前言

前面系列章节我们分析了Spring的IOC的启动流程，包括：容器创建，配置加载，配置解析，Bean注册等几个阶段，所以Bean注册其实就是把Bean的相关属性,依赖关系等封装成BeanDeafinition对象，然后注册到一个ConcurrentHashMap中。要注意的是这个BeanDeafinition只是对Bean的一个定义封装而已，并不是真正的Bean的实例，那Bean的实例是在什么时候创建的？有三种情况

如果是单利Bean，且lazy-init=false 急切初始(即时，立即，迫切，饥饿都是一个意思)的情况，在IOC容器启动之后就会根据BeanDeafinition对Bean进行实例化。
如果某个Bean还没被实例化，通过BeanFacotry.getBean 获取Bean的时候,就会触发Bean的实例化。
通过自动注入的时候，会触发Bean的实例化

虽然有多种情况会触Bean的发实例化，但是实例化的过程都是一样的，这篇文章就讲一下IOC启动之后，单利Bean的实例化过程吧。

refresh() 单利Bean实例化

代码还得回到 AbstractApplicationContext#refresh 方法中，在《IOC启动流程》一文中我们对容器刷新refresh方法已经有过研究

@Overridepublic void refresh() throws BeansException, IllegalStateException {synchronized (this.startupShutdownMonitor) {// Prepare this context for refreshing.//准备刷新工作 ,记录开始时间，初始化属性，校验配置文件，准备事件的存储SetprepareRefresh();// Tell the subclass to refresh the internal bean factory.//告诉子类，刷新Bean工厂，销毁旧beanFactory，创建新beanFactory，默认DefaultListableBeanFactory//从子容器的refreshBeanFactory方法中载入Bean的资源文件ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();...省略...// Instantiate all remaining (non-lazy-init) singletons.//实例化所有剩余的（非延迟初始化）单例的BeanfinishBeanFactoryInitialization(beanFactory);...省略...}

在 obtainFreshBeanFactory方法中完成了Bean的资源文件加载，Bean的解析，BeanDefinition的注册等一些列过程。最终把BeanDefinition保存到了DefaultListableBeanFactory管理的一个ConcurrentHashMap中。

我们接下来要探讨的是finishBeanFactoryInitialization; 方法，它里面实现了急切初始化的单例的Bean的创建流程。

/*** Finish the initialization of this context's bean factory,* initializing all remaining singleton beans.* 完成此上下文的 bean 工厂的初始化，初始化所有剩余的单例 bean。*/protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {// Initialize conversion service for this context.//用于类型转换的服务接口if (beanFactory.containsBean(CONVERSION_SERVICE_BEAN_NAME) &&beanFactory.isTypeMatch(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class)) {beanFactory.setConversionService(beanFactory.getBean(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class));}// Register a default embedded value resolver if no bean post-processor// (such as a PropertyPlaceholderConfigurer bean) registered any before:// at this point, primarily for resolution in annotation attribute values.if (!beanFactory.hasEmbeddedValueResolver()) {beanFactory.addEmbeddedValueResolver(strVal -> getEnvironment().resolvePlaceholders(strVal));}// Initialize LoadTimeWeaverAware beans early to allow for registering their transformers early.String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false);for (String weaverAwareName : weaverAwareNames) {getBean(weaverAwareName);}// Stop using the temporary ClassLoader for type matching.//停止使用临时类加载器进行类型匹配beanFactory.setTempClassLoader(null);// Allow for caching all bean definition metadata, not expecting further changes.//允许缓存所有 beandefinition 元数据,防止更改beanFactory.freezeConfiguration();// Instantiate all remaining (non-lazy-init) singletons.//[重要]入口在这 ，实例化 lazy-init= false的单利BeanbeanFactory.preInstantiateSingletons();}

这里初始化Bean Factory后，调用 beanFactory.preInstantiateSingletons 方法实例化 lazy-init= false的单利Bean, DefaultListableBeanFactory#preInstantiateSingletons源码如下：

