Spring Ioc代碼閱讀_ZenDei技術網路在線

1，理解控制反轉以前一直說著這個詞，然後把它等於上ioc這個詞，再等於上代碼里一個bean里依賴了其他bean，不用new，用註解，用xml去描述，就可以了。能用就行了，實際理論的不管也不影響編碼，其實能用了內心也是理解是怎麼回事的，知識理論上說不好而已。我覺得只要理解一個事情就好了，ioc所謂 ...

1，理解控制反轉

以前一直說著這個詞，然後把它等於上ioc這個詞，再等於上代碼里一個bean里依賴了其他bean，不用new，用註解，用xml去描述，就可以了。能用就行了，實際理論的不管也不影響編碼，其實能用了內心也是理解是怎麼回事的，知識理論上說不好而已。我覺得只要理解一個事情就好了，ioc所謂的控制翻轉，它控制翻轉的是什麼？ 依賴對象的獲得被反轉 　　把一個bean中對其他bean的依賴這個事提取出來，統一由一個容器管理，解耦了bean管理和業務的代碼（工廠方法模式不就乾這事嘛）最後，Spring提供了一個IoC容器來管理對象的生命周期、依賴關係，這也是spring的最核心的基礎組件。其實控制反轉也是一種設計模式，的確，從項目層面抽取了管理bean依賴關係，徹底解耦bean註入和業務代碼，也是一種軟體開發中的思想。

2，IoC容器它要完成什麼事？通過解析bean依賴關係的描述，容器需要創建出項目中所有需要管理的bean，然後註入。那麼拆成打的兩步：1，把依賴關係描述解析成bean；2，把bean註入但是，既然是bean之間有依賴關係，當生產一個bean時，它依賴的bean也需要已經產生，所以在一個軟體中，觸發開始初始化容器的時候是把項目中用到的bean看成樹結構的樣子，然後一次遞歸的生產bean，註入。有點像目錄結構遍歷。比如：A依賴B B依賴C，那麼A生產時會觸發B生產，然後觸發C生產。所以，我們閱讀IoC源碼時也會看到這樣的實現。

3，IoC容器的實現 BeanFactory和ApplicationContext BeanFactory是容器基礎設計，ApplicationContext增加了針對企業軟體一些擴展功能，應該說後者是前者的高級形態。

public interface BeanFactory {

    /**
     * 用來獲取FactoryBean本身的轉義符
     */
    String FACTORY_BEAN_PREFIX = "&";

    /**
     * 獲取容器維護的bean是用名稱來指定的
     */
    Object getBean(String name) throws BeansException;

    <T> T getBean(String name, Class<T> requiredType) throws BeansException;

    <T> T getBean(Class<T> requiredType) throws BeansException;

    Object getBean(String name, Object... args) throws BeansException;

    <T> T getBean(Class<T> requiredType, Object... args) throws BeansException;

    /**
     * 查看容器中是否包含指定name的bean
     */
    boolean containsBean(String name);

    /**
     * 判斷指定name的bean是否為單例
     */
    boolean isSingleton(String name) throws NoSuchBeanDefinitionException;

    /**
     * 判斷指定name的bean是否為多例
     */
    boolean isPrototype(String name) throws NoSuchBeanDefinitionException;

    boolean isTypeMatch(String name, ResolvableType typeToMatch) throws NoSuchBeanDefinitionException;

    boolean isTypeMatch(String name, Class<?> typeToMatch) throws NoSuchBeanDefinitionException;

    Class<?> getType(String name) throws NoSuchBeanDefinitionException;

    /**
     * 獲得bean的所有別名
     */
    String[] getAliases(String name);

}

BeanFactory設計定義了ioc的規範。

DefaultListableBeanFactory <— XmlBeanFactory DefaultListableBeanFactory有容器的完整實現，而XmlBeanFactory是對它的一種擴展，擴展什麼看名字就知道了，支持xml數據源的解析。我們看下XmlBeanFactory的代碼：

public class XmlBeanFactory extends DefaultListableBeanFactory {
    /**
     * 換了一個reader
     */
    private final XmlBeanDefinitionReader reader = new XmlBeanDefinitionReader(this);

    public XmlBeanFactory(Resource resource) throws BeansException {
        this(resource, null);
    }


    public XmlBeanFactory(Resource resource, BeanFactory parentBeanFactory) throws BeansException {
        super(parentBeanFactory);
        this.reader.loadBeanDefinitions(resource);
    }

