Spring IoC的初始化过程

本文会保留原注解

我们知道 IoC 容器最常用的就是 ApplicationContext 和 BeanFactory 这两个接口。接下来我们就以 Spring4.3.14 中的 org.springframework.context.support.FileSystemXmlApplicationContext 为例，来深入了解下 IoC 的初始化过程。

IoC 初始化过程分为 BeanDefinition 的 Resource 定位、加载和注册三个部分。

1. Resource定位

这个阶段的目的就是对 BeanDefinition 资源的 path 进行解析，然后生成一个org.springframework.core.io.Resource 对象供后面载入和解析。

看一下 FileSystemXmlApplicationContext 的继承关系以及其内部方法。

看到 FileSystemXmlApplicationContext 中的 getResourceByPath(String) 是否心生疑惑，这个方法有什么用？什么时候会被调用？如果你了解过模板设计模式应该就了解其中的奥妙。

我们就从 getResourceByPath(String) 的方法调用栈入手。下面就跟着这张方法调用栈图来看看每个方法的实现。

从上述的方法调用栈可以看出，getResourceByPath(String) 将在 refresh() 中被调用，同时这个方法也是 IoC初始化 的入口。

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

	super(parent);
	
	// 设置BeanDefinitions路径
	setConfigLocations(configLocations);
	if (refresh) {
		refresh();
	}
}

setConfigLocations(String[]) 设置 Resource 路径。

public void setConfigLocations(@Nullable String... locations) {
	if (locations != null) {
		Assert.noNullElements(locations, "Config locations must not be null");
		this.configLocations = new String[locations.length];
		for (int i = 0; i < locations.length; i++) {
			this.configLocations[i] = resolvePath(locations[i]).trim();
		}
	}
	else {
		this.configLocations = null;
	}
}

接下来看 org.springframework.context.support.AbstractApplicationContext.refresh() 方法。从上面的继承图可以看出，refresh 这个方法是实现自 org.springframework.context.ConfigurableApplicationContext 这个接口。

/**
 * Load or refresh the persistent representation of the configuration,
 * which might an XML file, properties file, or relational database schema.
 * <p>As this is a startup method, it should destroy already created singletons
 * if it fails, to avoid dangling resources. In other words, after invocation
 * of that method, either all or no singletons at all should be instantiated.
 * @throws BeansException if the bean factory could not be initialized
 * @throws IllegalStateException if already initialized and multiple refresh
 * attempts are not supported
 */
void refresh() throws BeansException, IllegalStateException;

refresh() 方法详细的描述了 ApplicationContext 初始化的整个过程。

@Override
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.
			invokeBeanFactoryPostProcessors(beanFactory);

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

			// Initialize message source for this context.
			initMessageSource();

			// Initialize event multicaster for this context.
			initApplicationEventMulticaster();

			// Initialize other special beans in specific context subclasses.
			onRefresh();

			// Check for listener beans and register them.
			registerListeners();

			// Instantiate all remaining (non-lazy-init) singletons.
			finishBeanFactoryInitialization(beanFactory);

			// Last step: publish corresponding event.
			finishRefresh();
		}

		catch (BeansException ex) {
			if (logger.isWarnEnabled()) {
				logger.warn("Exception encountered during context initialization - " +
						"cancelling refresh attempt: " + ex);
			}

			// Destroy already created singletons to avoid dangling resources.
			destroyBeans();

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

			// Propagate exception to caller.
			throw ex;
		}

		finally {
			// Reset common introspection caches in Spring's core, since we
			// might not ever need metadata for singleton beans anymore...
			resetCommonCaches();
		}
	}

obtainFreshBeanFactory 这个方法是告诉 AbstractApplication 的子类(这里是FileSystemXmlApplicationContext)刷新内部的Bean Factory。

protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {
	refreshBeanFactory();
	ConfigurableListableBeanFactory beanFactory = getBeanFactory();
	if (logger.isDebugEnabled()) {
		logger.debug("Bean factory for " + getDisplayName() + ": " + beanFactory);
	}
	return beanFactory;
}

先调用了 refreshBeanFactory() 根据 BeanDefinition 的位置来刷新 Bean Factory。

思考：

为什么说这个方法会去读取 BeanDefinition 的位置？虽然这个方法没有参数，但是根据方法名可知，其内部实现一定会有去读取BeanDefinition位置的步骤。

继续往下看，这里 refreshBeanFactory() 根据继承关系调用的是 org.springframework.context.support.AbstractRefreshableApplicationContext 中的 refreshBeanFactory 方法。

@Override
protected final void refreshBeanFactory() throws BeansException {
	// 先判断Bean Factory是否已经存在
	// 如果存在就销毁存在的Bean Factory
	if (hasBeanFactory()) {
		destroyBeans();
		closeBeanFactory();
	}
	try {
		// 创建一个Bean Factory
		DefaultListableBeanFactory beanFactory = createBeanFactory();
		beanFactory.setSerializationId(getId());
		customizeBeanFactory(beanFactory);
		// 加载BeanFactory
		loadBeanDefinitions(beanFactory);
		
		synchronized (this.beanFactoryMonitor) {
			this.beanFactory = beanFactory;
		}
	}
	catch (IOException ex) {
		throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
	}
}

