源码分析：Spring是如何把Bean注册到IOC容器中的？

前言

提到Spring就会想到IOC、DI等概念，这是Spring的核心思想，只要使用过Spring框架的人都知道这些概念，但要问到Spring具体是怎么实现IOC的，恐怕只能看Spring的源码才能找到答案，在我看来只要搞清楚两个问题，就能对Spring的整体脉络有个整体的认识

• Bean是如何注册到IOC注册中的？
• Bean是如何从IOC容器中get出来的？

一个简单的例子

我们先从一个简单的例子开始，基本上在最开始学习Spring的时候都会从xml配置开始，把你需要交给Spring管理的类配置到xml文件中，你就可以不用管对象的创建了，下面来看一下代码

1、首先定义一个User类

@Data
public class User {
    private String userName;
    private String password;
}

很简单的一个JavaBean

2、增加一个xml配置

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd">
    <bean id="userBean" class="org.kxg.springDemo.User">
        <property name="userName" value="jack" />
        <property name="password" value="123456" />
    </bean>

</beans>

3、读取配置，并运行

public class XmlBeanTest {
    public static void main(String[] args){
        ClassPathXmlApplicationContext applicationContext = new ClassPathXmlApplicationContext("bean.xml");
        User user = (User) applicationContext.getBean("userBean");
        System.out.println(user.getUserName());
    }
}

可以看到例子很简单，堪称Spring入门的HelloWorld

从上面几行代码可以看出，首先读取bean.xml中的配置，然后就可以从applicationContext中获取到User对象，那么肯定会有User对象注册到IOC容器中这个步骤

下面我们一起通过源码来看一下Bean是如何注册到Spring IOC容器中的

源码解析

从ClassPathXmlApplicationContext开始

public ClassPathXmlApplicationContext(String configLocation) throws BeansException {
		this(new String[] {configLocation}, true, null);
	}
	
public ClassPathXmlApplicationContext(String... configLocations) throws BeansException {
	this(configLocations, true, null);
}

从ClassPathXmlApplicationContext的构造方法入手，构造方法传入xml配置文件的路径，这里可以传入单个或多个配置文件

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

		super(parent);
		//将传入的xml配置位置信息设置到configLocations
		setConfigLocations(configLocations);
		if (refresh) {
		    //核心方法
			refresh();
		}
	}

这里看到，我们传入的配置文件设置到configLocations，然后调用了一个Spring最核心的方法refresh()，这个方法包括了容器启动的所有内容，是我们学习Spring源码的一个入口，可以说你只要把这个方法里面的内容研究清楚了，对于Spring框架的整个脉络会有一个全新的认识，下面我们来看看这个方法里面有些什么

public void refresh() throws BeansException, IllegalStateException {
		synchronized (this.startupShutdownMonitor) {
			// Prepare this context for refreshing.
			prepareRefresh();

			// 注意看这个方法
			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();
			}
		}
	}

refresh()方法里面的内容很丰富，从各个方法名称就大致可以看出来其作用，这里我们主要看Bean注册的过程，将目光聚焦到

ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

这一行是获取BeanFactory，里面进行了Bean的注册逻辑

protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {
		refreshBeanFactory();
		return getBeanFactory();
	}

这里调用的是AbstractRefreshableApplicationContext类的refreshBeanFactory()方法，需要注意一下在看Spring源码的时候，同一个方法可能会有多个子类都实现了，需要注意区分一下，你当前实例化的是哪个子类

protected final void refreshBeanFactory() throws BeansException {
		if (hasBeanFactory()) {
			destroyBeans();
			closeBeanFactory();
		}
		try {
		    //创建一个BeanFactory
			DefaultListableBeanFactory beanFactory = createBeanFactory();
			beanFactory.setSerializationId(getId());
			customizeBeanFactory(beanFactory);
			//这里进行Bean的加载
			loadBeanDefinitions(beanFactory);
			synchronized (this.beanFactoryMonitor) {
				this.beanFactory = beanFactory;
			}
		}
		catch (IOException ex) {
			throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
		}
	}

因为我们使用的是xml的配置，所以这里调用的是AbstractXmlApplicationContext这个抽象类中的loadBeanDefinitions方法

protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) throws BeansException, IOException {
		// 构造一个XmlBeanDefinitionReader，用于读到xml中配置的bean
		XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(beanFactory);

		// 配置XmlBeanDefinitionReader
		beanDefinitionReader.setEnvironment(this.getEnvironment());
		beanDefinitionReader.setResourceLoader(this);
		beanDefinitionReader.setEntityResolver(new ResourceEntityResolver(this));

		//初始化XmlBeanDefinitionReader
		initBeanDefinitionReader(beanDefinitionReader);
		//加载Bean
		loadBeanDefinitions(beanDefinitionReader);
	}