}

XmlBeanFactory在初始化時調用loadBeanDefinitions方法，使用XmlBeanDefinitionReader去解析元數據，這也是最初的一步。

FileSystemXmlApplicationContext源碼：

public class FileSystemXmlApplicationContext extends AbstractXmlApplicationContext {
    public FileSystemXmlApplicationContext() {
    }
    public FileSystemXmlApplicationContext(ApplicationContext parent) {
        super(parent);
    }
    public FileSystemXmlApplicationContext(String configLocation) throws BeansException {
        this(new String[] {configLocation}, true, null);
    }
    public FileSystemXmlApplicationContext(String... configLocations) throws BeansException {
        this(configLocations, true, null);
    }
    public FileSystemXmlApplicationContext(String[] configLocations, ApplicationContext parent) throws BeansException {
        this(configLocations, true, parent);
    }
    public FileSystemXmlApplicationContext(String[] configLocations, boolean refresh) throws BeansException {
        this(configLocations, refresh, null);
    }

    /**
     * setConfigLocations 將路徑字元串解析到自己存資源配置的configLocations里
     * 然後調用refresh()，這也是容器初始化的入口
     * 
     */
    public FileSystemXmlApplicationContext(String[] configLocations, boolean refresh, ApplicationContext parent)
            throws BeansException {

        super(parent);
        setConfigLocations(configLocations);
        if (refresh) {
            refresh();
        }
    }

    /**
     * 這是這個FileSystemXml自己的實現部分，getResourceByPath必然是哪一個父類模版方法需要調用的方法
     * 而這個方法自己實現提供一個FileSystemResource出來。
     * Resource這個就是將元數據轉化成容器可以統一可以解析的資源，根據不同的元數據類型，必然需要不同的演算法去讀取這些數據。
     */
    @Override
    protected Resource getResourceByPath(String path) {
        if (path != null && path.startsWith("/")) {
            path = path.substring(1);
        }
        return new FileSystemResource(path);
    }

}

調用AbstractApplicationContext的refresh方法：

標準的模版方法，這裡註意繼承鏈路，嵌套的模版方法調用下來。

    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.
                // 擴展點1
                invokeBeanFactoryPostProcessors(beanFactory);

                // Register bean processors that intercept bean creation.
                // 擴展點2
                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) {
                // Destroy already created singletons to avoid dangling resources.
                destroyBeans();

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

                // Propagate exception to caller.
                throw ex;
            }
        }
    }

調用AbstractApplicationContext 的obtainFreshBeanFactory方法：

protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {
    // 這要調用子類的實現=>AbstractRefreshableApplicationContext
        refreshBeanFactory();
        ConfigurableListableBeanFactory beanFactory = getBeanFactory();
        if (logger.isDebugEnabled()) {
            logger.debug("Bean factory for " + getDisplayName() + ": " + beanFactory);
        }
        return beanFactory;
    }

AbstractRefreshableApplicationContext的refreshBeanFactory

protected final void refreshBeanFactory() throws BeansException {
        if (hasBeanFactory()) {
            destroyBeans();
            closeBeanFactory();
        }
        try {
            DefaultListableBeanFactory beanFactory = createBeanFactory();
            beanFactory.setSerializationId(getId());
            customizeBeanFactory(beanFactory);
            // 子類實現
            loadBeanDefinitions(beanFactory);
            synchronized (this.beanFactoryMonitor) {
                this.beanFactory = beanFactory;
            }
        }
        catch (IOException ex) {
            throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
        }
    }

AbstractXmlApplicationContext的loadBeanDefinitions

protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) throws BeansException, IOException {
        // Create a new XmlBeanDefinitionReader for the given BeanFactory.
        // 指定Reader
        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.
        initBeanDefinitionReader(beanDefinitionReader);
        //
        loadBeanDefinitions(beanDefinitionReader);
    }
    protected void loadBeanDefinitions(XmlBeanDefinitionReader reader) throws BeansException, IOException {
        Resource[] configResources = getConfigResources();
        if (configResources != null) {
            reader.loadBeanDefinitions(configResources);
        }
        String[] configLocations = getConfigLocations();
        if (configLocations != null) {
            reader.loadBeanDefinitions(configLocations);
        }
    }

