bd.getPropertyValues().addPropertyValue(pv);
        }
        finally {
            this.parseState.pop();
        }
    }

19. 解析獲取property值

    public Object parsePropertyValue(Element ele, BeanDefinition bd, @Nullable String propertyName) {
        String elementName = (propertyName != null) ?
                        "<property> element for property '" + propertyName + "'" :
                        "<constructor-arg> element";

        // Should only have one child element: ref, value, list, etc.
        // 獲取<property>的所有子元素，只能是其中一種型別:ref,value,list等
        NodeList nl = ele.getChildNodes();
        Element subElement = null;
        for (int i = 0; i < nl.getLength(); i++) {
            Node node = nl.item(i);
            // 子元素不是description和meta屬性
            if (node instanceof Element && !nodeNameEquals(node, DESCRIPTION_ELEMENT) &&
                    !nodeNameEquals(node, META_ELEMENT)) {
                // Child element is what we're looking for.
                if (subElement != null) {
                    error(elementName + " must not contain more than one sub-element", ele);
                }
                else {// 當前<property>元素包含有子元素
                    subElement = (Element) node;
                }
            }
        }
        // 判斷property的屬性值是ref還是value，不允許既是ref又是value
        boolean hasRefAttribute = ele.hasAttribute(REF_ATTRIBUTE);
        boolean hasValueAttribute = ele.hasAttribute(VALUE_ATTRIBUTE);
        if ((hasRefAttribute && hasValueAttribute) ||
                ((hasRefAttribute || hasValueAttribute) && subElement != null)) {
            error(elementName +
                    " is only allowed to contain either 'ref' attribute OR 'value' attribute OR sub-element", ele);
        }
        // 如果屬性是ref，建立一個ref的資料物件RuntimeBeanReference，這個物件封裝了ref資訊
        if (hasRefAttribute) {
            String refName = ele.getAttribute(REF_ATTRIBUTE);
            if (!StringUtils.hasText(refName)) {
                error(elementName + " contains empty 'ref' attribute", ele);
            }
            // 一個指向執行時所依賴物件的引用
            RuntimeBeanReference ref