0. Bean 加载原理

加载过程： 通过 ResourceLoader和其子类DefaultResourceLoader完成资源文件位置定位，实现从类路径，文件系统，url等方式定位功能，完成定位后得到Resource对象，再交给BeanDefinitionReader，它再委托给BeanDefinitionParserDelegate完成bean的解析并得到BeanDefinition对象，然后通过registerBeanDefinition方法进行注册，IOC容器内ibu维护了一个HashMap来保存该BeanDefinition对象，Spring中的BeanDefinition其实就是我们用的JavaBean。

什么是BeanDefinition对象

BeanDefinition是一个接口，描述了一个bean实例，它具有属性值，构造函数参数值以及具体实现提供的更多信息。

在开始之前需要认真阅读和理解这个过程，有了这个过程，阅读源码难度就小了一半。

大多源码都进行了注释，有的是官方英文注释。

1. bean.xml

一个普通的bean配置文件，这里我要强调的是它里面的格式，因为解析标签的时候会用到。它有<beans>``<bean>``<import>``<alias>等标签，下文会对他们进行解析并翻译成BeanDefinition对象。

<beans>  <!-- this definition could be inside one beanRefFactory.xml file -->  <bean id="a.qualified.name.of.some.sort"      class="org.springframework.context.support.ClassPathXmlApplicationContext">    <property name="configLocation" value="org/springframework/web/context/beans1.xml"/>  </bean>  <!-- while the following two could be inside another, also on the classpath,	perhaps coming from another component jar -->  <bean id="another.qualified.name"      class="org.springframework.context.support.ClassPathXmlApplicationContext">    <property name="configLocation" value="org/springframework/web/context/beans1.xml"/>    <property name="parent" ref="a.qualified.name.of.some.sort"/>  </bean>  <alias name="another.qualified.name" alias="a.qualified.name.which.is.an.alias"/></beans>

加载资源的策略接口(策略模式)。

DefaultResourceLoader is a standalone implementation that is usable outside an ApplicationContext, also used by ResourceEditor

An ApplicationContext is required to provide this functionality, plus extended ResourcePatternResolver support.

public interface ResourceLoader {	/** Pseudo URL prefix for loading from the class path: "classpath:". */	String CLASSPATH_URL_PREFIX = ResourceUtils.CLASSPATH_URL_PREFIX;               // 返回一个Resource 对象 (明确配置文件位置的对象)	Resource getResource(String location);        // 返回ResourceLoader的ClassLoader	@Nullable	ClassLoader getClassLoader();}

然后我们看看DefaultResourceLoader对于getResource()方法的实现。

	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;			}		}               // 如果location 以 / 开头		if (location.startsWith("/")) {			return getResourceByPath(location);		}                // 如果location 以classpath: 开头		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);			}		}	}

可以看到，它判断了三种情况：/ classpath: url格式匹配， 然后调用相对应的处理方法，我只分析classpath:，因为这是最常用的。所以看一看ClassPathResource实现：

	public ClassPathResource(String path, @Nullable ClassLoader classLoader) {		Assert.notNull(path, "Path must not be null");		String pathToUse = StringUtils.cleanPath(path);		if (pathToUse.startsWith("/")) {			pathToUse = pathToUse.substring(1);		}		this.path = pathToUse;		this.classLoader = (classLoader != null ? classLoader : ClassUtils.getDefaultClassLoader());	}

看了上面的代码，意味着你配置静态资源文件路径的时候，不用纠结classpath:后面用不用写/,因为如果写了它会给你过滤掉。

那url如何定位的呢?

跟踪getResourceByPath(location)方法:

	@Override	protected Resource getResourceByPath(String path) {		if (path.startsWith("/")) {			path = path.substring(1);		}		// 这里使用文件系统资源对象来定义bean文件		return new FileSystemResource(path);	}

好了，很明显…跑偏了，因为我们想要的是xml文件及路径的解析，不过还好，换汤不换药。下文中会涉及到。

触发bean加载

回到正题，我们在使用spring手动加载bean.xml的时候，用到：

ApplicationContext applicationContext = new ClassPathXmlApplicationContext("bean.xml");