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);
		}
	}

这里进行了两种不同方式的加载，调用的是不同的方法，我们传入的是configLocations

@Override
public int loadBeanDefinitions(String... locations) throws BeanDefinitionStoreException {
	Assert.notNull(locations, "Location array must not be null");
	int count = 0;
	for (String location : locations) {
		count += loadBeanDefinitions(location);
	}
	return count;
}

	public int loadBeanDefinitions(String location, @Nullable Set<Resource> actualResources) throws BeanDefinitionStoreException {
		ResourceLoader resourceLoader = getResourceLoader();
		if (resourceLoader == null) {
			throw new BeanDefinitionStoreException(
					"Cannot load bean definitions from location [" + location + "]: no ResourceLoader available");
		}

		if (resourceLoader instanceof ResourcePatternResolver) {
			// Resource pattern matching available.
			try {
				Resource[] resources = ((ResourcePatternResolver) resourceLoader).getResources(location);
				int count = loadBeanDefinitions(resources);
				if (actualResources != null) {
					Collections.addAll(actualResources, resources);
				}
				if (logger.isTraceEnabled()) {
					logger.trace("Loaded " + count + " bean definitions from location pattern [" + location + "]");
				}
				return count;
			}
			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 count = loadBeanDefinitions(resource);
			if (actualResources != null) {
				actualResources.add(resource);
			}
			if (logger.isTraceEnabled()) {
				logger.trace("Loaded " + count + " bean definitions from location [" + location + "]");
			}
			return count;
		}
	}

这个里面主要方法是loadBeanDefinitions()，我们继续往下走

中间省略了一些简单调用

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 {
			InputStream inputStream = encodedResource.getResource().getInputStream();
			try {
				InputSource inputSource = new InputSource(inputStream);
				if (encodedResource.getEncoding() != null) {
					inputSource.setEncoding(encodedResource.getEncoding());
				}
				//进行BeanDefinations加载
				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();
			}
		}
	}

protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource)
			throws BeanDefinitionStoreException {

		try {
		    //构造xml的Document结构，解析DOM结构
			Document doc = doLoadDocument(inputSource, resource);
			//注册BeanDefinition
			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);
		}
	}

从上面的代码可以看出来，Spring是将xml的DOM结构解析后注册到IOC容器中的

public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
		BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();
		int countBefore = getRegistry().getBeanDefinitionCount();
		documentReader.registerBeanDefinitions(doc, createReaderContext(resource));
		return getRegistry().getBeanDefinitionCount() - countBefore;
	}

这个方法构造了一个BeanDefinitionDocumentReader，进行注册BeanDefinition，并且返回了本次注册Bean的数量

	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;
				}
			}
		}

		preProcessXml(root);
		//进行BeanDefinition转换，将DOM结构的对象转换成BeanDefinition
		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)) {
					    //Spring默认元素转换
						parseDefaultElement(ele, delegate);
					}
					else {
					    //xml中自定义的Element进行解析
						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)) {
			processBeanDefinition(ele, delegate);
		}
		else if (delegate.nodeNameEquals(ele, NESTED_BEANS_ELEMENT)) {
			// recurse
			doRegisterBeanDefinitions(ele);
		}
	}

可以看到xml中默认的配置元素包括import、alias、bean、beans，这些也是最常用的，我们主要看一个bean的转换

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));
		}
	}

public static void registerBeanDefinition(
			BeanDefinitionHolder definitionHolder, BeanDefinitionRegistry registry)
			throws BeanDefinitionStoreException {

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

// DefaultListableBeanFactory
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 existingDefinition = this.beanDefinitionMap.get(beanName);
		if (existingDefinition != null) {
			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 + "]");
				}
			}
			//将beanDefinition放入beanDefinitionMap中
			this.beanDefinitionMap.put(beanName, beanDefinition);
		}
		else {
			if (hasBeanCreationStarted()) {
				// Cannot modify startup-time collection elements anymore (for stable iteration)
				synchronized (this.beanDefinitionMap) {
				    //将beanDefinition放入beanDefinitionMap中
					this.beanDefinitionMap.put(beanName, beanDefinition);
					List<String> updatedDefinitions = new ArrayList<>(this.beanDefinitionNames.size() + 1);
					updatedDefinitions.addAll(this.beanDefinitionNames);
					updatedDefinitions.add(beanName);
					this.beanDefinitionNames = updatedDefinitions;
					removeManualSingletonName(beanName);
				}
			}
			else {
				// Still in startup registration phase
				//将beanDefinition放入beanDefinitionMap中
				this.beanDefinitionMap.put(beanName, beanDefinition);
				this.beanDefinitionNames.add(beanName);
				removeManualSingletonName(beanName);
			}
			this.frozenBeanDefinitionNames = null;
		}

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

源码跟到这里，整个流程基本清楚了，最终beanDefinition存到一个beanDefinitionMap中，key为Bean的名称，value为beanDefinition对象

private final Map<String, BeanDefinition> beanDefinitionMap = new ConcurrentHashMap<>(256);

beanDefinitionMap是一个ConcurrentHashMap，所以本质上Bean最终是被注册到一个Map中

上面我们说了很多次beanDefinition，最后注册到容器中的也是这个对象，那它到底是个啥对象?