@Overridepublic void preInstantiateSingletons() throws BeansException {if (logger.isTraceEnabled()) {logger.trace("Pre-instantiating singletons in " + this);}// Iterate over a copy to allow for init methods which in turn register new bean definitions.// While this may not be part of the regular factory bootstrap, it does otherwise work fine.//获取到所有Bean的名字List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);// Trigger initialization of all non-lazy singleton beans...for (String beanName : beanNames) {//获取Bean的BeanDefinition 定义对象RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);//如果不是抽象类，如果是单利，如果lazyInit为false,触发实例化逻辑if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {//判断是否是FactoryBeanif (isFactoryBean(beanName)) {//调用 getBean 方法得到FactoryBean工厂类实例。//【重要】getBean方法是实例化Bean的核心，后面我们要重点分析这个方法Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);//如果是FactoryBeanif (bean instanceof FactoryBean) {FactoryBean<?> factory = (FactoryBean<?>) bean;//是否要马上初始化boolean isEagerInit;if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {isEagerInit = AccessController.doPrivileged((PrivilegedAction<Boolean>) ((SmartFactoryBean<?>) factory)::isEagerInit,getAccessControlContext());}else {isEagerInit = (factory instanceof SmartFactoryBean &&((SmartFactoryBean<?>) factory).isEagerInit());}if (isEagerInit) {//【重要】 如果需要马上实例化，就调用getBean方法进行Bean的实例化getBean(beanName);}}}else {//【重要】 如果是普通的单利Bean，就调用getBean方法进行Bean的实例化getBean(beanName);}}}// Trigger post-initialization callback for all applicable beans...// 触发所有的Bean的生命周期方法的 init初始化方法.for (String beanName : beanNames) {//获取到单利Bean的实例Object singletonInstance = getSingleton(beanName);//判断Bean有没有实现 SmartInitializingSingleton接口，SmartInitializingSingleton是针对单利Bean的初始化接口if (singletonInstance instanceof SmartInitializingSingleton) {SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;if (System.getSecurityManager() != null) {AccessController.doPrivileged((PrivilegedAction<Object>) () -> {smartSingleton.afterSingletonsInstantiated();return null;}, getAccessControlContext());}else {//触发afterSingletonsInstantiated方法的调用smartSingleton.afterSingletonsInstantiated();}}}}

该方法中先是获取到IOC容器注册的所有的Bean的名字，然后循环拿到每个Bean的BeanDefinition定义对象，然后做了三个判断：不能是抽象，必须是单利，必须是急切初始化，如果满足条件再看是普通的Bean还是FactoryBean，最终都会调用 getBean(beanName); 方法，该方法调用 AbstractBeanFactory#doGetBean 该方法创建Bean。

创建好Bean之后，会判断Bean是否实现了 SmartInitializingSingleton 初始化接口,这个是针对单利Bean的初始化接口，然后调用afterSingletonsInstantiated进行Bean的初始化。