那就从ClassPathXmlApplicationContext开始：

3. ClassPathXmlApplicationContext.java

这个类里面只有构造方法(多个)和一个getConfigResources()方法，构造方法最终都统一打到下面这个构造方法中(Spring源码经常这样，适配器模式):

	public ClassPathXmlApplicationContext(			String[] configLocations, boolean refresh, @Nullable ApplicationContext parent)			throws BeansException {	// 动态的确定用哪个加载器去加载 配置文件		1.super(parent);	// 告诉读取器 配置文件在哪里， 定位加载配置文件		2.setConfigLocations(configLocations);	// 刷新		if (refresh) {			// 在创建IOC容器前，如果容器已经存在，则需要把已有的容器摧毁和关闭，以保证refresh			//之后使用的是新的IOC容器			3.refresh();		}	}

注意: 这个类非常关键，我认为它定义了一个xml加载bean的一个Life Cycle:

super() 方法完成类加载器的指定。setConfigLocations(configLocations);方法对配置文件进行定位和解析，拿到Resource对象。refresh();方法对标签进行解析拿到BeanDefition对象，在通过校验后将其注册到IOC容器。(主要研究该方法)

我标记的1. 2. 3. 对应后面的方法x, 方便阅读。

先深入了解下setConfigLocations(configLocations);方法:

方法2. setConfigLocations()

	// 解析Bean定义资源文件的路径，处理多个资源文件字符串数组	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++) {				// resolvePath 为同一个类中将字符串解析为路径的方法				this.configLocations[i] = resolvePath(locations[i]).trim();			}		}		else {			this.configLocations = null;		}	}

然后我们继续上面看ClassPathXmlApplicationContext的refresh()方法:

方法3. refresh()

	public void refresh() throws BeansException, IllegalStateException {		synchronized (this.startupShutdownMonitor) {			// 为refresh 准备上下文			prepareRefresh();			// 通知子类去刷新 Bean工厂			ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();			// 用该 上下文来 准备bean工厂			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();			}		}	}

**注:**下面的方法全都是围绕refresh()里深入阅读，该方法套的很深，下面的阅读可能会引起不适。

然后看看refresh()方法中的obtainFreshBeanFactory()方法:

方法3.1 obtainFreshBeanFactory()

	// 调用--刷新bean工厂	protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {		// 委派模式：父类定义了refreshBeanFactory方法，具体实现调用子类容器		refreshBeanFactory();		return getBeanFactory();	}

然后看obtainFreshBeanFactory()的 refreshBeanFactory()方法

方法3.1.1 refreshBeanFactory()

       // 刷新bean工厂	protected final void refreshBeanFactory() throws BeansException {		// 如果存在容器，就先销毁并关闭		if (hasBeanFactory()) {			destroyBeans();			closeBeanFactory();		}		try {			// 创建IOC容器			DefaultListableBeanFactory beanFactory = createBeanFactory();			beanFactory.setSerializationId(getId());			// 对容器进行初始化			customizeBeanFactory(beanFactory);			// 调用载入Bean定义的方法，(使用了委派模式)			loadBeanDefinitions(beanFactory);			synchronized (this.beanFactoryMonitor) {				this.beanFactory = beanFactory;			}		}		catch (IOException ex) {			throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);		}	}

然后再跟进refreshBeanFactory() 的loadBeanDefinitions()方法：

方法3.1.1.1 loadBeanDefinitions()

通过 XmlBeanDefinitionReader 加载 BeanDefinition

	// 通过 XmlBeanDefinitionReader 加载 BeanDefinition	@Override	protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) throws BeansException, IOException {		// Create a new XmlBeanDefinitionReader for the given BeanFactory.		// 为beanFactory 创建一个新的 XmlBeanDefinitionReader		XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(beanFactory);		// Configure the bean definition reader with this context's		// resource loading environment.		beanDefinitionReader.setEnvironment(this.getEnvironment());		// 为 Bean读取器设置Spring资源加载器 (因为祖父类是ResourceLoader的子类，所以也是ResourceLoader)		beanDefinitionReader.setResourceLoader(this);		//  为 Bean读取器设置SAX xml解析器DOM4J		beanDefinitionReader.setEntityResolver(new ResourceEntityResolver(this));		// Allow a subclass to provide custom initialization of the reader,		// then proceed with actually loading the bean definitions.		// 初始化 BeanDefinition读取器		initBeanDefinitionReader(beanDefinitionReader);		// 真正加载 bean定义		loadBeanDefinitions(beanDefinitionReader);	}

