本文是针对Srping的XMLBeanFactory来进行解析xml并将解析后的信息使用GenericBeanDefinition作为载体进行注册,xmlBeanFactory已经在Spring 3.1中被标记为不建议使用，但是我们分析源码不影响，因为源码并未改变，并ApplicationContext依旧使用XmlBeanDefinitionReader和DefaultListableBeanFactory进行xml的解析和注册工作，本篇博客是跟源码一步步看spring怎么实现bean的注册，源码为spring5.X,源码已经在每一行上加了注释，方便读者学习。

GitHub: https://github.com/lantaoGitHub/spring-framework.git

• 首先我们从XMLBeanFactory入手,直接上代码：

package org.springframework.lantao;

import org.springframework.beans.factory.BeanFactory;
import org.springframework.beans.factory.xml.XmlBeanFactory;
import org.springframework.core.io.ClassPathResource;

public class XmlBeanFactoryTest {

    public static void main(String[] args) {
        // 资源加载
        ClassPathResource classPathResource = new ClassPathResource("spring-bean.xml");
        // XmlBeanFactory 加载资源并解析注册bean
        BeanFactory beanFactory = new XmlBeanFactory(classPathResource);
        // BeanFactory.getBean();
        UserBean userBean = (UserBean) beanFactory.getBean("userBean");
        System.out.println(userBean.getName());
	}
}

• XmlBeanFactory解析Xml是使用了XmlBeanDefinitionReader.loadBeanDefinition()方法,源码如下：

@Deprecated
@SuppressWarnings({"serial", "all"})
public class XmlBeanFactory extends DefaultListableBeanFactory {

	private final XmlBeanDefinitionReader reader = new XmlBeanDefinitionReader(this);


	/**
	 * Create a new XmlBeanFactory with the given resource,
	 * which must be parsable using DOM.
	 * @param resource the XML resource to load bean definitions from
	 * @throws BeansException in case of loading or parsing errors
	 */
	public XmlBeanFactory(Resource resource) throws BeansException {
		//调用构造方法  79行
		this(resource, null);
	}

	/**
	 * Create a new XmlBeanFactory with the given input stream,
	 * which must be parsable using DOM.
	 * @param resource the XML resource to load bean definitions from
	 * @param parentBeanFactory parent bean factory
	 * @throws BeansException in case of loading or parsing errors
	 */
	public XmlBeanFactory(Resource resource, BeanFactory parentBeanFactory) throws BeansException {
		//ignoreDependencyInterface 忽略自动装配
		//主要功能就是当有忽略的接口类，自动装配会忽略这部分类的初始化装配，因为某种情况下，此时的接口实现类不能初始化，列如BeanNameAware,要想装配这个接口的实现对象，可以实现这个接口。
		super(parentBeanFactory);
		//这段代码是真正的资源加载
		this.reader.loadBeanDefinitions(resource);
	}

}

• 我们直接看loadBeanDefinition方法，源码：

/**
 * 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 {
	// 对EncodedResource进行封装，设置String encoding, Charset charset
	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 {
	//encodedResource 不可以为空
	Assert.notNull(encodedResource, "EncodedResource must not be null");
	if (logger.isTraceEnabled()) {
		logger.trace("Loading XML bean definitions from " + encodedResource);
	}

	// 通过属性来记录已经加载的资源
	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 {
		// 从encodedResource已经封装的Resource获取InputStream
		InputStream inputStream = encodedResource.getResource().getInputStream();
		try {
			//InputSource 并不是spring的，而是 org.xml.sax
			InputSource inputSource = new InputSource(inputStream);
			//如果encodedResource 中的 Encoding 不是 null 则同步设置 InputSource的 Encoding
			if (encodedResource.getEncoding() != null) {
				inputSource.setEncoding(encodedResource.getEncoding());
			}
			//加载bean的Definitions 将xml中的信息加载到Definition中，并且在内存中注册的也是key+definitions
			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();
		}
	}
}