Bean实例化流程图

先上一个流程图，待会儿你可以根据这个图来看代码。

AbstractBeanFactory#doGetBean

protected <T> T doGetBean(String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly)throws BeansException {//得到Bean的名字,如果指定的是别名，将别名转换为规范的 Bean 名称String beanName = transformedBeanName(name);Object bean;// Eagerly check singleton cache for manually registered singletons.//从缓存中获取单利的Bean，对于单利只会创建一次，创建好之后就会缓存到map中//默认情况下Bean都是迫切加载的，在容器启动的过程中就实例化好缓存到Map中Object sharedInstance = getSingleton(beanName);if (sharedInstance != null && args == null) {if (logger.isTraceEnabled()) {//判断bean是否在  singletonsCurrentlyInCreation set集合中，该集合中存储的是正在创建的单利Beanif (isSingletonCurrentlyInCreation(beanName)) {logger.trace("Returning eagerly cached instance of singleton bean '" + beanName +"' that is not fully initialized yet - a consequence of a circular reference");}else {logger.trace("Returning cached instance of singleton bean '" + beanName + "'");}}//获取 Bean 实例的对象，主要是完成 FactoryBean 的相关处理//即：naem是是以  $ 开头 ，就返回FactoryBean工厂bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);}else {//代码走到这里，说明单利缓存的map中是没有正在创建的bean。// Fail if we're already creating this bean instance:// We're assumably within a circular reference.//如果缓存中没有单利Bean,但是缓存中有一个多利的Bean，目前正在创建，由于循环依赖问题，抛出异常if (isPrototypeCurrentlyInCreation(beanName)) {throw new BeanCurrentlyInCreationException(beanName);}// Check if bean definition exists in this factory.//得到容器IOC工厂BeanFactory parentBeanFactory = getParentBeanFactory();//通过beanName检查，如果容器中不包含这个Bean//如果容器中没有Bean,沿着继承体系交给父级去找if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {// Not found -> check parent.//得到Bean的原始名字，将别名解析为规范名称String nameToLookup = originalBeanName(name);if (parentBeanFactory instanceof AbstractBeanFactory) {//根据名字，类型，参数找Beanreturn ((AbstractBeanFactory) parentBeanFactory).doGetBean(nameToLookup, requiredType, args, typeCheckOnly);}else if (args != null) {//委托父工厂去找Bean ， 根据名字和参数// Delegation to parent with explicit args.return (T) parentBeanFactory.getBean(nameToLookup, args);}else if (requiredType != null) {// 无参数 -> 委托给标准的 getBean 方法，根据名字和类型// No args -> delegate to standard getBean method.return parentBeanFactory.getBean(nameToLookup, requiredType);}else {//根据名字找Beanreturn (T) parentBeanFactory.getBean(nameToLookup);}}//创建Bean是否要类型检查if (!typeCheckOnly) {//标记Bean被创建markBeanAsCreated(beanName);}try {//合并Bean，主要解决Bean继承时子类和父类的公共属性RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);checkMergedBeanDefinition(mbd, beanName, args);// Guarantee initialization of beans that the current bean depends on.//保证当前 bean 所依赖的 bean 的初始化。String[] dependsOn = mbd.getDependsOn();//如果当前Bean依赖了其他Beanif (dependsOn != null) {for (String dep : dependsOn) {//if (isDependent(beanName, dep)) {throw new BeanCreationException(mbd.getResourceDescription(), beanName,"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");}//为给定的 bean 注册一个依赖 beanregisterDependentBean(dep, beanName);try {//递归，从容器中获取依赖的BeangetBean(dep);}catch (NoSuchBeanDefinitionException ex) {throw new BeanCreationException(mbd.getResourceDescription(), beanName,"'" + beanName + "' depends on missing bean '" + dep + "'", ex);}}}// Create bean instance.//创建单利Beanif (mbd.isSingleton()) {//根据Bean名字获取单利Bean，这里是真正创建Bean的地方，通过ObjectFactory创建//这里是匿名内部类sharedInstance = getSingleton(beanName, () -> {try {//创建Beanreturn createBean(beanName, mbd, args);}catch (BeansException ex) {// Explicitly remove instance from singleton cache: It might have been put there// eagerly by the creation process, to allow for circular reference resolution.// Also remove any beans that received a temporary reference to the bean.//销毁实例，从缓存的map中清除单利BeandestroySingleton(beanName);throw ex;}});//返回bean实例，主要处理FactoryBean相关bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);}//如果是多利else if (mbd.isPrototype()) {// It's a prototype -> create a new instance.//如果是prototype模式，每次都会创建新的实例Object prototypeInstance = null;try {beforePrototypeCreation(beanName);//创建指定 Bean 对象实例 ,如果是prototype模式，每次都会创建新的实例prototypeInstance = createBean(beanName, mbd, args);}finally {//默认实现将原型标记为不再创建afterPrototypeCreation(beanName);}//返回bean实例，主要处理FactoryBean相关bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);}else {String scopeName = mbd.getScope();//如果不是单利，不是多利，Bean没有 scope属性，则不合法if (!StringUtils.hasLength(scopeName)) {throw new IllegalStateException("No scope name defined for bean ´" + beanName + "'");}Scope scope = this.scopes.get(scopeName);if (scope == null) {throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");}try {Object scopedInstance = scope.get(beanName, () -> {beforePrototypeCreation(beanName);try {return createBean(beanName, mbd, args);}finally {afterPrototypeCreation(beanName);}});bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);}catch (IllegalStateException ex) {throw new BeanCreationException(beanName,"Scope '" + scopeName + "' is not active for the current thread; consider " +"defining a scoped proxy for this bean if you intend to refer to it from a singleton",ex);}}}catch (BeansException ex) {cleanupAfterBeanCreationFailure(beanName);throw ex;}}//检查所需类型是否与实际 bean 实例的类型匹配。// Check if required type matches the type of the actual bean instance.if (requiredType != null && !requiredType.isInstance(bean)) {try {T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);if (convertedBean == null) {throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());}return convertedBean;}catch (TypeMismatchException ex) {if (logger.isTraceEnabled()) {logger.trace("Failed to convert bean '" + name + "' to required type '" +ClassUtils.getQualifiedName(requiredType) + "'", ex);}throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());}}return (T) bean;}