再跟进loadBeanDefinitions(DefaultListableBeanFactory beanFactory)方法中的loadBeanDefinitions(XmlBeanDefinitionReader reader)方法:

方法3.1.1.1.1 loadBeanDefinitions()

XMLBean读取器加载BeanDefinition 资源

	// XMLBean读取器加载Bean 定义资源	protected void loadBeanDefinitions(XmlBeanDefinitionReader reader) throws BeansException, IOException {		// 获取Bean定义资源的定位		Resource[] configResources = getConfigResources();		if (configResources != null) {			// XMLBean读取器调用其父类 AbstractBeanDefinitionReader 读取定位的Bean定义资源			reader.loadBeanDefinitions(configResources);		}		// 如果子类中获取的bean定义资源定位为空，		// 则获取 FileSystemXmlApplicationContext构造方法中 setConfigLocations 方法设置的资源		String[] configLocations = getConfigLocations();		if (configLocations != null) {			// XMLBean读取器调用其父类 AbstractBeanDefinitionReader 读取定位的Bean定义资源			reader.loadBeanDefinitions(configLocations);		}	}

	@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;	}

再跟下去loadBeanDefinitions(): 这只是一个抽象方法，找到XmlBeanDefinitionReader子类的实现：

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

再深入loadBeanDefinitions:

通过明确的xml文件加载bean

    // 通过明确的xml文件加载bean	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 == 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 {			// 将资源文件转为InputStream的IO流			InputStream inputStream = encodedResource.getResource().getInputStream();			try {				// 从流中获取 xml解析资源				InputSource inputSource = new InputSource(inputStream);				if (encodedResource.getEncoding() != null) {					// 设置编码					inputSource.setEncoding(encodedResource.getEncoding());				}				// 具体的读取过程				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();			}		}	}

再深入到doLoadBeanDefinitions():

真正开始加载 BeanDefinitions

	protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource)			throws BeanDefinitionStoreException {		try {			// 将xml 文件转换为DOM对象			Document doc = doLoadDocument(inputSource, resource);			// 对bean定义解析的过程，该过程会用到 Spring的bean配置规则			int count = registerBeanDefinitions(doc, resource);			if (logger.isDebugEnabled()) {				logger.debug("Loaded " + count + " bean definitions from " + resource);			}			return count;		}        ...  ...  ..}

doLoadDocument()方法将流进行解析，返回一个Document对象:return builder.parse(inputSource);为了避免扰乱思路，这里的深入自己去完成。

还需要再深入到:registerBeanDefinitions()

注册 BeanDefinitions

	public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {		BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();		// 得到容器中注册的bean数量		int countBefore = getRegistry().getBeanDefinitionCount();		// 解析过程入口，这里使用了委派模式		documentReader.registerBeanDefinitions(doc, createReaderContext(resource));		// 统计解析的bean数量		return getRegistry().getBeanDefinitionCount() - countBefore;	}

再深入registerBeanDefinitions()方法（该方法是委派模式的结果）:

	@Override	public void registerBeanDefinitions(Document doc, XmlReaderContext readerContext) {		// 获得XML描述符		this.readerContext = readerContext;		doRegisterBeanDefinitions(doc.getDocumentElement());	}

再深入doRegisterBeanDefinitions(doc.getDocumentElement());：

真正开始注册 BeanDefinitions :

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;				}			}		}		// 在bean解析定义之前，进行自定义解析，看是否是用户自定义标签		preProcessXml(root);		// 开始进行解析bean定义的document对象		parseBeanDefinitions(root, this.delegate);		// 解析bean定义之后，进行自定义的解析，增加解析过程的可扩展性		postProcessXml(root);		this.delegate = parent;	}

