Spring源码-IOC部分-Xml Bean解析注册过程【3】

实验环境：spring-framework-5.0.2、jdk8、gradle4.3.1

• Spring源码-IOC部分-容器简介【1】
• Spring源码-IOC部分-容器初始化过程【2】
• Spring源码-IOC部分-Xml Bean解析注册过程【3】
• Spring源码-IOC部分-自定义IOC容器及Bean解析注册【4】
• Spring源码-IOC部分-Bean实例化过程【5】
• Spring源码-IOC部分-Spring是如何解决Bean循环依赖的【6】

本文以Xml Bean解析为例，步骤如下

Bean解析过程发生在AbstractApplicationContext#refresh()方法的第2步：obtainFreshBeanFactory()

AbstractApplicationContext#obtainFreshBeanFactory方法

    /**
	 * 初始化BeanFactory，在这一步完成了loadBeanDefinitions的加载
	 */
	protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {
		// 抽象方法，交给子类实现。
		// 子类刷新beanFactory，比如子类AbstractRefreshableApplicationContext会在refreshBeanFactory方法里createBeanFactory，并且执行loadBeanDefinitions加载资源配置。
		// 而GenericApplicationContext只会对已经创建好的beanFactory实例设置一下id即可，loadBeanDefinitions放在了外部去做。
		refreshBeanFactory();

		// getBeanFactory也是抽象方法，由子类创建beanFactory
		ConfigurableListableBeanFactory beanFactory = getBeanFactory();
		if (logger.isDebugEnabled()) {
			logger.debug("Bean factory for " + getDisplayName() + ": " + beanFactory);
		}
		return beanFactory;
	}

其中refreshBeanFactory()是个抽象方法，其子类AbstractRefreshableApplicationContext实现如下

AbstractRefreshableApplicationContext#refreshBeanFactory方法

    /**
	 * 刷新BeanFactory，实现父类AbstractApplicationContext的抽象方法
	 */
	@Override
	protected final void refreshBeanFactory() throws BeansException {
		// 如果已经存在BeanFactory，销毁容器里的bean，关闭BeanFactory
		if (hasBeanFactory()) {
			destroyBeans();
			closeBeanFactory();
		}
		try {
			// 创建beanFactory
			DefaultListableBeanFactory beanFactory = createBeanFactory();
			beanFactory.setSerializationId(getId());

			// 对IOC容器进行定制化，如设置启动参数、开启注解对自动装配等
			customizeBeanFactory(beanFactory);

			// 调用载入bean定义对方法，loadBeanDefinitions(beanFactory)在本类是个抽象方法，交给子类实现
			loadBeanDefinitions(beanFactory);

			// 加锁，写的时候不允许读写
			synchronized (this.beanFactoryMonitor) {
				this.beanFactory = beanFactory;
			}
		}
		catch (IOException ex) {
			throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
		}
	}

其中loadBeanDefinitions(beanFactory)是解析的入口，这里也是抽象方法，其子类AbstractXmlApplicationContext实现如下

AbstractXmlApplicationContext#loadBeanDefinitions方法

/**
	 * 实现AbstractRefreshableApplicationContext的抽象方法
	 */
	@Override
	protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) throws BeansException, IOException {
		// Create a new XmlBeanDefinitionReader for the given BeanFactory.
		// 创建xml定义的bean读取器，容器使用它来读取bean配置资源
		XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(beanFactory);

		// Configure the bean definition reader with this context's
		// resource loading environment.
		// 设置资源加载器
		beanDefinitionReader.setEnvironment(this.getEnvironment());
		beanDefinitionReader.setResourceLoader(this);
		// 为bean读取器设置SAX xml解析器
		beanDefinitionReader.setEntityResolver(new ResourceEntityResolver(this));

		// Allow a subclass to provide custom initialization of the reader,
		// then proceed with actually loading the bean definitions.
		// 当bean读取器读取bean定义的xml资源文件时，启动xml的校验机制
		initBeanDefinitionReader(beanDefinitionReader);