上面这个方法中代码量有点多，这里做一下总结

transformedBeanName(name)：转换Bean的名字，如果传入的name是别名需要找到真实的Bean的名字，如果传入的name是FactoryBean，比如“&bean” ,要去掉 “&”。
getSingleton(name)：从缓存中查找Bean（ConcurrentHashMap中），单利Bean只会在容器中创建一次，后续都从缓存中加载,如果缓存中没有就就会进行创建。默认情况下Bean都是迫切创建的，即在容器启动的过程中就会根据BeaDefinition创建好Bean的实例存储到缓存中。
在创建单利Bean的时候会存在依赖注入的情况，为了解决循环依赖（A依赖B，B依赖A），在创建Bean的时候会提前将ObjectFactory 加入缓存，如果下个Bean创建时需要依赖上一个Bean就会直接使用ObejctFactory(后面讲循环依赖)
createBean ：Bean的实例化，从BeanDefinition的缓存中获取当前Bean的BeanDefinition定义对象，根据BeanDefinition进行Bean的创建。如果是通过工厂Bea 方式实例化，则需要得到工厂Bean中的factory-method方法返回的的Bean的实例，通过getObjectForBeanInstance 处理的。
isPrototypeCurrentlyInCreation：Prototype依赖检查，单利情况下才会解决循环依赖，如果A中依赖了B,B中依赖了A，在创建依赖的时候就会产生循环依赖。
如果是多利Bean直接走createBean创建一个新的Bean

DefaultSingletonBeanRegistry#getSingleton(String, ObjectFactory)

对于 getSingleton从缓存中获取单利Bean的方法，这里有两个重载方法第一个方法是在 Object sharedInstance = getSingleton(beanName);
时候调用，源码如下：

//allowEarlyReference ：是否创建早期应用，主要用来解决循环依赖@Nullableprotected Object getSingleton(String beanName, boolean allowEarlyReference) {// Quick check for existing instance without full singleton lock//从Map中 singletonObjects = new ConcurrentHashMap<>(256); 获取单利Bean//singletonObject缓存中是否有BeanObject singletonObject = this.singletonObjects.get(beanName);//如果singletonObjects中没有Bean，但是 Set<String> singletonsCurrentlyInCreation中有Bean，代表Bean正在创建if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {//从早期单例对象的缓存 earlySingletonObjects 中获取singletonObject = this.earlySingletonObjects.get(beanName);if (singletonObject == null && allowEarlyReference) {synchronized (this.singletonObjects) {// Consistent creation of early reference within full singleton locksingletonObject = this.singletonObjects.get(beanName);if (singletonObject == null) {singletonObject = this.earlySingletonObjects.get(beanName);if (singletonObject == null) {//获取ObjectFactory ， 对象创建工厂ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);if (singletonFactory != null) {//获取对象实例singletonObject = singletonFactory.getObject();//把对象存储到缓存中this.earlySingletonObjects.put(beanName, singletonObject);this.singletonFactories.remove(beanName);}}}}}}return singletonObject;}

这个代码看着就晕，先理一下这几个缓存

singletonObjects:保存beanName和Bean的关系
singletonFactories：保存beanName和Bean的工厂之间的关系，ObjectFactory
earlySingletonObjects：存放beanName和Bean组件的关系，这个Map中的Bean代表正在创建中，用来检测循环依赖用的
registeredSingletons：保存所有已经注册成功的Bean