XmlBeanDefinitionReader繼承的基類AbstractBeanDefinitionReader的實現：

public int loadBeanDefinitions(String... locations) throws BeanDefinitionStoreException {
        Assert.notNull(locations, "Location array must not be null");
        int counter = 0;
        for (String location : locations) {
            counter += loadBeanDefinitions(location);
        }
        return counter;
    }
    public int loadBeanDefinitions(String location) throws BeanDefinitionStoreException {
        return loadBeanDefinitions(location, null);
    }
    public int loadBeanDefinitions(Resource... resources) throws BeanDefinitionStoreException {
        Assert.notNull(resources, "Resource array must not be null");
        int counter = 0;
        for (Resource resource : resources) {
            counter += loadBeanDefinitions(resource);
        }
        return counter;
    }
    /*
    *最終調用到的實現，這裡需要產生Resource，既然用FileSystemXmlApplicationContext
    * 那在讀取文件產生Resource的過程有它自定義的部分，也就是我們在FileSystemXmlApplicationContext中看到的getResourceByPath會被使用。
    */
    public int loadBeanDefinitions(String location, Set<Resource> actualResources) throws BeanDefinitionStoreException {
        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 {
              // 要產生Resource了
                Resource[] resources = ((ResourcePatternResolver) resourceLoader).getResources(location);
                // 這裡是解析Resource的入口
                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;
        }
    }

回到AbstractApplicationContext調用getResources方法，resourcePatternResolver用預設的PathMatchingResourcePatternResolver

public Resource[] getResources(String locationPattern) throws IOException {
        return this.resourcePatternResolver.getResources(locationPattern);
    }
public AbstractApplicationContext() {
        this.resourcePatternResolver = getResourcePatternResolver();
    }
    //預設PathMatchingResourcePatternResolver
protected ResourcePatternResolver getResourcePatternResolver() {
        return new PathMatchingResourcePatternResolver(this);
    }

PathMatchingResourcePatternResolver的getResources實現

public Resource[] getResources(String locationPattern) throws IOException {
        Assert.notNull(locationPattern, "Location pattern must not be null");
        // 一種是classpath開頭 一種沒有
        if (locationPattern.startsWith(CLASSPATH_ALL_URL_PREFIX)) {
            // a class path resource (multiple resources for same name possible)
            if (getPathMatcher().isPattern(locationPattern.substring(CLASSPATH_ALL_URL_PREFIX.length()))) {
                // a class path resource pattern
                return findPathMatchingResources(locationPattern);
            }
            else {
                // all class path resources with the given name
                return findAllClassPathResources(locationPattern.substring(CLASSPATH_ALL_URL_PREFIX.length()));
            }
        }
        else {
            // Only look for a pattern after a prefix here
            // (to not get fooled by a pattern symbol in a strange prefix).
            int prefixEnd = locationPattern.indexOf(":") + 1;
            if (getPathMatcher().isPattern(locationPattern.substring(prefixEnd))) {
                // a file pattern
                return findPathMatchingResources(locationPattern);
            }
            else {
                // a single resource with the given name
                // 使用ResourceLoader來產生Resource
                return new Resource[] {getResourceLoader().getResource(locationPattern)};
            }
        }
    }
    
    // 預設使用DefaultResourceLoader來解析路徑
    public PathMatchingResourcePatternResolver(ClassLoader classLoader) {
        this.resourceLoader = new DefaultResourceLoader(classLoader);
    }

    public ResourceLoader getResourceLoader() {
        return this.resourceLoader;
    }

DefaultResourceLoader的getResource，而此時返回的Resource是FileSystemResource類型

public Resource getResource(String location) {
   Assert.notNull(location, "Location must not be null");
   if (location.startsWith("/")) {
      // 最終調用子類的getResourceByPath，而這個方法正是FileSystemXmlApplicationContext實現的
      return getResourceByPath(location);
   }
   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 new UrlResource(url);
      }
      catch (MalformedURLException ex) {
         // No URL -> resolve as resource path.
         return getResourceByPath(location);
      }
   }
}