		// bean读取器真正实现加载的方法
		loadBeanDefinitions(beanDefinitionReader);
	}

    /**
	 * bean读取器真正实现加载的方法
	 */
	protected void loadBeanDefinitions(XmlBeanDefinitionReader reader) throws BeansException, IOException {
		// 获取bean配置资源的定位，比如 ClassPathXmlApplicationContext(String[] paths, Class<?> clazz)使用类加载资源，会走到这里
		Resource[] configResources = getConfigResources();
		if (configResources != null) {
			// xml bean读取器调用其父类AbstractBeanDefinitionReader读取定位的bean配置资源
			reader.loadBeanDefinitions(configResources);
		}

		// 获取bean配置资源的定位，比如 ClassPathXmlApplicationContext(String... configLocations)输入路径的构造，会走这里
		String[] configLocations = getConfigLocations();
		if (configLocations != null) {
			reader.loadBeanDefinitions(configLocations);
		}
	}

这里的AbstractBeanDefinitionReader将资源路径转换成了Resource对象，然后传给子类XmlBeanDefinitionReader来具体解析

AbstractBeanDefinitionReader#loadBeanDefinitions方法

@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;
	}

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

    public int loadBeanDefinitions(String location, @Nullable Set<Resource> actualResources) throws BeanDefinitionStoreException {
		// 获取在IOC容器初始化过程中设置的资源加载器
		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 {
				// 加载多个资源
				Resource[] resources = ((ResourcePatternResolver) resourceLoader).getResources(location);
				// loadBeanDefinitions(resources)在本类没有实现，交给子类实现
				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.
			// 加载单个资源
			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;
		}
	}

XmlBeanDefinitionReader是具体的解析过程，在这里将Resource对象转成了Document对象，传给BeanDefinitionDocumentReader来解析注册

XmlBeanDefinitionReader#loadBeanDefinitions方法

    /**
	 * 加载资源的入口方法
	 */
	@Override
	public int loadBeanDefinitions(Resource resource) throws BeanDefinitionStoreException {
		// 将读入的xml资源进行特殊编码处理
		return loadBeanDefinitions(new EncodedResource(resource));
	}

    /**
	 * 加载xml形式的bean配置
	 */
	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 {
			// 资源文件转为io流
			InputStream inputStream = encodedResource.getResource().getInputStream();
			try {
				// 从InputStream得到xml的解析源
				InputSource inputSource = new InputSource(inputStream);
				if (encodedResource.getEncoding() != null) {
					inputSource.setEncoding(encodedResource.getEncoding());
				}

				// 具体的读取流程
				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();
			}
		}
	}

    /**
	 *
	 * 从xml文件加载资源，具体执行
	 */
	protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource)
			throws BeanDefinitionStoreException {
		try {
			// xml文件转化为DOM对象
			Document doc = doLoadDocument(inputSource, resource);
			// 注册BeanDefinitions
			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);
		}
	}

    /**
	 * 将bean配置信息解析成容器内部数据结构
	 */
	public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
		// 获取一个解析器，这里用的是DefaultBeanDefinitionDocumentReader
		BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();

		// 获取已经注册的bean数量，getRegistry()获取的就是GenericApplicationContext
		int countBefore = getRegistry().getBeanDefinitionCount();

		// 解析过程的入口，具体解析过程交给DefaultBeanDefinitionDocumentReader
		documentReader.registerBeanDefinitions(doc, createReaderContext(resource));

		// 返回本次解析数量
		return getRegistry().getBeanDefinitionCount() - countBefore;
	}

DefaultBeanDefinitionDocumentReader是专门针对Document对象的解析器。会从根节点递归的解析，封装成beanDefinition注册到beanDefinitionMap里

DefaultBeanDefinitionDocumentReader#registerBeanDefinitions方法

/**
	 * DefaultBeanDefinitionDocumentReader类 解析DOM资源
	 */
	@Override
	public void registerBeanDefinitions(Document doc, XmlReaderContext readerContext) {
		this.readerContext = readerContext;
		logger.debug("Loading bean definitions");
		Element root = doc.getDocumentElement();