这个方法首先从 singletonObjects 获取Bean，如果没有就从earlySingletonObjects中获取正在创建的Bean，如果也没有就从singletonFactories中得到ObjectFactory，通过ObjectFactory.getObject 来创建Bean,并且放到earlySingletonObjects中。且把ObjectFactory从 singletonFactories移除。

如果上面 getSingleton 返回实例为空，就会走到 if (mbd.isSingleton()) 然后调用另外一个 getSingleton方法，这个方法需要传入ObjectFactory，源码如下：

public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {Assert.notNull(beanName, "Bean name must not be null");synchronized (this.singletonObjects) {//从singletonObjects获取BeanObject singletonObject = this.singletonObjects.get(beanName);if (singletonObject == null) {...省略...//把 beanName 添加到 singletonsCurrentlyInCreation 中，用来做循环依赖检查beforeSingletonCreation(beanName);try {//通过工厂获取BeansingletonObject = singletonFactory.getObject();newSingleton = true;}...省略...if (newSingleton) {//把Bean添加到缓存addSingleton(beanName, singletonObject);}}return singletonObject;}}//注册到registeredSingletonsprotected void addSingleton(String beanName, Object singletonObject) {synchronized (this.singletonObjects) {this.singletonObjects.put(beanName, singletonObject);this.singletonFactories.remove(beanName);this.earlySingletonObjects.remove(beanName);this.registeredSingletons.add(beanName);}}

这里先从singletonObjects获取Bean，如果没有，就调用ObjectFactory.getObject方法创建Bean的实例，然后调用addSingletion 把Bean添加到 singletonObjects ，和registeredSingletons中。且从 singletonFactories和 earlySingletonObjects移除该Bean。

上面讲的都是走缓存获取Bean，当缓存没有Bean就会走 ObjectFactory.getObject 创建Bean。

AbstractAutowireCapableBeanFactory#createBean

创建Bean的工作是交给 ObjectFactory#createBean方法完成，代码中采用了匿名内部类的方式实现。而又把具体的实例化过程委派给 AbstractAutowireCapableBeanFactory#createBean 方法来完成

下面是 AbstractAutowireCapableBeanFactory#createBean 创建Bean方法源码

@Overrideprotected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)throws BeanCreationException {...省略...try {//创建Bean【重要】Object beanInstance = doCreateBean(beanName, mbdToUse, args);if (logger.isTraceEnabled()) {logger.trace("Finished creating instance of bean '" + beanName + "'");}return beanInstance;}...省略...}