上述源码可能看着比较长,但实际上这里并不是真正解析的地方，在这里做了如下：

1:从encodedResource已经封装的Resource获取InputStream；

2:如果encodedResource 中的 Encoding 不是 null 则同步设置 InputSource的 Encoding；

3:将解析动作委托给doLoadBeanDefinitions实现；

• 接下来我们继续看doLoadBeanDefinitions方法内容：

/**
 * 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 {
		//加载 Document
		Document doc = doLoadDocument(inputSource, resource);
		//注册 bean
		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);
	}
}

当我们看着这个方法的时候，依旧不是真正的解析或注册的方法，在这里只是做了Document的加载，并将后续工作委托给了registerBeanDefinitions,registerBeanDefinitions方法的返回时注册Bean的个数；

• 我们继续看registerBeanDefinitions的源码：

/**
 * 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 {
	//实例化 BeanDefinitionDocumentReader
	BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();
	//获取之前的beanDefinition加载个数
	int countBefore = getRegistry().getBeanDefinitionCount();
	//加载xml及注册bean
	documentReader.registerBeanDefinitions(doc, createReaderContext(resource));
	//记录本次加载个数
	return getRegistry().getBeanDefinitionCount() - countBefore;
}

在registerBeanDefinitions方法具体实现：

1：通过BeanUtils.instantiateClass(this.documentReaderClass)的方法实例化BeanDefinitionDocumentReader；

2：通过DefaultListAbleBeanFactory中的beanDefinitionMap.size()获取之前注册bean的个数，（beanDefinitionMap是存储最终的xml解析后信息的载体，xml解析后信息是由GenericBeanDefinition进行存储，beanDefinitionMap的存储格式是key:String value:GenericBeanDefinition）

3:将解析xml和注册的工作委托给BeanDefinitionDocumentReader的registerBeanDefinitions方法；

4：记录本次加载个数并返回；

• 继续看BeanDefinitionDocumentReader的registerBeanDefinitions方法：

/**
 * 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) {
	//实例化 ReaderContext
	this.readerContext = readerContext;
	//注册
	doRegisterBeanDefinitions(doc.getDocumentElement());
}

• registerBeanDefinitions并没有做什么，我们继续看doRegisterBeanDefinitions方法：

/**
 * 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);

	//验证xml namespace, BeanDefinitionParserDelegate.BEANS_NAMESPACE_URI
	if (this.delegate.isDefaultNamespace(root)) {
		//获取Attribute
		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;
			}
		}
	}

	//解析前处理， 内容null 留个子类实现
	preProcessXml(root);
	//解析
	parseBeanDefinitions(root, this.delegate);
	//解析后处理， 内容null 留个子类实现
	postProcessXml(root);

	this.delegate = parent;
}

在doRegisterBeanDefinitions方法中验证xml的namespace,最重要的方法是parseBeanDefinitions，parseBeanDefinitions方法进行了解析操作；

• parseBeanDefinitions方法的源码：

/**
 * 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) {
	//验证xml namespace, BeanDefinitionParserDelegate.BEANS_NAMESPACE_URI
	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)) {
					//对默认标签处理
					parseDefaultElement(ele, delegate);
				}
				else {
					//对自定义标签处理
					delegate.parseCustomElement(ele);
				}
			}
		}
	}
	else {
		//对自定义标签处理
		delegate.parseCustomElement(root);
	}
}

parseBeanDefinitions方法中已经开始对标签进行解析，区分默认标签和自定义标签，我们本次只对默认标签的源码进行解析，自定义标签自行DeBug，

• parseDefaultElement方法的源码：

private void parseDefaultElement(Element ele, BeanDefinitionParserDelegate delegate) {
	//解析import标签
	if (delegate.nodeNameEquals(ele, IMPORT_ELEMENT)) {
		importBeanDefinitionResource(ele);
	}
	//解析alias标签并注册
	else if (delegate.nodeNameEquals(ele, ALIAS_ELEMENT)) {
		processAliasRegistration(ele);
	}
	//解析bean标签并注册
	else if (delegate.nodeNameEquals(ele, BEAN_ELEMENT)) {
		processBeanDefinition(ele, delegate);
	}
	//解析beans标签
	else if (delegate.nodeNameEquals(ele, NESTED_BEANS_ELEMENT)) {
		// recurse
		doRegisterBeanDefinitions(ele);
	}
}