		// 具体解析注册过程
		doRegisterBeanDefinitions(root);
	}

    /**
	 *
	 * 从根节点递归解析
	 */
	protected void doRegisterBeanDefinitions(Element root) {
		// 具体解析过程由BeanDefinitionParserDelegate实现，其内部定义了xml文件的各种元素

		// 父级指向当前
		BeanDefinitionParserDelegate parent = this.delegate;
		// 当前新建一个
		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;
				}
			}
		}

		// 在解析之前，进行自定义解析，支持扩展
		preProcessXml(root);

		// 从文档的根元素进行bean定义的解析
		parseBeanDefinitions(root, this.delegate);

		// 在解析之后，进行自定义解析，支持扩展
		postProcessXml(root);

		this.delegate = parent;
	}

    /**
	 * 使用spring的bean规则从文档的根元素开始解析
	 */
	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)) {

						// 使用spring的bean规则解析元素节点
						parseDefaultElement(ele, delegate);
					}
					else {
						// 使用用户自定义的bean规则解析元素节点
						delegate.parseCustomElement(ele);
					}
				}
			}
		}
		else {
			// 使用用户自定义的bean规则解析元素节点
			delegate.parseCustomElement(root);
		}
	}

	// 具体解析元素，并注册beanDefinition
	private void parseDefaultElement(Element ele, BeanDefinitionParserDelegate delegate) {
		// import导入元素解析
		if (delegate.nodeNameEquals(ele, IMPORT_ELEMENT)) {
			importBeanDefinitionResource(ele);
		}
		// alias别名元素解析
		else if (delegate.nodeNameEquals(ele, ALIAS_ELEMENT)) {
			processAliasRegistration(ele);
		}
		// 普通bean元素解析
		else if (delegate.nodeNameEquals(ele, BEAN_ELEMENT)) {
			processBeanDefinition(ele, delegate);
		}
		else if (delegate.nodeNameEquals(ele, NESTED_BEANS_ELEMENT)) {
			// 递归调用
			doRegisterBeanDefinitions(ele);
		}
	}

    /**
	 *
	 * 封装成beanDefinition注册到beanDefinitionMap里
	 */
	protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
		// 会把解析出来的bean封装成BeanDefinitionHolder
		BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
		if (bdHolder != null) {
			bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);
			try {
				// 在这一步把beanDefinition注册到了DefaultListableBeanFactory的beanDefinitionMap里
				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));
		}
	}

具体的注册过程由BeanDefinitionReaderUtils类实现

BeanDefinitionReaderUtils#registerBeanDefinition方法

/**
	 * BeanDefinitionReaderUtils类 注册过程
	 */
	public static void registerBeanDefinition(
			BeanDefinitionHolder definitionHolder, BeanDefinitionRegistry registry)
			throws BeanDefinitionStoreException {

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

		// BeanDefinition的具体注册，这个registry是个注册器，DefaultListableBeanFactory就是注册器的一种实现
		registry.registerBeanDefinition(beanName, definitionHolder.getBeanDefinition());

		// Register aliases for bean name, if any.
		String[] aliases = definitionHolder.getAliases();
		if (aliases != null) {
			for (String alias : aliases) {
				// 注册别名
				registry.registerAlias(beanName, alias);
			}
		}
	}

DefaultListableBeanFactory注册beanDefinition的具体逻辑，其实就是this.beanDefinitionMap.put(beanName, beanDefinition)

DefaultListableBeanFactory#registerBeanDefinition方法

/**
	 * DefaultListableBeanFactory类 注册beanDefinition实现
	 */
	@Override
	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;

		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 {
			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);
		}
	}

总结：bean解析的代码相对来说比较多，各种调用关系链路比较长。总的来说就是父类先做抽象，把一些公共的逻辑实现，具体的解析细节交给子类来做。子类解析好后再调用注册器将BeanDefinition注册进去。

参考资料：

《Spring5核心原理与30个类手写》作者 谭勇德

《Spring源码深度解析》作者 郝佳

《Spring技术内幕》作者 计文柯