下面是 AbstractAutowireCapableBeanFactory#doCreateBean 方法源码

protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)throws BeanCreationException {// Instantiate the bean.//对Bean的装饰BeanWrapper instanceWrapper = null;if (mbd.isSingleton()) {//如果是单利，清除缓存instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);}if (instanceWrapper == null) {//创建Bean方法【重要】instanceWrapper = createBeanInstance(beanName, mbd, args);}Object bean = instanceWrapper.getWrappedInstance();Class<?> beanType = instanceWrapper.getWrappedClass();if (beanType != NullBean.class) {mbd.resolvedTargetType = beanType;}//允许后处理器修改合并的 bean 定义。// Allow post-processors to modify the merged bean definition.synchronized (mbd.postProcessingLock) {if (!mbd.postProcessed) {try {//调用后置处理器applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);}catch (Throwable ex) {throw new BeanCreationException(mbd.getResourceDescription(), beanName,"Post-processing of merged bean definition failed", ex);}mbd.postProcessed = true;}}//急切地缓存单例，以便能够解析循环引用//是否允许提前曝光：单利&&允许循环依赖&&Bean在创建中// Eagerly cache singletons to be able to resolve circular references// even when triggered by lifecycle interfaces like BeanFactoryAware.boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&isSingletonCurrentlyInCreation(beanName));if (earlySingletonExposure) {if (logger.isTraceEnabled()) {logger.trace("Eagerly caching bean '" + beanName +"' to allow for resolving potential circular references");}//匿名内部类，防止循环引用，尽早持有对象的引用//为了解决循环依赖，在Bean初始化完成前把ObjectFactory放入工厂addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));}//初始化Bean，处理依赖注入// Initialize the bean instance.Object exposedObject = bean;try {//填充Bean ， 主要是赋值属性，使用 bean 定义中的属性值填充给定 BeanWrapper 中的 bean 实例。//如果依赖其他Bean，会进行递归初始化依赖的BeanpopulateBean(beanName, mbd, instanceWrapper);//初始化Bean，如果Bean实现了InitializingBean接口，就会在这里调用afterPropertiesSet方法//同时会调用init-method方法exposedObject = initializeBean(beanName, exposedObject, mbd);}catch (Throwable ex) {if (ex instanceof BeanCreationException && beanName.equals(((BeanCreationException) ex).getBeanName())) {throw (BeanCreationException) ex;}else {throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Initialization of bean failed", ex);}}if (earlySingletonExposure) {//获取已经注册的单利的BeanObject earlySingletonReference = getSingleton(beanName, false);if (earlySingletonReference != null) {if (exposedObject == bean) {exposedObject = earlySingletonReference;}else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {String[] dependentBeans = getDependentBeans(beanName);Set<String> actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);for (String dependentBean : dependentBeans) {if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {actualDependentBeans.add(dependentBean);}}if (!actualDependentBeans.isEmpty()) {throw new BeanCurrentlyInCreationException(beanName,"Bean with name '" + beanName + "' has been injected into other beans [" +StringUtils.collectionToCommaDelimitedString(actualDependentBeans) +"] in its raw version as part of a circular reference, but has eventually been " +"wrapped. This means that said other beans do not use the final version of the " +"bean. This is often the result of over-eager type matching - consider using " +"'getBeanNamesForType' with the 'allowEagerInit' flag turned off, for example.");}}}}// Register bean as disposable.try {//注册一个 DisposableBean 实现销毁回调//注册其 DisposableBean 接口和/或在工厂关闭时调用的给定销毁方法registerDisposableBeanIfNecessary(beanName, bean, mbd);}catch (BeanDefinitionValidationException ex) {throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);}return exposedObject;}

总结一下方法的步骤

如果是单利，清除缓存
createBeanInstance：调用了createBeanInstance方法创建实例，转换成BeanWrapper
createBeanInstance ，创建BeanDean的实例
addSingletonFactory：添加ObjectFactory到缓存中
依赖处理：当A中有属性B,B中有属性A的时候，实例化A时发现依赖了B,此时会实例化B，而实例化B时发现依赖了A，此时不会直接实例化A，而是通过缓存中的ObjectFactory来创建，目的是解决循环依赖。
populateBean，对Bean的属性进行依赖注入.
initializeBean，对Bean进行初始化，如果Bean实现了InitializingBean接口，就会在这里调用afterPropertiesSet方法
registerDisposableBeanIfNecessary ：销毁方法支持，如果Bean实现了DisposableBean接口，通过 DefaultSingletonBeanRegistry 把Bean包装成一个DisposableBeanAdapter，注册到一个disposableBeans = new LinkedHashMap 中。当容器关闭如：applicaiton.close() ,就会触发 AbstractApplicationContext#destroyBeans方法遍历该集合中的所有Bean，执行Bean的destroy方法。

下面是 AbstractAutowireCapableBeanFactory#createBeanInstance 创建实例的代码，该方法中使用了策略模式，

protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {// Make sure bean class is actually resolved at this point.//获取到Bean的classClass<?> beanClass = resolveBeanClass(mbd, beanName);Supplier<?> instanceSupplier = mbd.getInstanceSupplier();if (instanceSupplier != null) {return obtainFromSupplier(instanceSupplier, beanName);}//如果BeanDefinition中有 工厂方法，通过工厂方式实例化if (mbd.getFactoryMethodName() != null)  {//通过工厂方法实例化Beanreturn instantiateUsingFactoryMethod(beanName, mbd, args);}//使用自动装配功能实例化Bean// Shortcut when re-creating the same bean...boolean resolved = false;boolean autowireNecessary = false;if (args == null) {synchronized (mbd.constructorArgumentLock) {if (mbd.resolvedConstructorOrFactoryMethod != null) {resolved = true;autowireNecessary = mbd.constructorArgumentsResolved;}}}if (resolved) {if (autowireNecessary) {//如果配置了自动装配属性，通过自动装配方式实例化//构造器函数主动注入return autowireConstructor(beanName, mbd, null, null);}else {//使用默认的无参构造方法实例化return instantiateBean(beanName, mbd);}}...省略...//确定构造器，用来创建实例//需要根据参数解析构造函数// Candidate constructors for autowiring?Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);if (ctors != null || mbd.getResolvedAutowireMode() == AUTOWIRE_CONSTRUCTOR ||mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {return autowireConstructor(beanName, mbd, ctors, args);}...省略...//实例化Bean[重要]//使用默认的无参构造方法实例化// No special handling: simply use no-arg constructor.return instantiateBean(beanName, mbd);}

