/**
 * 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 {
	// 先判斷是否有BeanFactory
	if (hasBeanFactory()) {
		// destroy Beans
		destroyBeans();
		// 關閉BeanFactory
		closeBeanFactory();
	}
	try {
		// 實例化DefaultListableBeanFactory
		DefaultListableBeanFactory beanFactory = createBeanFactory();
		// 設置序列化id
		beanFactory.setSerializationId(getId());
		// 定義BeanFactory的一些屬性（是否允許Bean覆蓋，是否允許循環依賴）
		customizeBeanFactory(beanFactory);
		// 載入應用中的BeanDefinition
		loadBeanDefinitions(beanFactory);
		this.beanFactory = beanFactory;
	}
	catch (IOException ex) {
		throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
	}
}

{@link org.springframework.context.support.AbstractXmlApplicationContext#loadBeanDefinitions}

/**
 * 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.
	// 給BeanFactory實例化一個XmlBeanDefinitionReader實例
	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.
	// 給子類提供初始化XmlBeanDefinitionReader的模板方法
	initBeanDefinitionReader(beanDefinitionReader);
	// 重點在這，繼續跟
	loadBeanDefinitions(beanDefinitionReader);
}

// org.springframework.context.support.AbstractXmlApplicationContext#loadBeanDefinitions(org.springframework.beans.factory.support.DefaultListableBeanFactory)
protected void loadBeanDefinitions(XmlBeanDefinitionReader reader) throws BeansException, IOException {
	Resource[] configResources = getConfigResources();
	if (configResources != null) {
		// 繼續跟
		reader.loadBeanDefinitions(configResources);
	}
	String[] configLocations = getConfigLocations();
	if (configLocations != null) {
		reader.loadBeanDefinitions(configLocations);
	}
}

{@link org.springframework.beans.factory.support.AbstractBeanDefinitionReader#loadBeanDefinitions}

@Override
public int loadBeanDefinitions(Resource... resources) throws BeanDefinitionStoreException {
	Assert.notNull(resources, "Resource array must not be null");
	int counter = 0;
	// 變數資源對象，載入BeanDefinition
	for (Resource resource : resources) {
		// 在這裡，經過debug，調到XmlBeanDefinitionReader.loadBeanDefinitions(resource)
		counter += loadBeanDefinitions(resource);
	}
	// 返回統計數量，表示總共有多少個BeanDefinition
	return counter;
}

XmlBeanDefinitionReader類

{@link org.springframework.beans.factory.xml.XmlBeanDefinitionReader#loadBeanDefinitions}

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

// 省略...

/**
 * 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.isInfoEnabled()) {
		logger.info("Loading XML bean definitions from " + encodedResource);
	}
	// 用ThreadLocal存放資源對象，目的是保證執行緒安全
	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 {
		InputStream inputStream = encodedResource.getResource().getInputStream();
		try {
			InputSource inputSource = new InputSource(inputStream);
			if (encodedResource.getEncoding() != null) {
				inputSource.setEncoding(encodedResource.getEncoding());
			}
			// 核心，往下跟
			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();
		}
	}
}


/**
 * 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 {
		// 讀取xml資訊，將xml資訊保存在Document對象
		Document doc = doLoadDocument(inputSource, resource);
		// 封裝BeanDefinitions並註冊，繼續解析Document對象，往下跟
		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);
	}
}


/**
 * 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();
	// 獲取已有的BeanDefinition數量
	int countBefore = getRegistry().getBeanDefinitionCount();
	// 註冊BeanDefinition，關注點：registerBeanDefinitions、createReaderContext
	documentReader.registerBeanDefinitions(doc, createReaderContext(resource));
	// 返回新註冊的BeanDefinition統計數量
	return getRegistry().getBeanDefinitionCount() - countBefore;
}

{@link org.springframework.beans.factory.xml.DefaultBeanDefinitionDocumentReader#registerBeanDefinitions}

/**
 * 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;
	logger.debug("Loading bean definitions");
	Element root = doc.getDocumentElement();
	// 往下跟，從xml根節點開始解析文件
	doRegisterBeanDefinitions(root);
}

/**
 * Register each bean definition within the given root {@code <beans/>} element.
 */
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)) {
		// profile(環境校驗)
		String profileSpec = root.getAttribute(PROFILE_ATTRIBUTE);
		if (StringUtils.hasText(profileSpec)) {
			String[] specifiedProfiles = StringUtils.tokenizeToStringArray(
					profileSpec, BeanDefinitionParserDelegate.MULTI_VALUE_ATTRIBUTE_DELIMITERS);
			// 不是當前profile的，跳過
			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模板方法
	preProcessXml(root);
	// 重點是parseBeanDefinitions
	parseBeanDefinitions(root, this.delegate);
	// postProcessXml 同樣是模板方法 過
	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)) {
					// 解析default namespace下面的元素
					parseDefaultElement(ele, delegate);
				}
				else {
					// 解析自定義標籤元素
					delegate.parseCustomElement(ele);
				}
			}
		}
	}
	else {
		delegate.parseCustomElement(root);
	}
}


private void parseDefaultElement(Element ele, BeanDefinitionParserDelegate delegate) {
	// import元素處理
	if (delegate.nodeNameEquals(ele, IMPORT_ELEMENT)) {
		importBeanDefinitionResource(ele);
	}
	// alias元素處理
	else if (delegate.nodeNameEquals(ele, ALIAS_ELEMENT)) {
		processAliasRegistration(ele);
	}
	// bean元素處理
	else if (delegate.nodeNameEquals(ele, BEAN_ELEMENT)) {
		processBeanDefinition(ele, delegate);
	}
	// 嵌套beans處理
	else if (delegate.nodeNameEquals(ele, NESTED_BEANS_ELEMENT)) {
		// recurse
		doRegisterBeanDefinitions(ele);
	}
}

bean元素處理，本文挑beana元素的處理進行講述：

/**
 * Process the given bean element, parsing the bean definition
 * and registering it with the registry.
 */
protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
	//解析Bean元素為BeanDefinition，再通過BeanDefinitionHolder封裝成BeanDefinitionHolder對象
	BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
	if (bdHolder != null) {
		// 有自定義屬性會進行相應的解析
		bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);
		try {
			// Register the final decorated instance.
			// BeanDefinition的註冊過程
			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));
	}
}

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