从 refreshBeanFactory() 中可以获取两个信息：

  1. 这里的 BeanFactory 使用的是 DefaultListableBeanFactory 。从这里也可以看出，其实 ApplicationContext 是封装了BeanFactory初始化的步骤。
  2. 开始加载 BeanDefinitions。既然要加载，必然需要先定位BeanDefinition的位置。

这里调用的是 org.springframework.context.support.AbstractXmlApplicationContext.loadBeanDefinitions() 。

protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) throws BeansException, IOException {
	// 为给定的BeanFactory创建出一个XmlBeanDefinitionReader
	XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(beanFactory);

	// 给这个BeanDifinitionReader配置加载资源
	beanDefinitionReader.setEnvironment(this.getEnvironment());
	beanDefinitionReader.setResourceLoader(this);
	beanDefinitionReader.setEntityResolver(new ResourceEntityResolver(this));

	// 允许AbstractXmlApplicationContext的子类自定义reader的初始化过程
	initBeanDefinitionReader(beanDefinitionReader);
	// 开始加载BeanDefinition
	loadBeanDefinitions(beanDefinitionReader);
}

loadBeanDefinitions(XmlBeanDefinitionReader) 开始加载 BeanDefinition。

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);
	}
}

可以看到里面提供了两个获取BeanDefinition的定位方式。

  1. getConfigResources()
  2. getConfigLocations()

根据 getResourceByPath(String) 的方法调用栈来看，我们会调用getConfigLocations()。

@Nullable
protected String[] getConfigLocations() {
	// 在FileSystemXmlApplicationContext的构造函数中曽调用过setConfigLocations(String[])
	// 所以这里直接就是返回了当时设定的BeanDefinition的路径
	return (this.configLocations != null ? this.configLocations : getDefaultConfigLocations());
}

接着开始调用 reader.loadBeanDefinitions(configLocations);

@Override
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;
}

loadBeanDefinitions - 根据 location 加载 BeanDefinition。

public int loadBeanDefinitions(String location) throws BeanDefinitionStoreException {
	return loadBeanDefinitions(location, null);
}

经过上面的层层调用终于到了真正加载 BeanDefinition 的方法 org.springframework.beans.factory.support.AbstractBeanDefinitionReader.loadBeanDefinitions(String, Set) 。

public int loadBeanDefinitions(String location, @Nullable 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 {
			Resource[] resources = ((ResourcePatternResolver) resourceLoader).getResources(location);
			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;
	}
}

这里走 else 分支，调用 resourceLoader.getResource(location) 进行获取Resource对象。

public Resource getResource(String location) {
	Assert.notNull(location, "Location must not be null");

	for (ProtocolResolver protocolResolver : this.protocolResolvers) {
		Resource resource = protocolResolver.resolve(location, this);
		if (resource != null) {
			return resource;
		}
	}

	if (location.startsWith("/")) {
		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 (ResourceUtils.isFileURL(url) ? new FileUrlResource(url) : new UrlResource(url));
		}
		catch (MalformedURLException ex) {
			// No URL -> resolve as resource path.
			return getResourceByPath(location);
		}
	}
}

因为 FileSystemXmlApplicationContext 重写了getResourceByPath(String)这个方法，所以最后调用了重写的方法，返回了一个 FileSystemContextResource 对象。

protected Resource getResourceByPath(String path) {
	if (path.startsWith("/")) {
		path = path.substring(1);
	}
	return new FileSystemContextResource(path);
}

至此，我们已经完整的了解了BeanDefinition的定位过程。

2. BeanDefinition的载入和解析。

将用户定义好的 Bean 表示成 IoC 容器内部的数据结构，这个容器内部的数据结构就是 BeanDefinition。

上面的 getResourceByPath(String) 返回了 Resource 对象，接着就开始了BeanDefinition的载入。这里将调用 org.springframework.beans.factory.xml.XmlBeanDefinitionReader.loadBeanDifinitions(Resource)

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

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 {
		// 获取到XML文件，并得到IO的InputSource准备进行加载
		InputStream inputStream = encodedResource.getResource().getInputStream();
		try {
			InputSource inputSource = new InputSource(inputStream);
			if (encodedResource.getEncoding() != null) {
				inputSource.setEncoding(encodedResource.getEncoding());
			}
			//加载BeanDefinition
			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();
		}
	}
}

通过调用 doLoadBeanDefinitions(inputSource, encodedResource.getResource()) 来解析

protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource)
		throws BeanDefinitionStoreException {
	try {
		//取得XML文件的Document对象
		Document doc = doLoadDocument(inputSource, resource);
		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);
	}
}