上面代码体现出了几种实例化方式，通过工厂方法实例化，或者通过自动装配特性实例化Bean,这需要调用对应的构造器完成，但是我们通常使用的是默认无参构造，这就要用到了反射机制来实例化了，见： instantiateBean 方法。下面是 AbstractAutowireCapableBeanFactory#instantiateBean 方法源码如下：

protected BeanWrapper instantiateBean(String beanName, RootBeanDefinition mbd) {try {Object beanInstance;if (System.getSecurityManager() != null) {beanInstance = AccessController.doPrivileged((PrivilegedAction<Object>) () -> getInstantiationStrategy().instantiate(mbd, beanName, this),getAccessControlContext());}else {//重要的代码在这里，通过 InstantiationStrategy 实例化BeanbeanInstance = getInstantiationStrategy().instantiate(mbd, beanName, this);}BeanWrapper bw = new BeanWrapperImpl(beanInstance);initBeanWrapper(bw);return bw;}catch (Throwable ex) {throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Instantiation of bean failed", ex);}}

方法中通过 InstantiationStrategy 实例化策略来实例化Bean,单利的Bean使用的是 SimpleInstantiationStrategy策略来实例化。得到实例对象后包装到BeanWrapper对象中。

SimpleInstantiationStrategy#instantiate

下面是 SimpleInstantiationStrategy#instantiate() 方法源码

@Overridepublic Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner) {// Don't override the class with CGLIB if no overrides.//如果Bean定义没有覆盖方法，不要用 CGLIB 覆盖类。if (!bd.hasMethodOverrides()) {Constructor<?> constructorToUse;synchronized (bd.constructorArgumentLock) {constructorToUse = (Constructor<?>) bd.resolvedConstructorOrFactoryMethod;if (constructorToUse == null) {//反射，得到Bean的类型final Class<?> clazz = bd.getBeanClass();//接口没法实例化if (clazz.isInterface()) {throw new BeanInstantiationException(clazz, "Specified class is an interface");}try {if (System.getSecurityManager() != null) {//匿名内部类，得到构造器constructorToUse = AccessController.doPrivileged((PrivilegedExceptionAction<Constructor<?>>) clazz::getDeclaredConstructor);}else {//反射，获取到构造器constructorToUse = clazz.getDeclaredConstructor();}bd.resolvedConstructorOrFactoryMethod = constructorToUse;}catch (Throwable ex) {throw new BeanInstantiationException(clazz, "No default constructor found", ex);}}}//使用Bean工具,通过反射创建Bean，通过构造方法.newInstance(arg)创建Beanreturn BeanUtils.instantiateClass(constructorToUse);}else {// Must generate CGLIB subclass.//使用CGLIB 实例化Bean//调用子类 CglibSubclassingInstantiationStrategy.instantiateWithMethodInjection 来实例化return instantiateWithMethodInjection(bd, beanName, owner);}}public static <T> T instantiateClass(Constructor<T> ctor, Object... args) throws BeanInstantiationException {Assert.notNull(ctor, "Constructor must not be null");try {ReflectionUtils.makeAccessible(ctor);//通过  ctor.newInstance 构造器实例化Beanreturn (KotlinDetector.isKotlinType(ctor.getDeclaringClass()) ?KotlinDelegate.instantiateClass(ctor, args) : ctor.newInstance(args));}

方法中做了一个判断如果Bean有方法被覆盖，就使用JDK反射来创建Bean的实例，否则使用CGLIB来实例化。 CGLIB调用的是子类 CglibSubclassingInstantiationStrategy 来完成的，如果一个类没有接口，就只能使用CGLIB