可見FileSystemXmlApplicationContext最終體現在產生的Resource不同實現。

Resource相當於定位到資源的抽象，下一步就是不同Resource解析資源。

回到AbstractBeanDefinitionReader的loadBeanDefinitions。定位後下一步就是解析Resource的入口：

int loadCount = loadBeanDefinitions(resource);

子類XmlBeanDefinitionReader實現：

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<EncodedResource>(4);
            this.resourcesCurrentlyBeingLoaded.set(currentResources);
        }
        if (!currentResources.add(encodedResource)) {
            throw new BeanDefinitionStoreException(
                    "Detected cyclic loading of " + encodedResource + " - check your import definitions!");
        }
        try {
          // 終於到了java讀取文件熟悉的InputStream了，Recoure－>InputStream
            InputStream inputStream = encodedResource.getResource().getInputStream();
            try {
                InputSource inputSource = new InputSource(inputStream);
                if (encodedResource.getEncoding() != null) {
                    inputSource.setEncoding(encodedResource.getEncoding());
                }
                // 繼續解析：InputStream -> Document
                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();
            }
        }
    }

調用到DefaultDocumentLoader實現解析xml

protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource)
            throws BeanDefinitionStoreException {
        try {
          // 繼續解析：InputStream -> Document
            Document doc = doLoadDocument(inputSource, resource);
            // 轉化：Document->BeanDefinition
            // 顯然，從一個Document變成一個個bean的描述（BeanDefinition），就涉及到我們在使用spring時經常用到的bean定義時的各種配置規則，如果查看源碼，也許會更好的理解。
            return registerBeanDefinitions(doc, resource);
        }
        catch (BeanDefinitionStoreException ex) {
            throw ex;
        }
        catch (SAXParseException ex) {
            throw new XmlBeanDefinitionStoreException(resource.getDescription(),
                    "Line " + ex.getLineNumber() + " in XML document from " + resource + " is invalid", ex);
        }
        catch (SAXException ex) {
            throw new XmlBeanDefinitionStoreException(resource.getDescription(),
                    "XML document from " + resource + " is invalid", ex);
        }
        catch (ParserConfigurationException ex) {
            throw new BeanDefinitionStoreException(resource.getDescription(),
                    "Parser configuration exception parsing XML from " + resource, ex);
        }
        catch (IOException ex) {
            throw new BeanDefinitionStoreException(resource.getDescription(),
                    "IOException parsing XML document from " + resource, ex);
        }
        catch (Throwable ex) {
            throw new BeanDefinitionStoreException(resource.getDescription(),
                    "Unexpected exception parsing XML document from " + resource, ex);
        }
    }
    // 具體documentLoader來實現，DefaultDocumentLoader實現解析xml
    protected Document doLoadDocument(InputSource inputSource, Resource resource) throws Exception {
        return this.documentLoader.loadDocument(inputSource, getEntityResolver(), this.errorHandler,
                getValidationModeForResource(resource), isNamespaceAware());
    }

DefaultDocumentLoader使用JAXP方式解析：

public Document loadDocument(InputSource inputSource, EntityResolver entityResolver,
            ErrorHandler errorHandler, int validationMode, boolean namespaceAware) throws Exception {

        DocumentBuilderFactory factory = createDocumentBuilderFactory(validationMode, namespaceAware);
        if (logger.isDebugEnabled()) {
            logger.debug("Using JAXP provider [" + factory.getClass().getName() + "]");
        }
        DocumentBuilder builder = createDocumentBuilder(factory, entityResolver, errorHandler);
        return builder.parse(inputSource);
    }
    protected DocumentBuilder createDocumentBuilder(
            DocumentBuilderFactory factory, EntityResolver entityResolver, ErrorHandler errorHandler)
            throws ParserConfigurationException {

        DocumentBuilder docBuilder = factory.newDocumentBuilder();
        if (entityResolver != null) {
            docBuilder.setEntityResolver(entityResolver);
        }
        if (errorHandler != null) {
            docBuilder.setErrorHandler(errorHandler);
        }
        return docBuilder;
    }

registerBeanDefinitions方法：

public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
        BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();
        int countBefore = getRegistry().getBeanDefinitionCount();
        //BeanDefinitionDocumentReader具體實現
        documentReader.registerBeanDefinitions(doc, createReaderContext(resource));
        return getRegistry().getBeanDefinitionCount() - countBefore;
    }

