Spring BeanDefinition

定义

/**
 * A BeanDefinition describes a bean instance, which has property values,
 * constructor argument values, and further information supplied by
 * concrete implementations.
 *
 * <p>This is just a minimal interface: The main intention is to allow a
 * {@link BeanFactoryPostProcessor} to introspect and modify property values
 * and other bean metadata.
 *
 * @author Juergen Hoeller
 * @author Rob Harrop
 * @since 19.03.2004
 * @see ConfigurableListableBeanFactory#getBeanDefinition
 * @see org.springframework.beans.factory.support.RootBeanDefinition
 * @see org.springframework.beans.factory.support.ChildBeanDefinition
 */
public interface BeanDefinition extends AttributeAccessor, BeanMetadataElement {

解读：

BeanDefinition相当于一个数据结构，Spring容器对bean的管理是通过BeanDefinition来实现的。

方法

BeanDefinition中的方法如下图所示：

类图

解读：

从上述类图可以得知BeanDefinition有哪些实现类。

调用链路

本部分准备探究一下Spring中解析bean的调用链路，涉及到几个层次，譬如：ApplicationContext -> BeanDefinitionReader -> BeanDefinitionDocumentReader。

可以带着问题阅读：

• 这几个层次之间是怎么关联起来的？
• 最终是如何解析bean的？

ApplicationContext

AbstractApplicationContext

    @Override
    public void refresh() throws BeansException, IllegalStateException {
        synchronized (this.startupShutdownMonitor) {
            StartupStep contextRefresh = this.applicationStartup.start("spring.context.refresh");

            // 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);

                StartupStep beanPostProcess = this.applicationStartup.start("spring.context.beans.post-process");
                // Invoke factory processors registered as beans in the context.
                invokeBeanFactoryPostProcessors(beanFactory);

                // Register bean processors that intercept bean creation.
                registerBeanPostProcessors(beanFactory);
                beanPostProcess.end();

                // 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();
                contextRefresh.end();
            }
        }
    }

 解读：

上述AbstractApplicationContext中的refresh方法调用的obtainFreshBeanFactory方法是整个调用链路的入口。

    /**
     * Tell the subclass to refresh the internal bean factory.
     * @return the fresh BeanFactory instance
     * @see #refreshBeanFactory()
     * @see #getBeanFactory()
     */
    protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {
        refreshBeanFactory();
        return getBeanFactory();
    }

    /**
     * Subclasses must implement this method to perform the actual configuration load.
     * The method is invoked by {@link #refresh()} before any other initialization work.
     * <p>A subclass will either create a new bean factory and hold a reference to it,
     * or return a single BeanFactory instance that it holds. In the latter case, it will
     * usually throw an IllegalStateException if refreshing the context more than once.
     * @throws BeansException if initialization of the bean factory failed
     * @throws IllegalStateException if already initialized and multiple refresh
     * attempts are not supported
     */
    protected abstract void refreshBeanFactory() throws BeansException, IllegalStateException;

解读：

此处refreshBeanFactory方法是抽象方法，由子类实现。

AbstractRefreshableApplicationContext

    /**
     * This implementation performs an actual refresh of this context's underlying
     * bean factory, shutting down the previous bean factory (if any) and
     * initializing a fresh bean factory for the next phase of the context's lifecycle.
     */
    @Override
    protected final void refreshBeanFactory() throws BeansException {
        if (hasBeanFactory()) {
            destroyBeans();
            closeBeanFactory();
        }
        try {
            DefaultListableBeanFactory beanFactory = createBeanFactory();
            beanFactory.setSerializationId(getId());
            customizeBeanFactory(beanFactory);
            loadBeanDefinitions(beanFactory);
            this.beanFactory = beanFactory;
        }
        catch (IOException ex) {
            throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
        }
    }

    /**
     * Load bean definitions into the given bean factory, typically through
     * delegating to one or more bean definition readers.
     * @param beanFactory the bean factory to load bean definitions into
     * @throws BeansException if parsing of the bean definitions failed
     * @throws IOException if loading of bean definition files failed
     * @see org.springframework.beans.factory.support.PropertiesBeanDefinitionReader
     * @see org.springframework.beans.factory.xml.XmlBeanDefinitionReader
     */
    protected abstract void loadBeanDefinitions(DefaultListableBeanFactory beanFactory)
            throws BeansException, IOException;