CglibSubclassingInstantiationStrategy#instantiateWithMethodInjection

下面是 CglibSubclassingInstantiationStrategy#instantiateWithMethodInjection CGLIB创建对象的代码

@Overrideprotected Object instantiateWithMethodInjection(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner,@Nullable Constructor<?> ctor, Object... args) {// Must generate CGLIB subclass...return new CglibSubclassCreator(bd, owner).instantiate(ctor, args);}public Object instantiate(@Nullable Constructor<?> ctor, Object... args) {//【重要】创建代理子类，返回classClass<?> subclass = createEnhancedSubclass(this.beanDefinition);Object instance;if (ctor == null) {//通过反射把class变成对象instance = BeanUtils.instantiateClass(subclass);}else {try {//代理子类构造器Constructor<?> enhancedSubclassConstructor = subclass.getConstructor(ctor.getParameterTypes());//通过构造器创建实例instance = enhancedSubclassConstructor.newInstance(args);}catch (Exception ex) {throw new BeanInstantiationException(this.beanDefinition.getBeanClass(),"Failed to invoke constructor for CGLIB enhanced subclass [" + subclass.getName() + "]", ex);}}// SPR-10785: set callbacks directly on the instance instead of in the// enhanced class (via the Enhancer) in order to avoid memory leaks.Factory factory = (Factory) instance;factory.setCallbacks(new Callback[] {NoOp.INSTANCE,//查找覆盖方法拦截器//CGLIB MethodInterceptor 覆盖方法，将它们替换为返回在容器中查找的 bean 的实现。new LookupOverrideMethodInterceptor(this.beanDefinition, this.owner),//CGLIB MethodInterceptor 来覆盖方法，将它们替换为对通用 MethodReplacer 的调用new ReplaceOverrideMethodInterceptor(this.beanDefinition, this.owner)});return instance;}private Class<?> createEnhancedSubclass(RootBeanDefinition beanDefinition) {Enhancer enhancer = new Enhancer();//增强类(代理类)将会把当前类作为父类enhancer.setSuperclass(beanDefinition.getBeanClass());enhancer.setNamingPolicy(SpringNamingPolicy.INSTANCE);if (this.owner instanceof ConfigurableBeanFactory) {ClassLoader cl = ((ConfigurableBeanFactory) this.owner).getBeanClassLoader();enhancer.setStrategy(new ClassLoaderAwareGeneratorStrategy(cl));}enhancer.setCallbackFilter(new MethodOverrideCallbackFilter(beanDefinition));enhancer.setCallbackTypes(CALLBACK_TYPES);return enhancer.createClass();}

上面就是CGLIB创建代理的代码，对于JDK动态代理而言被代理类必须有接口，JDK动态代理会为被代理类生成一个代理类和被代理类实现相同的接口，从而实现相同的方法，只不过在代理类的方法中可以持有并调用被代理类，从而实现对被代理类的方法做了增强的目的。但是如果被代理类么有接口，那就必须切换成CGLIB代理方式。

这种方式生成的代理类会把被代理类作为父类，对被代理类的方法进行复写和增强。如果没有接触过CGLIB动态代理这一块的可能看起来就比较晕菜了。

总结

到这里，我已经把IOC容器创建Bean的具体过程分析完毕，下面做个小结

我们讨论了Bean的实例化时机，IOC启动之后对单利Bean实例化，手动getBean时触发实例化，自动注入时触发实例化
Bean的实例化是通过AbstractBeanFactory#doGetBean方法完成的。如果是单利的Bean,从单利Bean缓存的map中查找（ConcurrentHashMap中），找到直接返回，保证了单利，如果缓存中没有就就会进行创建。如果是多利Bean则每次都会创建一个新的Bean
创建Bean的过程主要是通过Bean的BeanDeifnition对象，使用JDK反射，或者CGLIB方式创建Bean。
当Bean创建成功之后会触发Bean的依赖注入，以及Bean的初始化方法。
最后注册了DisposableBean 来支持Bean的销毁方法调用

好了，本篇文章就到这吧，喜欢的话就给个好评吧，如果好评上500，我就是头发掉光，也出下章!!!

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