其实到这里，已经完成了加载的阶段。

解析 BeanDefinition 有两个过程：

  1. 将XML文件转换成document对象。
  2. spring再对document对象按照Bean的规则进行解析。

接下来看看spring是如何按照Bean的规则对Bean进行解析。

public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
	BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();
	int countBefore = getRegistry().getBeanDefinitionCount();
	//spring按照Bean的规则开始解析BeanDefinition
	documentReader.registerBeanDefinitions(doc, createReaderContext(resource));
	return getRegistry().getBeanDefinitionCount() - countBefore;
}

public void registerBeanDefinitions(Document doc, XmlReaderContext readerContext) {
	this.readerContext = readerContext;
	logger.debug("Loading bean definitions");
	Element root = doc.getDocumentElement();
	//开始进行解析并且注册BeanDefinitions
	doRegisterBeanDefinitions(root);
}

protected void doRegisterBeanDefinitions(Element root) {
	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;
			}
		}
	}

	preProcessXml(root);
	//解析BeanDefinitions
	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)) {
										//采用默认的解析方式
					parseDefaultElement(ele, delegate);
				}
				else {
					delegate.parseCustomElement(ele);
				}
			}
		}
	}
	else {
		delegate.parseCustomElement(root);
	}
}

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)) {
		//解析BeanDefinitions
		processBeanDefinition(ele, delegate);
	}
	else if (delegate.nodeNameEquals(ele, NESTED_BEANS_ELEMENT)) {
		// recurse
		doRegisterBeanDefinitions(ele);
	}
}

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));
	}
}

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)) {
										//进行默认解析
					parseDefaultElement(ele, delegate);
				}
				else {
					delegate.parseCustomElement(ele);
				}
			}
		}
	}
	else {
		delegate.parseCustomElement(root);
	}
}

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)) {
		// 解析BeanDefinition
		processBeanDefinition(ele, delegate);
	}
	else if (delegate.nodeNameEquals(ele, NESTED_BEANS_ELEMENT)) {
		// recurse
		doRegisterBeanDefinitions(ele);
	}
}

protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
		// 开始解析
	BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
	if (bdHolder != null) {
		bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);
		try {
			// 将解析好的BeanDefinition，注册到IoC容器中去。
			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));
	}
}

经过上面一系列的方法调用终于到了真正开始解析BeanDefinition的步骤了。

public AbstractBeanDefinition parseBeanDefinitionElement(
		Element ele, String beanName, @Nullable BeanDefinition containingBean) {

	this.parseState.push(new BeanEntry(beanName));

	String className = null;
	if (ele.hasAttribute(CLASS_ATTRIBUTE)) {
		className = ele.getAttribute(CLASS_ATTRIBUTE).trim();
	}
	String parent = null;
	if (ele.hasAttribute(PARENT_ATTRIBUTE)) {
		parent = ele.getAttribute(PARENT_ATTRIBUTE);
	}

	try {
		AbstractBeanDefinition bd = createBeanDefinition(className, parent);

		parseBeanDefinitionAttributes(ele, beanName, containingBean, bd);
		bd.setDescription(DomUtils.getChildElementValueByTagName(ele, DESCRIPTION_ELEMENT));

		parseMetaElements(ele, bd);
		parseLookupOverrideSubElements(ele, bd.getMethodOverrides());
		parseReplacedMethodSubElements(ele, bd.getMethodOverrides());

		parseConstructorArgElements(ele, bd);
		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;
}

可以看到上面对bean的各种属性进行解析。

3. 将BeanDefinition注册到容器中。

org.springframework.beans.factory.support.BeanDefinitionReaderUtils.registerBeanDefinition(BeanDefinitionHolder, BeanDefinitionRegistry) 中开始注册

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);
		}
	}
}

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");

	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;
	
		// 先判断当前这个BeanDefinition是否已经注册过
	oldBeanDefinition = this.beanDefinitionMap.get(beanName);
	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 + "]");
			}
		}
		this.beanDefinitionMap.put(beanName, beanDefinition);
	}
	else {
		//检查Bean Factory创建阶段是否已经启动，例如，同时是否有任何bean被标记为以创建。
		//如果没有启动，执行true分支；否则，执行false分支
		if (hasBeanCreationStarted()) {
			// Cannot modify startup-time collection elements anymore (for stable iteration)
			synchronized (this.beanDefinitionMap) {
				this.beanDefinitionMap.put(beanName, beanDefinition);
				List<String> updatedDefinitions = new ArrayList<>(this.beanDefinitionNames.size() + 1);
				updatedDefinitions.addAll(this.beanDefinitionNames);
				updatedDefinitions.add(beanName);
				this.beanDefinitionNames = updatedDefinitions;
				if (this.manualSingletonNames.contains(beanName)) {
					Set<String> updatedSingletons = new LinkedHashSet<>(this.manualSingletonNames);
					updatedSingletons.remove(beanName);
					this.manualSingletonNames = updatedSingletons;
				}
			}
		}
		else {
			// Still in startup registration phase
			this.beanDefinitionMap.put(beanName, beanDefinition);
			this.beanDefinitionNames.add(beanName);
			this.manualSingletonNames.remove(beanName);
		}
		this.frozenBeanDefinitionNames = null;
	}

	if (oldBeanDefinition != null || containsSingleton(beanName)) {
		resetBeanDefinition(beanName);
	}
}

参考：

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