解读：

此处loadBeanDefinitions方法是抽象方法，由子类实现。

AbstractXmlApplicationContext

    /**
     * Loads the bean definitions via an XmlBeanDefinitionReader.
     * @see org.springframework.beans.factory.xml.XmlBeanDefinitionReader
     * @see #initBeanDefinitionReader
     * @see #loadBeanDefinitions
     */
    @Override
    protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) throws BeansException, IOException {
        // Create a new XmlBeanDefinitionReader for the given BeanFactory.
        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);
    }

    /**
     * Load the bean definitions with the given XmlBeanDefinitionReader.
     * <p>The lifecycle of the bean factory is handled by the {@link #refreshBeanFactory}
     * method; hence this method is just supposed to load and/or register bean definitions.
     * @param reader the XmlBeanDefinitionReader to use
     * @throws BeansException in case of bean registration errors
     * @throws IOException if the required XML document isn't found
     * @see #refreshBeanFactory
     * @see #getConfigLocations
     * @see #getResources
     * @see #getResourcePatternResolver
     */
    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);
        }
    }

解读：

上述loadBeanDefinitions方法最终是调用了AbstractBeanDefinitionReader中的loadBeanDefinitions方法，参数是数组类型。

问题：此处loadBeanDefinitions方法的返回值为空，而reader是new出来的变量，reader做什么事情看似跟ApplicationContext没啥关系啊。

实则不然，构造reader时传递了beanFactory，在最终解析bean时用到了该参数，所以说beanfactory承担了关联解析bean的几个层次的作用。

BeanDefinitionReader

类图：

解读：

分析后续流程时可以参照类图继承层次。

AbstractBeanDefinitionReader

    @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;
    }

解读：

此处算是真正开始进入了解析bean的流程，使用了for循环。

BeanDefinitionReader

    /**
     * Load bean definitions from the specified resource.
     * @param resource the resource descriptor
     * @return the number of bean definitions found
     * @throws BeanDefinitionStoreException in case of loading or parsing errors
     */
    int loadBeanDefinitions(Resource resource) throws BeanDefinitionStoreException;

XmlBeanDefinitionReader

    /**
     * Load bean definitions from the specified XML file.
     * @param resource the resource descriptor for the XML file
     * @return the number of bean definitions found
     * @throws BeanDefinitionStoreException in case of loading or parsing errors
     */
    @Override
    public int loadBeanDefinitions(Resource resource) throws BeanDefinitionStoreException {
        return loadBeanDefinitions(new EncodedResource(resource));
    }

    /**
     * Load bean definitions from the specified XML file.
     * @param encodedResource the resource descriptor for the XML file,
     * allowing to specify an encoding to use for parsing the file
     * @return the number of bean definitions found
     * @throws BeanDefinitionStoreException in case of loading or parsing errors
     */
    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.add(encodedResource)) {
            throw new BeanDefinitionStoreException(
                    "Detected cyclic loading of " + encodedResource + " - check your import definitions!");
        }

        try (InputStream inputStream = encodedResource.getResource().getInputStream()) {
            InputSource inputSource = new InputSource(inputStream);
            if (encodedResource.getEncoding() != null) {
                inputSource.setEncoding(encodedResource.getEncoding());
            }
            return doLoadBeanDefinitions(inputSource, encodedResource.getResource());
        }
        catch (IOException ex) {
            throw new BeanDefinitionStoreException(
                    "IOException parsing XML document from " + encodedResource.getResource(), ex);
        }
        finally {
            currentResources.remove(encodedResource);
            if (currentResources.isEmpty()) {
                this.resourcesCurrentlyBeingLoaded.remove();
            }
        }
    }

    /**
     * Actually load bean definitions from the specified XML file.
     * @param inputSource the SAX InputSource to read from
     * @param resource the resource descriptor for the XML file
     * @return the number of bean definitions found
     * @throws BeanDefinitionStoreException in case of loading or parsing errors
     * @see #doLoadDocument
     * @see #registerBeanDefinitions
     */
    protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource)
            throws BeanDefinitionStoreException {

        try {
            Document doc = doLoadDocument(inputSource, resource);
            int count = registerBeanDefinitions(doc, resource);
            if (logger.isDebugEnabled()) {
                logger.debug("Loaded " + count + " bean definitions from " + resource);
            }
            return count;
        }
        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);
        }
    }