BeanDefinitionDocumentReader具體實現：

public void registerBeanDefinitions(Document doc, XmlReaderContext readerContext) {
        this.readerContext = readerContext;
        logger.debug("Loading bean definitions");
        Element root = doc.getDocumentElement();
        doRegisterBeanDefinitions(root);
    }
    
    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;
        // 指定好BeanDefinitionParserDelegate
        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)) {
                    return;
                }
            }
        }
    //可擴展入口
        preProcessXml(root);
        parseBeanDefinitions(root, this.delegate);
        postProcessXml(root);

        this.delegate = parent;
    }
    
    protected void parseBeanDefinitions(Element root, BeanDefinitionParserDelegate delegate) {
        if (delegate.isDefaultNamespace(root)) {
            NodeList nl = root.getChildNodes();
            for (int i = 0; i < nl.getLength(); i++) {
                Node node = nl.item(i);
                if (node instanceof Element) {
                    Element ele = (Element) node;
                    if (delegate.isDefaultNamespace(ele)) {
                      // 一次解析每個節點
                        parseDefaultElement(ele, delegate);
                    }
                    else {
                        delegate.parseCustomElement(ele);
                    }
                }
            }
        }
        else {
            delegate.parseCustomElement(root);
        }
    }
    //標簽解析
    private void parseDefaultElement(Element ele, BeanDefinitionParserDelegate delegate) {
        if (delegate.nodeNameEquals(ele, IMPORT_ELEMENT)) {
          //import
            importBeanDefinitionResource(ele);
        }
        else if (delegate.nodeNameEquals(ele, ALIAS_ELEMENT)) {
          //alias
            processAliasRegistration(ele);
        }
        else if (delegate.nodeNameEquals(ele, BEAN_ELEMENT)) {
          //bean
            processBeanDefinition(ele, delegate);
        }
        else if (delegate.nodeNameEquals(ele, NESTED_BEANS_ELEMENT)) {
            // beans recurse
            doRegisterBeanDefinitions(ele);
        }
    }

如果是beans的標簽則進行了遞歸，查看解析bean標簽的代碼：

protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
    // 終於是解析入口了,Element－>BeanDefinitionHolder
        BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
        if (bdHolder != null) {
            bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);
            try {
                // Register the final decorated instance.
                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));
        }
    }
    public BeanDefinitionHolder parseBeanDefinitionElement(Element ele) {
        return parseBeanDefinitionElement(ele, null);
    }

這裡的BeanDefinitionHolder包裝了BeanDefinition以及bean和alias：

private final BeanDefinition beanDefinition;
    private final String beanName;
    private final String[] aliases;

BeanDefinitionReaderUtils.registerBeanDefinition：

public BeanDefinitionHolder parseBeanDefinitionElement(Element ele, BeanDefinition containingBean) {
  // bean id
        String id = ele.getAttribute(ID_ATTRIBUTE);
        // bean name
        String nameAttr = ele.getAttribute(NAME_ATTRIBUTE);

        List<String> aliases = new ArrayList<String>();
        if (StringUtils.hasLength(nameAttr)) {
            String[] nameArr = StringUtils.tokenizeToStringArray(nameAttr, MULTI_VALUE_ATTRIBUTE_DELIMITERS);
            aliases.addAll(Arrays.asList(nameArr));
        }

        String beanName = id;
        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");
            }
        }

        if (containingBean == null) {
            checkNameUniqueness(beanName, aliases, ele);
        }
// beanDefinition產生 此時已經解析了bean中的其他欄位
        AbstractBeanDefinition beanDefinition = parseBeanDefinitionElement(ele, beanName, containingBean);
        if (beanDefinition != null) {
            if (!StringUtils.hasText(beanName)) {
                try {
                    if (containingBean != null) {
                        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);
            return new BeanDefinitionHolder(beanDefinition, beanName, aliasesArray);
        }

        return null;
    }

拿到holder後進行註冊：

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

        //<