到这里我们可以看到，spring对import/bean/alias/beans的解析过程，对于beans的解析无法就是解析beans中的bean标签，spring直接又重新调用了doRegisterBeanDefinitions方法，我们接下来进行对bean标签的解析；

• processBeanDefinition方法：

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

	//委托BeanDefinitionParserDelegate的parseBeanDefinitionElement方法进行元素解析并返回
	//BeanDefinitionHolder实例，BeanDefinitionHolder已经包含了配置文件中的各种属性

	BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
	//当BeanDefinitionHolder返回不null的情况，若存在默认标签下的子标签再有自定义的属性，还需要再次解析
	if (bdHolder != null) {
		//解析默认标签中的自定义标签
		bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);
		try {
			// Register the final decorated instance.
			// 进行实例注册注册操作是BeanDefinitionReaderUtisl.registerBeanDefinition进行处理
			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));
	}
}

在processBeanDefinition方法中，spring做了两件事情：

1：委托BeanDefinitionParserDelegate的parseBeanDefinitionElement方法进行元素解析并返回BeanDefinitionHolder实例，BeanDefinitionHolder已经包含了配置文件中的各种属性

2：通过上获得的BeanDefinitionHolder进行bean的注册操作，通BeanDefinitionReaderUtils.registerBeanDefinition方法；

• 通过delegate.parseBeanDefinitionElement方法进行xml解析：

/**
 * 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) {
	//解析id属性
	String id = ele.getAttribute(ID_ATTRIBUTE);
	//解析name属性
	String nameAttr = ele.getAttribute(NAME_ATTRIBUTE);

	//分割name属性
	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);
	}
	//将信息封装到 beanDefinition中
	AbstractBeanDefinition beanDefinition = parseBeanDefinitionElement(ele, beanName, containingBean);
	if (beanDefinition != null) {
		if (!StringUtils.hasText(beanName)) {
			try {
				//beanname不存在则使用默认规则创建
				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;
}

在parseBeanDefinitionElement方法中做了三件事：

1：解析id/name；

2：检查name的唯一性；

3：将信息封装到 beanDefinition中，接下来直接看parseBeanDefinitionElement方法；

• parseBeanDefinitionElement源码：

/**
 * Parse the bean definition itself, without regard to name or aliases. May return
 * {@code null} if problems occurred during the parsing of the bean definition.
 */
@Nullable
public AbstractBeanDefinition parseBeanDefinitionElement(
		Element ele, String beanName, @Nullable BeanDefinition containingBean) {
	this.parseState.push(new BeanEntry(beanName));

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

	try {
		//创建用于承载属性的AbstractBeanDefinition类型的
		AbstractBeanDefinition bd = createBeanDefinition(className, parent);

		//解析bean的各种属性
		parseBeanDefinitionAttributes(ele, beanName, containingBean, bd);
		//提取description
		bd.setDescription(DomUtils.getChildElementValueByTagName(ele, DESCRIPTION_ELEMENT));

		//解析meta （元数据）
		parseMetaElements(ele, bd);

		//解析Lookup-method 书中53页有使用方法
		parseLookupOverrideSubElements(ele, bd.getMethodOverrides());
		//解析replaced-method 书中55页有使用方法
		parseReplacedMethodSubElements(ele, bd.getMethodOverrides());

		//构造函数 参数
		//解析constructor-arg 书中replaced-method后边
		parseConstructorArgElements(ele, bd);
		//解析Property 书中replaced-method后边
		parsePropertyElements(ele, bd);
		//解析Qualifier 书中Property后边
		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;
}