    /**
     * Register the bean definitions contained in the given DOM document.
     * Called by {@code loadBeanDefinitions}.
     * <p>Creates a new instance of the parser class and invokes
     * {@code registerBeanDefinitions} on it.
     * @param doc the DOM document
     * @param resource the resource descriptor (for context information)
     * @return the number of bean definitions found
     * @throws BeanDefinitionStoreException in case of parsing errors
     * @see #loadBeanDefinitions
     * @see #setDocumentReaderClass
     * @see BeanDefinitionDocumentReader#registerBeanDefinitions
     */
    public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
        BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();
        int countBefore = getRegistry().getBeanDefinitionCount();
        documentReader.registerBeanDefinitions(doc, createReaderContext(resource));
        return getRegistry().getBeanDefinitionCount() - countBefore;
    }

    @Override
    public final BeanDefinitionRegistry getRegistry() {
        return this.registry;
    }

解读：

此处的registry是什么？

本文前面构造reader变量的代码语句的含义请参见如下代码：

    /**
     * Create new XmlBeanDefinitionReader for the given bean factory.
     * @param registry the BeanFactory to load bean definitions into,
     * in the form of a BeanDefinitionRegistry
     */
    public XmlBeanDefinitionReader(BeanDefinitionRegistry registry) {
        super(registry);
    }

AbstractBeanDefinitionReader

    /**
     * Create a new AbstractBeanDefinitionReader for the given bean factory.
     * <p>If the passed-in bean factory does not only implement the BeanDefinitionRegistry
     * interface but also the ResourceLoader interface, it will be used as default
     * ResourceLoader as well. This will usually be the case for
     * {@link org.springframework.context.ApplicationContext} implementations.
     * <p>If given a plain BeanDefinitionRegistry, the default ResourceLoader will be a
     * {@link org.springframework.core.io.support.PathMatchingResourcePatternResolver}.
     * <p>If the passed-in bean factory also implements {@link EnvironmentCapable} its
     * environment will be used by this reader.  Otherwise, the reader will initialize and
     * use a {@link StandardEnvironment}. All ApplicationContext implementations are
     * EnvironmentCapable, while normal BeanFactory implementations are not.
     * @param registry the BeanFactory to load bean definitions into,
     * in the form of a BeanDefinitionRegistry
     * @see #setResourceLoader
     * @see #setEnvironment
     */
    protected AbstractBeanDefinitionReader(BeanDefinitionRegistry registry) {
        Assert.notNull(registry, "BeanDefinitionRegistry must not be null");
        this.registry = registry;

        // Determine ResourceLoader to use.
        if (this.registry instanceof ResourceLoader) {
            this.resourceLoader = (ResourceLoader) this.registry;
        }
        else {
            this.resourceLoader = new PathMatchingResourcePatternResolver();
        }

        // Inherit Environment if possible
        if (this.registry instanceof EnvironmentCapable) {
            this.environment = ((EnvironmentCapable) this.registry).getEnvironment();
        }
        else {
            this.environment = new StandardEnvironment();
        }
    }

解读：

至此可知在构造XmlBeanDefinitionReader类型的变量时，实质是将beanFactory赋值给AbstractBeanDefinitionReader中的registry变量。

BeanDefinitionDocumentReader

纵观前述代码调用流程可知此流程的核心是前述标记为粉色的代码

    /**
     * Create the {@link BeanDefinitionDocumentReader} to use for actually
     * reading bean definitions from an XML document.
     * <p>The default implementation instantiates the specified "documentReaderClass".
     * @see #setDocumentReaderClass
     */
    protected BeanDefinitionDocumentReader createBeanDefinitionDocumentReader() {
        return BeanUtils.instantiateClass(this.documentReaderClass);
    }

解读：

上述方法中documentReaderClass的定义如下

    private Class<? extends BeanDefinitionDocumentReader> documentReaderClass =
            DefaultBeanDefinitionDocumentReader.class;

registerBeanDefinitions方法

上述方法最终是调用了registerBeanDefinitions方法，对应代码如下

    /**
     * This implementation parses bean definitions according to the "spring-beans" XSD
     * (or DTD, historically).
     * <p>Opens a DOM Document; then initializes the default settings
     * specified at the {@code <beans/>} level; then parses the contained bean definitions.
     */
    @Override
    public void registerBeanDefinitions(Document doc, XmlReaderContext readerContext) {
        this.readerContext = readerContext;
        doRegisterBeanDefinitions(doc.getDocumentElement());
    }