A BeanDefinition describes a bean instance, which has property values,
constructor argument values, and further information supplied by
concrete implementations.

从注释中可以看出来BeanDefinition是一个用来描述带有属性值、构造方法、还有一些其他进一步信息的Bean实例（半调子英语，也不知道翻译的对不对~）

BeanDefinition是对Bean的抽象，因为配置文件中的Bean是多种多样的，BeanDefinition是对Bean的公共属性进行抽象，在BeanDefinition中很多属性是用来描述xml配置中bean的配置属性的

所以，下面来总结一下整个流程

• 注册xml配置文件到configLocations
• 调用refresh()进行整个Context的刷新，实际上就是整个Context的启动
• Bean的加载会读到配置文件，解析成DOM对象
• 将DOM对象转换成beanDefinition
• 将beanDefinition存入beanDefinitionMap，完成整个Bean的注册

没看明白的同学，可以对照这个流程再回头去看看，整个流程下来还是挺清晰的

注解方式下Bean的注册

前面我们讲到xml配置文件进行Bean的注册，xml配置是Spring早期常用的配置方式，现在基本上大部分场景上都推荐使用注解的方式，尤其是SpringBoot时代的来临，进一步推动了注解方式的全面使用，下面我们来看看注解方式下的Bean注册，还是从个简单的例子入手

@Component
public class AnnotionConfig {
    @Bean(name = "userBean")
    public User getUserBean(){
        User user = new User();
        user.setUserName("Lucy");
        return user;
    }
}

public class AnnotionBeanTest {
    public static void main(String[] args){
        AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext("org.kxg.springDemo");
        User user = (User) applicationContext.getBean("userBean");
        System.out.println(user.getUserName());
    }
}

这里用到AnnotationConfigApplicationContext，是另外一种容器的实现，传入一个包名，会自动扫描包下面的Spring注解，然后将其注册到容器中

public AnnotationConfigApplicationContext(String... basePackages) {
		this();
		//主要是scan方法完成bean的注册
		scan(basePackages);
		//又到了这个方法，有没有很熟悉~~~
		refresh();
	}

下面我们重点看一**解方式的Bean注册

public int scan(String... basePackages) {
		int beanCountAtScanStart = this.registry.getBeanDefinitionCount();
        //扫描包，进行Bean注册
		doScan(basePackages);

		// Register annotation config processors, if necessary.
		if (this.includeAnnotationConfig) {
			AnnotationConfigUtils.registerAnnotationConfigProcessors(this.registry);
		}

		return (this.registry.getBeanDefinitionCount() - beanCountAtScanStart);
	}

//ClassPathBeanDefinitionScanner
protected Set<BeanDefinitionHolder> doScan(String... basePackages) {
		Assert.notEmpty(basePackages, "At least one base package must be specified");
		Set<BeanDefinitionHolder> beanDefinitions = new LinkedHashSet<>();
		for (String basePackage : basePackages) {
		    //扫描包下打了注解的类，并将其转换成BeanDefinition
			Set<BeanDefinition> candidates = findCandidateComponents(basePackage);
			for (BeanDefinition candidate : candidates) {
				ScopeMetadata scopeMetadata = this.scopeMetadataResolver.resolveScopeMetadata(candidate);
				candidate.setScope(scopeMetadata.getScopeName());
				String beanName = this.beanNameGenerator.generateBeanName(candidate, this.registry);
				if (candidate instanceof AbstractBeanDefinition) {
					postProcessBeanDefinition((AbstractBeanDefinition) candidate, beanName);
				}
				if (candidate instanceof AnnotatedBeanDefinition) {
					AnnotationConfigUtils.processCommonDefinitionAnnotations((AnnotatedBeanDefinition) candidate);
				}
				if (checkCandidate(beanName, candidate)) {
					BeanDefinitionHolder definitionHolder = new BeanDefinitionHolder(candidate, beanName);
					definitionHolder =
							AnnotationConfigUtils.applyScopedProxyMode(scopeMetadata, definitionHolder, this.registry);
					beanDefinitions.add(definitionHolder);
					//进行BeanDefinition注册
					registerBeanDefinition(definitionHolder, this.registry);
				}
			}
		}
		return beanDefinitions;
	}

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);
			}
		}
	}

看到这个方法，有没有点眼熟的感觉

上面xml方式进行Bean注册也调用到这个方法了，所以后面的流程都是一样的，注解方式和xml配置方式从本质上来讲，并没有什么不同，只是Bean的描述不同而已，最终都会被解析成BeanDefinition，注册到容器中，至此整个Bean的注册流程就已经完了。

当然在整个过程中，忽略了很多细节，只看了主线流程。

读源码的时候，很容易陷入细节中，尤其是像Spring这样通用的框架，它为了通用性和扩展性，会把代码写的很“绕”，如果你过于关注细节很容易让自己陷入实现的细节中，一开始看源码的时候，建议不用太关注细节，把主线功能先看完，知道大概的原理后再逐个去击破~

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