接下来看parseBeanDefinitions(root, this.delegate);:

document的根元素开始进行解析翻译成BeanDefinitions

	// 从document的根元素开始进行解析翻译成BeanDefinitions	protected void parseBeanDefinitions(Element root, BeanDefinitionParserDelegate delegate) {		// bean定义的document对象使用了spring默认的xml命名空间		if (delegate.isDefaultNamespace(root)) {			// 获取bean定义的document对象根元素的所有字节点			NodeList nl = root.getChildNodes();			for (int i = 0; i < nl.getLength(); i++) {				Node node = nl.item(i);				// 获得document节点是xml元素节点				if (node instanceof Element) {					Element ele = (Element) node;					// bean定义的document的元素节点使用的是spring默认的xml命名空间					if (delegate.isDefaultNamespace(ele)) {						// 使用spring的bean规则解析元素 节点						parseDefaultElement(ele, delegate);					}					else {						// 没有使用spring默认的xml命名空间，则使用用户自定义的解析规则解析元素节点						delegate.parseCustomElement(ele);					}				}			}		}		else {			delegate.parseCustomElement(root);		}	}	private void parseDefaultElement(Element ele, BeanDefinitionParserDelegate delegate) {		// 解析 <import> 标签元素，并进行导入解析		if (delegate.nodeNameEquals(ele, IMPORT_ELEMENT)) {			importBeanDefinitionResource(ele);		}		// alias		else if (delegate.nodeNameEquals(ele, ALIAS_ELEMENT)) {			processAliasRegistration(ele);		}		// bean		else if (delegate.nodeNameEquals(ele, BEAN_ELEMENT)) {			processBeanDefinition(ele, delegate);		}		// beans		else if (delegate.nodeNameEquals(ele, NESTED_BEANS_ELEMENT)) {			// recurse			doRegisterBeanDefinitions(ele);		}	}

importBeanDefinitionResource(ele);``processAliasRegistration(ele);``processBeanDefinition(ele, delegate);这三个方法里分别展示了标签解析的详细过程。

这下看到了，它其实使用DOM4J来解析import bean alias等标签，然后递归标签内部直到拿到所有属性并封装到BeanDefition对象中。比如说processBeanDefinition方法:

给我一个element 解析成 BeanDefinition

	// 给我一个element 解析成 BeanDefinition	protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {		// 真正解析过程		BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);		if (bdHolder != null) {			bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);			try {				// Register the final decorated instance.				// 注册： 将db注册到ioc，委托模式				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));		}	}

继续深入registerBeanDefinition():

注册BeanDefinitions 到 bean 工厂

	// 注册BeanDefinitions 到 bean 工厂	// definitionHolder ： bean定义，包含了 name和aliases	// registry： 注册到的bean工厂	public static void registerBeanDefinition(			BeanDefinitionHolder definitionHolder, BeanDefinitionRegistry registry)			throws BeanDefinitionStoreException {		// Register bean definition under primary name.		String beanName = definitionHolder.getBeanName();		// 真正注册		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);			}		}	}

再深入registry.registerBeanDefinition(beanName, definitionHolder.getBeanDefinition());

注册BeanDefinitions 到IOC容器

注意:该方法所在类是接口，我们查看的是DefaultListableBeanFactory.java所实现的该方法。

	// 实现BeanDefinitionRegistry接口,注册BeanDefinitions 	@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");		// 校验是否是 AbstractBeanDefinition)		if (beanDefinition instanceof AbstractBeanDefinition) {			try {				// 标记 beanDefinition 生效				((AbstractBeanDefinition) beanDefinition).validate();			}			catch (BeanDefinitionValidationException ex) {				throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,						"Validation of bean definition failed", ex);			}		}		// 判断beanDefinitionMap 里是否已经有这个bean		BeanDefinition existingDefinition = this.beanDefinitionMap.get(beanName);		//如果没有这个bean		if (existingDefinition != null) {			//如果不允许bd 覆盖已注册的bean， 就抛出异常			if (!isAllowBeanDefinitionOverriding()) {				throw new BeanDefinitionOverrideException(beanName, beanDefinition, existingDefinition);			}			// 如果允许覆盖， 则同名的bean， 注册的覆盖先注册的			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 + "]");				}			}			// 注册到容器，beanDefinitionMap 就是个容器			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 (existingDefinition != null || containsSingleton(beanName)) {			resetBeanDefinition(beanName);		}	}

这个方法中对所需要加载的bean进行校验，没有问题的话就put到beanDefinitionMap中，beanDefinitionMap其实就是IOC.这样我们的Bean就被加载到IOC容器中了。