解读：

BeanDefinitionReader与BeanDefinitionDocumentReader之间的关联是通过readerContext维护的。

后续流程是真正的解析bean配置文件的过程，代码如下：

    /**
     * Register each bean definition within the given root {@code <beans/>} element.
     */
    @SuppressWarnings("deprecation")  // for Environment.acceptsProfiles(String...)
    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;
                }
            }
        }

        preProcessXml(root);
        parseBeanDefinitions(root, this.delegate);
        postProcessXml(root);

        this.delegate = parent;
    }

    /**
     * Parse the elements at the root level in the document:
     * "import", "alias", "bean".
     * @param root the DOM root element of the document
     */
    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)) {
                        parseDefaultElement(ele, delegate);
                    }
                    else {
                        delegate.parseCustomElement(ele);
                    }
                }
            }
        }
        else {
            delegate.parseCustomElement(root);
        }
    }

解读：

查看此方法的注释部分即可明确方法的目的。

    private void parseDefaultElement(Element ele, BeanDefinitionParserDelegate delegate) {
        if (delegate.nodeNameEquals(ele, IMPORT_ELEMENT)) {
            importBeanDefinitionResource(ele);
        }
        else if (delegate.nodeNameEquals(ele, ALIAS_ELEMENT)) {
            processAliasRegistration(ele);
        }
        else if (delegate.nodeNameEquals(ele, BEAN_ELEMENT)) {
            processBeanDefinition(ele, delegate);
        }
        else if (delegate.nodeNameEquals(ele, NESTED_BEANS_ELEMENT)) {
            // recurse
            doRegisterBeanDefinitions(ele);
        }
    }

解读：

观察上述代码的一个分支，代码如下：

    /**
     * Process the given bean element, parsing the bean definition
     * and registering it with the registry.
     */
    protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
        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));
        }
    }

解读：

注意上述代码中着色（蓝色）部分代码，此处拿到了最前面传递的beanFactory。

BeanDefinitionReaderUtils

    /**
     * Register the given bean definition with the given bean factory.
     * @param definitionHolder the bean definition including name and aliases
     * @param registry the bean factory to register with
     * @throws BeanDefinitionStoreException if registration failed
     */
    public static void registerBeanDefinition(
            BeanDefinitionHolder definitionHolder, BeanDefinitionRegistry registry)
            throws BeanDefinitionStoreException {

        // Register bean definition under primary name.
        String beanName = definitionHolder.getBeanName();
        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);
            }
        }
    }

BeanDefinitionRegistry

    /**
     * Register a new bean definition with this registry.
     * Must support RootBeanDefinition and ChildBeanDefinition.
     * @param beanName the name of the bean instance to register
     * @param beanDefinition definition of the bean instance to register
     * @throws BeanDefinitionStoreException if the BeanDefinition is invalid
     * @throws BeanDefinitionOverrideException if there is already a BeanDefinition
     * for the specified bean name and we are not allowed to override it
     * @see GenericBeanDefinition
     * @see RootBeanDefinition
     * @see ChildBeanDefinition
     */
    void registerBeanDefinition(String beanName, BeanDefinition beanDefinition)
            throws BeanDefinitionStoreException;

解读：

此处registerBeanDefinition方法为抽象方法，由子类（即BeanFactory的具体实现类）实现。

Note：

从上面的分析可知，最终bean的解析是由BeanDefinitionHolder实现的。

    /**
     * Parses the supplied {@code <bean>} element. May return {@code null}
     * if there were errors during parse. Errors are reported to the
     * {@link org.springframework.beans.factory.parsing.ProblemReporter}.
     */
    @Nullable
    public BeanDefinitionHolder parseBeanDefinitionElement(Element ele, @Nullable BeanDefinition containingBean) {
        String id = ele.getAttribute(ID_ATTRIBUTE);
        String nameAttr = ele.getAttribute(NAME_ATTRIBUTE);

        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;
        if (!StringUtils.hasText(beanName) && !aliases.isEmpty()) {
            beanName = aliases.remove(0);
            if (logger.isTraceEnabled()) {
                logger.trace("No XML 'id' specified - using '" + beanName +
                        "' as bean name and " + aliases + " as aliases");
            }
        }

        if (containingBean == null) {
            checkNameUniqueness(beanName, aliases, ele);
        }

        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.isTraceEnabled()) {
                        logger.trace("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;
    }