通过上述代码我们可以看到这里首先是实例化了一个AbstractBeanDefinition来承载各种xml属性，接下来通过parseBeanDefinitionAttributes方法解析了xml中的各种你属性值，然后在解析lookUp-method(方法注入)，replaced-method(替换方法或方法返回值)，构造函数参数constructor-arg，property属性，Qualifier属性等；上述方法的源码就不一一展示了，无非都是通过Element进行解析；

• 接下来看真正注册的代码 BeanDefinitionReaderUtils.registerBeanDefinition

@Override
public void registerBeanDefinition(String beanName, BeanDefinition beanDefinition)
		throws BeanDefinitionStoreException {

	//beanName不可为空
	Assert.hasText(beanName, "Bean name must not be empty");
	//beanDefinition不可为空
	Assert.notNull(beanDefinition, "BeanDefinition must not be null");

	if (beanDefinition instanceof AbstractBeanDefinition) {
		try {
			//校验 MethodOverrides，MethodOverrides在解析并组装beanDefinition时有提到
			((AbstractBeanDefinition) beanDefinition).validate();
		}
		catch (BeanDefinitionValidationException ex) {
			throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
						"Validation of bean definition failed", ex);
		}
	}

	//beanDefinitionMap 存储实例的全局Map 使用ConcurrentHashMap 线程安全
	BeanDefinition existingDefinition = this.beanDefinitionMap.get(beanName);
	//如果已经注册 处理内容
	if (existingDefinition != null) {
		//是否覆盖
		if (!isAllowBeanDefinitionOverriding()) {
			throw new BeanDefinitionOverrideException(beanName, beanDefinition, existingDefinition);
		}
		else if (existingDefinition.getRole() < beanDefinition.getRole()) {
			// e.g. was ROLE_APPLICATION, now overriding with ROLE_SUPPORT or ROLE_INFRASTRUCTURE
			if (logger.isInfoEnabled()) {
				logger.info("Overriding user-defined bean definition for bean '" + beanName +
						"' with a framework-generated bean definition: replacing [" +
						existingDefinition + "] with [" + beanDefinition + "]");
			}
		}
		else if (!beanDefinition.equals(existingDefinition)) {
			if (logger.isDebugEnabled()) {
				logger.debug("Overriding bean definition for bean '" + beanName +
						"' with a different definition: replacing [" + existingDefinition +
						"] with [" + beanDefinition + "]");
			}
		}
		else {
			if (logger.isTraceEnabled()) {
				logger.trace("Overriding bean definition for bean '" + beanName +
						"' with an equivalent definition: replacing [" + existingDefinition +
						"] with [" + beanDefinition + "]");
			}
		}
		this.beanDefinitionMap.put(beanName, beanDefinition);
	}
	else {
		//判断是否已经至少创建过一次 使用AbstractBeanFactory.alreadyCreated来判断
		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 {
			// 仍处于启动注册阶段
			// 注册 beanDefinitionMap 新实例 beanName + beanDefinition
			this.beanDefinitionMap.put(beanName, beanDefinition);
			// 增加beanDefinitionNames
			this.beanDefinitionNames.add(beanName);
			// 清除缓存
			this.manualSingletonNames.remove(beanName);
		}
		// 清除缓存
		this.frozenBeanDefinitionNames = null;
	}

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

上述代码中首先验证了beanName和BeannDefinition不可为空,然后继续校验了MethodOverridesMethodOverrides在解析并组装beanDefinition时lookup-method和recpse-method的源码中有提到，继续判断beanDefinitionMap是否存在该bean，如果bean已经存在，通过allowBeanDefinitionOverriding属性判断是否可覆盖，反之则抛出异常；如果不存在则需要判断本次是否是第一次注册bean，如果是则初始化beanDefinitionMap后进行put操作，反之直接put beanDefinitionMap完成注册；

至此我们已经看完了整个XmlBeanFactory的xml解析和注册的源码部分，相信看本篇文章无法真正理解，还需要读者下载源码使用debug运行，再结合本篇文章的注释，相信会很容易理解，码字不易，转发请注明出处。

原文：https://www.cnblogs.com/technology-blog/p/14543685.html